GB1587182A - Control of alternating current motors - Google Patents

Description

(54) IMPROVEMENTS IN AND RELATING TO CONTROL OF ALTERNATING CURRENT MOTORS (71) We, NORTHERN ENGINEERING INDUSTRIES LIMITED, a British Company, of NEI House, Regent Centre, Newcastle on Tyne NE3 3SB, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to control apparatus employing semi-conductor controlled rectifiers by controlling the speed of A.C. induction motors including synchronous and reluctance synchronous motors by direct conversion (that is without an intermediate D.C. link stage) of an A.C. supply voltage to an A.C.

output voltage.

It is a common requirement for motor drives to provide one or more low speed settings in addition to a normal full speed, or optimum high running speed.

Equipment employing such a drive may be set up or calibrated, for example, at one of the low speeds in preparation for a high speed run. such low speeds in a drive are commonly referred to as "creep" speeds. It is also a-common requirement in motor drives of this type to provide a smooth starting characteristic without undue snatching or jerking (for example to avoid damage to material in a mill), preferably without excessive motor currents being demanded when using direct-on-line starting equipment.

In accordance with the present invention there is provided control apparatus for controlling the speed of A.C. induction motors by direct conversion of an A.C.

supply voltage to an A.C. output voltage comprising for the or each motor stator phase an A.C. switching arrangement comprising two semiconductor rectifiers one being a controlled rectifier for the purpose of providing voltage control, the apparatus further comprising firing means in which means for providing a ramp waveform voltage are included, and control means for providing an oscillating control voltage of variable frequency, said firing means providing control voltage pulses for firing angle control of the or each controlled rectifier in response to the interaction between said voltages, the ramp waveform voltage comprising only slopes of common inclination and the frequency of the component of the A.C.

output voltage from the semiconductor rectifiers of the or each phase which produces the dominant current component in the phase is the difference between the A.C. supply frequency and the frequency of the oscillating control voltage.

The means for providing an oscillating voltage preferably include means to vary the amplitude of the voltage in addition to its frequency and in preferred apparatus according to the invention frequency and amplitude variations are jointly controlled to provide a desired motor control function.

The oscillating voltage waveform is preferably of sinusoidal profile, but rectangular or other suitable profiles may be used.

Provision of variable direct current biassing means for the oscillating voltage is required in most cases, depending on the type of oscillator used, in order to achieve the desired interaction between the oscillating voltage and the ramp waveform voltage.

Accordingly, apparatus in accordance with the invention may include means for controlling in combination the frequency of an oscillating voltage together with a direct current biassing voltage level for the oscillating voltage.

The combined control is preferably effected by a ramp voltage and in preferred apparatus according to the invention two different control ramp voltages are provided for different ranges of firing control.

Combined control of frequency, amplitude and direct current biassing voltage of the oscillating control voltage whether governed by controlling ramp voltages or other means, is employed in preferred embodiments of the invention.

The firing means may include a thyristor control system of the type described in British Patent Specification No. 1,227,952.

In the preferred embodiment to be described, control of a three-phase alternating current motor supplied by a three-phase supply system is achieved by voltage control using apparatus according to the invention.

An anti-parallel arrangement of a thyristor and a semiconductor diode is used in each phase of the supply in the preferred embodiment, but alternative arrangements of thyristors are possible. Anti-parallel arrangements of thyristors only, for example, may be used for the preferred and other polyphase embodiments, and it is essential that such an arrangement be used in the application of the invention to single phase voltage control.

The invention is particularly applicable to three-phase alternating current motor control where the A.C. supply voltage is required to produce a rotating vector field, as is the case for example in induction motors, including synchronous motors and reluctance synchronous motors. In such applications the invention provides an effective, readily applicable and relatively cheap method of obtaining creep speed operation and reversal of drive as compared with previous more sophisticated controls for commonly used alternating current motors.

The interaction between the ramp waveform voltage and oscillating voltage in the firing means is in some respects at least analogous to a mixing effect. Thus, the most important aspects of the interaction are that the predominant frequency of the output voltage achieved by the invention is the difference frequency between the oscillating control voltage and the three-phase A.C. supply frequency, and that in a three-phase application the phase differences between the three resultant voltage waveforms of the output achieved are 120 .

Mathematical Analysis As is well known, a 3-phase, balanced sine wave voltage supply can be represented as A = V sin (W t) ..... (i) c c c

(where Ac, Bc, Cc are the three phases of the supply of voltage amplitude Vc).

