The invention relates to a procedure as defined in the preamble of
claim 1 and to an apparatus as defined in claim 5 for compensating the slip of
an induction machine.
The speed of rotation of an induction machine deviates from the frequency
of the supplying network in a known manner by a certain slip. When operated as
a motor, the speed of rotation is somewhat lower than the supplying frequency divided
by the number of pole pairs, i.e. the synchronous speed in motor operation and,
correspondingly, the speed of rotation in generator operation is somewhat higher
than the network frequency divided by the number of pole pairs. The magnitude
of the slip depends on the load of the machine, which is why adjusting the speed
of rotation often leads to a complicated control system, because the variable quantities
cannot always be measured accurately. For this reason, it is difficult to maintain
a constant speed of rotation of an induction machine, especially when the load
There are several previously known control systems for adjusting
the speed of rotation of an asynchronous motor to a level corresponding to a reference
value. These solutions have often led to the use of complicated and expensive regulating
apparatuses which could seldom be realized in the case of small motors. Besides,
the proposed apparatuses have required the measurement of the speed of rotation
directly from the motor shaft or the measurement of the load current of the motor.
The object of the invention is to achieve a new method for regulating
the speed of rotation of an induction machine by suitably compensating in a control
loop the deviation caused by the slip between the supplying frequency and the actual
and reference values of the speed of rotation. To implement this, the procedure
of the invention is characterized by the features presented in the characterization
part of claim 1. The apparatus of the invention is characterized by the features
presented in the characterization part of claim 5. Other embodiments of the invention
are defined in the subclaims.
The solution of the invention makes it possible to achieve an advantageous
and reliable system for controlling the speed of rotation of an induction machine.
The motor speed can be determined e.g. using a simple inductive detector without
a separate tachometer or optic pulse sensor mounted on the motor shaft. Especially
in cases where the axial length of the motor is limited, this provides a considerable
advantage. In the system of the invention, the slip is compensated digitally via
speed feedback, resulting in fast and accurate operation. The feedback is implemented
using a few cheap integrated circuits.
In the following, the invention is described by the aid of one of
its embodiments, referring to the drawings, in which
- Fig. 1 presents a block diagram of the control system of the invention,
- Fig. 2 presents the circuit implementing the compensation,
- Fig. 3 presents the pulse shapes of the signals, and
- Fig. 4 illustrates the arrangements for the measurement of the speed of rotation
of the motor.
Fig. 1 illustrates the system of the invention for the compensation
of the slip of an asynchronous motor. A three-phase asynchronous motor 1 is fed
through conductors 2, 4 and 5 by a frequency converter 2, which converts the constant-frequency
voltage of the supply network connected to its input terminals R, S and T into
a motor supply voltage determined by the control, the frequency and voltage of
which are adjustable. The frequency converter 2 consists e.g. of a PWM converter
6 and a controller 7 which generates a reference voltage us and a reference
frequency fs for it.
The structure and operation of the motor 1 and the frequency converter
2 controlling it are previously known systems in the art and their details are
irrelevant to the implementation of the present invention.
The speed reference fR for the motor 1 is connected to
the reference input of the frequency converter 2 or it is generated within the
frequency converter in accordance with external control. The signal C compensating
the slip of the asynchronous motor is passed via conductor 10 to the control terminal
11 of the frequency converter so that it is summed with the speed reference fR,
producing a reference frequency fs. The system controlling the frequency
converter determines, in a manner known in itself, the magnitude of the motor
supply voltage on the basis of the frequency reference. The motor speed is measured
by means of a pulse sensor 12, which outputs a frequency signal nm proportional
to the speed of rotation of the motor. The pulse sensor 12 is preferably implemented
by providing the motor with an inductive sensor responding to the brake wheel toothing,
the cooling ribs of the rotor or e.g. to the fan blades. Thus, no separate tachometer
or optic sensor is needed. The frequency reference fs of the frequency
converter is divided by the number of pole pairs p by a divider 8, and the signal
obtained from the divider, which is proportional to the synchronous
speed of the motor, and the frequency signal nm
obtained from the pulse
sensor, which is proportional to the speed, are passed to the slip compensator
15 via conductors 13 and 14. The output c of the slip compensator 15 is applied
to the input of the frequency converter 2 as described above. The frequency converter
2, the motor 1 and the speed detector 12 measuring the speed of rotation constitute
the outer control loop of the system of the invention, controlling the speed of
The slip compensator 15 consists of a summing element 16, a voltage/frequency
converter 17, a phase comparator 18 and an integrator 9. The frequency signal nm
obtained from the pulse sensor 12 is passed via conductor 14 to one of the inputs
of the phase comparator 18. The synchronous speed ns is obtained as
a voltage signal from the frequency converter 2 and passed to the summing element
16, which subtracts from its value the signal ds given by the phase comparator
18. The output signal of the summing element 16 is passed to the voltage/frequency
converter 17, which forms a frequency signal ns' corresponding to the
reference speed fR of the motor. The phase comparator 18 forms a phase
difference signal ds on the basis of the phase difference between its input signals
nm and ns' in the manner illustrated by Fig. 3. The phase
difference signal ds is conveyed to the summing element 16 and to the integrator
9, which generates from the phase difference signal ds a correction signal C,
which is applied to the correcting reference input 11 of the frequency converter
2. The summing element 16, the voltage-controlled oscillator 17 and the phase comparator
18 constitute the inner loop of the control system, which is structured as a so-called
The speed control system of the invention works as follows. The signal
ns obtained from the frequency converter 2 is proportional to the output
frequency of the frequency converter and to the synchronous speed of the motor.
