GB2224171A

GB2224171A - Operating AC motors

Info

Publication number: GB2224171A
Application number: GB8920231A
Authority: GB
Inventors: Ronald Alex Cottrell; Russell Stuart King; John Buttifant; Paul Lloyd
Original assignee: De la Rue Systems Ltd
Current assignee: De la Rue Systems Ltd
Priority date: 1988-09-13
Filing date: 1989-09-07
Publication date: 1990-04-25
Anticipated expiration: 2009-09-07
Also published as: GB8821444D0; GB2224171B; GB8920231D0

Abstract

An AC motor operating circuit comprises a first circuit 6 for connection to an AC supply 7 to apply an AC potential to stator windings 3, 4 of the motor 1 to cause the rotor 2 to rotate. A second, braking circuit 8, 9, 10 can be selectively connected to at least one of the stator windings 3, 4 to apply a DC potential to the stator winding in place of the AC potential. Circuit 6 is a zero voltage switching element that switches off when the applied voltage is zero (12, 13 Fig 2). The motor 1 is a single phase induction motor but could be a polyphase motor. The braking circuit includes a rectifier 8, a capacitor 9 and a switching element 10. A control system 11 controls the switching elements 6 and 8. On receiving a stop command signal from 11 switching element 6 is caused to open and a set period after the stop command, eg 75ms, the switching element 10 is closed so as to discharge the capacitor 9 through the motor windings 3, 4 causing the motor to brake. <IMAGE>

Description

DE LA RUE SYSTEMS LIMITED 30/2731/01 OPERATING AC MOTORS The invention relates to methods and apparatus for operating an AC motor.

There are many AC motors currently available and these have wide application. One particular application is in banknote feeding apparatus in which an AC motor is used to control the feed mechanism to a shredder system.

In this application, and indeed in many other applications where intermittent feed is required, it is important to be able to control accurately the braking of the motor. Conventionally, this braking is achieved by applying a brake member to a shaft coupled with the motor but this does not achieve consistency.

In the field of electrical braking GB1050585 describes a circuit with a diode and capacitor the capacitor storing charge in the AC phase and applying a dc potential to brake the motor when the AC is disconnected. This suffers from RF interference and stress in the switching element.

In accordance with one aspect of the present invention, a method of operating an AC motor having a stator and a rotor comprises applying an AC potential to the stator windings to cause rotation of the rotor; braking the rotor by terminating the application of the AC potential and applying a DC potential to at least one stator winding further that the termination cf the AC potential is controlled via switch comprising a zero voltage switching element.

We have devised a new method of braking an AC motor in which a DC potential voltage is applied across the stator windings to lock the rotor. It has been found that this leads to improved consistency in the stopping time of the motor and hence the stopping distance of a transport system controlled by the motor. In this wav the position is known relative te when a stop command was received. The advantage of a zero voltage switching device is that the switch will operate at only one of two points on the AC cycle at which the potential is zero and thus problems of RF interference emissions and stress on the switching element are minimised. This in turn improves stopping consistency.

Preferably, a time interval is introduced between terminating the application of the AC potential and applying the DC potential so that the capacitor which produces the magnetic field in the stators by charging and discharging will be fully discharged before application of the dc voltage.

In accordance with a second aspect of the present invention, an AC motor operating circuit comprises a first circuit for connection to an AC supply to apply an AC potential to the or each stator winding of the motor to cause the motor rotor to rotate wherein the first circuit has a switch comprising a zero voltage switching element; and a second, braking circuit which can be selectively connected to at least one of the stator windings to apply a DC potential to the at least one stator winding in place of the AC potential.

Preferably, each of the first apd second circuIts has a respective switch to enable the circuit to be connected to and disconnected from the AC motor. In the preferred arrangement, the operating circuit includes a ccntrol system for controlling operation of the switches such that when the rotor is to be braked, the first circuit switch is opened and the second circuit switch closed, possibly after a time interval has elapsed.

Preferably, the second circuit includes a capacitor which is discharged to apply the DC potential to the at least one stator winding.

In the preferred arrangement, the second circuit also has a rectifier connected in series with the capacitor, the rectifier also being connected to the first circuit so that during normal operation in which an AC potential is applied to the stator windings, the capacitor of the second circuit is charged.

The AC motor is preferably a single phase induction motor but could also comprise another polyphase motor.

