GB1595963A - Motor control circuits - Google Patents

Description

(54) MOTOR CONTROL CIRCUITS (71) We, LUCAS INDUSTRIES LIMITED, a British Company located at Great King Street, Birmingham B19 2XF, 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 a motor control circuit of the kind comprising a pair of leads for connection in use, to the positive and negative terminals of a source of d.c.

supply, a motor having an armature connected to one of said leads and a series field winding, a contactor for effecting reversal of the direction of current flow through the field winding in order to obtain reverse torque, a transistor having its collector emitter path connected in series with the contactor and the other lead, and a pair of diodes constituting a plugging diode and a free wheel diode respectively and a pulse source for switching the transistor on and off.

Such a circuit is well known and it has been the practice to connect the plugging diode in parallel with the armature and the free wheel diode between said one lead and the terminal or terminals of the contactor which are connected to the transistor. In this manner irrespective of the setting of the contactor other than the case where it forms an open circuit, the free wheel diode is connected in parallel with the armature and field winding. The contactor normally employs a pair of coils one of which is energised to achieve one direction of rotation of the motor and the energisation of the coils is controlled by switch means the power required being obtained from the aforesaid leads.

There is one problem with this arrangement and it is that if the leads sould be incorrectly connected to the source of supply then even when the contactor is open circuit, the free wheel diode provides a path which applies the reverse battery voltage across the transistor. This could lead to the destruction of the transistor.

The object of the invention is to provide a circuit of the kind specified in a simple and convenient form.

According to the invention in a power circuit of the kind specified said diodes are directly connected between said one lead and the ends of the field winding respectively, said contactor being arranged to connect one end of the field winding to the armature and the other end to the other lead through the transistor and vice versa, the operating coil or coils for the contactor having reverse voltage protection whereby the contactor cannot be closed when the leads are incorrectly connected to the source of supply.

One example of a motor control circuit in accordance with the invention will now be described with reference to the accompanying drawings in which there is provided a pair of leads 10, 11 for connection respectively to the positive and negative terminals of a source of d.c. supply indicated as a storage accumulator 12. A motor is provided and which has an armature 13 one brush associated with which being connected to the lead 10 by way of a series resistor 14. The other brush connection is connected to one set of contacts of a pair of switches 15, 16 forming part of a contactor. The contactor also includes a further pair of switches 17, 18 and contacts of these two switches are connected to the collector of an n.p.n. transistor 19 having its emitter connected to the lead 11.

The other contacts of the switches 15 and 17 are connected together and to one end of the field winding 20 of the motor and also to the lead 10 through the anode cathode path of a diode 21. The other contacts of the switches 16 and 18 are also connected together and to the other end of the field winding 20 and also to the lead 10 by way of the anode cathode path of a diode 22.

The switches 15 and 18 are mechanically coupled together so as to be closed when an actuating coil 23 is energised and the switches 16 and 17 are similarly mechanically coupled together and are arranged to be closed when an actuating coil 24 is energised.

One end of each of the actuating coils 23, 24 is connected to the lead 11 and the other ends are connected to switch means indicated at 25 the supply to the switch means being taken from the lead 10 by way of the anode cathode path of a diode 26. The switch means 25 may be a solid state switch associated with control circuits for the system. The solid state switch includes protection means whereby it is impossible for the two actuating coils to be energised at the same time.

The base of the transistor 19 receives control pulses of variable length from a pulse source 27 and this in turn receives a control signal from an operator controlled member including a potentiometer indicated at 28. As the operator moves his control to increase the speed of operation of the motor or increase the power developed by the motor, the pulses provided by the pulse source 27 increase in length so that the transistor 19 is switched on for a longer period of time.

Assuming that the switches 15 and I8 are closed then when the transistor 19 is switched on the motor will operate in one direction.

If now the aforesaid switches are opened and the other switches namely 16 and 17 closed the motor will operate in the opposite direction.

Assuming for the moment that switches 15 and 18 are closed then the diode 21 is connected in parallel with the armature 13 of the motor and the diode 22 is connected in parallel with the series combination of the armature and field winding of the motor.

In this situation therefore the diode 22 constitutes the free wheel diode and serves when the transistor 19 is switched off, to maintain the current flow in the armature and field winding of the motor at least for a portion of the time the transistor is turned off.

When the switches 16 and 17 are closed then diode 21 becomes the free wheel diode.

During plugging the diode which is connected in parallel with the armature of the motor forms the plugging diode.

With the arrangement described above if the leads 10 and 11 should be inadvertently connected to the opposite terminals of the battery then since the actuating coils cannot be energised because the diode 26 will be reverse biassed, the switches 15, 16, 17, 18 will remain in the open position and therefore neither of the diodes 21, 22 can provide a path for the incorrectly connected battery voltage to appear across the transistor 19.

The transistor is therefore protected.

It is usual in such systems to provide for current sensing of the motor current so as to ensure that the transistor and/or the motor is not overloaded particularly during plugging.

This is usually done by sensing the voltage across the plugging diode. In the circuit described however the plugging diode/motor armature loop includes a pair of contacts and as is well known, the voltage drop across a pair of contacts does in practice vary.

