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using a VFD with over-rated motor

jbbenni

Plastic
Joined
Jan 8, 2017
I'm looking at pairing a KBAC-27D VFD fed by 115v with a 2HP, 3450 RPM, 3phase motor. This theoretically exceeds limits, but as a practical matter will it work?

The VFD can drive this 2HP motor when supplied by 230v, but the VFD is limited to 1.5HP when supplied by 115v. (There are jumpers to configure the VFD with all the motor particulars.)

If I configure the VFD for 1.5HP but hang a 2HP motor on it, what will go wrong and how can I avoid serious damage? I expect the VFD to detect a sustained over current limit and fault in a non-destructive way. I want to be sure that it won't cook the VFD or the motor. Is that correct?

For what it's worth, my application is a belt grinder so there is no startup load. I'll be running at reduced speed much of the time, and probably be running with lighter loads when I do operate at full speed. When I'm grinding hard, it tends to be about 50% duty cycle - that is, only for a few seconds at a time with a few seconds between.

As a practical matter, can I "fly under the radar" of the VFD limit circuits and expect this setup to operate decently?

Thanks!
 
I'm looking at pairing a KBAC-27D VFD fed by 115v with a 2HP, 3450 RPM, 3phase motor. This theoretically exceeds limits, but as a practical matter will it work?

The VFD can drive this 2HP motor when supplied by 230v, but the VFD is limited to 1.5HP when supplied by 115v. (There are jumpers to configure the VFD with all the motor particulars.)

If I configure the VFD for 1.5HP but hang a 2HP motor on it, what will go wrong and how can I avoid serious damage? I expect the VFD to detect a sustained over current limit and fault in a non-destructive way. I want to be sure that it won't cook the VFD or the motor. Is that correct?

For what it's worth, my application is a belt grinder so there is no startup load. I'll be running at reduced speed much of the time, and probably be running with lighter loads when I do operate at full speed. When I'm grinding hard, it tends to be about 50% duty cycle - that is, only for a few seconds at a time with a few seconds between.

As a practical matter, can I "fly under the radar" of the VFD limit circuits and expect this setup to operate decently?

Thanks!

Sounds as if you are making a problem out of a solution.

Where did you even find a 2 HP 3-Phase 115 VAC motor?

Moreover one that was NOT capable of being strapped for 230 VAC?

??

Before trying it on 115 VAC, go and put on a pair of too-damned-small ladies high-heeled shoes and walk about in them for an hour.

Painful even to THINK about, yah?

Trying to get by with an overloaded KB VFD will be much the same, and you will feel just as much the fool for it.

Bill
 
Thanks for the candor, although now I can't get that image out of my mind.

I guess my question wasn't clear - the motor is, as you would expect, 3ph 230 VAC. The VFD I mentioned handles conversion from 1ph 115v or 1ph 230v to 3ph 208/230v (and higher voltages if needed). The VFD is rated for 2HP load when the VFD input is 230v. But the VFD is only rated at 1.5 HP load if VFD input is 115v. (Doing the math, it looks like 2HP motor driven by the VFD from 115v would draw about 23 Amps - not generally available through standard residential outlets on 12 gauge copper.)

Would I still have to grind in those darn silly shoes?
 
I think you will as you suspect, run out of 115V current before you exceed the VFD rating.

Without considering power factor of the VFD input, you are already over 20A at 115V with 1.5 HP. And, that may be part of the rating.

The other part of the rating is that the voltage doubler system puts a big stress on the capacitors in the power supply. Their ripple current rating may also be part of the 1.5 HP limit. If exceeded the time for them to fail will be much less.

I would expect the ultimate 115V current to be about 26 to 27A at 1.5 HP, IF you could actually pull that current from the 115V circuit, which normally are only 20A capable.
 
Thanks for the reply. I think you're right that the 115v supply power may be the limiting factor. So the mode of failure would probably be tripping a breaker. But it's worse than that.

The manufacturer (a reputable US brand) indicates the max current for 1.5HP is 22 Amps, and so the circuit should have a 25A breaker (Not common, but available), with a properly rated plug and outlet (e.g., a NEMA 5-30), and appropriate cord. Not to mention that 22 Amps is over the limit for the #12 wires in the wall.