The voltage-controlled oscillator 17 produces a frequency signal ns'
proportional to the reference speed of the motor. The pulse sensor 12 produces
a pulse train nm of a frequency proportional to the speed of rotation
of the motor 1. The phase comparator 18 compares the pulse trains of signals ns'
and nm. As a result of this comparation, the phase difference signal
ds increases if the pulses received from the motor are retarded. The phase difference
signal ds is subtracted from signal ns, causing the output frequency
ns' of the oscillator 17 to fall and the oscillator to be synchronized
with the frequency nm obtained from the pulse sensor. The mean value
of the phase difference signal causing the synchronization is proportional to the
difference between the synchronous speed and the speed of rotation of the motor,
i.e. to the slip. The integrator 9 generates the correction signal C by filtering
the phase difference signal ds. When the correction signal C is added to the original
frequency reference fR, a closed speed control circuit is formed which
determines the slip of the motor and corrects its supply frequency correspondingly.
Thus, the static state speed of the motor is not dependent on the load but is adjusted
to a value corresponding to the reference frequency fR as long as the
motor is able to generate a torque corresponding to the load.
Fig. 2 presents a circuit designed to perform the function of the
slip compensator shown in Fig. 1. The voltage signal ns (0...10 V) proportional
to the motor supply frequency fs is taken through amplifiers 21 and
22 to the voltage-controlled oscillator 23. Via conductor 24, a phase difference
signal corresponding to the slip is applied to the summing point at amplifier
22, which subtracts it from the signal ns. The oscillator 23 produces
at its output a frequency signal ns' proportional to the reference speed,
which is passed to the phase comparator 25. The signal nm obtained
from the pulse sensor connected to the motor is applied to the other input of the
phase comparator. The output 26 of the phase comparator 25 is passed to an integrator
27 acting as a filter, whose output gives the correction signal C via amplifier
28. The signal ZS keeps the integrator output at zero when the machine has been
stopped. The phase comparator 25 controls the input of the integrator 27 on the
basis of the phase difference between the frequency signals ns' and
nm so that the integrator output increases when the pulse of signal
nm lags behind the pulse of signal ns'.
Fig. 3 represents the input and output pulses of the phase comparator.
The pulse sensor produces e.g. 24 pulses per revolution, in which case a motor
speed of 1800 r/min corresponds to a pulse frequency of 720 Hz. The phase comparator
is implemented using a CMOS circuit with a three-state output. In the area indicated
by arrow A, signal ns' causes the differential signal ds to go into
the state +1, and signal nm causes ds to return into the 0-state. As
the phase difference increases, the pulse duration of ds also increases. Similarly,
if the motor speed tends to increase, the pulse of signal nm causes ds to change
into the -1-state and the pulse of signal ns' back into the 0-state as is illustrated
by the area indicated by arrow B.
Fig. 4a shows how the pulse sensor is mounted on the motor. The inductive
sensor 41 is attached with a fixing element 42 to the body of a brake installed
at the end of the motor. The brake wheel 43 is provided with a toothing 44 and
the sensor is fitted in the immediate vicinity of the teeth. Fig. 4b presents
a section through Fig. 4a along line A-A.
The invention has been described above by the aid of one of its embodiments.
However, the presentation is not to be regarded as limiting the sphere of protection
of the invention, but the embodiments of the invention may vary within the limits
defined by the following claims.