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RVAT Starters

I have a 100 hp 480 volt motor with a reduced voltage auto transformer starter, Square D. We have the taps set at 65% to achieve the torque required to start the motor.

I need to set the time delay between the start of the reduced voltage to the switch to full voltage. We have set it to about 5 seconds. We listened to the motor while starting and have it switch over before the speed at the reduced voltage levels out. It seems ok but I want it to be set perfectly.

I am going to use a RPM meter and I need to know at what speed should the starter switch over to full voltage?

Some of the literature that came with the starters say to set to switch over at 80% to 85% full speed. Or to do it by ear as we have already done.

Now if the nameplate RPM is 1750 rpm and 85% of that is 1400 rpm, then the switch over would be at 1400 rpm?

Or should it switch over at 85% of reduced voltage full speed? The nameplate RPM 1750 at 65% transformer tap is 1138 rpm this is the reduced voltage full speed. At 85% of the reduced voltage the speed would be 967 rpm.

Or do I have it wrong and 1400 rpm is the switch over speed. Will the motor get to nameplate RPM at the reduced voltage?

I'm so confused

t
Reply to
t
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There is no such thing as a "perfect" speed for switchover. In any reduced voltage starting scheme, the torque reductuion can never be lower than the required accelerating torque. Accelerating torque is the amount of torque necessary to both rotate the motor and load, overcome inertia AND a little bit more so that it continues to increase speed. If you wait for the motor to stop increasing in speed, you are wasting accelerating energy. In the case of an Autotransformer starter you also have the added concern of not overheating the transformer itself. Most RVAT starters have a temperature switch imbedded in the transformer winding that will drop out the control circuit if you over do it. The transformers are typically designed for a maximum of 15 seconds, and only 2 or 3 times per hour. Speed is irrelevant.

Reply to
Bob

If you are to gain the full benefit from using a reduced voltage starter, you should not step to full voltage until the impedance of the motor has risen to at least half of the full load impedance. In most standard induction motors, that means that you nead to be at 85 - 95% rated speed. ( you can check this out by looking at the full voltage start current curve for the motor. You want to transition where the full voltage current is less than 200%) This does require that the torque is high enough to accelerate the motor to this speed. In many applications, the motor will run out of torque before it reaches 90% speed and so will operate at reduced speed until the transition occurs, or the protection operates, or the motor fails. You may need to set the starter to the 80% tap in order to reach 90% speed. If you transition at much less than 90% speed, you will have full voltage start current which could be 6 - 8 times the rated current and you may as well just use direct on line starting, it is cheaper!! For more information, check out my web site at

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regards, Mark Empson.

Reply to
Mark Empson

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