Motor problem

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mickeyrench

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edison, n.j.
What can be done to 3 phase 480v 9 lead motor to not start it up across the line . It is a large motor that causes lights and and other equipment to dropout.
A vfd or soft start is not an option. Right now its using a starter. It is about 100 hp. Not sure. Thanks for any input.
 
mickeyrench said:
A vfd or soft start is not an option.
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Why not.

The only two ways to reduce the motor starting current are: start it unloaded or reduce the applied voltage. You have ruled out to methods of reducing the voltage (soft start and VFD) so all you are left with is reducing the the load.
 
actually the real cost does make a softstart an option.

By the time you add in all the materials and labor a sort start actually is your best option.

once you start MCC modification and or extra gear the accelartion of costs are going to equal a soft start that you can install in relatively short time and with relatively minor cost.

Now figure in the energy savings and the incidental shortened life expectancy of other equipment on the circuit that suffer brown outs every time this motor starts. Payback is quickly achieved.
 
jim dungar said:
Why not.

The only two ways to reduce the motor starting current are: start it unloaded or reduce the applied voltage. You have ruled out to methods of reducing the voltage (soft start and VFD) so all you are left with is reducing the the load.
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Even if you started the motor unloaded the system would still see the full LRC of the motor which may still cause issues. Starting unloaded would only make the acceleration time of the motor quiker wouldn't it?
 
mull982 said:
Even if you started the motor unloaded the system would still see the full LRC of the motor which may still cause issues. Starting unloaded would only make the acceleration time of the motor quiker wouldn't it?
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For 'across the line starting', you will always see the motor inrush current as that is a function of magnetizing the motor. The locked rotor current exists as long as the rotor is not turning and it decreases as the motor accelerates. "Lights dim and equipment drops out" usually due to extended low voltage conditions, so it is probable that an unloaded motor will cause few problems.

Generally, it is impossible to be able to 100% unload a motor unless you employ some type of clutch system.
 
mickeyrench said:
What can be done to 3 phase 480v 9 lead motor to not start it up across the line . It is a large motor that causes lights and and other equipment to dropout.
A vfd or soft start is not an option. Right now its using a starter. It is about 100 hp. Not sure. Thanks for any input.
Click to expand...
If you have (for some reason) ruled out a soft starter or a VFD, then your only other options remain an electro-mechanical means of Reduced Voltage starting. With a 9 lead motor, you should be able to use Wye-Delta starting, but it would require running 6 conductors to the motor instead of the 3 you probably already have, so that may entail running new conduit etc. the only other method would be a Reduced Voltage AutoTransformer (RVAT) starter. You didn't post the size, but you can find them available for larger motors, i.e. 100HP and up, fairly easily. If it is smaller than 100HP, chances are you will have to have one made to order. But RVATs are physivally larger and more expensive than Soft Starters now, so I'm not sure why you say a Soft Starter is out of the question.
 
Here is somethiing else you may consider if you really don't want to use a soft starter or a VFD. We recently installed a few and are still evaluating the benefits/drawbacks. I have no idea what they cost since it was not my project.

http://www.magnadrive.com/
 
At 100HP, a soft start is going to be cheaper than a reduced voltage starter.

If the lights are dimming because of the initial motor unrush, a larger transformer or a soft start is the answer.

If the problem is the long acceleration time to get the load up to speed, doing something with the load is the answer.
 
mickeyrench said:
thanks for all the information, i guess a soft start is the way to go for us.
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If your load inertia is large a soft start or ASD may not be your solution.

  1. The first thing you should do is check your feeder wire size to make sure you don't have an excessively long wire with undersized conductors that actually cause your dimming problem.
  2. Next get with a mechanical engineer to determine if the load inertia and the motor torque are a match.
  3. Calculate the runup acceleration voltage drop to see if the reduced voltage starter will solve the issue.
 
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