Methods of Start-Up in Asynchronous Motor
The easiest way to start a cage induction motor is to connect the motor directly to the mains. The required starting equipment is only the direct starter. In this most preferred method, attention should be paid to the rules and limitations of electrical administrations due to the high starting current.
The starting methods are divided into two.
1. Indirect Starting Method of Induction Motors
If the starting current of the motor is greater than the line limit value, star delta starting can be used. In delta connection, a motor wound according to the mains phase-phase voltage (eg 380V, 400V) is started in star connection. In this method, the starting current and torque decrease to approximately 1/3 of the direct starting value. In order to limit current and torque pulses during the star-to-delta transition, the transition should be performed when the motor is as close to its rated speed as possible (93...95%). However, it is possible to reduce the high starting current of large motors a little more with stepped star delta (Y / ∆ / ∆) connection.
2. Soft Starting Method for Induction Motors
Soft Starting is used when starting current is not important. Then a soft starter can be used. The starting time can be adjusted for a soft start and the motor operation can be continuously monitored and adjusted according to the voltage requirement, thus minimizing the losses. When a soft starter is used, the torque characteristic of the motor must match the specifications of the work machine.
Electrical Protection of Induction Motors
In Induction Motors, the winding temperatures should not be allowed to exceed the prescribed values. Therefore, the thermal protection of the windings should be chosen to be most suitable for the operating conditions. In general, motors are protected by bimetallic circuit breakers or overload relays that provide delayed overcurrent protection. But this protection is especially effective during the take-off process. In addition, the motors are protected against excessive temperature rises that may occur for any reason, with the help of thermostats, which are bimetallic switches, and thermistors, which are semiconductor temperature sensors, placed in their windings. Thermistor protection is safer than other motor protection schemes, as it controls the temperature in the winding, which is the most critical point, regardless of external factors and operating mode. Fuses, as a rule, do not protect the motor, but only the system.
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