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Starting of Multiple Fractional HP Motors

Starting of Multiple Fractional HP Motors

Starting of Multiple Fractional HP Motors

(OP)
I'm using SKM Powertools to study/size an electrical distribution system consisting of several blocks of multiple LV fractional HP motors.

The system consist of several 240V, 3-Ph power panels each feeding a group of 25 - 3/4 HP, 230V, 3-Ph motors (Code Letter - K) with local starters.  The system is to be designed to allow all 25 motors to start simultaneously.  The 240V power panel is currently supplied by a 75kVA, 3-ph, 3-wire 480V-240V transformer (oversized due to starting inrush of all 25 motors).

In a typical distribution system (where simultaneous starting of motors is limited) I would typically model the motor(s) as starting (using actual inrush current and 0.2PF) and perform a steady state loadflow to determine the maximum (worst case) voltage dip during starting.

In the above system with the large quantity of motors starting simultaneously, if I follow the above approach, I need to oversize the transformer and cables in the system to limit the motor terminal voltage dip to a reasonable value.  Since these are fractional HP motors feeding a low intertial pump (anticipate fast starting time - this is being confirmed) are there assumptions that I can make to limit oversizing the electrical distribution system (transformers, cables, etc) due to the short duration inrush?

RE: Starting of Multiple Fractional HP Motors

From the information provided it's hard to say.  How much torque is required to get the pump up to speed?  Remember that torque drops off with voltage squared so 80% voltage gives 64% or so torque.    The motor manufacturer should have start up torque curves for the motors.

------------------------------------------------------------------------
If it is broken, fix it.  If it isn't broken, I'll soon fix that.

RE: Starting of Multiple Fractional HP Motors

(OP)
Thanks TurbineGen - I think you have answered my question.

I have requested the starting curves from the mfg and I am waiting to receive those.

Provided my starting case does not result in a voltage or torque below the minimum required to accelerate the load I should be able reduce any fat in the distribution system to the point where I have some margin (to account for variations in source voltage, other loads operating on the system, tolerances, etc) available over the minimum required values provided by the mfg.

Guess sometimes you have to take a step back to see the path through the forest.

RE: Starting of Multiple Fractional HP Motors

If you are starting them all at the same time I think you would just add them all as if they were one motor of the combined HP.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Starting of Multiple Fractional HP Motors

Sorry Keith, been there, done that, got the Tee shirt.
I was faced with that issue once and rather than assume that it was so, I did some investigation. I found that as motors get smaller, the ratio of starting current to running current tends to get larger.
In my case, starting a number of smaller motors with one starter, I had to use one size larger starter than if the motor ratings were just added together. Then I had to justify my choice.
I'm not familiar with SKM. It may or may not handle this issue correctly.
The application was reversing 10 or 12 motors at about 5 HP in a lumber dry kiln. They reverse every 2 to 4 hours, 24-7. There is typically a generous coast time, but the possibility exists for the motors to be still turning when they are reversed. I would call the starting duty moderate to severe. Not the place to use a fudge factor on starter sizing.
Another interesting factor was the ambient temperature. When the fans are running the ambient may be close to 212 F. When the fans are timing out prior to reversing, the heat cook-off from the steam coils may raise the ambient to 250 F. I once recorded 230 F at chest height. The fans were about 10 or 12 feet higher and above the steam coils. Much hotter. At 120 psig the steam temperature could approach 350 F.
Yours, Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

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