The invention relates to a method for measuring and evaluating
energy consumption in electrical drive according to the preamble of claim 1, said
electrical drive including a control unit and an alternating current motor. The
invention also relates to an arrangement according to claim 7 for realizing the
method.
At present, machines, devices and processes in general,
particularly in the industrial sector, are used by electrical drives, where an essential
element is an electric motor. Electrical drive is composed of a suitable electricity
supply circuit, an electric motor and a control unit suited for controlling and/or
regulating said motor. The machine or the like functions as the load in the electrical
drive. The most common electric motor used in industrial processes is an alternating
current motor, AC motor, particularly an induction motor.
Often the control unit employed in an AC motor is a frequency
converter owing to the advantages achieved thereby. As an alternative, the electric
motor can be realized as a direct current motor, DC motor, that is regulated by
a suitable control unit. Another alternative is that the electric motor is not particularly
regulated, but a separate regulation arrangement is made between the electric motor
and the machine, or the regulation is realized as a procedure separate from the
electrical drive in connection with the actual process.
Electrical drive is designed according to the process,
machine or device that should be used by the electrical drive. The electric motor
can be considered as a source of moment. The motor must be able to generate a certain
moment, it must resist an overload of the process, but the motor must not be thermally
overloaded.
Most advantageously an electrical drive provided with a
frequency converter is operated by alternating current, AC. This kind of electrical
drive comprises an electricity supply circuit, a frequency converter and an AC motor,
advantageously an induction motor. A frequency converter includes a rectifier, a
direct voltage intermediate circuit and an inverter. Through electricity supply,
electric energy is obtained from an electric mains or a corresponding source of
electricity, and the electric motor is run by means of said electric energy in order
to drive an actuator, such as a machine or a device, connected to its axis. By means
of the frequency converter, the electric motor is regulated, for instance as regards
its speed of rotation and/or momentum, for driving the working machine and often
also the process connected to said actuator.
The load in the electrical drive can be a machine running
the process. Among these kind of machines, there are for example various pumps for
transfer processes of liquids or the like, fans for air conditioning applications,
various working machines, such as conveyors, feeders and machine tools, whereby
the material belonging in the process is treated by transporting or by mechanically
converting. At present, a common feature for nearly all processes is that they must
be regulated and controlled.
There are many simple control methods and arrangements,
such as throttling, bypass control and onoff control. The constructing of such
arrangements is usually easy, and the investment in the control equipment may often
look cost effective. However, simple control arrangements have remarkable drawbacks.
An optimal process capacity, which ensures the best quality of the process, is extremely
difficult to achieve with simple control arrangements. Moreover, it is pointed out
that an increase in the production capacity generally requires reconstruction of
the whole production process. Apart from this, with each direct online startup,
there is a risk of electrical and/or mechanical damage. An extremely remarkable
drawback with simple control arrangements is that they consume a lot of energy,
and energy is particularly consumed in various losses.
Electrical drives provided with a frequency converter are
becoming more and more common in controlling industrial processes. Particularly
such machines and devices where the speed control (speed of rotation) is an essential
factor, have obtained electrical drive provided with a frequency converter. The
employed frequency converter drives an AC motor, generally a squirrel cage motor,
which means that mechanical control systems are no longer necessary. The speed of
an electric motor is regulated by a frequency converter that converts the frequency
of the voltage fed into the motor. The frequency converter itself is controlled
with suitable electric control signals. Moreover, it is pointed out that through
the frequency converter, an optimal quantity of electric power is fed in the electric
motor and the process, and energy losses are thus avoided.
Four of the most common variable speed drives in the industrial
sector are: mechanical variable speed control, hydraulic coupling, DC drive and
frequency converter (AC drive). Mechanical variable speed control usually uses belt
drives, and the controlling takes place by moving conical pulleys manually or with
positioning motors. Hydraulic coupling applies the turbine principle, where the
volume of oil in the coupling is changed, so that the speed difference between the
driving and driven shafts changes. The oil amount is controlled with pumps and valves.
In the DC drive, a DC converter changes the motor supply voltage fed to the DC motor
in order to change the rotating speed. In the motor, a mechanical inverter, a commutator,
changes direct current to alternating current. In the frequency converter, a standard
squirrel cage motor is used. The speed of the motor is regulated by a frequency
converter that changes the frequency of the voltage fed into the motor, as was already
maintained above.
In mechanical and hydraulic variable speed drives, the
control device is located between the electric motor and the working machine. This
means that energy losses are created in the control device. Also the maintenance
of the device is often difficult. In electrical drive provided with a frequency
converter, i.e. electrical variable speed drive, VSD, all control systems are situated
in an electric equipment room, and only the driving motor is in the process area.
