GB1562340A - Starting arragnement for at least one single-phase or polyphase motor - Google Patents

Description

(54) STARTING ARRANGEMENT FOR AT LEAST ONE SINGLE-PHASE OR POLYPHASE MOTOR (71) We, SIEMENS AKTIENGESELL SCHAFT, a German company of Berlin and Munich, Germany, 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 starting arrangement for at least one single-phase or polyphase motor.

The field of application of motors having a short-circuited rotor is often limited by the relatively uneven torque characteristic during starting. Either the pull-out torque is too great or the starting torque is too small or too great. It is possible to a limited extent to adapt the starting torque to the desired torque characteristic by the choice of rotors of different classes, that is to say, ranges of rotors which differ from one another in respect of the construction and/or the arrangement of the rotor winding. If this possibility no longer exists, there are employed instead of motors having a shortcircuited rotor motors having a slip-ring rotor, the torque of which can be adapted to the necessary torque variation by the choice of different resistance stages.In this case, however, disadvantages arise owing to the sudden torque changes at the changeover from one resistance level to another.

In regions in which firedamp or explosive gases may be encountered, however, the use of motors having a slipping rotor is as far as possible avoided owing to the danger of explosions, even if they occur only in the interior of the motor where pressureproof enclosures are employed.

It would be desirable to use squirrel-cage motors with a uniform class of rotor for the most varied applications and to produce the desired torque curve during the starting phase by controlling the supply voltage by means of thyristors, for example by means of alternating-current or three-phase-current choppers in phase-angle or full-wave control.

Owing to the harmonics set up in choppers, these are unsuitable for con tinuous speed regulation. In addition, the chopper, like the choke in a starting arrangement comprising a choke adapted to be cut into the circuit during starting, must be designed for full motor power operation.

The present invention aims to provide an improved starting arrangement.

According to the invention there is provided a starting arrangement for at least one single-phase or polyphase motor which incorporates a short-circuited rotor or which is constructed so that its rotor functions as a short-circuited rotor during starting of the motor, the starting arrangement comprising in at least one phase an iron choke, having an air gap and two windings isolated from one another, one of the said windings being connected in the associated said phase and the othre winding being provided with an alternating-current or polyphase chopper arranged to control the voltage drop in said other winding for controlling the phase voltage.

In a starting arrangement for starting a number of motors having different starting currents, preferably the reactance of the choke is adjustable so as to be matched to the starting current of any individual motor.

In one such arrangement, the choke is provided with taps corresponding in number to the number of motors present, an isolating switch is provided for each tap, and a circuit breaker and a further isolating switch are provided for each motor. Alternatively, the air gap of the choke is adjustable to enable the reactance of the choke to be matched to the starting current of any individualmotor.

For a better understanding of the inuen- tion and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing which shows one embodiment of the present invention.

Referring to the drawing, there is connected to a three-phase current supply system, for example a supply system having a voltage of 6 KV, three three-phase squirrel-cage motors M1, M2 and M3 and a choke 1, each by means of a circuit breaker 2, 3 respectively. In addition, the motors can be connected, each by means of an isolating switch 4, to a bus bar 5 whose voltage is controlled by the choke 1.

The choke 1 is an iron choke having an air gap and at least two windings 6, 7 isolated from one another, one (6) of which is connected to one phase of the supply system through the circuit breaker 3 and to the bus bar, while the other (7) is supplied through an alternating-current chopper 8.

Of course, the choke may alternatively be of three-phase construction. The size of the air gap of the choke is so chosen that, when the secondary circuit is open-circuited, i.e.

when the thyristor of the chopper is in the "off" condition and the motor connected, the necessary maximum voltage drop is obtained during the running-up phase.

When the secondary circuit of the choke is short-circuited, there flows in that circuit a current which produces a magnetic flux to compensate the main magnetic flux in the iron, so that the voltage drop across the choke is substantially zero. With the aid of the chopper in the secondary circuit, therefore the voltage drop across the choke can be smoothly and rapidly adjusted between a maximum voltage drop and zero value.

When a number of motors of different powers are to be started by the same arrangement, the choke may be provided with a number of taps, one for each motor, each of which taps can be selected by means of a respective isolating switch 9.

The taps enable the reactance of the choke to be matched to the starting current or to the starting power of any of the motors.

Instead of the taps, which render possible a variation of the reactance of the choke, the reactance may be varied by adjusting the air gap.

With the starting arrangement described, it is not necessary for the whole starting power of a short-circuited-rotor motor to be controlled by thyristors, but only the power corresponding to the desired voltage drop. Since only the smaller part of the starting power is attended by harmonics, a relatively small total proportion of harmonics is obtained, so that the thermal capacity of the motor on starting-up can be better utilised. Owing to the square-law dependence of the torque of the motor upon the voltage, small voltage reductions and hence small thyristor outputs are generally sufficient to produce any desired starting characteristic curve. Any torque changes which may be necessary are rapidly and smoothly attainable. The arrangement may also be employed for synchronous motors having a starting winding, which function as squirrel-cage motors during starting.

