GB2212348A

GB2212348A - Improvements in circuits for starting electric motors

Info

Publication number: GB2212348A
Application number: GB8826346A
Authority: GB
Inventors: Brian Hawker
Original assignee: HAWKER CONTROLS Ltd
Current assignee: HAWKER CONTROLS Ltd
Priority date: 1987-11-10
Filing date: 1988-11-10
Publication date: 1989-07-19
Anticipated expiration: 2008-11-10
Also published as: GB2212348B; GB8826346D0; GB8726287D0

Abstract

A circuit for starting induction motors in which iron cored inductors (L1, L2, L3) in series with the lines to the motor can be short-circuited by pairs of anti-parallel thyristors (TH1-TH6) to remove said inductors subsequent to the starting of the motor. A coil (L4) magnetically associated with one of the inductors (L1) energizes both a timer (T) and supplies power to a solenoid coil of a relay (RL) through a rectifier bridge (B) and a transistor (Q) whose conduction is controlled by the timer. The timer is energized on starting the motor and when the motor has run up to speed, the timer rendering the transistor conductive to energize the relay. Relay contacts (RL:1-RL:3) close a circuit to trigger the thyristors into conduction thus short-circuiting the inductors. <IMAGE>

Description

"IMPROVEMENTS IN CIRCUITS FOR STARTING ELECTRIC MOTORS" The present invention relates to circuits for starting motors, in particular induction motors either single or polyphase motors.

Induction motors are normally started "direct-on-line", since this only requires a contactor in addition to the motor. In the case of three phase induction motors, these are normally designed to have their windings connected in delta. With a direct-on-line start, the motor will take a current which is several times the current it will take under full load conditions when running at normal operating speed. As the motor runs up to its normal operating speed, the current reduces at ever increasing rate as the motor reaches its full load speed.

The starting current for a three phase induction motor having its windings connected in delta can be up to 6.5 times the current on full load. Even if the motor is started by initially connecting its windings in star, the starting current may be two or three times its full load current when running at normal speed. Moreover, at the instant the main contactors close, a large current surge occurs. Likewise, even with delta connected three phase motors which are started in star, there is both a surge when the contactors close to energize the windings connected in star, and again when the windings are switched from star to delta.Agreed that in the case of delta connected three phase motors which are started in star, the surges are of an order of magnitude of three times less than in the case of the same motor which is started directly in star, but nevertheless undesirable surges do occur.

In systems using a large number of motors, the surges on starting may cause problems in the supply network and the accumulative effect of these surges may have disadvantageous effects on other electrical systems associated therewith.

In order to secure a greater reduction in starting current than can be achieved by using a star-delta starter, one may use either an auto-transformer or a series impedance. In principle, both auto-transformers and series impedances are capable of reducing the starting current and torque by any desired amount. However both methods employ using a series of taps on the auto-transformer or series impedance, the motor being started with the whole autotransformer winding or series impedance in the circuit and then as the motor runs up to speed, gradually notching out sections of the auto-transformer or series impedance until the last section is removed when full speed is obtained.

However neither of these methods removes the surges which occur at each switching step. Agreed that one or two comparatively large surges have been replaced by a large number of small magnitude surges, but it cannot entirely remove the problems which may arise as a result of a large number of small magnitude surges providing an accumulative effect.

In more recent times so-called "soft-start" devices have become available. These devices have become available.

These devices are capable of giving a smooth run-up from standstill to full speed thereby eliminating any sudden step changes. These so-called "soft-start" devices use thyristor switches, which permit the voltage supplied to the motor to be varied continuously. Such devices also provide an "energy saving" facility in addition to "soft-starting".

However, such a system involves the use of a complicated control system which has to monitor the requirements of the motor and moreover has to be capable of reducing the motor voltage when the motor is operating on a light load.

It is therefore an object to overcome partially or wholly the above referred to disadvantages in known types of systems for starting electric motors, particularly three phase induction motors.

According to the present invention there is provided a circuit for starting electric motors, said circuit including means for initially inserting an impedance into the line to the motor winding upon energization of said motor from an electric supply; and means for removing said impedance subsequent to the starting of the motor at a point where any current surge will be substantially zero.

Preferably said impedance is an iron-cored inductor, the impedance being created by saturation of the inductor by the line current to the motor.

The means for removing the impedance subsequent to the starting of the motor may be one of the following: a single thyristor a pair of anti-parallel thyristors, a triac or a contactor connected across each impedance.

In the cases where thyristors, contactors, triacs or diodes are used, these are triggered or closed to conduct in order to short-circuit the impedance at the point of substantially zero current surge, i.e. when the motor reaches maximum energy output.

The thyristors or triacs may be triggered into conduction by a relay, or a relay clock.

