How does an induction motor start?

Answer 1

The emf induced in the rotor depends upon the rotor frequency which further depends on relative speed between rotor and synchronous speed of rotating magnetic field. The relative speed at the time of starting or standstill is maximum and hence large emf in induced in rotor conductors or winding due to which very high current flows in rotor which is generally 5 to 7 times of the full load or running current of an indication motor.

The starting current, sometimes called inrush current will be 8-10 times higher than the motor's rated current because the motor is at rest.

When at rest, the motor appears to be a transformer, with it's secondary winding short circuited, i.e. the rotor winding or squirrel cage is a closed circuit. This results in a low impedance to the system voltage and the motor has a "locked rotor" current of typically 6 times full load current, but also up to 8 and sometimes 10 times current.

This starting value is independent of any load attached, however, the inertia of the motor and load has to be overcome. This is especially true when the motor is connected to a load, like a conveyor belt with product on it. The larger the inertia, the longer the motor takes to reach full speed.

As the motor accelerates, part of the starting current power overcomes this inertia and is conveted to kinetic energy. The remaining power of the starting current heats the rotor, up to possibly 250 deg C for a "long" start (20 seconds!!).

During the first couple of cycles of AC current, transient currents make some of the phases have higher assymetrical values, which can cause nuisance tripping of the supply breakers to the motor, when protection settings are too low!

Equation for current: I = P / cos phi / ( 1.732 * V), (cos phi can be 0.3 during starting).

V (voltage) stays pretty much constant, therefore I (current) must rise (but is limited to the locked rotor value for the specific motor) as P (power) is required to start the motor and to keep it running.

Some voltage drop will occur for most power systems during a motor start, thus with a constant locked rotor impedance, the starting current will reduce proportionally.

I start = V / 1.732 / Z (locked rotor) for a three phase motor. (Z is fixed)

When nearly full running speed is reached, the current drops rapidly to full load current or less, depending on the actual load attached.

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Answer 2

There are two types of induction motors,the single phase and three phase induction motor. 1.Single phase induction motor=At switch on,an alternating current is applied to the stator windings inducing a magnetic field which moves forth and back. This is because the resistance of the starting windings is higher and the number of its turns are higher than that of the running winding thus causing a phase shift between the currents in the starting and running windings. Since the stator magnetic flux moves forward and backward,it cuts the rotor windings or squirrel cage thus inducing a current in it. This current produce a flux opposite to that of the stator magnetic flux. The two flux's interaction,pull&attract, causes the rotor to turn. 2. Three phase induction motor. There are three pairs of windings arranged 120 electrical degrees apart. When a three phase alternating current is applied to the stator windings, each pair of winding produce a flux which adds up with 2 others forming what's called a resultant flux which sweeps across the rotor windings. In so doing, it induces an emf in the rotor windings and a flux as a result of the induced current. The polarity of this induced rotor magnetic flux is such the rotor is pulled along by the resultant flux of the stator windings.
Three-phase motors are selfstarting; the rotor is simply dragged around by the rotating field. Single-phase motors aren't. They need an additional source of torque to start them rotating in a particular direction and bring them up to a speed where they'll run by themselves. Except for very small motors this is usually done by a starting winding which often but not always uses a capacitor to generate a phase-shift. The starting winding may or may not be switched out when the motor comes up to maybe 3/4 speed. Most motors can operate in either direction depending on which way the start winding is connected at startup.

Answer 3

An induction motor takes more current in the starting than the rated/running current as when the motor is starting the rpm of rotor is less and a high amount of current passes through stator because there is no back force which resists the movement of electrons in the stator windings. In the stop state the stator windings are just like simple wires from which current is passing and have resistant equal to the resistance of the wire only. As the rotor gradually speeds up the electromagnetic field generated by stator windings induces emf in the rotor and induced current begins to flow in the rotor. The induced current in the rotor generates its electromagnetic field which plays two roles in the running of the induction motor:

1) it interacts with the electromagnetic field of the stator and the resultant forces run motor.

2) it induces back emf in stator windings as well which produces back current in stator windings. Now there are two type of currents flowing in the stator windings a) forward current from the power source b) back current. These two currents add up and the sum is the reduced running current in the stator.

net current in stator windings = forwards current from power source - back current

Now more the speed of the rotor more the will be the back current and less would be the running current of the motor.

Answer 4

For 2 reasons:

*due to the inductance of the stator

*because, to start, a motor must 'beats' the resistant torque created by the intertia of the load, which requires a big force and thus a large current.

Answer 5

When a motor is started it is in effect a transformer with a dead short across the windings. As the motor starts to turn a voltage induced in the rotor opposes the inrush current in the armature winding. This is known as back EMF (electro motive force). On most motors this happens very rapidly usually with in five time cycles of the applied frequency.

Answer 6

to overcome repulsion due to magnetic field

Answer 7

induction motor start by magnetic motor