BACKGROUND OF THE INVENTION
The present invention relates to a method according to
the preambles of the independent claims in connection with a frequency converter,
and a frequency converter.
In certain applications of a frequency converter the thermal
stress directed towards the frequency converter varies periodically. Such applications
include centrifuges, cranes, metal rollers and elevators. A cyclic load is typically
formed of acceleration, smooth driving, braking and of an unloaded state, whereby
the power components of the frequency converter are considerably stressed during
acceleration and braking.
Dimensioning the frequency converters is carried out conventionally
in accordance with the highest temporary temperature or cyclic temperature variation.
The temperature of a power semiconductor is not allowed to exceed a certain limit,
since there is a danger of definitely damaging the power semiconductor. Dimensioning
for periodic loading is carried out using a set of curves provided by the component
manufacturer and estimates concerning the duration of an individual load in a periodic
load as well as the temporal density of loads. Such a dimensioning allows achieving
the reliability of the operation of the apparatus without over-dimensioning the
apparatus when the density of loads and the profile of a load remain substantially
within planned limits.
A problem with current frequency converters is that when
the temperature of the power semiconductor in the frequency converter exceeds the
allowed maximum limit while in use, the frequency converter interrupts the operation
thereof while still in operation in order to prevent damaging the frequency converter.
Then the process controlled by the frequency converter undesirably stops owing to
too frequent recurrent periodic use or the increase in temperature caused by the
damaged cooling in the frequency converter.
BRIEF DESCRIPTION OF THE INVENTION
It is an object of the invention to provide a method in
connection with a frequency converter and a frequency converter so as to solve the
above problem. The object of the invention will be achieved with a method and an
apparatus, characterized in what is disclosed in the independent claims. The preferred
embodiments of the invention are disclosed in the dependent claims.
The invention is based on the idea to observe the temperature
of the power semiconductor in the frequency converter during periodic use. What
may be determined on the basis of the dimensioned or calculated temperature variation
before a new loading period is whether loading is allowed to be started without
the danger of the temperature of the power semiconductor increasing excessively
during the following loading period.
An advantage of the method and apparatus according to the
invention is that the periodic loading controlled by the frequency converter cannot
in conventional use be stopped by overheat tripping, since the frequency converter
is not permitted to fall into such a situation. Close to overloading the method
and the apparatus allow changing the operation of the apparatus to ensure the faultless
operation of the frequency converter. Thus for instance a new loading period is
not initiated at all or the profile of the load is facilitated, if it is anticipated
that the process will be interrupted at a critical stage.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following the invention will be described in greater
detail by means of the preferred embodiments with reference to the accompanying
drawings, in which
DETAILED DESCRIPTION OF THE INVENTION
Figures 1 and 3 show schematic curve forms of torque in connection with periodic
Figures 2 and 4 show schematic curve forms of the temperature of a power semiconductor
in connection with periodic loading.
In a method according to the invention a frequency converter
carries out a specific operation periodically in periodic recurrent use. Figure
1 shows a schematic torque curve of such a loading period. In the initial state
to the process is stopped or provided with insignificant load and the torque provided
by the frequency converter is close to zero. From moment t1 forward the
process is accelerated with a considerable torque Tnim until the moment of time
t2. Between t2 to t3 the speed of the process remains
constant and the torque is typically smaller than the acceleration and braking torque.
At moment t3 the process starts to slow down, whereby the torque again
reaches the maximum value thereof, and maintains such a value until the moment of
time t4, when the torque is reduced again close to zero.
The current of the frequency converter is proportional
to the torque produced by the frequency converter. The current in turn causes the
power semiconductors to heat, and in Figure 2 such a heat is indicated in connection
with the load changes corresponding to Figure 1. When the motor is at the moment
of time t1 controlled with full torque, the temperature of the power
semiconductor starts to rise. The temperature increases during the entire torque
step, but starts to decrease at moment t2, when the process has achieved
the constant speed thereof. The temperature decreases until moment t3,
and thereafter during the braking torque the temperature increases again until moment
t4. When the torque decreases close to zero at moment t4 the
temperature starts to drop rapidly.
The curve form in Figure 2 is a substantially simplified
description of the actual temperature variation, which is affected by temperature
time constants of several thermal masses connected to one another. The curve form
in Figure 2 therefore shows that the temperature could change only if dictated by
one temperature time constant. However, this does not affect the understanding of
the operation of the method according to the invention.
The temperature is preferably determined by measuring the
temperature of a cooling element, and what is further generated from the temperature
in a way known per se is the temperature of a semiconductor component chip using
the thermal model of the semiconductor component.
The temperature varies periodically in connection with
the load as described above. The method according to the invention determines a
typical power semiconductor heating, i.e. the difference between the maximum and
minimum values in the example shown in Figure 2. In order to reliably obtain the
typical value of the heating, the differences between the minimum and maximum temperatures
are determined during several loading periods. A describing value is further formed
from the collected values for the minimum and maximum, and the difference between
them is calculated that shows a typical heating. The minimum and maximum values
may be calculated directly as an arithmetic mean. If there is no desire to use several
measurings for determining the value of the heating, the values describing the actual
situation can be obtained by employing a root-mean-square value, which reduces the
relation of individual differing values in the mean. Thus, the heating is most preferably
calculated as the difference between several measured mean values of the maximum
values and several measured minimum values. Measuring is further carried out in
such a manner that the magnitudes of the minimum value are temperatures at the moment,
when the cyclic loading starts, i.e. at moment t1 in the examples shown
in Figures 1 and 2. For instance 10 to 30 of the first loading cycles may be used
for determining the mean heating &Dgr;T, and thereafter the mean is calculated
and stored in the memory of the frequency converter.
