Hi, just wondering what the cause of the high inrush current is, when a motor starts ?

I think it has 2 sources :

1. Electromagnetics of stator and rotor.
2. Inertia of rotor.

Assuming this to be the case is there a percentage that could be attributed to each ?

It is probably different for each type of motor. I am particularly interested in single and three phase induction motors (squirrel cage).

I could also use a technical source, whether it is a book, video, publication etc.

Many thanks Derek

 

If you keep the motor from rotating, the inrush current becomes continuous. It simply follows Ohm’s law. Once the motor is spinning, it acts as a generator, producing a voltage countering the driving voltage and this reducing the current. That is called back EMF. So the extra current you see at startup is due to the fact that the motor is not yet up to speed.

 

There is some small difference in motor technologies that influence the starting current, for example a DC brushed motor, once started revolving, generates a DC voltage that opposes the applied voltage.
In a AC squirrel cage motor, the shorted rotor (squirrel cage) is the reason for the very high starting current, as the RPM increases and the rotor rpm to rotating field approaches synchronism, the current falls drastically, as the rotor can never achieve true synchronism, there is always some current in the rotor.

 

the inrush is the results of the starting method, being instantaneous. Think of it this way, you have a load that runs at some desired speed, resulting in some level of work being performed. With an across the line starter, speed is initially zero, and to perform that work, the basic formula will show that with zero speed, torque must be close to infinite. Luckily, the impedance of the motor will limit torque producing current to a more manageable value. Different rotor classes behave somewhat differently in that regards. Rotor inertia, and more likely load inertia, determines that time duration that elevated torque values are required to bring the load to speed.

 

I just explained this on another thread. Motors that have a starting inrush are those motors whose rotation generates a voltage that opposes the supply voltage once they are rotating. so at startup, they are not rotating and thus no counter-emf is generated to oppose the supply current.
Note that this explanation, while correct, fails to explain how that counter EMF is generated. THAT portion of the explanation is rather more complex, and the details depend on the motor type.

 

Note that this explanation, while correct, fails to explain how that counter EMF is generated. THAT portion of the explanation is rather more complex, and the details depend on the motor type.

There is a stark contrast between the DC brushed motor and the AC induction motor, the principles are quite different.

 

There is a stark contrast between the DC brushed motor and the AC induction motor, the principles are quite different.

Quite Righ, Max, but in each case the starting inrush iis due to the lack of the counter EMF, which is generated by the rotation.

 

Hi, just wondering what the cause of the high inrush current is, when a motor starts ?
I could also use a technical source, whether it is a book, video, publication etc.
Many thanks Derek

@derick007 With the advent of VFD's, , a motor that you rarely see now is the wound rotor AC induction motor.
This was often used on cranes for the hoist etc, they were somewhat inefficient but did the job at the time.
They also reduced the high inrush current.


A good explanation is