GB2159352A

GB2159352A - Arrangement for braking a motor having a short-circuited rotor

Info

Publication number: GB2159352A
Application number: GB08413296A
Authority: GB
Inventors: Seppo Suur-Askola
Original assignee: Elevator GmbH
Current assignee: Elevator GmbH
Priority date: 1984-05-24
Filing date: 1984-05-24
Publication date: 1985-11-27
Also published as: GB2159352B; GB8413296D0; DE3419757A1

Abstract

Braking takes place by controlling with pulse width modulation the amount of current going to the braking winding of the motor. In order that in the braking process the frequency of the current would be in a range where it causes no moment fluctuations in the motor and where its additional filtering is easy and economically profitable, the braking current is supplied by means of a d.c. chopper 1. Comprising SCRS T1, T2. The frequency of the braking current pulses going to the braking winding of the motor with short- circuited rotor is rather high, and adjustable. <IMAGE>

Description

SPECIFICATION Procedure and means for braking a motor with short-circuited rotor The present invention concerns a procedure and a means for braking a motor with shortcircuited rotor where braking is effected by regulating with the aid of pulse breadth modulation the amount of current going to the motor's braking winding.

When braking a motor with short-circuited rotor, very many different problem solutions can be applied in order to obtain the desired deceleration. However, those modern solutions which have to be taken seriously are all based on use of thyristor technology in operating the motor with short-circuited rotor, this technology being applied to control the current going to the motor's braking winding, by changing the lead angle. Since the thyristor bridges are supplied from a 50 Hz voltage, there occurs in the direct current circuit a low frequency current fluctuation.

From the comparatively low frequency amplitude oscillations of the current arise certain drawbacks. The low frequency current pulse also causes an equivalent oscillation of the braking moment, which manifests itself, outward, as a sound with the same frequency issuing from the motor, and as vibration. The sound, which has a frequency e.g. 100 Hz, is unpleasant, and difficult to damp out. Vibration of this frequency is also felt in the feet when for instance a lift motor is concerned.

Even more serious is in the case of a lift motor that very frequently the frequency of resonance of high-rise residential buildings has been found to be 50-100 Hz, and if this resonating frequency happens to be close to the frequency of the current pulses which are being supplied, the result is really objectionable noise and vibration. In conventional mains-commutated designs, the frequency of the current pulses supplied to the braking winding cannot be regulated. In addition, the frequency is low because it is bound to the mains frequency, whereby the filtering out of the amplitude variations in the current, or in the braking moment, is uneconomical and difficult to implement.

It is also a fact that in controllers of this type no linear relationship is achieved between the control voltage and the braking moment obtained, and this is an obstacle to creating pleasant braking characteristics.

The object of the invention is to eliminate the drawbacks mentioned. The procedure of the invention is characterized in that current is supplied with a means based on the chopper d.c. source principle, known in itself in the art, of which the governing has been so implemented that the frequency of the braking current pulses going to the motor is rather high and adjustable, whereby the amplitude variations of the braking current are small. By utilizing the above-mentioned principle in this connection, the frequency of the current fluctuations can be made adjustable within wide limits without influencing the amount of current that can be transferred, because at the same time the breadth of the current pulses is correspondingly changed.In this way, the frequency can be adjusted, on the site if need be, to be high enough to surpass the resonating frequency of the building and to be such that the frequency causes no observable vibration in the apparatus. Moreover, the control voltage/braking momeent relationship is linear.

The procedure of the invention is also characterized in that the pulsing of the load current of the means has been carried out by means of a series resonant circuit, known in itself, which has been connected autooscillatively in series with the motor, between the main thyristor supplying braking current and the auxiliary thyristor quenching it. Thanks to the mode of connecting the auxiliary thyristor and the series resonant circuit, the advantage is gained that quenching of the thyristor cannot be effected before the commutating capacitor is in the right state.

