GB2268010A - Electric motor speed control - Google Patents

Abstract

Speed control of a brushless DC motor is achieved by a circuit arrangement such as an integrated circuit 10 which has a brake input BR for regenerative braking by connecting the motor windings to earth. Speed reduction beyond that normally achieved is produced by applying a pulsed braking signal to the brake input BR from a pulse generator 18 capable of producing a pulsed signal with a variable mark-to-space ratio for speed control. The arrangement also includes an inhibit circuit (24, Fig. 3) for avoiding motor stall at switch-on when the arrangement is set for low speed operation. The pulse generator 18 is described in detail (Fig. 3). <IMAGE>

Description

ELECTRICAL MOTOR SPEED CONTROL APPARi9US This invention relates to electrical motor speed control apparatus for providing an improved control speed range, and has particular application to the control of a brushless DC motor using a simple semiconductor integrated circuit control device.

It is known to connect an electric motor having a plurality of stator windings to an integrated circuit control chip which provides electronic commutation of the supply currents and which allows the motor to be driven in a forward direction and a reverse direction, to be varied in speed, and to be dynamically braked by shorting the windings to an earth connection. Speed control is achieved by variation of a threshold voltage input to the integrated circuit. If the threshold voltage is reduced below a certain figure, the currents flowing through the motor are correspondingly reduced and the motor slows. Commutation of the supply currents is performed in response to inputs from a sensing assembly, which may comprise three Hall effect sensors associated with a magnetic element or elements on a rotating part of the motor.While such an integrated circuit yields a very simple and inexpensive control system, the range of speed control is limited due to the fact that the currents supplied to the motor at the lowest speed setting are such that the motor speed can often be controlled to no lower than 1500 rpm or some other value dependent on the parameters of both the drive circuit and of the particular motor design.

It is an object of this invention to provide improved apparatus which allows the motor to be run at slower speeds.

According to this invention, speed control apparatus for an electrical motor comprises a switching device for commutating current supplied to windings of the motor to cause the motor to run and for connecting the windings in a regenerative braking configuration to cause the motor to slow down and stop, and a pulse generator coupled to the switching device for operating the switching device in a manner in which the windings are repeatedly momentarily connected in the braking configuration when the motor is to be run at a reduced speed.In the preferred embodiment the switching device comprises an array of semiconductor switch or amplifier elements on an integrated circuit chip, the switch or amplifier elements being arranged to feed current from a supply pin to respective output pins intended for connection to the motor windings, each element being triggered by an input signal derived from sensors associated with the motor, for instance Hall effect sensors mounted on the motor to detect the rotational position of the motor armature. The integrated circuit has a brake input which, on receipt of a braking input signal, causes the outputs for the motor windings to be disconnected from the supply voltage and connected to an earth pin for regenerative braking of the motor. In this configuration the windings are effectively coupled in low resistance or nil resistance loops.It has been found that, by applying a pulsed signal to the brake input so that the windings are repeatedly connected to earth several times during each motor revolution, the motor can be operated reliably at much lower speeds than has been possible with such an integrated circuit using the known threshold control method described above.

To avoid the possibility of the motor stalling when starting with the pulse braking signal applied, the pulse generator includes an inhibit circuit for inhibiting generation of the pulsed braking signal upon starting.

Speed of the motor may be varied over a wide range by altering the mark-to-space ratio of the pulsed braking signal. This may be achieved by generating the signal using an oscillator which produces a triangular or sawtooth waveform, feeding this waveform to one input of a comparator the other input of which is fed with a constant voltage signal having a level determining the mark-to-space ratio.

The invention also includes a method of controlling the speed of an electric motor which has windings coupled to a supply by a switching device by commutating the current supply to the windings to cause the motor to run, wherein the method comprises repeatedly momentarily connecting the windings in a regenerative braking configuration in response to a pulsed control signal to cause the motor to run at reduced speed.

The invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a circuit diagram of prior motor speed control apparatus; Figure 2 is a circuit diagram of motor speed control apparatus in accordance with the invention; Figure 3 is a detail circuit diagram showing part of the apparatus of Figure 2; and Figure 4 is a waveform diagram.

Referring to Figure 1, a known brushless DC motor control circuit makes use of an inexpensive bipolar integrated circuit 10 including a switching device 10S in the form of commutation logic which operates in response to sensing signals fed to inputs H1, H2 and H3 of the integrated circuit from three Hall effect sensors 12A, 12B and 12C to feed currents from a power supply pin V+ in a predetermined sequence to three output pins A, B, and C connected to the windings 14A, 14B, and 14C of a brushless DC motor 14 via connections 15A, 15B and 15C. The integrated circuit 10 has three control inputs DIR, BR and THS.The first input DIR is for controlling the direction of rotation of the motor, and is a simple change of state input which, when taken to a high voltage causes the motor to rotate in one direction, and when taken to a low voltage causes the motor to rotate in the opposite direction. The second input BR is for dynamically braking the motor. Again, this is a simple change of state input and when activated switches the three outputs A, B, and C to an earth pin E. A third input THS is a threshold voltage input for controlling the speed of rotation of the motor. If the voltage applied to this input is reduced below a certain level, the current flowing through the motor will be reduced progressively as the voltage is reduced.However, the minimum motor current which can be achieved by reducing the threshold voltage is insufficiently low for many applications where the possibility of running the motor at speeds much less than 1500 rpm is required. In practice, it is difficult to reduce the motor current to less than one third of the maximum motor current. A typical integrated circuit device having these features is available from Allegro Microsystems, Inc. under type number UDN-2936W.

In the prior art circuit, regenerative braking is achieved by grounding the brake input BR using a switch SBR. The threshold input is commonly connected to the same low voltage supply 16 which feeds the Hall effect sensors 12A, 12B, and 12C as shown if constant speed operation is required, or may be coupled to a variable voltage source for variable speed operation.

