GB2036475A

GB2036475A - Starting AC Electric Motors

Info

Publication number: GB2036475A
Application number: GB7937753A
Authority: GB
Original assignee: Danfoss AS
Current assignee: Danfoss AS
Priority date: 1978-11-02
Filing date: 1979-10-31
Publication date: 1980-06-25
Also published as: IT7969108A0; GB2036475B; FR2441291B1; DE2847456A1; FR2441291A1; DK454979A; JPS55117090A; DE2847456C2

Abstract

An asynchromous motor 1 has a run winding 5 in series with the coil 6 of a starter relay 7 having a contact 10 which controls the start winding 8, opens when the current in the winding 5 drops below a preset value and is bridged by a PTC resistor 11, which prevents arcing at contact opening and subsequently heats up to reduce the current through the winding 8. The voltage across the PTC resistor may energise the coil (14) of a delay relay (13) Fig. 2 (not shown), whose contacts (15) close to connect to the AC supply U a second similarly- controlled motor (101), whereby the motors are sequentially started to reduce the peak current demand on the supply U. <IMAGE>

Description

SPECIFICATION Electric Motors This invention relates to an electric motor comprising a run winding and a start winding, a contact of a motor starting device being connected in the start winding.

The present invention provides an electric motor comprising a run winding and a start winding, a contact of a motor starting device being connected in the start winding, wherein the contact is bridged by a PTC resistor.

The present invention also provides, in combination, at least two motors according to the invention, wherein a connecting lead between the two motors comprises a normally open contact of a delay relay, the relay having a coil in shunt with the PTC resistor of the upstream motor, the motors being connected in parallel when the contact closes.

The present invention also relates to a starter circuit for a compressor means comprising at least one compressor which is drivable by a single-phase electric motor and has the contact of a starter device in the starter branch, particularly for a refrigerator.

The starter device may be a centrifugal switch.

However, it is usually a starter relay having a coil which is traversed by the current of the main (run) winding and of which the contact opens when, as a result of running up, the current in the main winding drops below a limiting value. When the contact of the starter device is opened, sparks are formed. These not only reduce the life of the starter device but they also give rise to malfunctioning.

It is also known to use a starter device for a single-phase motor in the form of a PTC resistor which is connected in series with the auxiliary (start) winding. It defines a particular period available for running up to speed. There after, it is so intensively heated by the current flowing through it that the current of the auxiliary winding decays to an insignificant value. Often, for example, if the motor has to start again a strong resistance, the motor has not run up to speed when the PTC resister assumes its high resistance. This results in a false start. Sometimes the PTC resistor is stili too warm if the motor is to be started again shortly after switch-off. In this case, again, there will be a false start.

Further, compressor means are known in which two or more compressors are connected in parallel on the pressure and suction side. The associated single phase motors are also connected in parallel on the supply side. In this way it is possible to produce a higher refrigeration effect without the need for using special equipment corresponding to larger compressors.

However, since the starting currents are too large, it is known to employ a follower control which applies the single phase motors to the energising voltage successively. In this case difficulties arise in switching on the follower in dependence on running up of the preceding single phase motor whilst ensuring an adequately large safety spacing, particularly if the energising voltage is subjected to fluctuations.

The present invention is based on the problem of providing a starter circuit of the aforementioned kind which has simple means for preventing spark formation during opening of the starter relay contacts.

This problem is solved according to the invention in that the contact of the starter device is bridged by a PTC resistor.

As long as the contact of the starter relay is closed, the PTC resistor has a very low resistance which does not increase even as a result of the current flowing through it because most of the starting current is led through the contact.

Consequently, on opening the contact the PTC resistor is able to take up the entire starting current and prevent spark formation. The PTC resistor subsequently becomes hot, its resistance increases and the current in the auxiliary winding drops to an insignificant value. The design can also be such that running up is not completely terminated on response of the starter device but is certainly concluded after the delay period given by the PTC resistor. After the motor is switched off, the PTC resistor cools down again. The starter circuit is particularly suitable for refrigerators because in these consecutive running periods are usually interspersed with down times which permit adequate cooling of the PTC resistor.However, should another starting operation take place before cooling has finished, this will not lead to starting problems because the current in the auxiliary winding can flow entirely by way of the contact of the starter relay.

For compressor means comprising at least one second compressor driven by a single phase motor, it is particularly favourable if the lead of the second and possible every further single phase motor contains a normally open contact of a delay relay of which the coil is in shunt with the PTC resistor of the preceding single phase motor.

