US 3562614 A
Description (OCR text may contain errors)
United States Patent Inventor Asger Gramkow Augustenborg, Denmark Appl. No. 743,725 Filed July 10, 1968 Patented Feb. 9, 1971 Assignee Danfoss A/S Nordborg, Denmark a corporation of Denmark STARTING SWITCHING MEANS FOR A SINGLE- PHASE ASYNCI-IRONOUS MOTOR  References Cited UNITED STATES PATENTS 3,414,789 12/1968 Prouty 318/227X FOREIGN PATENTS 1,042,126 9/1966 Great Britain 318/221 Primary ExaminerOris L. Rader Assistant ExaminerGene Z. Rubinson An0rney-Wayne B. Easton along with the electric motor, in the housing of a refrigeration unit.
1 Claim, 1 Drawing Fig.
U.S.Cl 318/221, 3 18/227, 318/229 Int. Cl H02p l/44 Field ofSearch 318/220, 221, 227, 229
I2 5 4 r a I l 13 l l I 1 l l l l l l STARTING SWITCHING MEANS FOR A SINGLE-PHASE ASYNCHRONOUS MOTOR This invention relates to a starting switching means for a single-phase asynchronous motor having a symmetrical controlled semiconductor valve as the starting switch.
A starting switching means of this kind is known wherein the semiconductor valve is controlled with the help of a transfonner in dependence upon the main winding current. Herein, the primary winding of the transformer is in series with the main winding, whilst the secondary winding connects a pole of the valve with its control electrode. For many purposes, however, such a transformer is too complicated and too expensive and occupies too much space.
it is also known to use a PTC resistor for starting a singlephase asynchronous motor. The FTC resistor is, for example, connected in series with the starting winding and is heated by the current flowing through it and brought to a high resistance value, so that the starting current is reduced to an insignificant value in a prescribed period of time. Here, the PTC resistor must in the first instance carry the whole of the starting current. This leads to thermal shock which often cannot be withstood by the PTC resistors which are usually formed as ceramic bodies.
The object of the invention is to provide a starting control meansincorporating a symmetrical controlled semiconductor valve, which means does not employ a transformer and is of very simple design.
According to the invention, this object is achieved by connecting the valve in series with an ohmic resistor in the starting winding and in connecting itscontrol electrode, by way of a P'TC resistor, with that end of the ohmic resistor remote from the valve.
in this switching means a fairly heavy control current flows through the initially cold PTC resistor and the control electrode, this current opening the valve at each half-wave and thus enabling the full starting current to flow through the valve and the connected series resistor. The voltage-drop at the ohmic resistor provides for the current being maintained throughout the control circuit. After some time, the PTC resistor becomes heated as a result of the current flowing through it and/or by means of an additional heating means, in
r such manner that the increasing resistance enables the control current to diminish. After a certain time, the control current is so low, that it no longer suffices to prime the valve. The starting current is thus interrupted. A small current, flowing continuously through the PTC resistor, maintains its temperature. In this arrangement, the PTC resistor is only required to carry a control current that is small in comparison with the starting current. Sudden thermal shock is therefore precluded. Furthermore, small changes in temperature suffice to reduce the control current to a value at which the valve is no longer primed. The ohmic resistor also serves as a series resistor, so that the switching means is particularly suitable for an asynchronous motor having a resistance auxiliary phase.
The "switching means here described is especially suitable for asynchronous motors which are frequently started up and wherein the life expectation of the starting switching means can be considerable reduced by the failure of moving parts and any burning of contacts that may occur. This applies in particular in the case of motors for small refrigerating machines.
In this connection a preferred arrangement results if the valve, the ohmic resistor and the PT C resistor are accommodated, together with the motor, inside the case of a small refrigerating machine, since the case then only needs to have passages for two-pole operation.
The invention will now be described in more detail by reference to the drawing, in which is illustrated a connection diagram for the starting switching means according to the invention.
A single-phase motor 1 having a main winding 2 and a starting winding 3 is connected through a main switch 4 to the terminals 5 and 6 of a supply system. A symmetrical controllable valve 7 and an ohmic resistor 8, connected in series, are connected ahead of the starting winding 3. From a point 9 beyond the resistor 8, a conductor runs to the control electrode 11 of the valve 7 by way of a PTC resistor 10. The motor and the parts 7-9 are accommodated in the interior of a case 12 of a small refrigerating machine, so that only two passages 13 and 14 are required for the supply leads.
in particular, the valve 7 can take the form of an element known under the trade name TRlAC. When the main switch 4 is thrown the PTC resistor 10 is initially cold. Consequently, an adequate control current flows through the electrode 11 and the PTC resistor 10. As a result, the valve 7 isopened at each half-wave and the full starting current flows through the starting winding 3. As soon as the PT C resistor has become heated above a prescribed temperature value, it acquires so great a resistance that the control current flowing through it no longer suffices to prime the valve 7. The starting current is therefore interrupted and only a small current flows through the PT C resistor 10 thereby maintaining its temperature.
The switching means also prevents attempts to switch on again frequently, since the FTC resistor 10 is then not sufficiently cooled to pass a control current adequate for the starting operation.
The FTC resistor 10 can be heated not only by the current flowing through it; it can also be heated by another source, for example, by the series resistor 8, a heating resistor contained in the main current circuit or by heat produced by the motor I or its windings 2 and 3.
1. Starting switching means for a single-phase asynchronous motor having a starting winding, comprising, a symmetrical controlled semiconductor valve having a transconductive path and a control electrode, said valve being in series with said starting winding, an ohmic resistor in series with said valve, and a P'TC resistor having one end thereof connected to said control electrode and the other end thereof connected between said ohmic resistor and said starting winding.