The ways to protect electric motors
Generally ways in protection of motor:
1 normal current when the motor is running as a reference value to be protected. 2 motor operation when the motor temperature rise protection. Measure the motor shaft diameter shock protection. Four electric motors to run three-phase current value requires balancing, testing whether the current balance, the use of electrical appliances, electronic devices to be protected. 5 by measuring motor-to-ground leakage current size of the protection. Characteristics of the three-phase zero-sequence transformers measure the motor line current is zero protection. In summary, the classification are in fact the current size of the motor running, the change to be protected.
Two methods in practical project:
1. measuring current of thermal relay is more than the motor rated current value, adjust the setting current value. Motor running overload thermal relay auxiliary head, normally closed disconnect normally open point of closure of the features to be protected. Control of the relay, normally closed and stop buttons in series, overload, stop the motor running, and gives the alarm signal. PLC control system, normally open or normally closed operation signal is introduced into the PLC signal input through the procedures to control the motor start and stop. Such as the introduction of the normally open as a stop signal X input point normally closed, and vice versa since.
2. the temperature motor protection within the plant thermal devices to protect. Temperature is too high given the signal to be protected by an external control loop. Thermal resistance, thermocouple measuring the temperature of the motor overheat signal is given by the temperature controller, thereby protecting the motor. Three current transformers measure the motor line three-phase zero-sequence is zero, and are generally used for large power motor. The specific method: according to the motor power to select the appropriate size of the current transformer installed in the motor control box into the line side. A phase B phase C phase to the placement, the ABC current transformer L1-side access to a thermal relay on the line side, the L2 termination of the ABC current transformer into a star, the star point of the introduction of zero line. When the three-phase unbalanced current is too large thermal relay, normally open, normally closed point of action. Action signals into the control loop in order to cut off the current to protect motor.