WHAT WE CLAIM IS: 1. A starting arrangement for at least one single-phase or polyphase motor which incorporates a short-circuited rotor or which is constructed so that its rotor functions as a short-circuited rotor during starting of the motor, the starting arrangement comprising in at least one phase an iron choke having an air gap and two windings isolated from one another, one of the said windings being connected in the associated said phase and the other winding being provided with an alternating-current or polyphase chopper arranged to control the voltage drop in said other winding for controlling the phase voltage.

2. A starting arrangement according to claim 1 for starting a number of motors having differing starting currents, wherein the reactance of the choke is adjustable so as to be matched to the starting current of any individual motor.

3. A starting arrangement according to claim 2, wherein the choke is provided with taps corresponding in number to the number of motors present, an isolating switch is provided for each tap, and a circuit breaker and a further isolating switch are provided for each motor.

4. A starting arrangement according to claim 2, wherein the air gap of the choke is adjustable to enable the reactance of the choke to be matched to the starting current of any individual motor.

5. A starting arrangement for at least one single-phase or polyphase motor, substantially as hereinbefore described with reference to the accompanying drawing.

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

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. carried into effect, reference will now be made, by way of example, to the accompanying drawing which shows one embodiment of the present invention. Referring to the drawing, there is connected to a three-phase current supply system, for example a supply system having a voltage of 6 KV, three three-phase squirrel-cage motors M1, M2 and M3 and a choke 1, each by means of a circuit breaker 2, 3 respectively. In addition, the motors can be connected, each by means of an isolating switch 4, to a bus bar 5 whose voltage is controlled by the choke 1. The choke 1 is an iron choke having an air gap and at least two windings 6, 7 isolated from one another, one (6) of which is connected to one phase of the supply system through the circuit breaker 3 and to the bus bar, while the other (7) is supplied through an alternating-current chopper 8. Of course, the choke may alternatively be of three-phase construction. The size of the air gap of the choke is so chosen that, when the secondary circuit is open-circuited, i.e. when the thyristor of the chopper is in the "off" condition and the motor connected, the necessary maximum voltage drop is obtained during the running-up phase. When the secondary circuit of the choke is short-circuited, there flows in that circuit a current which produces a magnetic flux to compensate the main magnetic flux in the iron, so that the voltage drop across the choke is substantially zero. With the aid of the chopper in the secondary circuit, therefore the voltage drop across the choke can be smoothly and rapidly adjusted between a maximum voltage drop and zero value. When a number of motors of different powers are to be started by the same arrangement, the choke may be provided with a number of taps, one for each motor, each of which taps can be selected by means of a respective isolating switch 9. The taps enable the reactance of the choke to be matched to the starting current or to the starting power of any of the motors. Instead of the taps, which render possible a variation of the reactance of the choke, the reactance may be varied by adjusting the air gap. With the starting arrangement described, it is not necessary for the whole starting power of a short-circuited-rotor motor to be controlled by thyristors, but only the power corresponding to the desired voltage drop. Since only the smaller part of the starting power is attended by harmonics, a relatively small total proportion of harmonics is obtained, so that the thermal capacity of the motor on starting-up can be better utilised. Owing to the square-law dependence of the torque of the motor upon the voltage, small voltage reductions and hence small thyristor outputs are generally sufficient to produce any desired starting characteristic curve. Any torque changes which may be necessary are rapidly and smoothly attainable. The arrangement may also be employed for synchronous motors having a starting winding, which function as squirrel-cage motors during starting. WHAT WE CLAIM IS:

1. A starting arrangement for at least one single-phase or polyphase motor which incorporates a short-circuited rotor or which is constructed so that its rotor functions as a short-circuited rotor during starting of the motor, the starting arrangement comprising in at least one phase an iron choke having an air gap and two windings isolated from one another, one of the said windings being connected in the associated said phase and the other winding being provided with an alternating-current or polyphase chopper arranged to control the voltage drop in said other winding for controlling the phase voltage.

2. A starting arrangement according to claim 1 for starting a number of motors having differing starting currents, wherein the reactance of the choke is adjustable so as to be matched to the starting current of any individual motor.

3. A starting arrangement according to claim 2, wherein the choke is provided with taps corresponding in number to the number of motors present, an isolating switch is provided for each tap, and a circuit breaker and a further isolating switch are provided for each motor.

4. A starting arrangement according to claim 2, wherein the air gap of the choke is adjustable to enable the reactance of the choke to be matched to the starting current of any individual motor.

5. A starting arrangement for at least one single-phase or polyphase motor, substantially as hereinbefore described with reference to the accompanying drawing.