In this arrangement the delay relay responds a certain time after opening of the contact of the starter device.

This delay period is determined by heating of the PTC resistor and the consequent voltage drop across it. It ensures that the second compressor runs up at a sufficient time interval after the first compressor.

Single phase electric motors constructed in accordance with the invention and starting circuits therefor will now be described by way of example only, with reference to the accompanying drawing, wherein Fig. 1 shows a first motor starting circuit; Fig. 2 shows a second such circuit, and Fig. 3 is a graph of current against time during starting.

Referring to the accompanying drawing, an asynchronous motor 1 of which the rotor 2 drives a refrigerant compressor 3 and may be driven by a single phase A.C. voltage U controlled by thermostatic switch 4 arranged a space to be refrigerated. The main (run) winding 5 is in series with a coil 6 of a starter relay 7. The auxiliary (start) winding 8 is in series with a starter condenser 9 and the contact 10 of the starter relay 7. This contact is bridged by a PTC (positive temperature co-efficient of resistance) resistor 11. An operating condenser 12 is in parallel with the starter condenser 9 and the shunt circuit of the contact 10 and PTC resistor 11.

When the thermostatic switch 4 is switched on, the full current flows through the main winding 5 and the auxiliary winding 8. On running up, the current in the main winding drops. As soon as the current in the main winding drops below a limiting value lg, the starter relay 7 opens at the instant tt. Thereafter, the current in the main winding flows through the PTC resistor 11 which becomes heated and increases its resistance until negligible current flows through through it.

Consequently, no arcing arises during opening of the contact 10. When the thermostatic switch 4 opens, the asynchronous motor 1 comes to a standstill. The PTC resistor 11 cools off so that, on the next starting, the original conditions obtain.

Fig. 2 shows a circuit for a compressor arrangement comprising at least two compressors 3, 103 which can be switched on successively. Corresponding integers are provided with the same reference numerals as in Fig. 1 or reference numerals increased by 100. The starter condenser 9 and the operating condenser 1 2 are omitted. Instead, there is a delay relay 1 3 of which the coil 14 is connected parallel to the PTC resistor 11 and the contact 1 5 is disposed in a conductor 1 6 which, as does a conductor 17, leads to a second asynchronous motor 101. The latter therefore starts only after the PTC resistor 11 has become so warm that there is an adequately large voltage drop across it to permit attraction of the delay relay 13.A similar delay relay 11 3 can be provided to operate a third asynchronous motor by way of conductors 11 6 and 117.

In this way the current load on the mains can be kept very low, as will be evident from the graph of Fig. 3. If both motors 1 and 101 were to be switched on simultaneously, the chain-dotted curve A would be obtained. If there were only one motor 1, one would obtain the curve B which is shown in full lines at the start and later in broken lines. After the current in the main winding drops below the limiting value Ig, the starter relay 7 responds at the instant t,. The PTC resistor 11 then becomes hot, whereby the voltage drop across it increases. At the instant t2, a value has been reached such that the delay relay 13 responds and the second asynchronous motor 101 starts. The switching on current pulse of curve B is superimposed so that the tot current load corresponds to curve C.It will be seen that the maximum current is only slightly higher than the maximum current obtaining in the presence of only one motor. This effect also occurs, albeit to a smaller extent, when the delay period t1-t2 is less than that shown. If one assumes that the maximum current for Curve C has a value of 1 7.5A and the maximum current for curve A has a value of 30A, this means that with a mains impedance of 1 ohm in the leads, i.e. with a voltage drop of 1 V/A, the compressors will have 1 5 V or 12.5 V more available at the instant of starting. This leads to an increase in motor torque in the order of about 15%.

Numerous modifications are possible. For example, the operating condenser 12 can be omitted from Fig. 1 and added to Fig. 2. Similarly, the starter condenser 9 can be omitted from Fig.

1 and added to Fig. 2.

Claims

1. An electric motor comprising a run winding and a start winding, a contact of a motor starting device being connected in the start winding, wherein the contact is bridged by a PTC resistor.

2. A single phase electric motor substantially as hereinbefore described with reference to, and as illustrated by the accompanying drawing.

3. In combination at least two motors each as claimed in claim 1, wherein a connecting lead between the two motors comprises a normally open contact of a delay relay, the relay having a coil in shunt with the PTC resistor of the upstream motor, the motors being connected in parallel when the contact closes.

4. A combination of motors as claimed in claim 3, and substantially as hereinbefore described with reference to, and as illustrated by the accompanying drawing.