15hp motor on 10hp vfd

We have some used air handlers that we are going to be putting back in service. In the pile of parts we have 2 vfd's

VFD1
ABB ACS550-U1-015A-4+B055

VFD2
ABB ACS401601132

Both of these appear to be 10hp units.

The fans in air handlers are
HP 15
V 230/460
FLA 37/18.5

The vfd's had previously driven the fans in the air handlers. Is there a problem running the 15hp fan on the 10hp drives? The drives will only produce so much amps at full load.

As you say, the motor can only draw the current that the vfd can deliver, you'll likely find with an air handler the motors don't need to run full speed so the current will be significantly less than rated any way.
I've used 132KW vfd's with 185 KW motors and the machines have been running day and night for years.
You will probably need to auto tune the drives (ID Run on ABB)

That is a loaded question.

If the motors do not attempt to use more Amps than the drive can provide you likely will not have an issue with them.

Russ is right, that is a loaded question. (ETA: just noticed Russ's pun. Must be a slow day )

A motor will draw as much current as it needs to match the load until something trips, burns, or stalls. None of these situations is good for a drive. If the motors are over sized for the fans and/or you are running at less than full motor speed then you MIGHT be OK. Emphasis on MIGHT. Don't underestimate the power needed to turn a fan. Fans are subject to affinity laws. Power is proportional to the cube of the fan speed. Increase speed by just 25% and you'll double the power you need. Drop the fan speed to 1/2 and you need 1/8th the power.

Most VFDs have a current limiting function available. This will clamp the VFD speed when the set output amps is reached, preventing overload.

Note that a fan's power draw is a function of the air flow and discharge pressure. When the air is warm, or the system dampers are closed, or the air flow is otherwise restricted the power draw will decrease. It is probable that the motors were oversized to account for a worst case condtion that you have not reached.

I personally don't like to have a motor sized larger than the VFD, but you can obviously get away with it.

Again, this is on a second hand air handler that we are starting to piece back together. Literally piece back together, they cut it into 4 pieces to remove. The drives ran the fans fine for many years. I guess if we get it all back up and running and find that we are running into current limits on the drive we can up size then.
Thanks for the info everyone.

Setting an under rated VFD

Using the V/F straight line, Select a maximum motor RPM that matches your available output watts from the VFD.

So if you have a 240V wired motor rated at 1750 rpm and 15HP
Program the VFD to hit 240V at 1150rpm.

Don't just limit the motor RPM, Set the voltage span 0-240 over the rpm range 0-1150.

Watts are watts, and the VFD will take care of not hurting the motor or it's self if it is set up (programmed) right.

Now you have a ten HP motor in a fifteen HP chassis. Turning a bit slower that full rated speed.

This works the other way too. If you have a 440V VFD and supply a 220V wired motor, you can turn double the rpms (120 hz) and make double the HP. This does take a VFD rated at both high V and High HP. The scheme is described in Toshiba VFD literature, and I have it configured that way on a machine tool. Works fine! Big power in a small motor package. (Of course the VFD and transformer are a bit bulky ;-) A large 3450 motor would not be a good candidate for this method. Might spin it's self apart. But a 1725 or less rated motor should have no problems.

Most all VFD's allow such programming, be sure to set your current limits as well.

CalG said:

This works the other way too. If you have a 440V VFD and supply a 220V wired motor, you can turn double the rpms (120 hz) and make double the HP.

Click to expand...

Most VFDs are limited to constant hp above 60 Hz, with the current and torque decreasing above that point. The output voltage of a VFD cannot exceed the input voltage.

I've done over-speed applications on centrifugal loads, but you have to engineer motor, load, and VFD sizes carefully.

TJ
You are absolutely correct. Several VFD's I have worked with are designed to output frequencies up to 400hz. If one were to apply these frequencies at constant VOLTAGE, it's very likely a 60cps motor would not develop enough power to keep it's self spinning.

The entire exercise is to maintain the V/f relationship.

One could think of it as a multi speed motor with "external electronic windings"

VFD's are wonderful devices! they may be a bit hard on old and weak insulation however.

Ok a lot of good theory but everything need to pass thru a complete study. Drive rating should be better match by amp rating than hp rating.
Overspeeding a motor need to check for speed rating of rotating mass capacity/balancing also. sending 440v to a 220v motor to increase it's torque would make it very hot for sure and will probably burn the wire inside...

In this case, if there is no initial torque required to break any big inertia the only limit will be speed according to fan load

Even though this discussion has diverged onto a path that isn't relevant to the OP, its time for a reality check. Over speeding the motor isn't a real option for driving fans. Running at 120 HZ will increase the load by a factor of 8. For our OPs fan that would mean 120 HP needed to turn the fan.

In general, a 15hp NEMA frame motor with cast aluminum rotor and with 4 or more poles (that's 1800rpm or less on 60hz) is good for 90hz right out of the box. Just check the manufacturers' torque-speed curves for verification. So, in the OP's application overspeed is possible but is not going to make his situation better because there isn't enough hp to get to 60hz much less above that.

If the OP desires to use the motor without any "funny stuff" or "tricks", then setting the current limit right at the drive's continuous output currect rating would be simple and adequate. The motor will be able to run up in speed until it reaches the drive current limit and that's all. Everything will be happy running right there. It might be 58.5hz or 57.5hz or somewhere right in that neighborhood. If the air is warm it will run a bit faster. If colder, it will run a bit slower.

