starting current

[jacib]

i have 2 questions

1.Normally a squirrel cage induction motor, is designed for a maximum starting current of 6-7 times its rated full load current.
What will be the starting current of the same motor, if it is started without any load using DOL(Direct On Line) starter?

2.Consider the case of a squirrel cage induction motor having a centrifugal pump (water application) as load. Normally before starting the delivery valve of the pump remains closed.
If the pump has a running load of X kW, what will be the starting load (kW) on the motor if it is started with a DOL starter?

 

[steve66]

Re: starting current

I think (but I wouldn't bet money on it):

1. The starting current will still be 6-7 times the running current. This is largely a function of the internal parameters of the motor including the slip (at startup the slip is maximum).

2. The KW will be almost 0 and equal to the losses in the motor and circuit. Pout = 0 since the motor can't pump any water, and Pin = Pout + Plosses.

In these extreme cases, power is not directly proportional to current since the PF may be very low.

Steve

 

[stew]

Re: starting current

I have always used 3-5 time for no load starts and 6-7 for loaded starts.The reason for this is that the wr2 or inetia required to start the load is lower. The maximum average peak value of current is less under no load conditions. An analogy would be if you were pulling an unloaded wagon it would take less effort and time to get up to speed tahn if the wagon had a tonof bricks in it. You may still be able to achieve the same speed but the duration and effort required to acheive it would be considerably increased. Just my thought anyhow

 

[gpedens]

Re: starting current

The peak starting current is the same loaded or unloaded. Once the rotor starts turning this peak current drops off rapidly as the rotor accelerates to running speed. If loaded the current drops off a little slower. This is where your 3 to 5 and 6 to 7 times currents may work for choosing your overload protection most of the time without nusiance trips. You can usually get motor curves from the manufacturer to show how the motor responds to voltage, current, time, torque, speed etc. The peak current depends on what point the contacts close on the voltage sine wave. At start the motor is like a transformer with the secondary shorted. The power factors are low 15 to 25 % versus 75 to 85 running. The NEMA code leter gives the range for the locked rotor KVA per horsepower and the design letter gives the range for the torque and slip.