GB2065337A

GB2065337A - Speed controller for an alternating current motor

Info

Publication number: GB2065337A
Application number: GB8037951A
Authority: GB
Original assignee: IFE GmbH
Current assignee: IFE GmbH
Priority date: 1979-12-06
Filing date: 1980-11-26
Publication date: 1981-06-24
Also published as: GB2065337B; SE8008422L; ES8201781A1; AT369934B; ES497484A0; ATA773479A; FR2471697B1; FR2471697A1; ZA807524B; DE3004456A1

Abstract

A speed controller for an alternating current motor comprising electronic switching means for starting and stopping the supply of current to the motor to control the motor speed, the controller comprising means for determining, during each revolution of the motor, the actual time taken for the rotor of the motor to rotate through an angle which is a sub-multiple of 2 pi , means for comparing the actual time with a predetermined reference time, and means for starting the supply of current to the motor when the actual time exceeds the reference time and for maintaining the supply of current until the actual time falls below the reference time.

Description

SPECIFICATION Speed controller for an alternating current motor The invention relates to a speed controller for alternating current motors in which control is effected by on-off operation of the motor current, starting and stopping of the current being undertaken by electronic switching means.

Speed controls are already known in which the current supply is stopped in dependence upon the indication of a tachometer. The current supply is restarted for example after the expiration of a period of time at zero current predetermined by a timer (German PS 1 5 38 577).

Speed controls are also known in which the difference between the voltage supplied by a tacho-generator (the actual value voltage proportional to the motor speed) and a predetermined voltage (reference value voltage) is determined and the phase angle of a phase-shifting control is controlled in dependence upon this voltage difference (GB PS 1 377 139). Furthermore, a phase-shifting control is also known, in which pulse trains supplied by a transducer connected to the drive motor and by a reference frequency generator are used for controlling the speed of a drive motor (German AS 12 89 871).

In contrast, the present invention relates to a method according to which for each revolution of the rotor, the time for the rotation of the rotor of the alternating current motor through a predetermined angle is monitored and compared with a predetermined time. If the time (actual time) necessary for this is greater than the predetermined time (reference time), the current supply to the motor is started or maintained. In the other case, the current supply is stopped.

The invention is based on the idea of providing the simplest possible speed controlled, which consists essentially of integrated components, which are commercially available world wide and therefore facilitate a circuit construction requiring the least possible expenditure.

According to the invention, there is provided a speed controlled for an alternating current motor comprising electronic switching means for starting and stopping the supply of current to the motor to control the motor speed, characterised by means for determining, during each revolution of the motor, the actual time taken for the rotor of the motor to rotate through an angle which is a sub-multiple of 271, means for comparing the actual time with a predetermined reference time, and means for starting the supply of current to the motor when the actual time exceeds the reference time and for maintaining the supply of current until the actual time falls below the reference time.

If it is intended to achieve a particulariy accurate speed control, then a plurality of time measurements should be carried out during one revolution of the motor. In this case, fixing the angle of rotation for the individual time measurements is undertaken so that with an uninterrupted supply of driving power at a level such that the time for successive full revolutions of the motor remains constant, the partial times for passing through the individual angles of rotation are the same.

In order to be able to achieve a particularly simple method of carrying out the time measurements, it seems appropriate to select the sum of the angles of rotation for the individual time measurements to be the same (2 7r, 360 ), i.e. to correspond to a full revolution of the motor.

For the actual time measurements, an inductive proximity switch is preferably provided on the motor casing, which is damped by small metal plates attached to the rotor of the motor, as they pass by.

A controller is described hereafter, which may serve to carry out the speed-control method according to the invention. In the drawings, Fig. 1 is a block circuit diagram and Fig. 2 is the associated signal-flow graph.

In the block circuit diagram of Fig. 1, the following components are illustrated: Component: Designation: Outputs: Inputs: 1. Input circuit with initiator I + E E-3, E-4 E-1. E-2 2. Edge-controlled time lag flip-flop FF FF-Q FF-S, FF-CP, FF-D 3. Edge-controlled mono flop with refreshable running time MF MF-Q MF-T, MF-A 4. Oscillator Oz Oz-2 Oz-1 5. Reference value P + So So-l, So-3, So-4, transmitter So-2 So-5 6. Mains signal transmitter NS NS-1 7. Power output part LT LT-2, LT-1 LT-3 8. Power supply N The signal-fiow paths are designated by the letters a, b, c, d, e, f.

Description of the individual components: 1) Input circuit with initiator (I + E): an inductive proximity switch I is located on the housing (stator) of the motor for monitoring the actual speed (actual value). One or more small metal plates attached to the rotor damp the proximity switch as they pass the active switching surface of the proximity switch. The input circuit E converts the pulses emitted by the initiator I to OV(Llevel) or HV(H-level) and emits them at the output E4.

The leads to the initiator I are monitored for interruption. If one of the two leads is interrupted, then an H-signal is emitted at the output E3.

2) Edge-controlled time lag flip-flop (FF): if there is an L-signal coming from the mains signal transmitter NS at the input FF-S, the flip-flop FF is inoperative. There is an L-signal at the output FF-Q.

If there is an H-signal at the input FF-S, on encountering a positive pulse at FF-CP, the flipflop FF emits the signal which is present at FF-D, in an inverted form at FF-Q.