In many processes the production volumes change. Changing production volumes by
mechanical means is usually very inefficient. With electrical drive provided with
a frequency converter (i.e. electrical VSD), production volumes can be changed by
changing the motor speed. This saves a lot of energy, particularly in pump and fan
applications, because the shaft power of the motor is proportional to the flow rate
to the power of three. Electrical VSD also has other advantages in comparison with
electrical drives using conventional control methods and devices. We can use pumping
as an example. In traditional methods, there is always a mechanical part and an
electric part. In the electric part of throttling, there are needed fuses, contactors
and reactors, and valves on the mechanical side. Onoff control needs the same electrical
components as throttling, as well as pressure tanks on the mechanical side. Mechanical
parts are not needed in electrical drive provided with a frequency converter, because
all control procedures are realized on the electrical side.
Many surveys and experiments have proved that with electrical
VSD, there are easily achieved 50% energy savings in comparison with other conventional
electrical drives and regulation systems. This means for example that if the power
requirements of a constant speed motor and throttling control system would be 0.7
kW, with electrical VSD they would be 0.37 kW. If a pump would be used 4,000 hours
per year, throttling control would need 3,000 kWh, and frequency converter drive
would need 1,500 kWh energy per year. In order to calculate the savings, the energy
consumption must be multiplied by the price of energy, which fluctuates depending
on the availability and supplier of electric energy.
When comparing the advantages on one hand of a conventional
constant speed electrical drive provided with a possible control arrangement connected
therein, and on the other hand of electrical drive provided with a frequency converter,
maintenance and upkeep expenses must also be taken into account. It has been estimated
that in the maintenance of throttling control, there is consumed even ten times
as much money as in the maintenance of a frequency converter drive. In many cases,
a frequency converter needs hardly any maintenance at all, whereas the mechanical
devices of conventional control systems require continuous maintenance.
Electrical drives provided with a frequency converter bring
savings in energy in most applications, for example in industrial processes, which
as such is a known an empirically proved fact. The problem is how to measure and/or
evaluate energy consumption and particularly the achieved energy savings in comparison
with other known electrical drives and control systems, particularly with electric
motor drives operated at a constant speed and with process regulations connected
thereto.
For a man skilled in the art, a known solution is to provide
an existing, traditional electric motor and electrical drive with suitable measurement
sensors, to measure power consumption at suitable intervals, and define energy consumptions
on the basis of this. In that case there is measured the power and voltage fed in
the electrical drive, and the power and energy consumption are defined according
to this principle. On the other hand, as regards an electrical drive provided with
a control unit, such as a frequency converter, it is possible to extract the data
of the electric motor state, and the power and energy consumption can be defined
on the basis thereof. A drawback of the measurement sensor arrangement is that the
targets of measurement, i.e. particularly electric motors, must be provided with
sensors. A drawback both in the measurement sensor arrangement and in the processing
of measurement data extracted directly from the control unit is that both the sensors
and the frequency converters must be connected by measurement wires to the measurement
device always when measurements are carried out. Another drawback is that in this
way, there is not obtained any direct comparison data for the energy consumption
of various electrical drives, particularly for electric motors and working machines
and the like, provided with various control arrangements. In practice, in one and
the same target, the energy consumption of the electrical drive connected in the
process is measured prior to replacing the drive (or when planning a replacement),
and the energy consumption of a new electrical drive provided with a frequency converter
is measured after replacing, i.e. after installing the electrical drive (or there
are made preliminary calculations of its energy consumption), and the results are
compared for proving that energy has been saved (or for estimating possible savings).
We refer to e.g. Japanese patent application JP2003228621A (or EP1085636A2).
The following publications generally describe the state
of the art: US 3,998,093, US 2003/0057904A1 and WO 86/05887.
The patent publication US 3,998,093 introduces a system
for monitoring and controlling the consumption of energy, where the actual consumed
energy is compared with an ideal or desired energy consumption, and both are displayed.
The system is particularly used for monitoring the consumption of electric energy
or gas in an industrial plant. The comparison between actual and ideal energy consumption
is displayed for the working staff as visual information on a suitable display.
When necessary, an alarm can be given, in case a predetermined threshold value between
the actual and ideal energy consumption is surpassed. The object of this system
is mainly to save energy, and particularly to monitor the ratio between the actual
and the ideal energy consumption in an easily readable way.
Patent abstract of Japan vol. 2003 no. 05,12 May 2003 and
Japanese publication JP 2003 006288A discloses a method and a system for calculation
of energy savings where an actual energy consumption of an actual fan drive/plant
is compared to an ideal energy consumption based on a fan drive/plant stimulation
model. The objective is to provide customers the energysaving effect resulting
from the operation of pumpdriven customer equipment by inverters in comparison
with driving with the commercial power source.
The patent application US 2003/0057904A1 discloses a control
system utilizing feedback, which system is applied in an electric motor running
a rotary machine, such as a pump or a fan. This control system makes use of a predetermined
specific curve of for example the capacity of a rotary pump and the motor intake
power for realizing feedback control. This procedure replaces the earlier used pressure
sensors or pressure difference sensors in the control system. The object of the
invention is to optimize energy consumption and to reduce noise.
