US 3600656 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
United States Patent Inventor Asger Gramkow Gammelgard pr. Augustenborg, Denmark Appl. No. 7,890 Filed Feb. 2, 1970 Patented Aug. 17, 1971 Assignee Danfoss A/S Nordborg, Denmark Priority Mar. 25, 1969 Germany P 19 15 135.0
STARTING MEANS FOR A SINGLE-PHASE References Cited Primary ExaminerGene Z. Rubinson Attorney-Wayne B. Easton ABSTRACT: The invention relates to a starting switch assembly for a single phase asynchronous motor. The assembly includes a symmetrically controlled semiconductor valve in series with the motor starting coil. The control electrode of the valve is connected by way of a PTC resistor to the end of the starting coil which is remote from the valve. In this arrangement the PTC resistor is controlled directly by the power supply voltage and the time required for the PTC resistor to reach the cutout temperature depends'substantially only on the characteristics of the PTC resistor and is independent of the particular construction of the motor.
ASYNCHRONOUS MOTOR 3 Claims, 1 Drawing Fig. 03. Cl 318/221 E, 318/227, 318/229 Int. Cl H02p 5/40, H02p 1/44 Field of Search 318/220, i 221, 227, 229
STARTING MEANS FOR A SINGLE-PHASE ASYNCHRONOUS MOTOR The invention relates to a starting switch for a single-phase asynchronous motor in the form of a symmetrically controlled semiconductor valve, arranged in series with the starting coil.
A starting switch of this kind is known in which the semiconductor valve is controlled with the help of a transformer in dependence upon the current passing through the main coil. Here, the primary winding of the transformer is arranged in series with the main coil, whereas'the secondary winding connects a contact on the valve to its control electrode. A transformer of this kind is however too complicated, expensive and bulky for many applications.
It is also known to use a FTC-resistor for starting a singlephase asynchronous motor. For example, the FTC-resistor is connected in series with the starting coil and is heated by the current that flows through and its resistance is raised, so that the starting current is reduced to an insignificant value in a prescribed period. In this arrangement, the FTC-resistor must initially carry the full starting current.'This leads to a thermal shock which FTC-resistors, usually formed as ceramic elements, cannot often withstand.
In accordance with an earlier proposal, the semiconductor valve is connected in series with an ohmic resistor in the starting coil, and the control electrode thereof is connected through a FTC-resistor to that end of the valve remote from the ohmic resistor. In the case of this inherently very useful switch it is however necessary for the ohmic resistor and the FTC-resistor to be precisely rated tosuit the data of the asynchronous motor, since the voltage-drop at the ohmic resistor, used for control purposes, is dependent upon the magnitude of the starting current. The object of the invention is to provide a semiconductor starting switch which can be operated using extremely simple means and which can be used for a large number of different motors.
According to the invention, this object is achieved by connecting the control electrode of the semiconductor valve to that end of the starting coil remote from the valve, by way of a FTC-resistor.
In this arrangement, the heating-up of the FTC-resistor is controlled by the constant mains voltage. The period required by the FTC-resistor to reach that temperature at which the control voltage blocks the semiconductor valve depends substantially only upon the characteristics of the FTC-resistor and is independent of the particular construction of the motor.
If the current flowing across the control gap of the semiconductor valve is not sufficient to hold the FTC-resistor at a sufficiently high level of resistance, i.e., at a sufficiently high temhas to provide systems operating at different powers, but, for reasons of rationalization tries to make do with one or just a few types of starting switch.
Here, in particular, the semiconductor valve, the PTC-resistor, the motor and, where required, the shunt resistor can be accommodated inside the case of a small refrigerating machine, the case being holed for two-pole connection only.
The invention will now be described in more detail by reference to the drawing which shows a connection diagram for the starting switch of the invention.
A single-phase motor 1, having a main coil 2 and a starting coil 3, is connected to the terminals 5 and 6 of the supply system through a main switch 4. Arranged in series with the starting coil 3 is a symmetrical semiconductor valve 7, for example a multilayer diode, marketed under the trade name Triac. The control electrode 8 of this valve is connected by way of a FTC-resistor 9 to a point 10 on that end of the starting coil 3 remote from the valve 7.
When the main switch 4 is switched on, the FTC-resistor 9 is at first cold. There is thus applied to the control electrode 8 a voltage which is only slightly below the voltage at the supply terminal 6. Consequently the semiconductor valve 7 strikes and the motor starts to run. Because of the flow of current, induced by the supply voltage, the FTC-resistor 9 heats up until it reaches so great a resistance that the voltage available at the control electrode 8 no longer suffices to strike the valve 7, so that the starting coil 3 is cut out. There is no difficulty in so rating the FTC-resistor 9 that the moment of cutout occurs when it is certain that the motor 1 is running at speed.
If required, a shunt resistor 11, bypassing the control gap of the semiconductor valve 7, can be provided. This, together with the FTC-resistor 9, constitutes a voltage divider at the tapping point 12 of which the control voltage can be tapped off.
i The FTC-resistor 9 also prevents any attempt at repeatedly switching on again, since then the FTC-resistor will not have cooled down sufficiently during the starting operation to make a control voltage sufficient for striking available.
The parts 1, 7, 9 and, possibly, 11 are accommodated within a case 13 containing only two connection orifices 14. This results in a much simpler construction for an encased refrigerating machine.
The expression symmetrical, controlled semiconductor valve is intended to cover such control elements that permit the current to pass in both directions when an adequate striking signal exists.
1. A motor and control assembly comprising a single-phase asynchronous motor having a main coil and a starting coil in parallel with said main coil, said coils having leads extending to a junction, a symmetrical controlled semiconductor valve in series with said starting coil on the opposite side of said starting coil from said junction, said valve having a control electrode, and a FTC-resistor between said control electrode and said junction.
2. An assembly according to claim 1 including a shunt resistor between said control electrode and the side of said valve remote from said starting winding.
3. An assembly according to claim 1, a refrigeration machine casing, said assembly being enclosed in said casing, and terminal means internally of said casing for the attachment thereto of power supply conductors.