GB1187759A - Electric Motor Heating Unit - Google Patents

Abstract

1,187,759. Switching circuits. M. & B. ELECTRIC CO. Inc. 24 Feb., 1969, No. 9722/69. Heading H2H. An arrangement for keeping an electric motor warm during the period when it is switched off includes a switching circuit for applying a reduced voltage to the windings of the motor, the switching being delayed until the motor has come to rest after switching off. As shown in Fig. 2, the motor 10 is energized by a threephase supply at terminals L1, L2, L3 through a main switch MS. This switch co-operates with a further switch PS so that when the switch MS is closed, switch PS is open and vice versa. Hence, when the motor is turned off by opening switch MS, switch PS closes to energize a transformer T. Connected across the secondary winding 14 of the transformer, there is a time delay device 17 which, after a predetermined time longer than the time taken for the motor to come to rest, closes a switch 17<1>. A relay R is now energized and switches S1-S4 are closed. Hence, a voltage is applied to lines 20, 21, 22 which are connected to the input terminals of the motor. This voltage is selected from tappings on the transformer secondary, and is chosen so that the current in the motor windings is of the order of one-fifth of the rated current. Sufficient resistive heat is now produced to maintain the temperature of the windings 15- 20‹ C. above the ambient temperature and to prevent atmospheric condensation. When the motor is next used, closure of the main switch MS immediately opens switch PS, de-energizing the transformer T and the relay R. Should any of the contacts S1-S4 stick, the circuit is protected by fuses F1, F2.