A voltage wave as described by each of the above expressions may be used as a carrier wave and modulated by a further voltage wave whose expression may be: Xm=P+Q sin (cornt) (4) (where P is a bias voltage amplitude and Q is the modulating voltage amplitude).

Three modulation products result whose values are given below:

Consider one of these modulation products, (5) which consists of three components,

and the effect of applying this voltage to one stator winding of an induction motor whose simplest equivalent circuit is a resistance and inductance in series.

Component (a) is at supply frequency and will cause a current to flow in the winding proportional to the magnitude of the voltage (VcP) and inversely proportional to the impedance of the winding at the supply frequency.

Component (b) is at the difference frequency between the supply and modulating oscillator and will cause a current to flow in the winding proportional to the magnitude of the voltage VcQ 2 and inversely proportional to the impedance of the winding at the difference frequency: Component (c) is at the sum frequency of the supply and modulating oscillator and will cause a current to flow in the winding proportional to the magnitude of the voltage VcQ ( ) 2 and inversely proportional to the imDedance of the winding at the sum frequency.

It should be noted that in any modulated wave the peak amplitudes of the carrier and the modulating wave are the same and this means that in this case VcP and VcQ 2 have the same value and therefore that the amplitudes of all the modulation products are equal.

Taking a practical case where fc=50 H2 and fm=45 Hz, the stator currents will be inversely proportional (approximately) to frequency only, and the three modulation products will produce currents in the following proportion: At 5 Hz (50-45) 100% At 50 Hz - 10% At 95 Hz (50+45) 5% Thus the motor torque, which is a function of current, will be largely that due to the 5 Hz component and the machine will revolve at the appropriate speed for that frequency.

From equations (5), (6) and (7) it should be noted that the phase-shift between the modulation products for each component of frequency is appropriate to that frequency, that is to say, the supply frequency components VcP sin & ct 27 VcP sin (coct ) 3 4,r VcP sin (w,t---) - 3 are phase shifted 1200 relative to each other at frequency (w,-w,) and similarly for the sum frequency components.

The difference frequency output components, which have been shown to produce the most significant current, are therefore correctly phased to provide normal drive conditions for an induction motor.

Increase of the modulation frequency so as to exceed the supply frequency causes the motor to reverse direction as the coincident frequency is passed and to run in the reverse direction at the subsequent difference frequency.

For low resultant frequencies ( < 2 Hz) applied to the motor, the motor is seen to "cog", that is, to move in step fashion in a series of 60 angular steps for a 4-pole machine.

In order that the invention may be more fully understood, preferred embodiments thereof will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an a.c. motor drive system including control apparatus in accordance with the present invention; Figure 2 is a diagram showing in more detail a control unit in the system shown in Figure 1; Figure 3 shows a circuit arrangement of part of the control unit shown in Figure 2; Figure 4 shows a firing angle/input voltage characteristic for the firing unit shown in Figure 1; Figure 5 shows a control unit output function for the unit shown in Figure 2 during a motor start operation; Figure 6 is a graph illustrating the interaction between control voltages in the firing unit in Figure 1; and Figure 7 shows an alternative thyristor controlled motor arrangement to which the invention is applicable.

Referring first to Figure 1, a three-phase A.C. induction motor 1 has its stator supplied by a 50 Hz three-phase voltage system via lines 2, the three phases being designated R(red), Y(yellow) and B(blue).

Stator voltage control of motor 1 is provided by a thyristor/diode phasecontrolled regulator comprising an A.C. switch arrangement of a thyristor 3 and diode 4 in an anti-parallel arrangement in each supply line. The regulator may be by-passed by contactor 5 to provide a full supply voltage to the motor stator phases.

Firing means for the thyristors 3 are provided in a firing unit 6. This unit initiates the firing of the thyristors in each phase, and thus their conduction angles, and is in turn controlled by the control unit 7.

The details of control unit 7 are shown in Figure 2, where it will be seen that its principal elements comprise a ramp generator 8, a voltage controlled oscillator 9, a current limiting unit 10 and a summing amplifier 11. The ramp generator 8 controls the conduction angles of thyristors 3 under the command of manual controls 13.

Synchronising transformers 14 for firing unit 6 are also utilised to provide low voltage direct current supplies for the control unit 7.