3) Edge-controlled mono-flop with refreshable running time (MF:) if the mono-flop is not triggered, the output MF-Q continuously emits an H-signal. If a pulse occurs at the input MF-A, the signal at the output MF-Q changes from H to L and is maintained for a running time 1, which is determined by the reference value voltage at the input MF-T. If a further pulse arrives at the input MF-A before the expiration of 1, the timing begins afresh.

4) Oscillator (Oz): if there is an L-signal at the input Oz-1 of the oscillator, the oscillator Oz emits a firing pulse train at a frequency of approximately 1 kHz at the output Oz-2.

If there is an H-signal at the input Oz- 1, no pulses are emitted by the oscillator.

5) Reference value transmitter (P + So): the reference value transmitter So emits a reference voltage at the output So-2, which voltage can be adjusted steplessly by the reference value potentiometer P.

The upper and lower adjusting limit value is fixed by two adjusting trimmers. The leads to the reference value potentiometer P are monitored for interruption. If one of the leads is interrupted, an H-signal is emitted at the output SO-1.

6) Mains signal transmitter (NS): if there is a connection to the power supply N, the mains signal transmitter NS emits an L signal at the output NS-1. After the expiration of approximately 1 second, the signal returns to H and is maintained until the power supply is switched off.

7) Power output part (LT): the power output part amplifies the firing pulses supplied by the oscillator Oz for firing triacs or thyristors.

8) Power supply (N): the power supply N supplies the d.c. supply voltage for the electronic components 1-7.

Method of operation of the individual components during operation: in the signal flow graph of Fig. 2, the function of the components is plotted against time.

There is a distinction between the following time intervals: 1-2 starting operation 2-3 running the motor speed to beyond the reference speed 3-4 reducing the motor speed to below the reference speed.

Time interval 1-2: switching on occurs at the instant 1a. NS-1 emits an L-signal, which is present at the input FF-S (a). An L-signal is present at the output FF-O and at the input Oz-1 (e). Oz-2 supplies firing pulses (f).

It is assumed that the active switching surface is not damped at the instant 1 a. Therefore E-4 supplies an H-signal to MF-A and FF-CP (b).

MF-O supplies an L-signal with the running time 1 at FF-D (d). At the instant 1 b, damping of the active switching surface begins (due to the passage of a small metal plate), E-4 supplies an L-signal (b).

The clamping ends at the instant ic, E-4 once more supplies an H-signal (b). At the instant 1 d, MF-Q once more supplies an L-signal, delayed by the time taken to travel through the components, for the duration of the running time 1 (instant 1e) (d).

Time interval 2-3: At the instant 2a, approximately 1 second has expired. NS-1 has an H-signal (a), E-4 returns from L to H at the end of the next damping (instant 2b) and thus emits an H-signal(b).

MF-Q supplies an L-signal for the duration of the running time 1 (d) at the instant 2c (once more delayed by the time taken to travel through the components). This operation is repeated until (by increasing the speed of the motor) the time interval between two positive edges of E-4 is shorter than the running time 1. See instant 2d, at which an L-signal with the running time 1 is present at MF-Q, delayed by the travelling time. Before the expiration of 1, the L-signal at E-4 changes to H and is transmitted to FF-CP and MF-A (b). On account of the running time 1 of MF, and L-signal is still present at FF-D up to instant 3. FF-Q changes from L to H (e), Oz-2 emits no pulses.

Time interval 3-4: The supply of current to the motor is interrupted, the motor speed begins to decrease and drops until the time interval between two positive edges at E-4 is longer than the running time 1 of MF; see instant 4.

The motor once more runs up until the interval between two positive edges at E-4 is shorter than the running time of MF. The motor is again stopped. This corresponds to the action up to instant 3.

The invention is not limited to the embodiment described. Thus, proximity switches and small metal plates can be arranged inversely as regards the rotor and stator of the motor. In place of an electrical or electro-magnetic signal transmitter, signal transmitters of other types, such as for example optical or acoustic signal transmitters can be used.

Claims

1. A speed controller for an alternating current motor comprising electronic switching means for starting and stopping the supply of current to the motor to control the motor speed, characterised by means for determining, during each revolution of the motor, the actual time taken for the rotor of the motor to rotate through an angle which is a sub-multiple of 2 7r, means for comparing the actual time with a predetermined reference time, and means for starting the supply of current to the motor when the actual time exceeds the reference time and for maintaining the supply of current until the actual time falls below the reference time.

2. A speed controller according to claim 1, characterised by the fact that an edge-controlled mono-flop with refreshable running time is provided to determine the reference time.

3. A speed controller according to claim 2, characterised by the fact that one or more inductive proximity switches are provided for monitoring the actual time pulses output from the proximity switch or switches being supplied to an edge controlled flipflop for comparison of the reference time and actual time.

4. A speed controller according to claim 3, characterised by the fact that triacs or thyristors are used for starting and stopping the supply of current, the triacs or thyristors being supplied with firing pulses by an oscillator actuable by the edge controlled flip-flop.

5. A speed controller according to claim 3, characterised by the fact that the or each inductive proximity switch is provided on the rotor casing, small metal plates attached to the rotor being provided to damp or excite the or each proximity switch at the limits of each measuring angle.

6. A speed controller according to claim 1, characterised by the fact that successive measuring angles follow each other without a gap.

7. A speed controller for an alternating current motor substantially as hereinbefore described with reference to the accompanying drawings.