I think I'm back to the drawing board. If I'm going to have to rewire, I might as well put in 230VAC at some decent ampacity and be done with it.

Case closed, thanks to all for the input.
 
Assuming that jbbenni can either configure the VFD to limit the maximum motor current or tune the acceleration and deceleration to values that the VFD is happy with, there shouldn't be any issue with using the VFD this way.

It's also important to note that the reactive current drawn by the motor (due to its power factor) is supplied by the VFD, not the circuit that feeds the VFD. This means that a 2hp motor fed from a VFD run from 110V, will draw about 14-16A from the supply not 27+A, if the acceleration is controlled.
 
......
It's also important to note that the reactive current drawn by the motor (due to its power factor) is supplied by the VFD, not the circuit that feeds the VFD. This means that a 2hp motor fed from a VFD run from 110V, will draw about 14-16A from the supply not 27+A, if the acceleration is controlled.

Not at all true, sorry.

First... The reactive current of the motor is all included in the basic motor draw per the NEC tables, which is what you use to select wiring etc.

So....

230 V 3 phase... 6.8A at 3 phase. if it was 115V three phase, that would be then 13.6A THREE PHASE (per phase).

But the OP wants to use 115V input with a voltage doubling circuit in the VFD to get the 230V output.

Moving that to 115V SINGLE PHASE, 1.73 x 13.6A = 23.5A at single phase.

THEN, the rectifier and capacitor circuit has a power factor also. The most optimistic number for that is probably 0.8. Often it is lower. So the 2 HP is likely to draw nearly 30A at 115V for a 2HP output, assuming that were even possible.

Now, if you limit the VFD output, by giving the VFD a lower motor rated current setting, etc, you CAN GET any max draw you want. And then that can be set to limit input current, at least roughly.
 
Although the reactive current affects the choice of wiring from VFD to motor and the rating of the VFD output it does not affect the input to the VFD. The reactive current is supplied by the output stage of the VFD and creates a net zero average current on the DC bus.

The input to the DC bus of the VFD does not and cannot supply reactive current It purely supplies the real power of the motor, which will be 1492W/motor_efficiency. Assume 90% efficiency for a 2hp motor and you get 1658W which would be 15A at 110V single phase equivalent.

The power factor seen by the supply will depend on the design of the input stage of the VFD, whether it's a straight rectifier, rectifier with filter or well designed pump circuit.


The manual for the KBAC-27D actually quotes a maximum 110V supply current of 22A, but this is for 150% rated current. If acceleration is limited to keep power within the 2HP rating, then the current comes down to 14.7A

and yes, 220V would make the life of the VFD easier!

regards
Mark
 
As a practical matter, it does not always work out quite like that, although theoretically that is correct. In many cases the math works out pretty much how I suggested. For instance, a 3HP motor with matching VFD for a military project I did drew 24 to 26 A from the mains at 3HP motor load 230V (motor drawing FLA). That works out just about exactly using straight motor current, and a rectifier PF of 0.7. It had to work on 50Hz 230V single phase, and we needed the voltage to stay up to get the 3HP, so there was a decent amount of filter capacitance.

OK, your point is that the internal DC supply forms a "buffer" or "isolator" that tends to insulate the mains from the direct effects of the motor current. And the out of phase current is "boosted" back into the supply in a manner similar to braking. I think I misunderstood what you were saying.

It does not, however, work out exactly like that in most cases I have to deal with, as noted above. I have found that with many VFDs, the net result of the losses due to reactive current, and the low power factor of the input supply make it work out pretty much as calculated.

That may be due to low PF on the rectifier side, and one could equally well figure the real power and run that back through, using a different estimate for the rectifier PF, or just use the motor current and a higher PF for the rectifier, which is what I did.

The KB may use smaller capacitors, which will improve the PF, but also lead to higher ripple voltage, and some limits on power output due to lower average voltage.