Another possibility for forming and storing the mean heating
is to feed it directly into the memory of the frequency converter as a parameter
provided by the user. The parameter may then be calculated or estimated based on
experience. By feeding the heating as a parameter value into the frequency converter,
the apparatus may start anticipatory protection without the identification presented
In accordance with the method of the invention a temperature
limit is determined from the generated heating and the highest allowed temperature
of the power semiconductor. The component manufacturers indicate the highest allowed
temperature Tmax for the semiconductors, which is not to be exceeded
in order for the apparatus to remain in working order. The temperature limit Tlim
according to the invention refers to a temperature, the exceeding of which, when
the loading step starts, would increase the temperature of the semiconductor component
excessively during the loading step. The temperature limit is therefore calculated
as a difference between the highest allowed temperature and the mean heating (Tlim
= Tmax - &Dgr;T). Preferably minor inaccuracies can be taken into account
when calculating the temperature limit by adding a temperature margin to the mean
heating. Such a margin allows observing a possible inaccuracy of the temperature
Also in accordance with the invention the temperature of
the power semiconductor is determined. The temperature of the semiconductor chip,
or the chip, of the power semiconductor is the most critical temperature in the
semiconductor. The temperature of the chip may be determined by measuring the temperature
of a part being in thermal connection with a semiconductor component, and by calculating
the temperature of the chip using the heating model. The temperature to be measured
may for instance be the temperature of a cooling element. It is also possible to
measure the temperature of the chip directly, if a possibility to do so is created.
In accordance with the method of the invention the operation
mode of the frequency converter is also changed when the temperature of the power
semiconductor exceeds the temperature limit, when transferring to the loading period.
The operation mode may be changed by delaying the start of the loading period or
by lightening the loading period (for instance by reducing the torque limit or maximum
speed of the apparatus).
Figures 3 and 4 show the slowing of the process. In Figure
3 the unbroken line shows the torque curve shown in Figure 1 and the broken line
shows the torque curve associated with the slowed process. Correspondingly in Figure
4 the unbroken line shows the temperature curve shown in Figure 2 and the broken
line shows the temperature curve associated with the slowed process.
Figure 4 is also provided with horizontal lines describing
the temperature limit Tlim and the maximum temperature Tmax.
The difference between the two temperatures (plus a possible margin) refers to the
heating defined above. As shown in Figure 4 the temperature of the semiconductor
component is at the start of the torque step above the temperature limit, i.e. the
maximum allowed starting temperature of the component. Thus, the loading period
implemented with a normal torque reference could cause the temperature of the power
component to exceed the maximum temperature. This point is indicated in the figure
with an arrow.
The heating of an average cycle can also be utilized for
determining the operating life of the frequency converter. The component manufacturers
provide the power semiconductor with an average estimation about how many cyclic
loads the component will endure at a particular average heating. When the heating
is known, then the operating life of the apparatus can be estimated in the frequency
converter based on the temporary density of the loading cycles. This piece of information
is advantageous for the operating staff of the apparatus and based on said information
the staff is able to estimate when the apparatus is to be renewed before the apparatus
may be subjected to malfunction while in use. The estimated operating life can be
shown directly in years and months to the operating staff in the operation panel
of the frequency converter or possibly on the display unit of the upper control
system communicating with the frequency converter.
What also affects the loading capacity of the frequency
converter is the condition of the cooling system. The cooling system typically comprises
a cooling element, which is thermally connected to the power semiconductor, and
a fan that intensifies the operation of the cooling element by transferring heat
from the element to the surrounding air. When the cooling system is subjected to
malfunction the cooling does not operate as desired and the temperature increases
more than has been intended. A possible malfunction may be caused by a broken fan
or by unclean cooling fins in the cooling plate.
The condition of the cooling system can be monitored by
determining the temperature of the cooling element, such as the cooling plate, when
the apparatus remains stationary or after a load change. The measured temperature
of the cooling plate can for instance be compared to the temperature according to
a model prepared of the cooling plate and thermal components associated therewith.
If the temperature according to the model decreases faster than the actual measured
temperature, then it may be assumed that the cooling system is subjected to malfunction.
The malfunction can also be observed in such a manner that a temperature time constant
is determined for the cooling plate when commissioning the apparatus. If the temperature
measured during use does not behave in accordance with the temperature time constant,
then the cooling system is subjected to failure. Concerning such a failure the frequency
converter may generate an alarm to the operating staff, or alternatively prevent
the apparatus from being loading. The condition of the cooling apparatus is significant
as regards the anticipated protection according to the invention, since the magnitude
of the average heating changes and the temperature is restored slowly below the
temperature limit when the cooling is inadequate.
It is apparent for those skilled in the art that as technology
advanced the basic idea of the invention can be implemented in various ways. The
invention and the preferred embodiments thereof are therefore not limited to the
examples above but may vary within the scope of the claims.