The discussion above about wiring the motor for 240V and setting up the drive for a 480V 120hz motor is not going to work because the drive doesn't have enough ampacity to operate the coils in parallel. This kind of configuration is a very clever way to get double hp our of a dual voltage motor but the drive has to have double the ampacity to pull it off. Since the drive is the weakest link here, it is not a solution.

Boy I feel like such a distraction The overspeed was only in example

DickDV said:

In general, a 15hp NEMA frame motor with cast aluminum rotor and with 4 or more poles (that's 1800rpm or less on 60hz) is good for 90hz right out of the box. Just check the manufacturers' torque-speed curves for verification. So, in the OP's application overspeed is possible but is not going to make his situation better because there isn't enough hp to get to 60hz much less above that.

If the OP desires to use the motor without any "funny stuff" or "tricks", then setting the current limit right at the drive's continuous output currect rating would be simple and adequate. The motor will be able to run up in speed until it reaches the drive current limit and that's all. Everything will be happy running right there. It might be 58.5hz or 57.5hz or somewhere right in that neighborhood. If the air is warm it will run a bit faster. If colder, it will run a bit slower.

The discussion above about wiring the motor for 240V and setting up the drive for a 480V 120hz motor is not going to work because the drive doesn't have enough ampacity to operate the coils in parallel. This kind of configuration is a very clever way to get double hp our of a dual voltage motor but the drive has to have double the ampacity to pull it off. Since the drive is the weakest link here, it is not a solution.

Click to expand...

The over speed, over voltage over hp example is just using the equipment in a favorable way. An extreme example in the opposite direction that is needed by the OP. But the example lays on the same line. The V/F line!

Skeptics might think of these aspects of electric motor manufacturing.

Many motors are dual voltage. 220/440 are very common. No rocket science insulation or wiring goes into these motors at either voltage.

Two pole or four pole motors (3450/1725 rpm) are also common. No special construction goes into building 3450 motors. Inertia is not that great even at the higher rpm. ("large" motors are excepted. 10hp is not large) Dual speed motors exist. two poles at 60 cycle would yield the same motor rpm as a four pole motor supplied at 120 cycles. (not exactly the case we are discussing, but an example of motor robustness and flexibility.)

Importantly, VFD's do NOT output Voltage independent of frequency (the V/F thing, this is different from running on mains without a VFD) Since Volts drive amps, there is a bit of self regulation going on. How much Resistance, (inductive reactance) the motor provides is accommodated in the VFD motor profile settings.

So all I'm saying is

If, I had a 10HP VFD and wanted to power a 15 hp motor (we could do this in watts) and didn't want a lot of nusiance tripping due to over amping, I would set the vfd to limit the motors ability to consume the amperage that would cause troubles. (Clear as mud!)

As the ABB-ACS550-U1-015A-BO55 VFD mentioned is a 380-440 volt device,

Wire the MOTOR for 440V
Provide line Voltage between 380-440 3ph power to the VFD
Set the V/f profile to 440 volts at 40 cycles per second (not the default 50-60)
set the motor amps to match the maximum rated output of the VFD (or a bit less) (I might use auto tune, but would go back in and reset some of the values. The VFD might not play well with a motor half again as big as it's supposed to see.

Done!

One could then monitor the actual amperage used while the motor was in service and bump up the maximum frequency at line voltage if possible.
Moving the end point at the high end of the V/F line.

There is always a time element in the over current protection to guard against nuisance faulting. Some playing around is always possible.
I might also slow the acceleration ramp on start up to keep the amp draw in check. What's 10 seconds to a fan? ;-)

Cheers

Cal

I'm sorry, CalG, but I beg to differ on several of your points.

First, the ACS550 product in the model number you provide is a 380-550VAC input drive. Not that any of that helps us because, assuming the OP's voltage is 480V, that's nicely in the middle of the range.

Second, your statement "volts drive amps" is not true for a motor circuit. In fact, Volts per Hertz ratio drives amps at any given shaft load.

Third, due to the second item above, your proposal to set motor voltage to 440 at 40hz is a ticket for rapid motor failure. That's a V/Hz ratio of 11 on a motor designed for 480/60 or a V/Hz ratio of 8. You can expect the motor to draw way too much current at all load points including no load. It would be unlikely that the ACS550 we have before us would even start the motor much less run the fan up to any useful speed.

Forth, I assure you that doing a motor ID run (you call it autotune) would fail with the motor data entered as you propose. The ABB motor model is very good and would surely reject any attempt to violate the motor's design V/Hz ratio by that much. And, what would it gain? The motor model in sensorless vector mode provides for more precise speed control over varying motor loadings and manages slip better in the lower frequency ranges under 5hz so you can get smooth shaft rotation down to about .7 to .8hz. Below that, you need ABB's DTC drive or the addition of an encoder. None of this is appropriate for a motor driving a centrifugal load like a fan.

I don't mean any offense here, CalG, but 24 years in the field with VFD's and motors has given me a pretty good feel for what will work and why. If you want to discuss this further, just say so.

Dick

In response

I did not provide the ABB model number, the OP did, I just looked up the specs and posted them for a rough voltage reference.

The "volts drives amps" is a general statement. perhaps you missed the comments regarding inductive reactance for motor applications


I didn't just fall off the pumpkin truck either

cheers