Virtually NO VFDs in common use (other than system drives with a common power supply) use a PFC front end. If they did, the line draw would be a good deal less. I think that will be changing at least in the EU, due to tightening rules on harmonic current.
 
not intending to argue but anyone can calculate 2 hp at 120vac is 12.5 amps
when you account for 75% motor efficiency its 16-17 amps.

my guess is you can't get better than .7 power factor for the input rectifier, so that's 25 amps (rms, which is what blows your breaker) at 120vac.

as far as power factor correction.
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my guess is you can incorporate it into a vfd for 20-30 dollars a kilowatt, maybe double that if you want the capacitors to last 20 years.

don't be too quick to say this should have already happened.. the power factor of a three phase rectifier is 94% and who needs a 20 hp spindle in their garage? /sarc.
 
not intending to argue but anyone can calculate 2 hp at 120vac is 12.5 amps
when you account for 75% motor efficiency its 16-17 amps.

my guess is you can't get better than .7 power factor for the input rectifier, so that's 25 amps (rms, which is what blows your breaker) at 120vac.
...

And, by (of course) complete coincidence, the NEC rating for a 115V 2 HP motor is 24A...... That is without having any VFD at all.
 
and who needs a 20 hp spindle in their garage? /sarc.

Meah. I have a 295 HP spindle in my garage. When it ain't carrying me to the supermart, anyway.

But I thot this thread had arrived at the wiser choice arredy of JF pulling 240 VAC service 'stead of beating dead equations?

??
 
Meah. I have a 295 HP spindle in my garage. When it ain't carrying me to the supermart, anyway.

But I thot this thread had arrived at the wiser choice arredy of JF pulling 240 VAC service 'stead of beating dead equations?

??

Seems reasonable to answer the original question, and use that as a reason to do what, as you remind us, has been suggested already.

1 HP is about the limit that ought to be used with 115V, anyhow, and 1.5 HP is about "all that will fit". Due to the higher current, losses are higher in the 115 V circuit, and a voltage doubler is a bit of a kludge.

If a power factor correcting circuit was used in the VFD, then one could get 230V with any sensible input voltage, and the circuit would be under less stress, the PFC would draw just the required POWER from the line, no reactive current at all. Depending on what was allowed as the minimum voltage, and how efficient the motor is, one could draw anything up to about 1900W, which would be approximately 2 HP at 80% efficiency.
 
Thanks for the good summary, I think JST hit the important points and provided supporting notes. I'm summarizing here to "put the matter to rest" - so please correct me if I've misunderstood.

My original question was, "as a practical matter, will [the configuration] work?". The best answer seems to be: "Not very well. It might work some of the time, but is very likely to disappoint." On a regular 20A 115v circuit, the configuration could work with a 1HP motor, may be okay at 1.5HP - but is asking for trouble with 2HP. It's never going to actually deliver 2HP, except possibly for very short intervals with a low duty cycle.

As for the other aspect of the question (about the mode of failure) no one explicitly addressed this, but I gather it is unlikely to smoke the motor, and may not smoke the VFD either, but it's quite likely to pop the 20A circuit breaker whenever the motor is loaded (including at startup, unless the ramping is generous).

Let me know if I've misconstrued. Again, thanks for the good info!
 
Close. Probably will work better than that.

Remember, we were basically discussing the max power situation. Use less power, have less draw. The VFD will happily operate an idling motor at low input power. The draw goes up with output, so if you do not work the motor hard, it will be fine.

If you already HAVE the stuff, I would use it, and see if you run into trouble with breakers, etc. You may not. If you do, you know what to do about it.
 
What you are not understanding here is that for a VFD to accept 115V and put out 230V 3 phase, the DC bus must be at about 330VDC. You don't get 330VDC from a 115VAC source, at least not without some "extra" parts. The extra parts are called a "voltage doubler" circuit, a totally separate set of diodes and capacitors on the input side of the drive that double the voltage on the DC side of the rectifier. THOSE components are the ones that have a more limited input current capacity. As a general rule, the 1-1/2HP limit is the common design limit for these components when a VFD is UL listed, because of the heat rise they add to the drive. On this drive, you use the same product for 115 or 230V input, but you move a jumper if it's 115V. That jumper puts the "voltage doubler" circuit components into play.

