Can a 55kW VFD Replace a 8kW RPC

I have recently got some VFD's, a 55kW Commander, a 45kW Commander, a 30kW Commander and a 30kW ABB.
All 380 to 480v. I have very little VFD installation experience. So with using a single phase isolation transformer to increase 240v to 415v. My motors range up to 5HP and the largest is 7 1/2 HP.
So would the general principal be to set the perameters to the combined motor load.
Just looking for ideas, is it doable, what is the down side. These VFD's cost me very little.

Jim

Jim Kennedy said:

I have recently got some VFD's, a 55kW Commander, a 45kW Commander, a 30kW Commander and a 30kW ABB.
All 380 to 480v. I have very little VFD installation experience. So with using a single phase isolation transformer to increase 240v to 415v. My motors range up to 5HP and the largest is 7 1/2 HP.
So would the general principal be to set the perameters to the combined motor load.
Just looking for ideas, is it doable, what is the down side. These VFD's cost me very little.

Jim

Click to expand...

So basicallly... you are trying to apply a "big enough hammer" of small-enough-money cheap VFD ... to duplicate what a very EXPENSIVE "Phase Perfect" does as to serving as a general purpose shop-wide source of 3-Phase power for -

"bring any combination of loads you happen to feel like switching on"
.. any at given moment of any given day?

It has been done. It CAN work.. after a fashion.

It is an uphill battle.


The design philosophy of a VFD and Phase-Perfect are diametrically OPPOSITE to begin with. That matters. A lot.

- A VFD is a HIGHLY "dynamic" device. It is meant to be "tuned to", then serve a SINGLE load, knowing in advance that load will have highly varying needs - to which it must respond. Dynamically. VERY! "fast" in other words.

- A Phase-Perfect is the reverse. it is meant to stand fast as the local Rock of Gibralter regardless of what combination of loads are switched onto or off of its output or how hard those loads are working or how fast their loading is changing.

The "broad shoulders" it needs to JF DO that simply do not exist in a VFD.

-- Easily four times, if not ten times the physical space to house more than one set of much larger capacitors, HP for HP, to survive whilst taking the beating.

-- A HEAVY ration of multiple big, fat Iron and Copper INDUCTORS ... that a VFD has ZERO of.

I did say "uphill battle"?

FWIW-not-much? An RPC is "in between".

Not very "dynamic", it benefits from "some" run-cap tuning.

But is otherwise dependent on the Brute Force and Bloody Ignorance of broad shoulders derived from ... ta da .. a large mass of.. Iron and Copper. "Rotating, even!"

Go figure an RPC JF works where a VFD tends to go bug-f**k nuts when operated out of its expected environment.

The controls get confused. And it has no "flywheel."

Compare by "gravity test". Weigh a 10 HP-capable VFD. Weigh a 10 HP Phase-Perfect.

Ski-boat. Container ship.

Pick any ONE!

Eh..... so long as you set the VFD to V/Hz mode, so it does not want to measure things about the motor. and just set it to 60 Hz (or 50 if needed), it can work. people on this forum do that every day.

Figure that the motor you want to start may draw 6x maybe 7x marked full load amps (FLA). Some are better, few are much worse (WEG may be).

Most VFD will supply 150% of max current for a shortish while, from a few seconds to a minute. Long enough to start a motor.

Then the largest motor you should expect to start would be about 125% to 150% of rated VFD current. You may indeed do better, but you cannot be certain of that.

If you run wires around the shop from the VFD, you probably will want to put the wires in conduit, and/or add at least a line inductor, if not a real filter, to get rid of radio noise.

It's been done, it does work, so long as you have a big enough VFD to handle the start currents of your largest motor, plus the draw from other things that can be expected to be "on" at the same time.

A 55kW VFD for 380V is around an 80A VFD, so you should be able to run any of the motors you mention.

JST said:

people on this forum do that every day.

Click to expand...

..within limits..

Figure that the motor you want to start may draw 6x maybe 7x marked full load amps (FLA). Some are better, few are much worse (WEG may be).

Most VFD will supply 150% of max current for a shortish while, from a few seconds to a minute. Long enough to start a motor.

Click to expand...

And there's part of that "uphill battle"

WHEN.. dedicated to a specific motor? One of the NICER things about a VFD is the ability to be optioned to slow-ramp a hard to start motor up off its knees ...even motors ABOVE the VFD's nominal rating ... when other AC power sources cannot easily do so.

Set it up "blind" to cover whatever comes along, and that option is no longer as useful. See below:

Then the largest motor you should expect to start would be about 125% to 150% of FLA. You may indeed do better, but you cannot be certain of that.
.
.