The voltage controlled oscillator 9 gives a sinusoidal voltage output the frequency of which varies inversely with the input voltage. A variable directcurrent biassing voltage is obtained from ramp generator 8 and both this voltage and the oscillator output voltage are fed to summing amplifier 11. A control ramp voltage from ramp generator 8 is fed to oscillator 9 and this, in combination with the varying biassing voltage, is arranged to yield an output from summing amplifier 11 consisting of a voltage of decreasing frequency which is increasingly offset from a zero reference level. This output is applied to the firing unit 6 where it is combined with a ramp waveform voltage generated in the firing unit.Firing of the thyristors in accordance with the resultant firing control voltage gives an output voltage to motor I the frequency of which is the difference frequency between the oscillating voltage frequency from oscillator 9 and the A.C. supply frequency at any instant during the frequency variation of the oscillator. Thus the ramp voltage from ramp generator 8 effects variation of speed of the motor 1.

The output from current limit unit 10 is also fed into the operational amplifier 11 to limit current into the motor windings. This limitation is achieved by means of feedback, the current limit unit 10 receiving load current signals from a current transformer arrangement 15 in the supply lines via line 16.

Figure 3 shows details of a ramp generating circuit suitable for ramp generator 8, to give a motor control facility whereby motor speed may be increased from zero to a creep speed corresponding to a 5Hz motor supply voltage, and then run up to a full operational speed of 50 Hz motor supply voltage, at which speed by pass contactors 5 (shown in Figure 1) are brought into operation by a relay RLI.

The ramp circuit comprises a resistor Rl and capacitor Cl and the latter is clamped with zero voltage drop across it at position Sl of a ganged switch 17, this being the "off' position of the control. The oscillator 9 is thus supplied with zero volts at this position, but has an inverse characteristic so that it is arranged to give a 50 Hz output with this input. Thus, as a rising ramp voltage is applied to the oscillator 9 its output frequency falls.

Two voltage ramps are available from the ramp circuit. The first ramp is initiated by moving the switch to position S2 and this causes the motor to start and run up to creep speed. The creep speed is set by a voltage V1, as indicated in Figure 3, and corresponds to a frequency of 45 Hz from oscillator 9 when the end of the ramp is reached. The motor thus runs steadily at creep speed with an applied voltage of 5 Hz frequency applied to its stator, as indicated by the level portion of the oscillator characteristic shown in Figure 4.

The motor is next run up to full speed by selecting position S3 of switch 17 which applies voltage V2 to the oscillator. This voltage corresponds to 0 Hz frequency output from the oscillator and thus provides a final 50 Hz frequency supply to the motor when the end of the second voltage ramp is reached, causing the motor to run at full speed.

The voltage at the junction of Rl and Cl when the full speed value is reached is arranged to be sufficient to allow a zener diode 18 to conduct and thus operate a relay RLI by way of a transistor 19. Operation of this relay initiates contactor 5 to allow the supply to bypass the thyristor control arrangement.

The lower connections of switch 17 in Figure 3 allow voltages Vla and V2a to be connected to the current limiting unit 10 in Figure 3 to limit current in the motor windings for the creep and running speed settings, respectively.

Control of the amplifier output voltage waveform in the embodiment described has been limited to oscillator frequency and current biassing directly from the ramp generator 8 and adequate motor control for many purposes may be achieved with such an arrangement. Improved motor control voltage waveforms may be obtained if required, however, by the additional control of oscillator amplitude. Such additional control can be provided without undue complication in the control circuitry.

Figure 5 illustrates the signals emitted by the amplifier 11 as referred to a firing circuit input voltage/conduction angle characteristic when both oscillator output parameters are varied. The basic characteristic curve of a firing unit (for example of the type described in British Patent Specification No. 1,227,952), is indicated by curve 20, which indicates the thyristor conduction angle variation for different settings of input voltage to the unit. The curves 21 and 22 represent the relevant voltage signals emitted by the amplifier 11 for creep and running speeds, respectively. The intersections of those curves with the curve 20 indicate the relative timing of the instants of firing applicable to each thyristor in turn in order to produce those speeds.

Figure 6 indicates the interaction between the oscillating voltage waveform from the control unit 7 and the ramp voltage waveform in the firing.unit 6.

The ramp voltage waveform repeats three times for each cycle of the 50 Hz supply, that is to say, every 1200, which is the standard arrangement for controlling a three-phase thyristor firing arrangement. In known control apparatus, firing angle control is achieved simply by interaction between this waveform and a direct current voltage level setting, causing all thyristors to conduct over the same set level. Thus, intersections due to a set d.c. level close to the lower end of the ramps of the sawtooth waveform would result in thyristors firing over almost the entire positive half-cycles of voltage applied to them, whilst intersections near the top of a ramp would give minimal periods of conduction.