The international patent application WO 86/05887 discloses
a power meter which is able to display the power consumed and the cost of power
consumed. The device provides a continuous measurement of the electrical power consumed,
which gives a more accurate measurement than measurement at discrete intervals of
time.
The object of the present invention is to eliminate the
drawbacks connected to the measurement and/or evaluation of the above described
energy consumption. Another object of the invention is to realize a new method and
arrangement for evaluating consumption and particularly savings of electric energy
between conventional electrical drive and an electrical drive provided with a control
unit in general and a frequency converter in particular.
The method according to the invention for measuring and
evaluating energy savings achieved by an electrical drive, where said electrical
drive comprises a control unit and an AC motor, and in which method data of the
control unit related to the state of the AC motor, said state data being the power
and frequency, are read and saved, is characterized by what is set forth in claim
1. The measurement and evaluation arrangement of an electrical drive according to
the invention, where said electrical drive comprises a control unit and an AC motor,
said control unit including a measurement unit for measuring state data of the AC
motor, said state data being power and frequency, is characterized by what is set
forth in claim 7. The dependent claims describe preferred embodiments of the invention.
The method according to the invention for measuring and
evaluating energy savings achieved by an electrical drive is characterized in that
 the data from the control unit related to the state of the AC motor is utilized
so that in one or several predetermined measurement time periods, at predetermined
measurement intervals, i.e. sample intervals, at the measurement points, said state
data is read, and the measurement values of power and frequency are saved in the
measurement register,
 the energy consumption of the electrical drive is calculated in said one or
several predeterimined measurement time periods,
 the power value of a virtual AC motor that is fed by constantfrequency input
current is defined on the basis of the power and frequency measurement values saved
in the measurement register,
 the energy consumption of said virtual AC motor are calculated in said one or
several predetermined measurement timeperiods, based on the power value defined
in the preceding step,
 the energy consumption of an alternative control arrangement is defined in said
one or several predetermined measurement time periods,
 the calculated energy consumptions of the virtual AC motor and the alternative
control arrangement are added together, and
 the energy consumption of the electrical drive is compared with the added total
energy consumption of the virtual AC motor and the alternative control arrangement
in order to evaluate the energy savings achieved by the electrical drive in the
measurement time period in question.
In an electrical drive provided with a control unit, such
as frequency converter, there is normally measured the state data of an AC motor,
preferably an induction motor, particularly feed currents and feed voltages as well
as frequency, for regulating the motor. In particular, the method according to the
invention utilizes the data fed in the control unit as regards the state of the
AC motor, i.e. its state data; in the case of a threephase motor and frequency
converter, there is especially utilized the voltage and current data of the phases,
as well as the frequency by which the voltage/current of the phases is changed.
In a method according to the invention, also the energy
consumption of an alternative control system with respect to the control unit, such
as a frequency converter, is taken into account. Thus the power losses of one or
several alternative control systems are modeled, and on the basis of said modeling,
the energy consumption of the control system is defined in the measurement period,
and the energy consumption of the control system is added to the energy consumption
of an electric motor fed by constantfrequency input power in one or several predetermined
periods of time. Among these alternative control systems are for example throttling,
onoff control and slip control.
In a preferred embodiment of the invention, the number
of measurement points in a measurement period is defined on the basis of the accepted
maximum error of the power value. The purpose of this is that the numerical integration
of energy consumption is made sufficiently accurate at the same time as the number
of the measurement points is optimized. Energy consumption can be calculated by
various numeric methods that also include an error estimation procedure or formula
for estimating the maximum error. For instance in a preferred embodiment of the
invention, energy consumption can be calculated on the basis of the measurement
data collected from the measurement register by using a trapezoid formula, in which
case the optimal number of the measurement points can be defined, on certain conditions,
on the basis of the error clause of the trapezoid formula.
In another preferred embodiment of the invention, the measurement
register is converted to a graphical powerfrequency pattern, advantageously a curve,
by means of which the power value of a virtual electric motor fed by constantfrequency
input current is defined. In that case the virtual electric motor is an AC motor
that is run at constant speed. The power value of the virtual electric motor is
defined by this method in a case where the frequency control range of a control
unit, such as a frequency converter, does not during the measurement period reach
said constant frequency, which is usually the frequency of the electric mains. In
case the frequency control range of the frequency converter reaches said constant
frequency, the power value of the virtual electric motor is obtained directly from
the measurement data and the measurement register.
In a preferred embodiment of the invention, the above presented
graphical powerfrequency pattern is advantageously defined mathematically as a
polynom of a third degree, so that the coefficients are matched with the measured
powerfrequency value pairs. In this matching operation, there can for example be
used the least squares method, in order to make the measurement values and the curve
as compatible as possible.
