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Will a 10 HP RPC burn up a 1.5 HP Grinder?

swatkins

swatkins

Titanium
Joined
Jul 24, 2011
Location
Navasota / Whitehall Texas
According to a motor shop that is the case.....

A friend of mine has been rebuilding an old 12" pedestal grinder that has a 1.5 220 volt motor... He has spent a lot of time painting, fixing and he also put in new bearings... Once together he started it up on his 10 HP American Rotary RPC and let it run.. After 10 minutes he smelled smoke and turned it off....

Today he took it to a motor shop to see about rewiring it and the repair man told him the motor was ok even though my friend saw it smoking...

The repairman asked about the RPC and said that was the problem as it was too large for the grinder's 1.5 HP motor. Never heard that before.. Repairman said a large company had machines running on a 30 HP RPC and after they took off a few machines they started burning up motors... An engineer told them they should add more idler motors back to the line, which they did, and the problem went away.

I run a 1.5 HP grinder on my 10 HP RPC and 5 HP machines on my 20 HP RPC all the time without problems...

What is the scoop on running a motor on a RPC three times the rating of the motor?
 
swatkins said:
According to a motor shop that is the case.....

A friend of mine has been rebuilding an old 12" pedestal grinder that has a 1.5 220 volt motor... He has spent a lot of time painting, fixing and he also put in new bearings... Once together he started it up on his 10 HP American Rotary RPC and let it run.. After 10 minutes he smelled smoke and turned it off....

Today he took it to a motor shop to see about rewiring it and the repair man told him the motor was ok even though my friend saw it smoking...

The repairman asked about the RPC and said that was the problem as it was too large for the grinder's 1.5 HP motor. Never heard that before.. Repairman said a large company had machines running on a 30 HP RPC and after they took off a few machines they started burning up motors... An engineer told them they should add more idler motors back to the line, which they did, and the problem went away.

I run a 1.5 HP grinder on my 10 HP RPC and 5 HP machines on my 20 HP RPC all the time without problems...

What is the scoop on running a motor on a RPC three times the rating of the motor?
Click to expand...

It only detects ANY difference at all if the overall Voltage is waay too high for its build-generation.... as it CAN be, but damned seldom, once under even a light load. back EMF thing. it is only going to take as much of the power ration as it needs..

So....far more likely.. the big RPC has been balanced as to "generated leg" too agressively capacitored for a lighter load.

NOW the mains can already be high (mine are 246, surely no longer 220!) AND the generated leg under too light a load is too HIGH even above that level.

Adding idlers suggestion implied that those added did NOT add more caps, so that takes the "edge" off the over-compensation and JFW.

Otherwise, an RPC idler can assuredly be too large to START or too large to SUSTAIN off the available service not being up for it.

But cannot be "too large" to a(ny) LOAD as it is only trying to be as natural and STABLE as if it were the utility grid itself.

Obviously, a BIG GRID doesn't harm motors, but then again grids are very well balanced compared to the average RPC! Most minutes of a given day, anyway.

NB: Motors CAN run rather hot. Rebuild one and fail to get it's innards all nice and CLEAN?

It will "smoke-off" whatever you didn't flush-off.

A "motorman" knows to use non-aggressive but fully-volatile solvents that leave no film and then get them OUT of any hidey-holes as well.

A mere "civilian" might think a light all-over film of WD-40 or such will protect the motor's guts.

He can be wrong enough to inspire a touch of "drama"?

:D
 
RPCs are often large motors with higher pole counts which means that given the increased inductance to resistance ratio, you can push the generated leg of the rpc high enough (with capacitors) that it does become possible to burn out a smaller 3 phase motor.

and that high voltage is also causing lots of extra losses in the RPC, but its proportionally less, so the motor doesn't burn up.


the fix for this problem is to put the capacitors at the load sized for each load. the problem with this is the relay contacts may burn up. capacitors typically have a 1% or less "loss" which means the transient current spike (for example if you simply connect a 50uf motor run cap directly on your line) is in the hundreds of amps.

most every single phase motor has a 10uf run cap per hp at 240vac, but that run capacitor is switched into the circuit in series with the motor's start winding, which limits the current to 5-15 times the nominal full load amps.. not 100 or 200 times the full load amps.


anyhow i'm willing to run an experiment if anyone wants me to:

leave a 200uF cap connected across 240 mains. suppose this represents a 20HP RPC with 200uf needed to pull its generated leg up to get ballanced voltage at no load.

then take a 50uF cap located 10 feet from the 200uf capacitor.. and randomly switch it in and out of circuit with a 15$, 30amp 2 pole contactor.

how many cycles is that contactor going to last before the contacts burn up.
 
johansen said:
then take a 50uF cap located 10 feet from the 200uf capacitor.. and randomly switch it in and out of circuit with a 15$, 30amp 2 pole contactor.

how many cycles is that contactor going to last before the contacts burn up.
Click to expand...

Dunno. I generally can find more than fifteen bucks when the job NEEDS it to last 20 years, no hassle.

So mine are either Mercury-Displacement.

Or monster-current rated, but small physical goods made by an outfit called "Gigavac".

GIGAVAC Power Products - Power Products Contactors RoHS Compliant

Old Fart trick?

How did you think we GOT to BE "old" .....and not starve TF to death ... off the back of foolish penury at the game of "false economy"?

Run what the tasking REQUIRES!

:(
 
Or look at it this way - for 16 years the 10 HP Phase Perfect has run 1 1/2 and 1/2 HP (and larger of course) three phase motors without issue
 
WHAT DOES AN RPC DO?

--What it does is to provide a third "phase wire" needed to make 3 phase from single phase.

What burns out a motor?