The invention is particularly applicable to motors which drive article feed systems, such as banknote transport systems, where intermittent and accurate feeding is required.

An example of a method and apparatus according to the invention will new be described with reference to the accompanying drawings, in which: Figure 1 is a bloc circuit diagram of the apparatus; and, Figure 2 illustrates graphically the operation of the switching element of the first circuit.

Figure 1 illustrates a single phase induction motor 1 comprising a rotor 2, and stator windings consisting of a start winding 3 and a run winding 4, and a capacitor 5.

The motor 1 is connected in a first circuit having a zero voltage switching element 6 coupled to an AC supply 7.

A second, braking circuit is provided comprising a rectifier 8 connected in series with a capacitor 9 and a switching element 10. The second circuit is connected in parallel with the first circuit as shown in Figure 1.

A control system 11 is provided coupled with the switching elements 6, 10 to control operation of those switching elements in response to external conditions.

Under normal running conditions, the switching element 6 is closed and the switching element 10 is open so that the motor 1 is connected across the S.C supply 7.

Rotation of the rotor 2 is achieved since during one half cycle of the AC supply the winding 4 will generate a magnetic field while the capacitor 5 is charged and during the other half cycle the capacitor 5 will discharge through the winding 3 which will generate a magnetic field thus causing the magnetic field to move and the rotor to rotate.

In addition, during normal running, the AC supplE.t is half-wave rectified by the rectifier 8 and is used to charge the capacitor 9 to the peak voltage Vp of the supply. Once the capacitor 9 has charged, it will stay in this condition as long as the motor 1 is being run.

When the control system receives a stop command, it causes the switching element 6 to open so as to disconnect the motor 1 from the AC supply 7. Since the switching elerent 6 is a zero voltage switch, it will only open when the applied voltage is zero which, as can be seen in Figure 2, can occur at two points within a single cycle. These points are labelled 12 and 13.

Switching at these points 12, 13 has the advantage of fixing the stopping conditions in the motor 1, reduces RF interference emissions and also reduces the stress on the switching element 6. Thus, in any one cycle, at the switching point, the voltage across the motor is either zero and falling at a known rate or zero and rising at a known rate.

A set period after the stop command is issued, for example 75ms, the switching element 10 is closed thus discharging the capacitor 9 through the motor windings 3, 4 causing the rotor to brake. The advantage of providing a time interval between opening the switching element 6 and closing the switching element 10 is that stopping consistency is improved by allowing any electrical energs stcred in the capacitor 5 to substantially discharge (ie. to a known state) to a point where it becomes negligible compared with the energy stored on the capacitor 9 so that its effect on the stopping time is always the same.

When a run command is received by the control system 11, the switch 10 is opened and the switch 6 closed to reconnect the motor and the braking circuit to the AC supply 7.

Claims

1. A method of operating an AC motor having a stator and a rotor, the method comprising applying an AC potential to the stator windings to cause rotation of the rotor; braking the rotor by terminating the application of the AC potential and applying a DC potential to at least one stator winding further that the termination of the AC potential is controlled via a switch comprising a zero voltage switching element.

2. A method according to claim 1, wherein a time interval is introduced between terminating the application of the AC potential and applying the DC potential such that a capacitor in series with the stator winding will be fully discharged before application of the dc voltage.

3. A method of operating an AC motor substantially as hereinbefore described with reference to the accompanying drawings.

4. An AC motor operating circuit comprising a first circuit for connection te an AC supply to apply an AC potential to the or each stator winding of the motor to cause the motor rotor to rctate wherein the first circuit has a switch comprising a zero voltage switching element; and a second, braking circuit which can be selectively connected to at least one of the stator windings to apply a DC potential to the at least one stator winding in place of the AC potential.

5. A circuit according te claim 4, wherein each of the first and second circuits has a switch.

6. A circuit according to claim 5, further comprising a control system for controlling operation of the switches such that when the rotor is to be braked, the first circuit switch is opened and the second circuit switch closed.

7. A circuit according to any of claims 4 to 6, wherein the second circuit includes a capacitor.

8. A circuit according to claim 7, wherein the second circuit has a rectifier in series with the capacitor and coupled to the first circuit to enable the capacitor to be charged.

9. A circuit according to any of claims 4 to 8, wherein the motor comprises a single phase induction motor.

10. An AC motor operating circuit substantially as hereinbefore described with reference to the accompanying drawings.