The means applying the invention is characterized in that it consists of a chopper current source and of its control circuit, whereby the frequency of the braking current pulses is comparatively high, and adjustable. The control circuit moreover includes the requisite monitoring and protective circuits.

The means applying the invention is furthermore characterized in that the main components of the braking current pulsing circuit are: a main thyristor, an auxiliary thyristor, a series resonant circuit connected between them in series with the braking winding, and a zero diode. The braking current is conducted to pass through the zero diode when the main thyristor is in "Off" state.

The invention is more closely described in the following with the aid of an example with reference to the attached drawing, wherein Figure 1 presents a braking means according to the invention, the chopper circuitry being presented at components level.

Figure 2 illustrates the operation of the most essential components of the chopper circuit, plotted over one and the same time axis.

The operational parts of the braking means are: the chopper current source 1, the control electronics 4, and the current source 5. In the chopper 1, the rectifying bridge 2 and the series resonant circuit 3 have been demarcated with interrupted lines. Essential components in the chopper are the main thyristor T1, the auxiliary thyristor T2, and the commutation capacitor C.

The operation of the chopper circuit is as shown in Fig. 2: Period Event t1-t2 T1 is ignited The load current transfers from the zero diode DO to the main thyristor T1 The polarity of the commutation capa citor C is reversed over T1, D2 and L1 t2-t3 The load current flows through T1 t3-t4 T2 is ignited The load current transfers from the main thyristor T1 to the auxiliary thy ristor T2 The communication capacitor C main tains the current in the load and pro duces across the main thyristor T1 a voltage in the blocking direction T1 is extinguished The extra energy in the communi cation capacitor C is returned over L2 and D1 to the communication capaci tor C t4-t5 The load current charges the commu tation capacitor C to its initial state t5-t6 The load current transfers from the auxiliary thyristor T2 to the zero diode DO.

In this way is obtained a high frequency pulsed direct current in the braking winding of the motor with short-circuited rotor. The pulse frequency is determined by the ignition frequency of T1, set in the control electronics 4, and the corresponding pulse breadth, by the phase shift of T2 with reference to T1. The actual set-point value for the deceleration of the motor is input to the control electronics 4.

The dependence of the braking moment on the set-point value is linear. -J It is obvious to a person skilled in the art that different embodiments of the invention are not confined merely to the example presented and may instead vary within the scope of the claims, stated below, For instance, the set-point loading value need not be a d.c.

voltage: it may be any suitable signal which can be expediently detected. No limitations are imposed on the implementation of the control electronics 4 and the current source 5 either.

Claims

1. Procedure for braking a motor with short-circuited rotor, wherein the braking takes place by controlling by means of pulse breadth modulation the amount of current going to the motor, characterized in that current is supplied with a means, known in itself in the art, based on the chopper direct current source principle, the controlling of which has been carried out so that the frequency of the braking current pulses going to the motor is rather high and is adjustable, whereby the amplitude variations of the braking current will be small.

2. Procedure according to claim 1, characterized in that pulsing of the braking current of the means has been implemented with a series resonant circuit (3) known in itself in the art, connected autooscillatively in series with the motor between main thyristor (T1) supplying braking current and,an auxiliary thyristor (T2) quenching it.

3. Means for braking a motor with shortcircuited rotor according to Claim 1, characterised in that the means consists of a chopper current source (1) and its control circuit (4), due to which the frequency of the braking current pulses is rather high and adjustable.

4. Means according to claim 3, characterized in that the main parts of the braking current pulsing circuit (1) are a main thyristor (T1), an auxiliary thyristor (T2), a series resonant circuit (3) connected between them in series with the braking winding, and a zero diode (3).

5. Procedure for braking a motor with short-circuited rotor, as claimed in Claim 1 and substantially as herein described.

6. Means for braking a motor with shortcircuited rotor as claimed in Claim 3 and substantially as herein described with reference to the accompanying drawings.