The Importance Of Overload Protection

Register to read the introduction… When the motor current exceed a safe value, the current through the heaters generates enough heat to activate the thermal device, which opens the normally closed contacts in the control circuit (Figure 163). The overload device is sensitive to current and time. Small currents take longer to open the contacts than do large currents. It is important to realize that the overload device does not provide short-circuit protection. Short circuit protection must be accomplished through the use of fuses, circuit breakers, or other devices. Each heating element has a specific rating. To provide adequate protection, the heater …show more content…
The motor (1) is connected to the three lines L1, L2, and L3 by means of the movable lever (2). When the lever is moved from the middle, or off position, to the start position, the autotransformers (3) are connected in the circuit by means of the oil-immersed contacts (4). The low-voltage taps from the autotransformer are connected to the motor and the motor starts. When the motor is up to speed, the lever is thrown from the start position to run position, disconnecting the autotransformer and connecting the supply directly to the motor by the contacts (5). The lever is held closed by the holding coil (6), which is connected across the phase of the supply through the relay contacts (7) of the relay and the stop button. The relays are built with dashpots, which delay the operation of the relay during the starting period and for momentary overload. Operation of the relays, or of the stop pushbutton, open the holding coil circuit and allow the lever to come to the off position, breaking the circuit made by the relay contacts (7).