GB2049242A

GB2049242A - Regulating a slip-ring induction motor

Info

Publication number: GB2049242A
Application number: GB8014403A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1979-05-16
Filing date: 1980-05-01
Publication date: 1980-12-17
Also published as: DE2919853C2; DE2919853A1; ZA802892B; GB2049242B

Abstract

Three-phase slip-ring induction motors with liquid starters 3 are frequently used as drives for winders. The position of the liquid starter 3 is controlled by means of a position control device 4 as a function of the respective desired resistance value. However, such adjustment of the starter 3 can lead to an actual resistance value which, as a result of the parameters of the starter 3, deviates to a considerable extent from the desired value. To obtain the actual resistance value required, therefore, a rotor current regulator 17 feeds the position control device 4. The product of a signal which is proportional to a desired resistance ie motor current set at 16 and of an actual value signal of the slip of the motor as derived from the measured induction motor speed are supplied to the rotor current regulator 17. <IMAGE>

Description

SPECIFICATION Control apparatus for regulating a slip-ring induction motor This invention relates to control apparatus for regulating a slip-ring induction motor, and is particularly although not exclusively concerned with control apparatus for regulating a three-phase slip-ring induction motor with direct current braking in the stator circuit, and with a liquid starter, its position controllable by means of a position. control device, in the rotor circuit.

With known arrangements of this latter type, the rotor resistance, which is produced as a result of adjustment of the liquid starter by means of the position control device, can deviate to a considerable extent from a respective desired value set by a control or regulating device, as a result of variations in the parameters of the starter, such as, for example, the concentration, temperature and level of brine as the liquid of the starter.

The present invention aims to provide a generally improved control apparatus.

According to the present invention, there is provided control apparatus for regulating a slip-ring induction motor having a liquid starter in its rotor circuit, the apparatus comprising: a position control device for controlling the setting of the liquid starter; first means for setting a desired value signal for the rotor current of the motor; second means for deriving a speed signal proportional to the speed of the motor in use; and a rotor current regulator for deriving from said signals a control value signal which is fed to the position control device in use.

At least in preferred embodiments, the starter may be adjusted in such a. way that the actual value of the rotor current corresponds to the desired value independently of brine (or other liquid) parameters. Furthermore, as a result of current regulation, the moments of several drives mechanically connected in parallel may be uniformly distributed between the drive motors when a common desired value is prescribed to the current regulators for the motors.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which Figures 1 and 2 are schematic circuit diagrams of respective embodiments of the invention.

In Fig. 1, there is provided for the purpose of starting a three-phase slip-ring induction motor 1, which, in the exemplary embodiment, drives a double winding drum 2, 2a, a liquid (brine) starter 3 in the rotor circuit, its position being controlled by way of a position control device 4. To this end, there is connected to the output of the position control device 4, which comprises a position regulator having a rotational speed and current regulator adjacent thereto, a servomotor 5 which drives both the position adjustment device of the starter and at the same time a potentiometer 6 as an actual position value generator. A tacho-generator 7 is provided to supply an actual rotational speed value of the servomotor, and a current converter 8 is provided to supply an actual current value of the servomotor.

To brake the three-phase slip-ring induction motor 1, the stator can be connected to a static converter 10 which is fed by way of a transformer 9, the static converter being controlled by a braking current regulator 11. The actual current value for this regulator is tapped by means of a current converter, whilst, for the purpose of setting a desired value, two resistances 12, 13, which are fixedly set at different values and which are connected to a direct voltage, are provided, a foot switch 14,15 respectively being co-ordinated to each for the purpose of selecting desired retardation rates.

Since the resistance of the liquid starter 3 is also dependent on the brine parameters apart from its position, a desired rotor resistance, set by a master controller 16, could deviate from the actual resistance set by the respective position of the starter 3, which is dependent on the position control device 4. Therefore, in order for the position of the starter 3 which is set by the position control device 4 to correspond to the desired rotor resistance, a rotor current regulator 1 7 superimposes a control value on the position control device 4, and. takes an input value by way of a transducer 1 9 from a shunt or a current converter 1 8 arranged in the rotor circuit of the slip-ring induction motor 1.To form another input value for the rotor current regulator 17, a voltage which is proportional to the setting of the master controller 1 6 is multiplied in a multiplier 20 with a variable which is proportional to the actual value of the slip of the slip-ring induction motor 1. This variable is produced from the comparison of an actual rotational speed value, supplied by a tachogenerator 21 which is coupled to the slip-ring induction motor 1, with a voltage which corresponds to the maximum slip and which is supplied to the comparison point by way of a switch 22. Advantageously, there is connected between the multiplier 20 and the rotor current regulator 1 7 a limiter stage 23 which allows the rotor current to be limited in a simple manner.

