6-Induction Motor

Lesson
Tools

5. Starting an asynchronous machine

5.1. Case of the starting torque

Method :

For an Induction Motor to start, the torque demanded by the load on startup must be less than the starting torque of the machine.
The machine produces the starting and stabilizes at the point F which is the point of operation of the system.
This point must be as close as possible to point N (nominal point of the machine) to obtain maximum energy efficiency.

Method : Direct On Line (DoL)

Direct startup of an asynchronous machine causes a high starting current. Id = 6 to 8 In.
Make sure that the protections are not tripped: fuse use aM.
To avoid the disadvantage of the fuse aM (melting delay), the motor is protected against overloads by a thermal tripping device

Method : Star – Delta Starting

This type of startup avoids the disadvantage of the strong current at startup. Indeed by using a reduced voltage at startup, the current is limited

and

The disadvantage lies in the weakness of the starting torque.
Star Delta Starter can only be used if the starter-resistant torque is zero or very low.

5.2. Stator Starter : Starting by eliminating Stator resistors

Always to eliminate the intensity peak at startup, work under reduced voltage. This reduced voltage is achieved by inserting stator resistors in series with the motor. We can achieve a start in several times

5.2.1. Torque and current curves

In this type of starting, the intensity absorbed by the motor is proportional to the voltage applied to the motor.

On the other hand, the motor torque is proportional to the square root of the voltage applied to the motor.

5.2.2. Wiring diagrams

5.3. Start by elimination of rotors resistors

This process, by the addition of resistors in the rotor at startup, reduces the current; the torque meanwhile, is translated (e.g. 3 steeps)

5.3.1. Torque and current curves

This type of start allows for a lower absorbed current to obtain a torque at the start.

5.3.2. Wiring diagrams

5.4. Variation of the opposite torque by the load

For correct motor operation, the torque opposite the load must not exceed the point M because it causes the machine to stop by "unhooking" of the rotor on the stator.

For a zero resistance torque, the speed of rotation of the motor is close to the speed of synchronism ( )

This gives the maximum variation of the speed as a function of the load.

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