An important advantage achieved by the invention is that
the method and arrangement according to it provides a possibility to measure and/or
evaluate energy consumption and particularly the achieved energy savings in an electrical
drive provided with one electrical drive, particularly in an electrical drive provided
with a frequency converter, in comparison with other known electrical drives and/or
control systems. The invention is based on the operationtime measurement of one
real electrical drive, in connection with which drive there is constructed a reference
electrical drive, and the energy consumption of said reference drive is defined
on the basis of the same operational time data. Thus the energy consumption of two
(or even several) electrical drives used in the same conditions can be mutually
compared. Earlier it has been extremely difficult to carry out similar energy consumption
comparisons, and therefore they have generally been left undone. A corresponding
measurement and evaluation arrangement has not been described in any of the above
mentioned reference publications representing the general state of the art.
Another advantage of the invention is that by means of
it, the benefits of an electrical drive that is particularly based on a frequency
converter and of applied speed regulation are found out in a simple and effective
way in comparison with other control systems. A particular advantage of the invention
is that thereby different electrical drives can be compared, and the best possible
electrical drive solution can be found from the point of view of both the application
target and energy consumption. Consequently, for example premade energy savings
calculations can be checked and verified, and the costefficiency of corresponding
new electrical drives, particularly electrical drives provided with a control unit,
particularly a frequency converter, can be estimated.
The invention and its further advantages are described
in more detail below, with reference to the appended drawing, where
 figure 1
 is an illustration in principle of a an electrical drive provided with a frequency
converter,
 figure 2
 is a schematical illustration of the frequency converter of figure 1,
 figure 3
 is a flowchart of a method according to the invention,
 figure 4
 is a schematical illustration of a powerfrequency pattern, particularly curve,
constructed on the base of the measurements, and
 figure 5
 is a block diagram illustrating an arrangement according to the invention.
Like reference numbers for like parts are used in the drawings.
An electrical drive, one example of which is illustrated
in figure 1, comprises an electricity supply 1, a control unit that is advantageously
realized as a frequency converter 2, an AC motor 3, in this case a threephase motor.
The electrical drive is operated for running, for example rotating, a load 4 that
can be for instance a pump. The electricity supply 1 comprises an alternating current
mains, such as a threephase mains, or a corresponding alternating current source
for feeding electric energy into the electrical drive. The AC motor 3 is advantageously
a squirrel cage motor, which is the most common electric motor used in industrial
processes.
In this embodiment the control unit, in this case a frequency
converter 2, comprises a rectifier 5, a direct voltage intermediate circuit 6 and
an inverter 7. The inverter 7 of the frequency converter 2 is illustrated as a block
diagram in figure 2. The inverter 7 comprises a switch group 9 and a controller
8. By means of the controller 8, the six switches of the switch group are controlled,
so that a current and voltage with a changing frequency and with a suitable magnitude
can be fed in the motor 3.
In an electrical drive provided with a frequency converter,
there are normally measured the state data of the electric motor, particularly the
input current and input voltage I, U of the different phases, as well as the frequency
f, for regulating the speed of the electric motor. The regulation is carried out
in the controller 8, which receives control instructions as a suitable electric
signal from outside the electrical drive, for instance from the process measurement
data, as a suitable speed instruction. By means of said currents and voltages I,
U, the power of the electric motor can be calculated for example in the controller
8, (or outside said controller, but preferably, however, in the control unit) at
each point of time, and this can be utilized in the frequency converter for regulating
the motor.
A method according to the invention, the flow diagram in
principle of which is illustrated in figure 3, relates to a measurement and evaluation
method of energy consumption, which method is applied in an electrical drive provided
with a control unit, particularly a frequency converter, and an AC motor, such as
in the electrical drive illustrated in figure 1.
In the first step 31, prior to the actual measurements,
certain definitions are carried out. The first definition period 301 relates to
the measurement time period T and the number of samples n. The measurement time
period T is chosen individually in each case. If the nighttime and daytime energies
have different grounds for invoicing, it is natural to choose at least 24 hours
as the measurement time period. Then again, if a week describes the rhythm of operations
with a load provided with electrical drive, such as a device or a plant, the selected
measurement time period is a week. In case the power of the electrical drive is
known to fluctuate owing to an external factor, such as the season, the measurement
can be arranged to be repeated during different seasons, for example in the cold
and warm season, in order to make the measurement time periods together describe
the total time of the energy calculation. In order to make reporting easier, the
length of the measurement time period is advantageously chosen in advance, so that
it is of the order of whole hours or whole days.
It is also advantageous to choose the number of the measurement
points t_{i}, i.e. the number of samples n before the measurement operation.
