An AC single phase motor-system comprises a rotor driven by a main winding 4 and an auxiliary winding 5 powered through a start capacitor Cs for driving the rotor during start-up. A potential relay 6 controls a relay-switch 7 which deactivates the start capacitor Cs when a control voltage across the relay 7 exceeds a threshold. The relay-switch 7 is located so that when the start capacitor Cs is connected, the control voltage depends on the voltage across the auxiliary winding 5, and when the capacitor Cs is disconnected, the control voltage is independent of the voltage across the auxiliary winding 5. A PTC 20 may be included to temporarily bypass the open relay 7. This prevents unstable switching. The motor system may be used in a compressor for a refrigerator.
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