Power interruption protection system for electric motors

The combination with a polyphase alternating current motor of

• (a) means including an electromagnetic contact for connecting and disconnecting said motor from a source of polyphase AC supply
• (b) an electromagnetic relay having normally closed contacts in circuit with the energizing coil of said contactor to disconnect said motor from said source of AC supply,
• (c) means including current transformers in the supply connections to the motor primary terminal for providing a unidirectional control signal proportional to motor current,
• (d) means connected to said source of AC supply and providing clock pulses each half cycle of the supply frequency,
• (e) a digital pulse counter subjected to said clock pulses and when active to register such clock pulses providing count output pulses in accordance with the number of clock pulses registered,
• (f) current level detector means rendered active as a result of energization of said electroresponsive contactor to reset and block said counter from registering clock input pulses whenever said control signal rises above a predetermined value, the last mentioned means unblocking said counter to provide count output pulses when said control signal decreases below said predetermined value, and
• (g) means including amplifier-driver means responsive to a fixed number of count output pulses from said counter to cause energization of said electroresponsive relay.

In an overload and power interruption protection system for an AC electric motor which is connectable to an AC supply source by electroresponsive means, the combination with an electroresponsive relay which when energized causes said electroresponsive means to disconnect said motor from said supply source, of

• (a) means providing a control signal which varies in accordance with motor current,
• (b) means responsive to connection of the motor to said supply source to provide a timed output signal upon elapse of a present timed interval,
• (c) current overload responsive means activated by connection of said motor to said supply source to be responsive to a first predetermined value of said control signal to provide a first overload output signal,
• (d) second current overload responsive means responsive when activated to provide a second overload output signal whenever said control signal exceeds a second predetermined value,
• (e) means responsive to said motor attaining its running speed for deactivating said means providing said timed output signal and for activating said second overload responsive means,
• (f) means responsive to said control signal decreasing to a third predetermined value for a given number of more of the half cycles of the AC supply frequency to provide a power disconnect output signal, and
• (g) means responsive alternatively to said timed output signal, either of said first and second overload output signals, or said power interruption output signal to energize said electroresponsive relay.