The figures below the waveforms of Figure 6 given at (i) indicate the approximate percentage conduction periods of thyristors when the single-phase oscillating waveform from the control unit 7 is applied to the firing unit 6 in accordance with the invention, the figures applying to red-phase thyristor conduction being shown along line (ii), and the yellow and blue phase thyristors at (iii) and (iv), respectively.

It is thus seen that the red-phase thyristor is allowed to conduct in cycles of pulses of conduction angles (expressed as percentages of full half wave conduction) 0, 100, 70%, 40 /" and 0. The envelope of these voltage pulses, whilst not sinusoidal, provides the desired alterating voltage waveform for motor control. It will be noted from Figure 6 that the envelope waveforms for the yellow and blue phases are displaced by 1200 with respect to each other and the red phase on the low frequency wave envelope cycle.

Whilst in most applications an anti-parallel thyristor and diode combination is necessary, Figure 7 shows a motor control arrangement in which only three thyristors are used in a three-phase system, the switching of the A.C. currents in each phase being effected by an A.C. switch arrangement comprising the two thyristors connected to the phase. In single phase applications, however, an antiparallel arrangement of two thyristors must be employed to give control of reverse half-cycle current.

From the foregoing it will be seen that a relatively simple means has been provided to enable a single selected, or a range of selected, lower operating speeds to be obtained from an A.C. induction motor. Such means also enable a graduated or 'soft' start to be achieved and enable reversal of the machine at low speeds to be electronically carried out in a precise manner.

Within the limitations imposed by machine heating, due to the relatively crude waveform and inefficient cooling at low rotor speeds, high torque can be developed at currents which are not excessive as the motor is operating on a variable frequency, variable voltage supply. A further advantage is that no rotating feedback element, such as a tachometer, is required for "off synchronous" running. The invention can, therefore, be applied to existing installations with minimum modifications.

The invention provides a measure of braking between run and creep speed settings for a motor which, in some applications, may allow conventional methods of braking to be reduced or dispensed with.

WHAT WE CLAIM IS: 1. Control apparatus for controlling the speed of A.C. induction motors by direct conversion of an A.C. supply voltage to an A.C. output voltage comprising for the or each motor stator phase an A.C. switch arrangement comprising, two semiconductor rectifiers one being a controlled rectifier for the purpose of providing voltage control, the apparatus further comprising firing means in which means for providing a ramp waveform voltage are included, and control means for providing an oscillating control voltage of variable frequency, said firing means providing control voltage pulses for firing angle control of the or each controlled rectifier in response to the interaction between said voltages, the ramp waveform voltage comprising only slopes of common inclination and the frequency of the component of the A.C. output voltage from the semiconductor rectifiers of the or each phase which produces the dominant current component in the phase is the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. settings of input voltage to the unit. The curves 21 and 22 represent the relevant voltage signals emitted by the amplifier 11 for creep and running speeds, respectively. The intersections of those curves with the curve 20 indicate the relative timing of the instants of firing applicable to each thyristor in turn in order to produce those speeds. Figure 6 indicates the interaction between the oscillating voltage waveform from the control unit 7 and the ramp voltage waveform in the firing.unit 6. The ramp voltage waveform repeats three times for each cycle of the 50 Hz supply, that is to say, every 1200, which is the standard arrangement for controlling a three-phase thyristor firing arrangement. In known control apparatus, firing angle control is achieved simply by interaction between this waveform and a direct current voltage level setting, causing all thyristors to conduct over the same set level. Thus, intersections due to a set d.c. level close to the lower end of the ramps of the sawtooth waveform would result in thyristors firing over almost the entire positive half-cycles of voltage applied to them, whilst intersections near the top of a ramp would give minimal periods of conduction. The figures below the waveforms of Figure 6 given at (i) indicate the approximate percentage conduction periods of thyristors when the single-phase oscillating waveform from the control unit 7 is applied to the firing unit 6 in accordance with the invention, the figures applying to red-phase thyristor conduction being shown along line (ii), and the yellow and blue phase thyristors at (iii) and (iv), respectively. It is thus seen that the red-phase thyristor is allowed to conduct in cycles of pulses of conduction angles (expressed as percentages of full half wave conduction) 0, 100, 70%, 40 /" and 0. The envelope of these voltage pulses, whilst not sinusoidal, provides the desired alterating voltage waveform for motor control. It will be noted from Figure 6 that the envelope waveforms for the yellow and blue phases are displaced by 1200 with respect to each other and the red phase on the low frequency wave envelope cycle. Whilst in most applications an anti-parallel thyristor and diode combination is necessary, Figure 7 shows a motor control arrangement in which only three thyristors are used in a three-phase system, the switching of the A.C. currents in each phase being effected by an A.C. switch arrangement comprising the two thyristors connected to the phase. In single phase applications, however, an antiparallel arrangement of two thyristors must be employed to give control of reverse half-cycle current. From the foregoing it will be seen that a relatively simple means has been provided to enable a single selected, or a range of selected, lower operating speeds to be obtained from an A.C. induction motor. Such means also enable a graduated or 'soft' start to be achieved and enable reversal of the machine at low speeds to be electronically carried out in a precise manner. Within the limitations imposed by machine heating, due to the relatively crude waveform and inefficient cooling at low rotor speeds, high torque can be developed at currents which are not excessive as the motor is operating on a variable frequency, variable voltage supply. A further advantage is that no rotating feedback element, such as a tachometer, is required for "off synchronous" running. The invention can, therefore, be applied to existing installations with minimum modifications. The invention provides a measure of braking between run and creep speed settings for a motor which, in some applications, may allow conventional methods of braking to be reduced or dispensed with. WHAT WE CLAIM IS:

1. Control apparatus for controlling the speed of A.C. induction motors by direct conversion of an A.C. supply voltage to an A.C. output voltage comprising for the or each motor stator phase an A.C. switch arrangement comprising, two semiconductor rectifiers one being a controlled rectifier for the purpose of providing voltage control, the apparatus further comprising firing means in which means for providing a ramp waveform voltage are included, and control means for providing an oscillating control voltage of variable frequency, said firing means providing control voltage pulses for firing angle control of the or each controlled rectifier in response to the interaction between said voltages, the ramp waveform voltage comprising only slopes of common inclination and the frequency of the component of the A.C. output voltage from the semiconductor rectifiers of the or each phase which produces the dominant current component in the phase is the

difference between the A.C. supply frequency and the frequency of the oscillating control voltage.

2. Control apparatus as claimed in Claim 1, in which the control means for providing an oscillating control voltage includes means to vary both the amplitude and frequency of the oscillating voltage.

3. Control apparatus as claimed in Claim 2, in which variations in the amplitude and frequency of the oscillating control voltage may be jointly controlled to provide a desired motor control function.

4. Control apparatus as claimed in any of Claims I to 3 in which variable direct current biassing means for the oscillating control voltage is provided.

5. Control apparatus as claimed in claim 1 in which means are provided for controlling in combination the frequency of the oscillating control voltage together with the level of direct current biassing voltage for the oscillating control voltage.

6. Control apparatus as claimed in Claim 5 in which the combined control is effected by a ramp voltage arranged in conjunction with the oscillating control voltage to give àn output voltage of decreasing frequency whilst being increasingly offset from a zero voltage reference level.

7. Control apparatus as claimed in Claim 6 in which two or more control ramp voltages of different slope are provided for different ranges of thyristor firing control.

8. Control apparatus according to any of Claims I to 7 in which combined control of the frequency, amplitude and direct current biassing voltage of the oscillating control voltage is provided.

9. Control apparatus as claimed in any of Claims 1 to 8 in which the firing means comprises a thyristor control system of the type described in British Patent Specification No. 1,227,952.

10. Control apparatus as claimed in any of Claims I to 9 applied to control the rotating field of an alternating current motor.

11. Control apparatus as claimed in any of Claims 1 to 9 applied in the form of a start and speed control system for a three-phase induction motor connected to a three-phase supply.

12. Control apparatus as claimed in Claim 11 in which each of the three phase windings of the motor field are unitary, non-tapped, windings.

13. Control apparatus as claimed in Claim 11 or 12 in which a reverse-parallel connected rectifier arrangement comprising at least one thyristor is provided in each supply phase whilst the control means for thyristor firing control provides control voltage pulses for three thyristors oly.

14. Control apparatus according to Claim I substantially as described herein and with reference to Figures 1 to 6 of the accompanying drawings.

15. Control apparatus according to Claim 1 substantially as described herein with reference to Figures 1 to 6 of the accompanying drawings but modified substantially as described herein with reference to Figure 7 of the accompanying drawings.