Auto TR strating or Star-Delta Starting

[kimseunghee]

I have problem with starting motor as auto tr. The starter is designed with auto tr 65% tap but motor is not deisgned with DOL. Motor supplier said that motor start up auto tr 70% as minimum. When motor reaches up to 60% of the rated speed at auto tr 65% tap, then starter changes over to full voltage. Is this process acceptable according to international standards such as IEC or others. It under stood that auot tr is changed over to full voltage at rated speed.
Another secenario is to change star-delta starting. It is also similar process i.e, motor reaches up to 60% of rated speed at star connection, then at this moment motor will be changed over to delta connection. Is this process acceptable according to international standards such as IEC or others.
I have never considered the above problems before.
Please advise me ASAP.

 

[jraef]

There are no standards that apply (as far as I know), you can do whatever you like. But from a "best practices" standpoint, neither solution is acceptable really. In the case of any 2-step process such as Star-Delta or Autotransformer starting, the transition to Full Voltage should only take place when the motor reaches at least 90% speed. Otherwise, the current / torque spike from transition will be as high as or greater than if you had started DOL. Only the duration of the spike will be reduced. If your motor's torque output is insufficient to accelerate the load beyond 60% speed at 65% voltage on the RVAT starter (the Star-Delta will be even worse by the way), then you have only 2 choices:
1) See if the 80% taps will start it;
2) Scrap all of the electro-mechanical options and use a Solid State Soft Starter. It will allow you to adjust it to whatever level it takes to accelerate it.

 

[LionelHutz]

When you start the motor the current will be high and it will drop off as the motor reaches full speed. You want to transition close enough to full-speed that the current does not go higher than the initial starting current. Each motor is different so the speed will vary somewhat but overall the speed needs to be close to full-speed for this transition.

If you transition at 60% speed you might as well just use a full-voltage starter.

When you use an autotransformer you will lower the motor voltage and current to the transformer tap percentage. Then, you have the autotransformer step down action lowering the input current by the autotransformer ratio times the output current (or motor current).

This means the input current of the autotransformer is;
line current = motor current x ratio x ratio
line current = motor current x ratio^2 (0.65^2)

You need to pick the speed where the full-voltage current is less than the reduced current at 0 speed. Doing this will ensure that the line current after the transition does not go higher than the line current during the acceleration.

So, take the current at 0 speed and multiply by the transformer tap ratio squared (65%^2). Then, pick the speed off the curve where the current is less than this amount. That is the minimum transition speed for best operation.

 

[kimseunghee]

This is to find easy way to resolve motor starting but finally I place reorder to soft starter.
Thanks a lots

 

[rbulsara]

What jraef said...