Speed control arrangement for electric motors

Abstract: In a speed control arrangement for an induction motor or an a.c. permanent magnet motor in which any one of a number of power frequencies may be selected for application to the motor, there are provided means to control the rate of change of frequency from one selected power frequency to another to control the acceleration or deceleration of the motor.

Description: The present invention relates to speed control arrangements for electric motors, and in particular, although not exclusively, to speed control arrangements for induction motors or a.c. permanent magnet motors, in which the speed of the motor is dependent upon the frequency or at least the fundamental frequency component of an energising signal applied to the motor. Such speed control arrangements are described for example in United States Patent No. US-A-5146148, in which a microcomputer is used to control the frequency and rate of change of frequency of an energising signal in dependence upon a speed-set input signal and a stator current feedback signal, and in European Patent Application No. EP-A-476588, in which means are provided for changing the rate of output frequency change. According to the present invention, there is provided a speed control arrangement for an electric motor the speed of rotation of which in operation is dependent at least in part on the frequency of an energising signal waveform applied thereto, the arrangement being provided with logic circuit means for generating an energising signal waveform for said motor of a frequency determined by a first digitally encoded signal value applied to said logic circuit means, storage means for storing a plurality of second digitally encoded signal values for determining a like plurality of predetermined different frequencies, means for selecting a said second digitally encoded signal value from said storage means, means responsive to a difference between said selected second digitally encoded signal value and said first digitally encoded signal value to change said first digitally encoded signal value towards said selected second signal value at a predetermined rate, and means for determining said predetermined rate. Preferably the predetermined frequencies are defined by means of a set of respective binary coded numbers and the logic circuit means comprises a binary up/down counter which is arranged to count at a predetermined rate towards a selected binary coded number. The energising signal waveform may be a pulse-width modulated pulse train, said frequency being that of the fundamental frequency component of the pulse-width modulated pulse train. A speed control arrangement in accordance with the present invention for controlling induction motors or a.c. permanent magnet electric motors will now be described with reference to the accompanying drawing, which shows a block schematic diagram of the arrangement. Referring to the drawing the arrangment, which may be formed as a single integrated circuit chip, comprises a logic circuit 1 for generating respective pulse-width modulated pulse trains which are provided to three pairs of outputs 2, 3 and 4, the fundamental frequency components of these three pulse trains being of the same frequency but mutually spaced in phase by 120°, as in a conventional three-phase a.c. supply. Each of these pulse trains is utilised to control the conduction state of a respective pair of switching devices, for example hybrid FET/bipolar semiconductor switching devices (not shown), by which positive and negative voltage sources are selectively connected to a respective phase input of an electric motor to be energised. The fundamental frequency component of the pulse-width modulated pulse trains, which may be termed the "power frequency", may be varied, to vary the speed of rotation of the electric motor, between any of, say, sixteen preselected values in either direction in response to an input signal applied by way of a speed select input path 5. The preselected values may be represented by fourteen-bit numbers stored in a read-only memory 6, the respective addresses of these numbers in the memory 6 being selectable by respective speed select input signals on the path 5. Of the fourteen bits at each address twelve bits represent the selected scalar speed of rotation effectively as a fraction of the maximum speed (represented by all "ones"), while the value of thirteenth bit dictates whether the "aiming" direction of rotation of the electric motor is to be determined internally by the value of the fourteenth bit or externally by way of a one-bit value presented on a path 7. The twelve speed of rotation bits from a selected address in the memory 6 are applied to one input of a twelve-bit magnitude comparator 8, while the two speed of direction bits are decoded at 9 to provide an indication of the "aiming" direction of rotation, that is, the direction in which the electric motor is next required to rotate, whether it is to be in the same direction as the motor is now running or the reverse. In the comparator 8 the selected twelve speed of rotation bits are compared with a twelve-bit output from a thirteen-bit up/down counter 10, which twelve-bit output is also applied to the logic circuit 1 to set the frequency of the fundamental component of the pulse-width modulated pulse trains. The thirteenth bit of the counter 10, which indicates the existing instantaneous direction of rotation, determines the sequency of the three phase outputs. If the selected "aiming" direction of rotation is the same as the existing direction, the acceleration or deceleration called for by any difference in the selected speed of rotation and the existing speed is effected by counting up or down respectively the twelve-bit value in the counter 10, by means of pulses from a divider chain 11 driven from a crystal controlled clock pulse generator 12, until it matches the selected twelve bits from the memory 6. The acceleration and deceleration rates of counting are separately selectable by means of a set of links 13, and need not be the same. If the selected "aiming" direction of rotation is the opposite to the existing direction, the value in the counter 10 has to be counted down to zero, to bring the motor to rest, and then counted up to the required speed in the opposite direction. A read-only memory 14 is arranged to store digitised sample values representing one quarter cycle of a sinusoidal or near-sinusoidal waveform, these sample values being read out in turn and in the required order, at a rate determined by the twelve-bit output from the counter 10, to generate a stepped waveform at the required fundamental frequency, this waveform being asynchronously double-edge sampled in known manner to derive the pulse-width modulated pulse trains to be supplied to the outputs 2, 3 and 4. The memory 14 may hold, for example, 384 8-bit amplitude values. Differing motor/load characteristics are allowed for by providing a further memory 15 storing a voltage/frequency profile, the twelve-bit instantaneous frequency output from the counter 10 being decoded to read from the memory 15 a value for the amplitude of the stepped waveform appropriate to that frequency for the particular motor/load combination. The voltage/frequency profile may consist of three different areas, voltage boosting at lower frequencies, proportional to frequency over middle range frequencies and constant voltage at the upper end of the frequency range. In view of the wide frequency range of which the arrangement is capable, the output of the counter 10 may be applied to the address decoder 18 by way of metallisation links (not shown) to ensure the best possible voltage/frequency resolution irrespective of the chosen frequency range. The minimum width of pulse in the pulse width modulated pulse trains delivered to the outputs 2, 3 and 4 may be defined in respective circuits 16, in which shorter pulses are deleted, while the delay time between switching of the complementary outputs used to control the pairs of devices (not shown) for each phase may be defined in respective delay circuits 17. By means of a "hold" signal on an input 19 to the accelerate/decelerate decoder 20, any acceleration or deceleration may be prevented, for example to prevent excessive deceleration rates from dumping too much power into into the switching circuitry and causing a potentially damaging over-voltage condition. The "hold" condition may be initiated, for example, if the d.c. supply voltage to the switching circuitry rises above a predetermined level. Similarly, a "decelerate" signal may be applied to the input 19 to decelerate the motor to rest, irrespective of the selected state, if it becomes necessary to prevent any overcurrent/overheating situation at the switching devices.