The relay may be energized after starting the motor from standstill by a timer after a preset elapse of time or by means of a rectified voltage generated by magnetic linkage with one of the inductors constituting the line impedance.

The present invention will now be described in greater detail by way of examples with reference to the accompanying drawings wherein Figure 1 is a circuit diagram showing the basic principle on which the invention is based for effecting a "soft-start" of a three phase delta connected induction motor; Figure 2 is a circuit diagram showing a first embodiment of a circuit utilizing a reed relay and timer for starting a three phase delta connected induction motor; Figure 3 is a circuit diagram showing a second embodiment of a circuit utilizing only a timer for starting a three phase delta connected induction motor; Figure 4 is a circuit diagram showing a third embodiment of a circuit utilizing a reed relay for starting a three phase star connected induction motor; and Figure 5 is a circuit diagram showing a fourth embodiment utilizing a current meter, a timer and a contactor in conjunction with a three phase delta connected induction motor.

Referring first to Figure 1, the basic "soft-start" principle on which this invention is based will be explained. The motor to be "soft-started" is a three phase delta connected induction motor having its three windings X, Y and Z permanently connected in delta. The three phase windings X, Y and Z are supplied from a three phase A.C.

supply over lines A, B and C, provided with ganged main contactor switches S1, S2 and S3 respectively. Iron cored inductors L1, L2 and L3 are connected in series with the three lines to the motor windings, the inductor L1 being connected to the junction between windings X and Y,the inductor L2 being connected to the junction between windings X and Z, and the winding L3 being connected to the junction between the windings Y and Z. Pairs of anti-parallel thyristors TH1 to TH6 are connected across the respective iron cored inductors L1 to L3, the thyristors THl and TH2 being connected across the inductor L1, the thyristors TH3 and TH4 being connected across the inductor L2, whilst the thyristors TH5 and TH6 are connected across the inductor L3.

When the gates of the three pairs of anti-parallel thyristors are supplied with a biasing potential which keeps them permanently non-conductive, the iron-cored inductors are connected in the three lines to the windings X, Y and Z of the delta connected induction motor. The thyristors are rendered conductive by a suitable positive biasing voltage applied to the gates thereof. In this case the iron-cored inductors L1, L2 and L3 are all short-circuited by the respective pairs of conducting thyristors TH1, TH2; TH3, TH4; and TH5, TH6, so that current can pass in both directions to the delta connected windings without experiencing any impedance.

When the induction motor is started, the three ironcored inductors are all in circuit with motor windings.

Thus, as soon as the main contactor switches S1, S2 and S3 are closed, the current flow to the motor windings gradually and smoothly increases from zero up to the current which the motor takes when it is running at full speed.

At the approximate instant that the current reaches its full speed value, the pairs of anti-parallel thyristors are all rendered conductive, thus short-circuiting the inductors L1 to L3. Accordingly, there are no sudden current surges on the lines to the induction motor and the current increases smoothly from zero up to a maximum value when the motor has reached full speed. The reason for this smooth increase will be clear to those skilled in the art from a study of Lenz's law of magnetic induction.

Referring now to the first embodiment of the circuit shown in Figure 2, for the sake of clarity, only the inductor L1 and the thyristors THI and TH2 are shown.

Moreover, in order to obtain the necessary control for all the pairs of anti-parallel thyristors, it is necessary to obtain a voltage sample from only one inductor.

The control circuit includes: a timer T, a semiconductor switch Q which in this example is a transistor, a reed relay RL having three contacts RL:1 to RL:3, a rectifier bridge B and a coil L4.which is magnetically coupled to the inductor L1. The rectifier bridge B rectifies the A.C. voltage developed across the coil L4. The coil of the relay RL and the emittercollector path of the transistor Q are connected in series across the rectified output of the rectifier bridge B. The energization of the timer T is obtained from a tap on the coil L4. The timer controls the conduction of the transistor Q. The transistor Q is initially held nonconductive by the application of a zero or negative bias applied to its base electrode.After the timing period has expired following energization of the timer, the base of the transistor Q receives a positive bias sufficient to render it fully conductive. The contacts RL:1 to RL:3 of the relay RL connect a suitable biasing potential through respective resistors R1, R2 and R3 to the gates of respective pair of anti-parallel thyristors TH1, TH2; TH3,TH4; and TH5, TH6.

In operation, when the main contactors S1, S2 and S3 are closed, current is supplied to the motor windings X, Y and Z through the inductors L1, L2 and L3. As the current starts to flow through the inductor L1, a voltage is induced in the coil L4. This energizes the timer T, which starts to count down from its preset value. Since the transistor Q is held non-conductive during this count down period, no current flows through the coil of relay RL. When the timer T has counted down to its preset count which is set to correspond to approximately the time it takes the motor to run up to full speed, the transistor 9 is rendered conductive, so that the coil of relay RL is energized with rectified current from the bridge B.Energization of the relay causes the contacts RL:l to RL:3 to close and supply a bias to the gates of all the thyristors. The thyristors thus become conductive and the inductors L1 to L3 are short-circuited which permit the motor windings X, Y and Z to be directly connected to the three phase mains supply.

