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Starting 250 HP Motor

Starting 250 HP Motor

Starting 250 HP Motor

I am working on a project that will include a 250 HP motor (460v; 60Hz; 1185 RPM; 292 FLA). I will have a pump and a hydraulic motor attached to this dual shaft motor. The purpose is to test the pump at HP's above 250 by using the hydraulic motor as a regenerative power source using the output fluid of the pump. Because I will have this hydraulic motor on electric motor, I will have the opportunity to bring the electric motor to 1185 RPM before a DOL start. Is there anyway I can match the phase of the motor with my shore power to accomplish this?

I've read a few things about starting a coasting motor, and it seems it is OK after the rotor flux has dropped? Is this a similar situation to starting a motor that's being externally driven? Do I need to reach a constant speed.

Thanks for the help. It's been a long time since I studied motors.

RE: Starting 250 HP Motor

The only way I have seen is using a VFD programmed for "flying start".

Harmless flakes working together can unleash an avalanche of destruction

RE: Starting 250 HP Motor

Not an expert, but this sounds similar to a Motor Bus transfer system application. Beckwith makes relays/controls for these specific applications Beckwith Link. If nothing else, you can navigate to the application notes and there is some background on this type of system, used at large thermal plants that need to transfer over large spinning motors to a different source quickly.

RE: Starting 250 HP Motor

The whole fast transfer business is about transferring a motor that was running on one source to another so it can keep running. In this case the motor may be spinning but it wasn’t running. No field to worry about, at least not that you can measure. VFDs can figure out where a spinning motor is, but I’m pretty sure that a DOL start will just be what it is.

RE: Starting 250 HP Motor

I'm with David on this. If the motor is turning at between 1185 RPM and 1215 RPM your current should be at or below rated full load current.
Disclaimer. Apart from the motor starting surge, DOL motors often experience a transient current during the first 1/2 cycle that may be several times greater than the normal 6X starting surge.
This transient is so short lived that it is ignored by most protective gear and fuses.
You may have a similar transient and not even know it.
It has to do with establishing the initial magnetic field and the point on wave at which the power is applied.

"Why not the best?"
Jimmy Carter

RE: Starting 250 HP Motor

Per what David and Bill said, something similar to this is (was) done with large motors, it's referred to as a "Pony motor starting method". You accelerate a large (typically 1000HP and up) motor unloaded using a smaller (pony) motor mechanically coupled to the shaft, then once you are at full speed you uncouple the pony motor and energize the large motor as a way to keep the starting current down. Since the large motor was not energized, there was no field synchronizing necessary. It would have to be ensured that there is a "restart delay" of a few seconds on your application, time to allow the motor fields to collapse. That was inherent in the pony motor system because of the time it would take to reconfigure the mechanical coupling.

I've seen some very "funky" methods of coupling and uncoupling the pony motor, most of them dangerous, so it has fallen out of favor now and in fact most of the ones I saw were because I was being brought in to apply modern methods of accelerating the large motor that didn't involve someone risking life and limb. But at one time it seemed to have been somewhat common, probably in the 1930s to 1950s maybe. All of the ones I saw were either on large "whole log chippers" or for paper mill refiner motors.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Starting 250 HP Motor

Jeff - why decouple the pony motor at all?

We used this type of system on our gas turbine starters, with (from memory - it's been a year or ten since I left) a 90kW pony motor bringing a 1.5MW main motor up to speed. Once the main motor was energised the pony motor supply was disconnected and the pony motor was taken along for the ride by the main motor.

RE: Starting 250 HP Motor

On the ones I saw like that, the pony motor was coupled to the large motor's coupling via a tire that was driving the coupling by being pressed against it, i.e. small wheel driving a larger wheel to get mechanical advantage over the inertia. But once the main motor was powered up and got to speed, that would make the pony motor over speed by a factor of maybe 10.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Starting 250 HP Motor

That sounds more like turning gear than a pony motor Jeff.
Getting up to 10% of speed won't do much for starting but it will bring the oil up into the sleeve bearings.

"Why not the best?"
Jimmy Carter

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