The measurement points t_{i} follow each other in timewise succession i
= 1, 2, 3,...n at certain measurement intervals, i.e. sample intervals &Dgr;t,
which interval depends on the number of samples n. The longer the measurement time
period T is, the larger the number of the measurement points n is. For practical
reasons, it is in that case sensible to choose the calculation method and the sample
interval so that the numeric integration applied in the calculation of energy consumption
over the measurement points becomes sufficiently accurate, but the number of measurement
points is not unnecessarily increased. There are several alternative methods, and
for each of them there exist clauses for estimating the maximum error, as far as
there is obtained information of the derivatives of the power signal. For example
the error of a trapezoid formula is in the form
$$\left(1\right)\begin{array}{}\end{array}\mathit{\&egr;}=\frac{{\left({T}_{b},,{T}_{a}\right)}^{3}}{12{n}^{2}}P\begin{array}{}\end{array}\u02ba\left(t\right)$$
where
 T_{b}
 the terminal moment of integration, i.e. the terminal point of the measurement
time period
 T_{a}
 the starting point of the integration, i.e. the initial point of the measurement
time period
 n
 number of samples
 P"(t)
 second derivative of power (and first derivative of power change speed)
 &egr;
 accepted error
The accepted error &egr; is defined and set. Then in
the equation (1) there is solved a necessary quantity of samples n, which are added
to the definitions.
The first definition period 301 described above is useful
for achieving a good and reliable energy consumption estimate. It also is advantageous
to define in the second definition period 302 the model of the graphical powerfrequency
pattern, advantageously a curve model describing the mutual codependence of these
two, which curve model is advantageously applied later, on certain conditions, when
processing the measurement register of the method.
In a general case, the shape of the powerfrequency pattern
or curve PF is in the form
$$\left(2\right)\begin{array}{}\end{array}\mathit{PF}=a\cdot {f}^{3}+b\cdot {f}^{2}+c\cdot f+d$$
where a, b, c and d are constants and f = frequency.
The form of the powerfrequency curve PF is a polynom of
a third degree, and the constants are its coefficients. This is based on the fact
that at least part of the electric motor power is consumed in the countermoment
a·f^{3} of the rotation speed, i.e. a countermoment proportional to
a quadrature of the frequency. This may in some cases represent only a small part
of the whole power, but for example in an electric motor running a centrifugal pump,
it represents the major part of the whole power. The constant countermoment results
in a third term b·f^{2}, which as such is fairly common. Another term
c·f is seen for instance in working machines, and it is needed for the sake
of perfection. The last term d reflects power independent of the frequency, which
as such is also present, at least in the form of losses.
It also is advantageous to define, in the third definition
period 303, the power losses of an alternative control arrangement with respect
to an electrical drive applying a frequency converter, and particularly its modeling
in order to evaluate energy consumption. This shall be dealt with below.
When the definitions sufficient for the measurements are
carried out in step 31, the measurement step 32 is advantageously started, for example
as controlled by the system clock, or by intermediation of a timer. In the measurement
step 32, in a predetermined one or several measurement time periods T, at predetermined
measurement intervals, i.e. sample intervals &Dgr;t, at measurement points t_{i},
the state data of the electric motor, particularly the measurement values P_{i},
f_{i} of power and frequency are read from the control unit of the frequency
converter and saved in the next step, i.e. the saving step 33, in the measurement
register. In case the power measurement value cannot be directly read, the required
voltage and current values of one or several steps are read in order to calculate
the power. For example the input power P_{i} of a threephase motor at the
measurement point t_{i} is obtained from the known formula
$$\left(3\right)\begin{array}{}\end{array}{P}_{i}=\sqrt{3}{U}_{i}{I}_{i}\mathrm{cos}\left(\mathrm{\&PHgr;}\right),$$
where U_{i} = phase voltage, I_{i} = phase current and &phgr;
= power coefficient. Each measuring event at each measurement point is given a time
stamp, reflected by the symbol t_{i} of the measurement point.
When the measurement step 32 and the measurement data saving
step 33 are carried out in the measurement time period, in a preferred embodiment
of the invention the measurement values P_{i}, f_{i} are matched
in the model of the powerfrequency pattern, cf. formula (2), or in a corresponding
predetermined pattern model, and the constants a, b, c and d are solved. This is
carried out in the modeling step 331 of the powerfrequency pattern. Thereafter
the measured power as the function of frequency can be represented for example as
a graphical curve on the display, and it can be used for defining the power consumption
of an AC motor operated at constant speed, as shall be described below.
In the first calculation period 341 of the calculation
step 34, the energy consumption of the electrical drive provided with a frequency
converter is calculated on the basis of the measurement results P_{i}, f_{i}
during said measurement period T. When the measurement and registering of power
(or of corresponding voltage and current values) is carried out by means of a frequency
converter, and not for example with a power meter connected in front of the frequency
converter, in addition to the power time distribution, there is obtained information
of the integral of the operation speed, which often describes the volume of a working
machine output. In nearly all cases, the output is proportional to speed, i.e. to
the rotary speed of the drive shaft, which means that in an electrical drive applying
a frequency converter, it is proportional to frequency. This kind of applications
are used for instance in paper machines, conveyors, pumps and various vehicles.
The profile and quantity of the output becomes important in comparison with onoff
type controls.