The above circuit also has provision for "off-load" running of the motor to cover cases where the load on the motor is removed at intervals. Thus once the relay has been energized to allow the thyristors to conduct to shortcircuit the inductors, if the induction is now taken off load, the line current drops to such a value that the reply RL is no longer held energized. The relay drops out, and the inductors are reconnected into the lines to the motor windings. Upon the load being reimposed on the motor, the current gradually increases without any surge occurring, causing a corresponding increase in the output voltage of the rectifier bridge B. This causes the relay RL to become energized once again in order to short-circuit the inductors L1 to L3, once full load conditions have been reestablished.

Referring now to Figure 3, the three phase delta connected motor is not provided with the off load running feature. In this second embodiment, the relay RL is energized with D.C. from a battery P. As before, the col of the reed relay RL is connected in series with the transistor Q and is energized by the battery P when the transistor is conductive. The timer T is energized by a coil L5 magnetically associated with the inductor L1. The operation of the circuit of the second embodiment should be clear from what has been explained concerning the operation of the first embodiment, except as noted above, the "off-load" running feature is omitted.

Referring now to Figure 4, the motor associated with the third embodiment is three phase star connected induction motor, its windings X, Y and Z being permanently connected in star. This circuit is provided with the "off-load" running feature referred to above, but since star connected motors have a low start up current, the timer feature is omitted. A coil L6 magnetically associated with the inductor Ll directly energizes the coil of the reed relay RL through the rectifier bridge B. Also in this circuit as a variant each pair of anti-parallel thyristors is replaced by respective triacs TRl to TR3. As will be appreciated by those skilled in the art, a triac is almost equivalent to two anti-parallel thyristors made in a single chip. It has the advantage that it needs only a single gate connection to enable it to be biased so as to permit current to be conducted in either direction.

In operation, when the main contactors S1, S2 and S3 are closed, current is supplied to the motor windings X, Y and Z through the respective inductors L1, L2 and L3. When the voltage induced in the coil L6 reaches a sufficient magnitude as a result magnetic interaction from current flow in the inductor L1, the relay RL is energized to close its contacts RL:1 to RL:3 for the supply of a bias voltage to the gates of respective triacs TR1 to TR3. The triacs are then turned on so that current can flow in both directions, thus short-circuiting the inductors L1 to L3.

Although the above embodiment disclosed in Figure 4 utilizes a triac, it will be appreciated that pairs of antiparallel thyristors may be used instead. Likewise triacs may be used in the embodiments of Figures 2 and 3 in place of the pairs of anti-parallel thyristors.

Moreover instead of the reed relay Rl, andy known form of electronic switch can be used instead. Likewise, the time may be any known form of mechanical or electronic timer which is capable of having its timing period manually settable, according to the size and characteristics of the electric motor with which the system is used.

Referring now to Figure 5, the motor associated with the fourth embodiment is a three phase delta connected induction motor, its windings X, Y and Z being permanently connected in delta. This circuit is provided with a presettable "current-meter" CM, a relay RL, and a timer T. The relay RL has contactors RL:1; RL:2 and RL:3 connected in parallel with the respective inductors L1 to L3. The inductor L1 has magnetically associated with it a winding L7 which energizes the timer T. The "current-meter" CM is effectively a shunt type moving coil ammeter having an electrically conductive pointer P whose moving end makes contact with a series of contacts K. The spindle of the pointer P is electrically connected to the line A through a resistor R4.A movable contact K1 which can be pre-set to make electrical contact with any of the contacts K is connected via a switch S4 controlled by the timer T to one end of the solenoid of the relay RL. The other end of the solenoid is connected to the neutral point N of the three phase supply.

For normal operation, the contact K1 of the "currentmeter" CM is set just below the maximum running line current of the induction motor. So long as the induction motor remains on normal load, a current will flow from the line A, through the resistor R4, the pointer P of the current meter, contact K1, the switch S4 which is closed when the motor has run up to speed, the solenoid of the relay RL to the neutral point N of the three phase system. The relay RL is held energized by this current flow so that the contactors RL:1; RL:2 and RL:3 are held closed, thus shortcircuiting the respective inductors L1, L2 and L3.