--Excessive current burns out a motor. That can result from too high a voltage, too low a voltage, too heavy a load, single phasing, or wrong frequency. RPCs provide 3 phase voltage, do not affect the load, and normally cannot affect frequency, so the only bad thing they can do is provide a bad voltage.

Why doesn't connection to an outlet capable of providing much more current than the device needs "burn it out"?

--Because the motor only takes what it needs as far as current, if operated at the proper voltage and frequency. That's a feature of how motors work, whether connected direct to a 3 phase outlet, or to an RPC.

So:

The takeaway is that the ONLY way an ordinary RPC can "burn out a motor" is if it is made wrong in such a way that it will provide a voltage that is higher than spec for the nominal mains voltage. That can happen with poorly "balanced" RPCs. (An RPC that can produce a different frequency might also do it, but are rare).

There is NOTHING about a properly made RPC of any power that will burn out a motor.

The basic adjustment (balancing) of an RPC is to connect sufficient "balance capacitors" to raise the unloaded voltage to no more than 10% higher than the nominal mains voltage (264V for a 240V nominal mains voltage).

There is a temptation to connect extra "balance" capacitors to raise the unloaded voltage to some high level in an attempt to provide a "balance" under heavy load. That's not a very good idea because light loads may then see a very high voltage. Many loads, including CNC machines, will not operate correctly with high voltage. The adjusted voltage will drop under load, but without enough information about the RPC and load to do a proper calculation, the drop for any particular load is not predictable.

The whole system of "balance capacitors" is inherently unstable when random loads are connected. That is why using a buck-boost transformer in boost mode provides a better correction. It is far more predictable.
 
johnoder said:
Or look at it this way - for 16 years the 10 HP Phase Perfect has run 1 1/2 and 1/2 HP (and larger of course) three phase motors without issue
Click to expand...

I'm TORN on that!

The bean-counter in me wants to run mine MORE than the RPC so as to better amortize the four-large or so it cost me, brand-new.

The country boy in me has been SAVING it, much as we'd go barefoot in warmer months so that year's pair of tennis-shoes could make it through the winter before needing the cardboard insert patches doubled.

Then Dad got a tiny bottle of "Goodyear Pliobond" and we may as well have struck gold!

And yes.

The bathrooms DO have dual holders for bum-fodder.

But I ain't YET gone so much the gambler as to load two different colours.... and risk not being able to exit over decisions, decisions, decisions...

:(
 
More..... My friend called American Rotary and asked them the question.... They said RPC's will not have that problem but a RFD will burn a too small motor out....

Who to believe......
 
swatkins said:
More..... My friend called American Rotary and asked them the question.... They said RPC's will not have that problem but a RFD will burn a too small motor out....

Who to believe......
Click to expand...

My 20 hp RPC will run my 1.5 hp SB for days and has done so for 15+ years with no issue. What is an RFD? Whatever, believe American Rotary
 
VFD? A mismatch in size is fine (and often recommended for heavy-duty applications). But a >10:1 mismatch and the drive and sensors can start having trouble making small enough pulses and measuring them.
 
swatkins said:
More..... My friend called American Rotary and asked them the question.... They said RPC's will not have that problem but a RFD will burn a too small motor out....

Who to believe......
Click to expand...

American Rotary also sells fancier RPC's (as in more expensive) for sensitive equipment. Better tolerances on the voltage.
So that right there should tell you that a basic RPC isn't quite as good as utility 3-phase.
On the other hand, there are also a lot of old, and homemade RPCs. Likely ones with capacitors that have gone bad but still in use. Or just the WRONG capacitors. So it is possible the motor shop guy is basing his experience/advice on a questionable RPC.

Motors "burn up" because of too much heat (duh). Lots of things will cause that, overloading them, built up dirt limiting cooling, as well as bad line voltage. Like anything, paying some attention to what is going on, and not pushing your equipment so hard it breaks, make sense. Grinders by nature are subject to being used aggressively. With a bad balanced RPC, you may get slightly less performance out of your grinder before overheating from overload. Effectively a 1.5HP grinder becomes a 1.3HP grinder. But if your friend let his grinder run for 10 minutes without actually doing any grinding, it is really unlikely that the RPC caused the smoke, I'd suspect stray oil in the motor.
 
rabler said:
Effectively a 1.5HP grinder becomes a 1.3HP grinder.
Click to expand...

^^^^ THIS ^^^^

May be irrelevant. This time. I'm in the "unclean motor" camp, actually!

But "too soon we forget" ..... that we are using a bit of a fiddle.

One that can only provide about 91% of OEM nameplate BEST CASE!
And has also already wasted some energy itself... even to get there at all.

That is waaaay TF BETTER.. than a "static" so-called-converter, which isn't.. really... a "CONVERT er". Just a direct diddle-fiddle on the load-motor's indulgence. Not to mention imbalanced heating effects., and drastic fall-off as it is tasked with getting even CLOSE to nameplate HP actually being demanded.

.. but still..

If an RPC were "perfect"?

Phase-Perfects would not command the prices they DO GET!
 
SomeoneSomewhere said:
VFD? A mismatch in size is fine (and often recommended for heavy-duty applications). But a >10:1 mismatch and the drive and sensors can start having trouble making small enough pulses and measuring them.
Click to expand...


Actually, the pulses are likely OK. The problem is that the current sensors, and the range of adjustment of the current settings can be too coarse to set the sort of protection that a very small motor (compared to VFD nominal current) needs. So you end up either with a setting that is considerably higher current than the motor should have, or limiting it to a fair amount less current than it needs to do its job.

Or, if there are several motors, the VFD gets set to the largest of the lot, and the others have to "like it or lump it". In that case there is no issue with relying on an old-fashioned motor protector. Yes, it disconnects the motor, and no, that will not hurt the VFD.
 
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