It is necessary therefore to sense the plugging current in some other fashion and for this purpose a pair of amplifiers 29, 30 are provided. Amplifier 29 has as its input, the voltage developed across the resistor 14 and amplifier 30 receives the voltage developed across the armature and the resistor 14.

The amplifier 30 senses when plugging is about to take place and its output enables the amplifier 29. The output of the amplifier 29 is representative of the plugging current and this is supplied to the pulse source or associated circuits so that the conduction of the transistor 19 during plugging is controlled.

WHAT WE CLAIM IS: 1. A motor control circuit of the kind specified in which said diodes are directly connected between said one lead and the ends of the field winding respectively, said contactor being arranged to connect one end of the field winding to the armature and the other end to the other lead through the transistor and vice versa, the operating coil or coils for the contactor having reverse voltage protection whereby the contactor cannot be closed when the leads are incorrectly connected to the source of supply.

2. A circuit as claimed in claim 1 in which the reverse voltage protection is afforded by a diode connected in series with said operating coil or coils.

3. A circuit as claimed in claim 1 including a resistor connected in series with the armature and said one lead, first means responsive to the voltage developed across the resistor and the armature to detect plugging and the second means responsive to the voltage developed across said resistor and which is enabled by said first means when plugging is detected said second means acting to control the length of the pulses produced by said pulse source thereby to prevent overload of the motor and/or transistor.

4. A motor control circuit comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying diagram.

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

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. at 25 the supply to the switch means being taken from the lead 10 by way of the anode cathode path of a diode 26. The switch means 25 may be a solid state switch associated with control circuits for the system. The solid state switch includes protection means whereby it is impossible for the two actuating coils to be energised at the same time. The base of the transistor 19 receives control pulses of variable length from a pulse source 27 and this in turn receives a control signal from an operator controlled member including a potentiometer indicated at 28. As the operator moves his control to increase the speed of operation of the motor or increase the power developed by the motor, the pulses provided by the pulse source 27 increase in length so that the transistor 19 is switched on for a longer period of time. Assuming that the switches 15 and I8 are closed then when the transistor 19 is switched on the motor will operate in one direction. If now the aforesaid switches are opened and the other switches namely 16 and 17 closed the motor will operate in the opposite direction. Assuming for the moment that switches 15 and 18 are closed then the diode 21 is connected in parallel with the armature 13 of the motor and the diode 22 is connected in parallel with the series combination of the armature and field winding of the motor. In this situation therefore the diode 22 constitutes the free wheel diode and serves when the transistor 19 is switched off, to maintain the current flow in the armature and field winding of the motor at least for a portion of the time the transistor is turned off. When the switches 16 and 17 are closed then diode 21 becomes the free wheel diode. During plugging the diode which is connected in parallel with the armature of the motor forms the plugging diode. With the arrangement described above if the leads 10 and 11 should be inadvertently connected to the opposite terminals of the battery then since the actuating coils cannot be energised because the diode 26 will be reverse biassed, the switches 15, 16, 17, 18 will remain in the open position and therefore neither of the diodes 21, 22 can provide a path for the incorrectly connected battery voltage to appear across the transistor 19. The transistor is therefore protected. It is usual in such systems to provide for current sensing of the motor current so as to ensure that the transistor and/or the motor is not overloaded particularly during plugging. This is usually done by sensing the voltage across the plugging diode. In the circuit described however the plugging diode/motor armature loop includes a pair of contacts and as is well known, the voltage drop across a pair of contacts does in practice vary. It is necessary therefore to sense the plugging current in some other fashion and for this purpose a pair of amplifiers 29, 30 are provided. Amplifier 29 has as its input, the voltage developed across the resistor 14 and amplifier 30 receives the voltage developed across the armature and the resistor 14. The amplifier 30 senses when plugging is about to take place and its output enables the amplifier 29. The output of the amplifier 29 is representative of the plugging current and this is supplied to the pulse source or associated circuits so that the conduction of the transistor 19 during plugging is controlled. WHAT WE CLAIM IS:

1. A motor control circuit of the kind specified in which said diodes are directly connected between said one lead and the ends of the field winding respectively, said contactor being arranged to connect one end of the field winding to the armature and the other end to the other lead through the transistor and vice versa, the operating coil or coils for the contactor having reverse voltage protection whereby the contactor cannot be closed when the leads are incorrectly connected to the source of supply.

2. A circuit as claimed in claim 1 in which the reverse voltage protection is afforded by a diode connected in series with said operating coil or coils.

3. A circuit as claimed in claim 1 including a resistor connected in series with the armature and said one lead, first means responsive to the voltage developed across the resistor and the armature to detect plugging and the second means responsive to the voltage developed across said resistor and which is enabled by said first means when plugging is detected said second means acting to control the length of the pulses produced by said pulse source thereby to prevent overload of the motor and/or transistor.

4. A motor control circuit comprising the combination and arrangement of parts substantially as hereinbefore described with reference to the accompanying diagram.