Motor Starting

[bob]

Hi,

 

I am more than convinced about the bad consequences, electrical and mechanical transients,of the star/delta transition mode of starting. This issue is marvellously illustrated throughout this wonderful forum.

However, I am a bit worried as I am now using a DOL starter for a fan application, 90 kW. I am due to start the fan this week. I am not at all worried about the time the fan will reach nominal speed and the starting current as I have a huge supply behind. I have got similar applications where open transition star/delta starters are being used. It took less than 5 seconds to reach nominal speed and the delta contactor is then closed.

I am scarced about the mechnical transient.

 

Regards.

 

Bob

[marke]

Hello Bob

 

The major issues with a DOL starter are that there in an initial inrush current to establish the magnetic field in the motor, followed by a start current of the locked rotor current of the motor. The start current is typically almost constant as the motor accelerates and then falls significantly as the motor approaches full speed.

 

The start torque jumps from zero to the locked rotor torque, and typically drops a small amount to a minimum torque at part speed the then increases to a maximum at almost full speed.

Apart from the initial step in torque, there is not real transient as there is with the open transition star delta starter.

If the fan, motor and coupling are able to withstand a star delta starter, I would not expect problems with DOL starting.

 

From my experience, star delta starting does more damage than DOL.

 

Best regards,

Mark.

[bob]

Hello Mark,

 

I just started the fan and we hardly notice the starting transient.

The inrush current to establish the field is independent of the mode of starting and is a factor of the applied voltage. I trust that for a DOL, the inrush is obviously greater than a DOL application. But what about the LRC, is it voltage dependent ?

 

Thanks.

 

Bob

 

Hello Bob

 

The major issues with a DOL starter are that there in an initial inrush current to establish the magnetic field in the motor, followed by a start current of the locked rotor current of the motor. The start current is typically almost constant as the motor accelerates and then falls significantly as the motor approaches full speed.

 

The start torque jumps from zero to the locked rotor torque, and typically drops a small amount to a minimum torque at part speed the then increases to a maximum at almost full speed.

Apart from the initial step in torque, there is not real transient as there is with the open transition star delta starter.

If the fan, motor and coupling are able to withstand a star delta starter, I would not expect problems with DOL starting.

 

From my experience, star delta starting does more damage than DOL.

 

Best regards,

Mark.