Instead of using the variable proportional to slip, which is multiplied by the master control ler setting, there may be employed the output quantity of the rotational speed regulator, a desired value of which is set by the master controller.

To initiate direct current braking, the master controller must firstly be brought into its zero position, and subsequently one of the two foot switches 14 or 1 5 must be actuated. At the same time as the master controller is set to zero or as one of the two foot switches is actuated, the switch 22 opens, a switch 31 separates the stator winding 1 of the motor from the power supply and a switch 27 connects the stator winding to the static converter 10.

In the embodiment of Fig. 2, in addition to the desired value set by the master controller 16, a speed regulator 24 is provided. A desired speed value is the difference from the output voltage of a tacho-generator 21 a which is coupled to the second winding drum 2a and the output quantity of a drive regulator 25 which sets a highest permissible speed. If the output signal of the speed regulator 24 exceeds a prescribed value, then a command stage 26 responds and initiates direct current braking by closing of switches 27 and 28 and by opening switches 22 and 31. Furthermore, the command stage causes the current regulators 1 7 and 34 to switch from PI- to Pbehaviour.The limiter input of the limiter stage 23 and an input of an amplifier 32 are further connected to the output of the speed regulator 24, with the result that, with a reduction in rotational- speed, limitation by the limiter stage and the control value supplied by the braking current regulator 11 from the amplifier 32 are reduced to zero. As a result of the switch 28 closing, a control variable derived from the rotational speed by means of a characteristic element 33 is fed to the position control device 4. This control variable causes the pull-out slip to constantly correspond to the actual rotational speed of the slip-ring induction motor.With a high level of thermal efficiency of the slip-ring induction motor, in place of this a control variable which prescribes a rated value of the rotor resistance is advantageously set as a linear function of the rotational speed, and the actual value of the rotor resistance is determined as the quotient of the rotor voltage and the rotor current.

An equalising regulator has a correcting effect on the rated value of the starter position if rated and actual values deviate from one another.

If several slip-ring induction motors are mechanically coupled with one another, then, as indicated in Fig. 2 for two motors operating in parallel, advantageously a common input value is fed to superimposed current regulators 1 7 and 34 by way of a connection provided with a switch 35, and the actual current values of both motors are prescribed by way of connections represented with a switch 29 and 30 respectively. Equalising regulation of the rotor currents may be thereby obtained, since any difference occuring between the rotor currents corrects the desired value of the starter position.

Claims

1. Control apparatus for regulating a slipring induction motor having a liquid starter in its rotor circuit, the apparatus comprising: a position control device for controlling the setting of the liquid starter; first means for setting a desired value signal for the rotor current of the motor; second means for deriving a speed signal proportional to the speed of the motor in use; and a rotor current regulator for deriving from said signals a control value signal which is fed to the position control device in use.

2. Apparatus according to claim 1, wherein, in use, said second means derives a signal proportional to the speed of the motor and compares that signal with a pre-set signal which corresponds to a maximum slip value, to derive a comparison signal which is multiplied by said desired value signal and then fed to said rotor current regulator.

3. Apparatus according to claim 1 or 2, including a current limiter through which said desired value and speed signals, or at least one signal proportional thereto, are fed to the rotor current regulator in use.

4. Apparatus according to claim 3, including means for setting a desired speed signal for the motor, and a speed regulator arranged to receive said desired speed signal and provide an output signal which is fed to said current limiter in use.

5. Apparatus according to any preceding claim, including braking means for effecting d.c. braking in the stator circuit of the motor.

6. Apparatus according to claims 4 and 5, wherein said braking means comprises a static converter for supplying a djc. braking current to the stator circuit for the motor, and a braking current regulator which is arranged to receive an output signal from said speed regulator and to control the static converter.

7. Apparatus according to any preceding claim, including means for calculating an actual value of the resistance of the liquid starter, wherein the position control device includes an equalising regulator responsive to the difference between a signal proportional to speed of the motor in use and said calculated actual value.

8. Control apparatus for regulating a slipring induction mqtor, the apparatus being substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

9. Apparatus according to any preceding claim in combination with the liquid starter.

10. An induction motor provided with control apparatus according to any of claims 1 to 8 or a combination according to claim 9.