downrating a motor hp/ speed/ vfd size

kd1yt · Feb 17, 2015

Let's say I have a chance to get an electrically driven hydraulic press on really appealing terms, but that the electric drive motor on it is vastly bigger than I need or want to accommodate (5HP) and the tonnage of the press is also more than I need.

Could I set a VFD with parameters that would assume full hp at 240V 60hz but then run the VFD at levels that are never any more than 1/5 speed/ frequency (and thus way down the v/f curve, so the lower voltage would also pull lower current) with a 1HP VFD (which I happen to already have several extra of)?

This would be for intermittent use, so the reduced cooling of the motor at lower RPM is not an issue to the degree that it would be for continuous use.

Thanks

atomarc · Feb 17, 2015

KD1YT,

Are you merely trying to utilize a drive you already have to power your 5hp press? The FLA of a 5hp motor on 230 volts is 15.2 amps. The FLA of a 1hp motor at the same voltage is only 4.2 amps. I don't think your smaller drive could be programmed to detune a 5hp motor to 1hp parameters...but I may be wrong.

My experience has been that the press components...pump,ram and motor are matched to produce a desirable ram speed and tonnage. Upsetting this combo may give you very unsatisfactory results.

What you save on the cost of the press you could put toward a proper inverter for the motor size.

Stuart

atomarc · Feb 17, 2015

Adding to Bill's post, although he and many members may not agree with me, you situation would be a perfect application for an inexpensive static converter. All you would in addition would be appropriate motor overload protection unless the press came with it.

mike_kilroy · Feb 17, 2015

kd1yt said:
1) Let's say I have a 5HP [motor]
2) Could I [use] a 1HP VFD
Thanks

Simple answer: NO.

Reason is given by atomarc altho he did not specify it explicitly:

5hp motor FLA=15amps (about 7amps magnetizing current)
1hp VFD output rating: 4amps.

the 1hp cannot magnetize the 5hp motor, let alone have more amps to make any torque. So no, you cannot do this.

Also the manual on your VFD will state how much lower amp motor it will allow parameters to be set down to - typically it is still over 30%....

Last, as others have said, presses rely on INERTIA in a big inertia wheel to do the actual curchunk; motor is just to add energy to it during the non-smack time; so slow it down and your energy available goes down fast.... E still =MC2=1/2mass*velocity^2

atomarc · Feb 17, 2015

No "curchunk" on a hydraulic press...or big old flywheel either.

Stuart

kd1yt · Feb 17, 2015

mike_kilroy said:
Simple answer: NO.

Reason is given by atomarc altho he did not specify it explicitly:

5hp motor FLA=15amps (about 7amps magnetizing current)
1hp VFD output rating: 4amps.

the 1hp cannot magnetize the 5hp motor, let alone have more amps to make any torque. So no, you cannot do this.

Also the manual on your VFD will state how much lower amp motor it will allow parameters to be set down to - typically it is still over 30%....

Even if run at a fractionally/proportionally derated max motor RPM driving a hydraulic pump (thus lower V and lower F on a V/F curve, and since V is lower, current would be lower)? I realize that the flow rate will go down and even the peak PSI may be limited, but assume for my application that I have no need for speed and that the pressure capacity of the press has so much headroom that a fractional amount will serve me fine.

I'm not just asking to be obstinate at persisting at a half-__ed idea; just interested in learning...

Thanks

Toms Wheels · Feb 17, 2015

The complete answer should be NO Way, but there is an out sort of. 5hp of motor matched to a pump is the issue. 5HP is 1 gallon per minute at 5000 psi, or 1/2 gallon minute at 10000psi. running a motor at less than base speed is constant torque, decreasing HP. So if a 1750 rpm motor run with a VFD at 350 rpm would produce 1 hp, and max pressure, with 1/5 the stroke speed. If the VFD can handle the starting amperage it would work fine. but Sloooow.

You can,t put a 1 hp motor at the same rpm as the 5hp on that pump, as you would have 1/5 the pressure before the motor overloads. I have a 3/4 hp pump 10000 psi 25 ton, its a two speed pump, up to 1000 psi it pumps about 4 times the oil for fast advance, then drops to the slow speed for pressing. If that pump has that feature which would be likely, that 1hp would likely be topped out at advance speed. Buy a bigger vfd and do it right.

mike_kilroy · Feb 17, 2015

my bad if this is just a hyd cylinder pushing. sorry for misunderstanding!

as for lower speed on 5hp motor, no. reason you will ALWAYS have 7+ amps is because you stay on the v/hz curve for the motor.

now you COULD lower v/hz ratio to voltage starve the motor.... but your torque goes down by square of the voltage. quickly your 5hp motor will become no more than a big amp eating heatsink.

mike_kilroy · Feb 17, 2015

kd1yt said:
Even if run at a fractionally/proportionally derated max motor RPM driving a hydraulic pump (thus lower V and lower F on a V/F curve, and since V is lower, current would be lower)?

just to be clear as mud, the current WILL BE THE SAME. 7AMPS plus MORE to make torque. upto same 15amps.

JST · Feb 17, 2015

Just to clarify.....

It is NOT impossible to use a smaller VFD with a larger motor.

It is just pretty much impossible to use THIS motor with THAT VFD.... reason being that it is just too big a mismatch, as detailed and re-detailed above.

You can likely use a 3HP with a 5HP motor, because it CAN supply the magnetizing amps for the 5HP. It just can't supply much MORE. So you can do it at (considerably) reduced power.

Tony Quiring · Feb 17, 2015

Harbor freight sells a marathon 5 hp motor for decent money.

You could swao to motor.

Or since yiu claim the press is way over your needs just put a static converter on it.

You loose 1/3 your power but so what.

Just be sure the motor protection is correct.

kd1yt · Feb 17, 2015

Thank you all; I get it now that you can only go so far down the voltage/current curve before the mass of iron fails to get enough magnetism to do useful work. Can do an RPC, now to see if I can make a deal on the press.

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