The energy consumption E of an electrical drive
provided with a frequency converter, where particularly rotary speed regulation
is applied, is obtained for example by a trapezoid formula as follows:
$$\left(4\right)\begin{array}{}\end{array}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}{E}_{\mathit{day}}={\displaystyle \sum _{i=\mathit{day}}}\left({P}_{i},+,{P}_{i+1},),/,2,\cdot ,(,{t}_{i+1},,{t}_{i}\right)$$
$$\left(5\right)\begin{array}{}\end{array}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}{E}_{\mathit{night}}={\displaystyle \sum _{i=\mathit{night}}}\left({P}_{i},+,{P}_{i+1},),/,2,\cdot ,(,{t}_{i+1},,{t}_{i}\right),$$
where

P_{i}
 = at the measurement point t_{i}
measured power

t_{i}
=
 measurement points of a daytime/nighttime measurement time period T
Daytime energy consumption E_{day} is obtained
from the formula (4), and nighttime energy consumption E_{night} is obtained
from the formula (5). The point of time and length of the measurement time period
T in the formulas (4) and (5) are defined so that i = daytime falls at a predetermined
time period in the daytime (for example between 6.00 am and 6.00 pm) and respectively
i = night falls at a predetermined time period in the nighttime (for example between
6.00 pm and 6.00 am). Preferably the sum of the daytime and nighttime measurement
time periods is 24 hours. By a corresponding basic formula, there is generally defined
the energy consumption of the measurement time period T.
The output volume O_{meas} during the measurement
time period is obtained as follows:
$$\left(6\right)\begin{array}{}\end{array}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}{O}_{\mathit{meas}}={\displaystyle \sum _{\mathit{i}}}\left({f}_{i},+,{f}_{i+1},),/,2,\cdot ,(,{t}_{i+1},,{t}_{i}\right),$$
where
f_{i}
= frequency measured at the measurement point t_{i}
In the second calculation period 342 of the calculation
step 34, the energy consumption of a virtual electric motor, particularly an AC
motor, fed by constantfrequency input current and consequently operating at a constant
speed, is calculated on the basis of the measurement time period data. In basic
drive (without frequency converter), alternating current is fed in the electric
machine at a constant frequency, i.e. generally at the mains frequency, from a suitable
source of electricity. The energy consumption of the basic drive of this type of
electric machine is obtained from the processed measurement data, either directly
from the data power vs. frequency, or by interpolating the powerfrequency curve
to constant frequency, at which the constantspeed motor is operated, and by reading
the power value therefrom.
In figure 4 the powerfrequency curve is formed by utilizing
the curve model according to formula (2), and thus it represents measurement, and
the point illustrated in the drawing represents the motor rotated at a constant
speed (and constant frequency).
In the third calculation period 343 of the calculation
step 34, there is calculated other possible energy consumption that is connected
to an electric motor fed by constantfrequency input current and operated at a constant
speed, and particularly to the control systems arranged in connection with it.
Frequency converter drive is applied for regulating alternating
current electric motors, particularly for regulating its rotation speed, and for
controlling it in many different targets both in industry and in the constructed
environment in general. Instead of a frequency converter drive, there were earlier
used, and are still widely used electric motor drives operated at a constant speed,
and for controlling these, other generally known control methods are applied. Among
the most common of these are throttling, onoff control and slip control. In case
this type feed control method is applied, it causes additional losses in the process.
This kind of alternative control method is generally known in the field, and the
energy consumption caused by it can be defined by calculatory means.
In the third definition period 303 of the definition step
31, the power losses/energy losses of a control arrangement alternative to an electrical
drive applying a frequency converter are converted into models that are then applied
in the third calculation period 343 of the calculation step 34, for calculating
the power losses/energy losses of the alternative control arrangement.
In the third definition step 303, there is defined an alternative
control arrangement and defined a calculatory model for calculating the energy losses
thereof, i.e. the power and/or energy losses of the alternative control arrangement
are modeled, as was maintained above. Next we shall observe particularly throttling,
onoff control and slip control, which are widely used control methods.
The energy consumption of throttling control is obtained
from the formula
$$\left(7\right)={E}_{t,\mathit{day}}={P}_{\mathrm{max}}*{\mathrm{\&Dgr;}\begin{array}{}\end{array}T}_{\mathit{day}}$$
where
 P_{max} =
 the power required by throttling control, i.e. the power of a virtual motor
fed by constant frequency
 &Dgr;T_{day} =
 the daytime energy time collected in the measurement, i.e. the daytime measurement
time period
 E_{t,day} =
 daytime energy consumption with throttling
The consumption of nighttime energy with throttling, or
in general the energy consumption of a predetermined measurement period with throttling,
is obtained in a similar way.
The realization of on/off control is affected by: a) planned
maximum output, b) volume of the "storage capacity" in the product and c) for the
sake of controllability, the ontime cannot be full 100%. For economical reasons,
ontimes that fall below 90% are disadvantageous for a maximum output, because then
the equipment would be outsized in all situations. As a result, the power demand
is higher than with a frequency converter input, and thus the output volume also
is larger from time to time, although it in average represents a similar quantity
as with frequency converter input.