If now the motor drops to idle running, the line current falls, so that the pointer P of the current-meter" CM moves onto a contact K, which is not in electrical contact with the pre-set movable contact K1. The current to the solenoid of the relay RL is interrupted and the relay is de-energized to open the contactors RL:1; RL:2 and RL:3 thus placing the inductor L1; L2 and L3 in series with the three phase current lines to the induction motor.

The timer T has the same function as in the embodiments of Figures 2 and 3, thus ensuring that the inductors L1, L2 and L3 remain in circuit until the motor has run up to speed whence it closes the switch S4.

With the above described circuits shown in the embodiments of Figures 2, 3, 4 and 5 current surges which are normally experienced on starting electric motors can be reduced to substantially zero, the in some applications the motor can be switched from off load to full load conditions again without any substantial current surge occurring.

In an alternative form of the circuit for starting delta connected three phase motors, the three iron cored inductors, instead of being connected in the three lines to the motor winding, are connected in star across the motor windings, such that inductors L1 and L2 are in series across the winding X, inductors L1 and L3 are in series across the winding Y and the inductors L2 and L3 are in series across the winding Z.

Furthermore, in any of the above embodiments only a single thyristor may be provided across any iron cored inductor, since the D.C. component provided by the thyristor conducting in only one direction will effect a collapse of the magnetic filed in the inductor.

Finally, the above described circuits can be used for starting any type of electric motor, although it is more particularly applicable to induction motors whether single phase or polyphase. "Off-load" running motors do not need any switch-gear, and likewise motors running at full load for short periods also do not need any switch-gear.

Claims

CLAIMS:

1. A circuit for starting electric motors, said circuit including means for initially inserting an impedance into the line to the motor winding upon energization of said motor from an electric supply; and means for removing said impedance subsequent to the starting of the motor at a point where any current surge will be substantially zero.

2. A circuit according to claim 1, wherein said impedance is an iron cored inductor, the impedance being created by saturation of the inductor by the line current of the motor.

3. A circuit according to claim 1 or 2, wherein the means for removing the impedance subsequent to the starting of the motor is a thyristor in parallel with the impedance the thyristor being triggered to conduct at the point of substantially zero current surge.

4. A circuit according to claim 1 or 2, wherein the means for removing the impedance subsequent to the starting of the motor is a pair of anti-parallel thyristors in parallel with the impedance, the thyristors being triggered to conduct at the point of substantially zero current surge.

5. A circuit according to claim 1 or 2, wherein the means for removing the impedance subsequent to the starting of the motor is a triac in parallel with the impedance, the triac being triggered to conduct at the point of substantially zero current surge.

6. A circuit according to claim 1 or 2, wherein the means for removing the impedance subsequent to the starting of the motor is a contactor in parallel with the impedance, the contactor being closed to short-circuit the impedance at the point of substantially zero current surge.

7. A circuit according to any one of the preceding claims 3 to 5, wherein the thyristor, the pair of anti-parallel thyristors or the triac are triggered by a relay controlled by a timer or a relay clock.

8. A circuit according to claim 4, wherein said pair of anti-parallel thyristors are triggered by means of a timer which controls the supply of rectified current to a relay solenoid having a contact in series with the gate electrodes of the thyristors.

9. A circuit according to claim 8, wherein the timer and the rectified supply to the solenoid are supplied from a coil magnetically coupled to said impedance.

10. A circuit according to claim 8 or 9, wherein said timer controls the conduction of a transistor whose emittercollector path is in series with the relay solenoid.

11. A circuit according to claim 4, wherein said pair of anti-parallel thyristors are triggered by means of a timer energized from a coil magnetically coupled to said impedance, said timer controlling the conduction of a transistor whose emitter-collector path is in series with a D.C. supply and a relay solenoid having a contact in series with the gate electrodes of the thyristors.

12. A circuit according to claim 5, wherein said triac is triggered by means of a relay solenoid having a contact in series with the control electrode of the triac, the solenoid being energized from rectified current derived from a coil magnetically coupled with said impedance, such that when said motor is run "off-load" said impedance is not shortcircuited by said triac.

13. A circuit according to claim 6, wherein said contactor is operated by means of a relay solenoid, said solenoid being energized through a "current-meter" and a switch controlled by a timer.

14. A circuit according to claim 13, wherein said timer is pre-settable to just below the maximum running current of the motor.

15. A circuit according to claim 13 or 14, wherein timer is energized from a coil which is magnetically coupled to said impedance.

16. A circuit according to any one of the preceding claims as applied to a three-phase delta connected or star connected induction motor, an impedance being provided in each of the three current lines, each impedance being provided with said means for removal thereof, the control to remove said impedances being provided with said means for removal thereof, the control to remove said impedances being associated with only one of said impedances.

17. A circuit for starting electric motors, constructed substantially as herein described with reference to any one of Figures 1 to 5 of the accompanying drawings.