It's been done, it does work, so long as you have a big enough VFD to handle the start currents of your largest motor, plus the draw from other things that can be expected to be "on" at the same time.

A 55kW VFD for 380V is around an 80A VFD, so you should be able to run any of the motors you mention.

Click to expand...

Could work OK.

For comparison, a 10 HP-load-rated Phase-Perfect:

- wants a 60 to 70 Amp breaker for a steady-state 50 A load off 240 VAC single-phase in...

- to deliver 29 A steady-state (36 A with service factor), of three-phase out

- has a 4 second momentary overload rating of 150 A.

And draws about 100 Watts on "no load standby". That goes up for larger units 240 W for 20 HP, 400 W for 30 HP.

(By contrast, a DC Drive is hard OFF when on standby .. so only the control logic is alive, at about half a Watt, HP / size immaterial). It also has adjustable ramping, but not because the drive OR the motor necessarily needs it. So the motor doesn't BREAK the stuff attached to it, rather!

Stick a VFD in V/Hz and don't run past the absolute maximum current, and it will happily start motors DOL. Yes, there are all kinds of fancy settings - but you don't have to enable them, and most drives probably aren't even used in vector mode.

I now want to go see what the standby losses of a big, decent quality drive are when outputting full frequency no load. Hmm.

Do I understand that you want to feed it single phase? I have never seen a drive that big be fed with single phase. Doable I guess, but you might run into problems. May need a different rectifier section and more caps. One big thing to consider is how/if the drive does phase loss protection - some are more defeatable than others.

SomeoneSomewhere said:

Stick a VFD in V/Hz and don't run past the absolute maximum current, and it will happily start motors DOL. Yes, there are all kinds of fancy settings - but you don't have to enable them, and most drives probably aren't even used in vector mode.

I now want to go see what the standby losses of a big, decent quality drive are when outputting full frequency no load. Hmm.

Click to expand...

The input side rectifier and capacitor bank is on the line even if the drive is not in "RUN". Even so, I doubt they "leak" much at all.

Idle VFD should be close to the DC Drive's very low figure for keeping the control logic alive.

Both are waaaay less than a Phase-Perfect.

The P-P has Iron & Copper inductors "cycling" even at idle so as to be always ready, not just the storage and tuning/filter caps.

ISTR the P-P "efficiency" is actually not much better than a well-tuned and balanced RPC?

It just doesn't need to BE "well tuned and balanced" - because it adapts, dynamically.

markz528 said:

Do I understand that you want to feed it single phase? I have never seen a drive that big be fed with single phase. Doable I guess, but you might run into problems. May need a different rectifier section and more caps. One big thing to consider is how/if the drive does phase loss protection - some are more defeatable than others.

Click to expand...

One normally needs to de-rate the drive to do the single-phase input, but with this situation, the drive is massively de-rated to begin with, in order to handle start current.

The phase-loss protection is an issue, but it may be able to be turned off. The manual will give that info, somewhere. And, it depends if it is a "voltage present" check, or if it is checking the phase of the voltage as well.

The "voltage present" variety normally is ok, just connect one of the input wires to two of the inputs. If it checks for proper 3 phase, that is more of an issue, unless it can be turned off.

Really large VFDs have protection against all sorts of things, and may check phase. The 55 kW is neither low nor really high power, and could go either way. The 200 kW units that a client had years ago did, as I recall, check phase.

JST said:

The 55 kW is neither low nor really high power, and could go either way. The 200 kW units that a client had years ago did, as I recall, check phase.

Click to expand...

As a general rule, my guideline has always been a drive rated 10x the motor rating is fine. The limiting factor is the resolution of the current sensing devices. In order to reduce the number of spare drives we carry, that is the number we use. Running in V/Hz makes that number a little less important.

Agree that the drive is nicely oversized and that is a good benefit. Was not sure if there were any other additional factors to consider that might affect the 50% de-rating with the larger drives or not.

If they use phase loss based on dc ripple, then phase loss should not be a problem. Otherwise????????

markz528 said:

Otherwise????????

Click to expand...

"Otherwise"? Throw a Sine Guard grade dv/dt 3-Phase inductor into series with the VFD's output, all legs covered in one go... and go TEST it.

Last three as came in the door here averaged under 80 bucks, new, or near-as-dammit, what with the free shipping I get from NRi. One of 'em is 120 A rated.

That'll make life easier for wiring as to radiating less s**t into the appliances, protect older motors, and Jim won't have much money nor even labour at any significant risk, will he?

And let PM know how you did it, what you found wise to alter, and how well it works.

If a patent on brains, igenuity, or desperation of simple economics was ever even issued, surely it has expired by this late date?

"Try it. If it doesn't work? Fix it! But fix it FAST!"

C. Alan Peyser