For example in the case of pumping, storage capacity is
represented by the size of the storage tanks. Because the purpose is not to make
this control method worse than it is, the starting frequency of the control is requested
from the maker of the analysis. However, starting intervals longer than the time
limits for nighttime rates are not sensible, i.e. it is sensible to start at least
once with a nighttime rate.
Consequently, calculation proceeds as follows: a) output
is calculated per start interval, b) it is calculated how long &Dgr;T_{on}
the constantspeed motor would be on (start interval) and c) finally there is calculated
the energy consumption E_{on/off}.
The output during the start interval is obtained from the
formula:
$$\left(8\right)\hspace{1em}{\mathrm{\&Dgr;}\begin{array}{}\end{array}T}_{\mathit{on}}={O}_{\mathit{meas}}*{\mathrm{\&Dgr;}\begin{array}{}\end{array}T}_{\mathit{start}}/{O}_{\mathit{on}\mathit{}\mathit{off}}$$
where
 O_{meas} =
 measured output
 O_{onoff} =
 maximum output of onoff control
 &Dgr;T_{start} =
 start interval
 &Dgr;T_{on} =
 ontime of onoff control
The energy consumed during the onstep, particularly the
daytime energy consumption E_{on/off}, day, is obtained from the formula:
$$\left(9\right)={E}_{\mathit{on}\mathit{}\mathit{off}\mathit{,}\mathit{day}}={P}_{\mathrm{max}}*{\mathrm{\&Dgr;}\begin{array}{}\end{array}T}_{\mathit{day}}$$
By a corresponding formula, there is defined the nighttime
energy consumption E_{on/off,} night.
Slip control is typically realized by hydraulic couplings
or eddycurrent break couplings. Generally the control device requires cooling,
which here must be ignored. It is assumed that when desired, the slip can be controlled
to zero. This is possible in some devices.
In this method, every sample point is recalculated, so
that the electric power is converted as follows:
$$\left(10\right)\begin{array}{}\end{array}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}}{P}_{\mathit{slip},i}={P}_{i}/{f}_{i}*{f}_{s}$$
where
 P_{i} =
 sample point power
 f_{i} =
 sample point frequency
 f_{s} =
 mains frequency
 P_{slip,i} =
 motor power at slip frequency
With slip control, the daytime and nighttime energy consumptions
E_{slip, day}, E_{slip, night}, are calculated separately from the
powers.
$$\left(11\right)\begin{array}{}\end{array}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}{E}_{\mathit{slip}\mathit{,}\mathit{day}}={\displaystyle \sum _{i=\mathit{day}}}\left({P}_{\mathit{slip},i},+,{P}_{\mathit{slip},i+1},),/,2,\cdot ,(,{t}_{i+1},,{t}_{i}\right)$$
$$\left(12\right)\begin{array}{}\end{array}\mathrm{\hspace{1em}\hspace{1em}\hspace{1em}}{E}_{\mathit{slip}\mathit{,}\mathit{night}}={\displaystyle \sum _{i=\mathit{night}}}\left({P}_{\mathit{slip},i},+,{P}_{\mathit{slip},i+1},),/,2,\cdot ,(,{t}_{i+1},,{t}_{i}\right)$$
When the alternative control method is known, and the power
and/or energy consumption resulting from said control method is appropriately modeled
and defined, the energy consumption can now be calculated in the third calculation
period 343 of the calculation step 34. Next, in the fourth calculation step 344,
the combined energy consumption of an electric motor operated at a constant speed
and of an alternative control method connected thereto can be calculated during
said measurement period.
In the comparison step 35, the calculated energy consumption
(step 341) of the frequency converter drive is compared with the energy consumption
of an electric motor drive operated at constant speed and the energy consumption
of the alternative control method (steps 342, 343), and the result from the comparison
is given for example as a value of absolute energy consumption, or in percentages
as energy savings of the electrical drive provided with a frequency converter in
comparison with an ordinary electrical drive.
In special cases, it may be desirable to obtain only the
electrical drive power as a function of frequency, irrespective of the output distribution.
In that case, the frequency converter can be controlled throughout the interesting
frequency range, from minimum to maximum frequency. This measurement can be carried
out in the course of a few minutes, and thus it differs from the collecting of the
actual survey material. As a result, there is obtained the power as a function of
the frequency in graphical illustration.
The invention also relates to an arrangement 50 for measuring
and evaluating energy savings achieved by an electrical drive, which electrical
drive includes a control unit, in this case a frequency converter, and an AC motor.
By means of this arrangement, the method according to the invention can be realized.