Claims: A speed control arrangement for an electric motor the speed of rotation of which in operation is dependent at least in part on the frequency of an energising signal waveform applied thereto, the arrangement being provided with logic circuit means (1, 10, 14, 16, 17) for generating an energising signal waveform for said motor of a frequency determined by a first digitally encoded signal value applied to said logic circuit means (1, 10, 14, 16, 17), storage means (6) for storing a plurality of second digitally encoded signal values for determining a like plurality of predetermined different frequencies, means (5) for selecting a said second digitally encoded signal value from said storage means (6), means (8, 20) responsive to a difference between said selected second digitally encoded signal value and said first digitally encoded signal value to change said first digitally encoded signal value towards said selected second signal value at a predetermined rate, and means (11, 13) for determining said predermined rate. A speed control arrangement in accordance with Claim 1 further characterised in that the digitally encoded signal values are binary coded, and that the logic circuit means (1, 10, 14, 16, 17) comprises a binary up/down counter (10) which is arranged to count at said predetermined rate towards a selected second signal value. A speed control arrangement in accordance with Claim 1 or Claim 2 wherein the energising signal waveform applied to the electric motor is a pulse-width modulated pulse train, said frequency being that of the fundamental frequency component of the pulse-width modulated pulse train.

Inventors: GILBERT DAVID PETER, GILBERT, DAVID PETER
Applicants: PLESSEY SEMICONDUCTORS LTD, PLESSEY SEMICONDUCTORS LIMITED
Application year: 1993
Main classification: H02P7
IPC codes: H02P27, H02M7, H02P23