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Adding Vsd To Motors And Bearing Failure

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I need to add VSDs to 2 x 150kW motors. They are currently started by old large orange cabineted Turnbul and Jones liquid starters. However the power company is disturbed with the voltage droop we are causing at start up in the incoming 11kV lines. Hence the need to go to VSD's. Now both these motors will run at 50Hz. do i need shaft earthing brushes to eliminate bearing failure caused by stray induced currents?

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Hello arcserv


The replacement of the existing starters with VSDs is certainly one way to start these motors with reduced current, but there may also be a good chance of starting them with good soft starters at a lower start current than with the liquid resistance starters.

One of the problems with the liquid resistance starters, is that they can not be easily tuned to provide the optimum starting conditions, plus as they heat up due to the very high power dissipated, the resistance goes down resulting in an increasing current. If the volume of solution is to low, the current can be close to LRC for a high percentage of the start time.

As you are operating at a fixed speed, the soft starter is worth considering provided that it can start the machine without disturbing the 11KV supply, because it is a much cheaper option, more efficient and will last a lot longer.


What is the application?

What is the start current at present?

How much voltage drop is the on the 11KV at present?


Best regards,

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Starting of these motors has been identified by lines company to be causing problems. They have stipulated we use a VSD and only allow 250% start current.

both applications are in the quarry industry. One is a Barmac crusher the other is a Gyrotary crusher.

Start current exceeds 1000 Amps causing 11% drop in 11kV.

I have used AB soft starts before and dont think that either motor will get away at 250%

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If you use a VSD, then the start current should not exceed 150%, so that will not be a problem, however, there will be a high harmonic current continuously and this could create other issues.

If the motor is a high starting efficiency motor, you can get a Barmac away at 350 - 300% current except for the very early Barmacs that were very high inertia and needed a high start torque just to accelerate them in a reasonable time.

One trick, is to use two motors to drive the crusher, soft start one motor and then connect the second motor when the crusher is up to full speed. The advantage of this is that you get a much higher starting efficiency than using a single motor of twice the size.

For example, if you use a single motor of locked rotor current 600% and locked rotor torque 180%, and you need to have 60% to accelerate the machine to full speed, you would need a start current of 346%.

If we now replace that motor with two motors of half the rating, but equal start characteristics, then the start current required would be 244% => lower start current for the same start torque.

We have done this many times on medium to large machines. The only thing to be careful of, is the starting dissipation is now in one motor rather than two, so the rotor selection is more important.


In answer to your question about bearing currents (EDM), this is a difficult question as there is no real exact answer here. Some motors are more susceptible than others, and some drives are also more susceptible than others. At 150KW, you are on the threshold where it is often considered that EDM could become a problem. I do not know of a "formula" that can be reliably applied to predetermine whether EDM will be a problem. Probably the best advice, is to talk to the supplier of the motors and the supplier of the VSDs to see what they recommend, but if in doubt, there is no negative to adding a brush.

The EDM effect is a function of the switching frequency and rate of rise of voltage (dv/dt), rather than the drive output frequency.


Best regards,

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