Referring now to Figure 2, motor control apparatus in accordance with the invention makes use of the same integrated circuit 10, but instead of connecting the brake input BR to ground via a switch as in Figure 1, this input BR is coupled to the output 180 of a pulse generator which generates a pulsed braking signal at a frequency such that the motor windings are connected in a regenerative braking configuration momentarily several times during each motor revolution. The threshold input THS is set at a continuous high level by means of a potential divider PD connected across the main DC supply V+. This potential divider also provides the supply 18S for the pulse generator 18. The direction input DIR is operable as before.In the diagram of Figure 2 it should be noted that the moto.r and Hall effect sensors of Figure 1 are not shown, but are considered to be connected to a group of terminals 20. Another group of terminals 22 provides for connection of the supply and for application of a signal to change the direction of rotation and a continuous brake signal to the input BR to stop the motor. The pulse generator 18 has a control input 18C for altering a characteristic of the pulsed braking signal to vary the level of regenerative braking and thereby the speed of the motor.

The circuitry of the pulse generator 18 is shown more clearly in Figure 3. This circuitry is particularly simple and inexpensive to construct since it can be produced from a single quad op-amp integrated circuit, two capacitors, a bipolar transistor, a diode, and nine resistors, one of which is variable. Referring to Figure 3, the pulse generator 18 comprises a simple dual op-amp triangular waveform generator IC1/1, IC1/2 which feeds the inverting input of a third op-amp IC1/3, the non-inverting input of op-amp IC1/3 being connected to the control input 18C, here shown connected to the wiper of a variable resistor VR1 connected between a low voltage supply rail and earth.The output of this op-amp IC1/3 is connected via a resistor R7 to a switching transistor TRi, the emitter of which provides an output for connection to the brake input of the integrated circuit 10 shown in Figure 2. Referring to Figure 4, the two inputs of the comparator op-amp IC1/3 are shown as waveform A and waveform B. It will be appreciated that the output waveform at the output of op-amp IC1/3 is a square wave as shown by waveform C in Figure 4, the mark-tospace ratio depending on the relative level of the signals applied to the comparator inputs.

Referring back to Figure 3, an inhibiting circuit for inhibiting the pulsed braking signal when the circuit is switched on comprises a fourth op-amp IC1/4 having one input connected to an RC timing network C1, R4 and its other input connected to a resistive potential divider. The output of op-amp IC1/4 is coupled via a diode D1 to the output of the comparator op-amp IC1/3. The effect of the inhibiting circuit 24 is to impose a lower limit on the voltage applied to the base of the transistor TR1 as shown by waveforms D and E. This has the effect of preventing the regenerative braking action for a period when the motor is started, so as to avoid the possibility of stalling.

It has been found that, by adjusting the voltage applied to the non-inverting input of the comparator op-amp IC1/3 using resistor VR1, the pulse widths of the pulsed braking signal (waveforms C and E in Figure 4) may be varied over a wide range and reliable operation of a motor at a speed less than 60 rpm has been possible. Typically, the frequency of the pulsed braking signal is such as to provide 10 braking operations for each phase cycle.

The effect of the inhibiting circuit depends on the time constant of the RC timing network C1, R4. In the preferred embodiment the sequence of operation is such that after switch-on, the timer circuit is active for a period sufficient to accelerate the motor to full power. After this period, the pulse width modulation circuit becomes active and the motor speed drops to a level set by variable resistor VR1.

Claims (11)

1. Speed control apparatus for an electric motor comprising a switching device for commutating current supplied to windings of the motor to cause the motor to run and for connecting the windings in a regenerative braking configuration to cause the motor to slow down or stop, and a pulse generator coupled to the switching device for operating the switching device in a manner in which the windings are repeatedly momentarily connected in the braking configuration when the motor is to be run at a reduced speed.

2. Apparatus according to claim 1, wherein the pulse generator includes an inhibit circuit operable to inhibit pulse generation by the pulse generator when the motor is started.

3. Apparatus according to claim 1 or claim 2, wherein the pulse generator has a speed control input and is arranged to allow variation of the mark-to-space ratio of a pulsed signal applied to the switching device in response to a signal fed to the speed control input in order to permit variation of the motor speed.

4. Apparatus according to claim 3, wherein the pulse generator comprises an oscillator, and a comparator for receiving an oscillating signal from the oscillator and a threshold signal from the speed control input, the comparator thereby producing the pulsed signal, and the pulsed signal having a mark-to-space ratio which depends on the level of the threshold signal.

5. Apparatus according to claim 4, wherein the oscillator is a triangular or sawtooth waveform generator.

6. Apparatus according to claim 2, and claim 3, claim 4, or claim 5, wherein the inhibit circuit includes a resistor and capacitor timing network and a comparator for generating an inhibit signal which overrides the pulsed signal for a predetermined period when power is initially switched to the apparatus.

7. Apparatus according to any preceding claim, wherein the pulse generator is operable at a frequency which causes the motor windings to be connected in the braking configuration a plurality of times during each motor revolution.

8. A method of controlling the speed of an electric motor which has windings coupled to a supply by a switching device for commutating the current supplied to the windings to cause the motor to run, the method comprising repeatedly momentarily connecting the windings in a regenerative braking configuration in response to a pulsed control signal to cause the motor to run at reduced speed.

9. A method according to claim 8, including the step of inhibiting the pulsed control signal when the motor is started.

10. Speed control apparatus for an electric motor, the apparatus being substantially as herein described and as shown in Figures 2 and 3 of the drawings.

11. A method of controlling the speed of an electric motor substantially as herein described.