The arrangement 50 according to the invention, figure 5,
includes at least a definition unit 51, a measurement unit 52, a memory unit 53
and a calculation unit 54. One or several measurement time periods T and its measurement
points t_{i} at predetermined measurement intervals, i.e. sample intervals
&Dgr;t, are defined in the definition unit 51. At least the start and end moments
of the measurement time period T, and the sample number n or sample interval &Dgr;t,
are given from outside the arrangement. By means of the measurement unit 52, at
the measurement points t_{i}, there are read during said measurement time
period the measurement values P_{i}, f_{i}, of the power and frequency
of the AC motor. The read measurement values are saved, together with the measurement
point data, in the memory unit 53. Of the measurement values A, there is created
a measurement register 531.
It can be maintained that in the definition unit 51, there
is generally realized the first or definition step 31 according to the method of
the invention, at least the first and third definition period 301 and 303, and respectively
in the measurement unit 52 there are realized the measurement steps 32, and in the
memory unit 53 there is realized the measurement data saving step 33 and other steps
that are alternatively connected thereto. In the calculation unit 54, there are
respectively realized the calculation steps 34 and the energy consumption comparison
step 35.
The calculation unit 54 comprises a first means 541 for
defining the power value of a virtual AC motor that is fed by constantfrequency
input current, and for calculating the energy consumption thereof in one or several
predetermined measurement time periods. The calculation unit 54 also comprises a
second means 542 for calculating the energy consumption of an electric motor provided
with frequency converter or a corresponding control unit i.e. applied electrical
drive in predetermined one or several measurement time periods. The calculation
unit 54 further comprises a third means 543 for calculating the energy consumption
of one or several alternative control systems in predetermined one or several measurement
time periods. The calculation unit 54 advantageously comprises also a fourth means
544, where firstly the energy consumption of said virtual AC motor and the energy
consumption of said alternative control arrangement are added together and where
secondly the calculated energy consumption of the applied electrical drive are compared
with the total added energy consumption of the virtual AC motor and the alternative
control system for estimating the energy savings brought by the electrical drive
in said measurement time period/periods.
The arrangement 50 also advantageously comprises means
532 for forming a graphical powerfrequency pattern, advantageously a curve, of
the measurement value data. The means 532 can be arranged for example in connection
with the memory unit 53 or, as an alternative, in connection with the calculation
unit 54.
The arrangement 50 also includes a modeling unit 533 of
the a powerfrequency pattern, advantageously a curve, where measurement value data
is converted into a powerfrequency pattern that in mathematical shape is a polynom
PF of a third degree, PF = af^{3} + bf^{2} + cf + d, so that the
polynom coefficients a, b, c and d are matched in the measured powerfrequency value
pairs P_{i}, f_{i}. The modeling unit 533 can be situated for instance
in connection with the memory unit 53, or as an alternative, in connection with
the calculation unit 54.
In connection with the calculation unit 541, there are
advantageously provided means 542 for modeling the energy consumption of the alternative
control arrangement, and on the basis of said modeling, the energy consumptions
in the control system are defined and calculated in the measurement period.
It can be maintained that in the calculation unit 54, there
are generally realized the calculation steps 34 of a method according to the invention,
as well as the energy consumption comparison step 35.
The collection and processing of the power and frequency
data of an electrical drive provided with a frequency converter is realized by means
of an arrangement 50, which is preferably realized as a data processing unit. The
data processing unit includes a microprocessor or the like. In addition, the data
processing unit of the arrangement is connected to or includes one or several memory
units for recording measurement data, intermediate results and end results. The
collecting of desired data is realized by means of the data processing unit, in
a predetermined way and in a controlled fashion from the control unit of the actual
electrical drive, for example from the controller 8 of the frequency converter,
as is illustrated in figure 5. The processing of the collected data and the calculation
of the energy quantities is preferably realized by means of software processing
units recorded in connection with the microprocessor, such as a definition unit
51, a measurement unit 52, and calculation unit 54. The results, i.e. the calculated
power values and their identifier data are recorded in one or several memory units,
such as a memory unit 53 in the data collection step, and the results from the energy
consumption comparison can also be recorded in a suitable memory unit.
It is pointed out that one or several electrical drives
can be simultaneously analyzed by one energy savings measurement and evaluation
arrangement according to the invention. The measurement and evaluation arrangement
can be fitted in the actual electrical drive control unit, such as a frequency converter,
as an integrated part thereof, or it can alternatively be realized as an external
arrangement of one or several electrical drives. In that case the arrangement is
for example integrated in a personal computer and connected to an electrical drive/electrical
drives by a suitable data transmission bus.
In the above specification, the invention is described
mainly with reference to an electrical drive that includes a frequency converter
as the control unit. However, for a man skilled in the art it is obvious to apply
the invention also to other types of electrical drive control units, such as cyclo
converters. The essential feature in these control units are the measurements of
the current, voltage and frequency of the electric motor for controlling and running
the motor, and particularly these measurement signals are made use of in the invention.
The invention is not restricted to the described embodiments
only, but many modifications are possible within the scope of the inventive idea
defined in the appended claims.