Output current and input current are integrally linked, so in theory you COULD do a current limit on the output in order to avoid pulling too much current on the input. But here's the rub. Normally, the way most drives accomplish a current limit function is to override the commanded speed; lower speed = lower current. So that's often a problem you can live with, unless it happens when you are not expecting it. be that as it may, THIS particular drive does NOT offer that feature. The only thing it has that they call "current limit" is essentially a way to shorten the trip time to limit the current. Probably not what you were thinking of, but if you are stuck with only 115V input and you motor is not fully loaded, that's one thing you could do. Here's what it says;

4.2 ELECTRONIC MOTOR OVERLOAD PROTECTION – The drive contains Modified I2t Overload
Protection.* Part of this function consists of a Current Limit (CL) circuit, which limits the drive current
to a preset level of 160% of the Motor Nameplate Rated Current setting. The factory setting
for motor nameplate current is the drive rated current, which must be set to the actual
motor nameplate current (see Important Application Information (Item 2), on page 4). See
Table 3, on page 10. Also see Function No. 0.01, on page 32.
Standard I2t is undesirable because it causes nuisance tripping. It allows a very high motor current
to develop and will turn the drive off after a short period of time. KB’s RMS Current Limit Circuit
avoids this nuisance tripping while providing maximum motor protection.

If the motor is overloaded to 120% of the Motor Nameplate Rated Current setting, the I2t
Timer starts. If the motor continues to be overloaded at the 120% level, the timer will shut
down the drive after 30 minutes. If the motor is overloaded to 160% of full load, the drive will
trip in 6 seconds.
*UL approved as an overload protector for motors.

So given that 1-1/2HP is roughly 75% of the drive's rating, and it CAN take 120% current for 3 minutes, turning it down to 90% current limit would effectively make that ratio correct. But you can't do that, there is no setting like that. All they really let you do is shorten the trip time.
 
Actually, the voltage doubler is IN PLACE AND USED ALL THE TIME. It just does not double voltage unless the jumper is in place.

It's the same old series pair of capaciitors that are already in the unit. The jumper simply shorts one input wire to the junction between the capacitors. That leaves two diodes in the circuit, and turns the already present parts into a voltage doubler. Instead of charging both series caps at a 120Hz rate, it charges each capacitor individually to the peak of the 115Vrms waveform, doing them alternately, each as a 60 Hz rate. Since the two are in series still, the total voltage across them is still about the needed 330V.

My suggestion was to basically lie to the drive about the motor and tell the drive it is smaller than it really is.

If you look at pages 23 and 24, it appears that the OP can set "jumper J2" to any of a number of motor HP settings: 1/2, 3/4, 1, 1 1/2, and 2, with 2 not being applicable to 115V input.

I am not quite clear on what the implications are for THIS drive, other than adjusting the current limit to fit the appropriate size motor, assuming the limit was calibrated to the 160% and not changed. And the limit does not seem to do any more than a regular motor overload does, i.e. shut down if the motor is predicted to go over temp.

I don't think it does any special frequency adjustment to limit current. It's apparently just like a motor overload unit, except that it may be faster to shut off for heavy overloads. This is a very simple drive with analog control, much like certain Minarik units.... (did KB and Minarik have an OEM agreement? This one is a LOT like the Minarik)

If the OP is concerned about inadvertently exceeding the motor limits, he could lie to the drive by jumpering it for, say ONE HP, and his motor would not be unhappy, and the line current would be OK as well. If the load exceeded the 1HP limits for too long, the drive would shut off, saving him from having to reset a breaker, and surely protecting both the drive and the machine motor from any risk of damage.
 
I did not read the whole thread but I use this exact drive every day on the grinders I make and sell. All the grinders ship with 2hp 3 phase motors regardless if the customer has only 110 volts. What you need to do is make sure the jumper in the drive is set for 1.5hp and not 2hp to use the motor you have. This will limit the amperage from the drive to the motor. I have spoken with both KB Electronics and Leeson and both have stated there is no issues doing this. It will effectively only give you 1.5hp out of your motor though. The reason I do this is so I do not have to stock two different horsepower motors.

Esteem Grinders
 








 
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