Best VFD to control 20 parallel motors ?

Hello everyone,

I have to find a VFD to control 6 to 20 parallel asynchronous 1/2 HP motors sharing a load in V/f mode. Does anyone have any advice on which VFD i could use ?

Thank you in advance

Basile

So long as all of the motors are close to identical, most any one should work.
No drive that I know of knows how many motors are out there, but most only have one place to give it motor data i.e. polls, slip, FLA, etc. Which is not easily changed on the fly.

Your best choice is the brand with the best and most available local support.

One word of caution: To size the drive, forget hp. Add up the motor FLA's and in the case of 6 motors add about 40% to the total of the FLA's and choose a drive with that output current rating. In the case of 20 motors, the drive should be at least twice the ampacity of the total of the FLA's.

Sizing smaller than this will result in Short Circuit or Overcurrent Faults when starting. This is due to the very very low impedance of many stationary motors in parallel. They look like a dead short to the VFD.

Thanks for the answers =).

No preference on the brand ? A lot of people have told me that the vendor would be critical. I mean, controlling 20 parallel motors is not a common task and some VFDs might perform better at it than others.

But yes, all the connected motors are identical.

DickDV said:

Your best choice is the brand with the best and most available local support.

One word of caution: To size the drive, forget hp. Add up the motor FLA's and in the case of 6 motors add about 40% to the total of the FLA's and choose a drive with that output current rating. In the case of 20 motors, the drive should be at least twice the ampacity of the total of the FLA's.

Sizing smaller than this will result in Short Circuit or Overcurrent Faults when starting. This is due to the very very low impedance of many stationary motors in parallel. They look like a dead short to the VFD.

Click to expand...

I dont understand that.
How is it that many small motors looks like a dead short, in relative to a single motor with the same FLA ?

I would like to learn. Right now I am sizing a project with 12 motors in parallel.
It is for low duty cycle (20 minutes stop, 1 minute run), and I need some extra power for accel and decel. So I figure that 100% overload relative to total FLA is enough for me. According to the manufacturers specifications I can easily overload the VFD 100% during the accel and decel time which I figure will be around 5 seconds.
What you write makes me nervous. Is the above not enough ?

Has it been mentioned that with parallel motors and/or long cables, an output filter should be mandatory ?

JesperMP said:

I dont understand that.
How is it that many small motors looks like a dead short, in relative to a single motor with the same FLA ?

Click to expand...

i think it's because you have 20 impedances, and when connecting impedance in parallel total is: Z(tot)=Z1*Z2/(Z1+Z2) < min(Z1,Z2) so the VFD will see smaller impedance. so if you 20 identical motor total impedance is 20 times smaller.

Appearantly if you dont spell it out, people misunderstand the question.

JesperMP said:

How is it that many small motors looks like a dead short, in relative to a single motor with the same FLA as the total of all the small motors FLA. ?

Click to expand...

I'd like to know that, too =)
And does anyone have a VFD preference when it comes to control that many motors simultaneously, and why ?

The equivalent circuit for an induction motor at zero speed is very close to a transformer with a shorted secondary. The shorted secondary is essentially the rotor bars which are very close to zero ohms impedance.

Putting 20 rotors in parallel is even closer to zero impedance, I would think.

As to 200% of the total of the FLA's, I would think that would be a bit small if that is short term overload current. For twenty motors it should be at least 200% continuous current unless you can start super soft, maybe S curve rounding on the accel ramp. But even then I would size up a bit.

Its all about current or isnt it ?
Generally, typical AC motors starting current is 7x that of FLA, because of the locked rotor effect.
The efficiency varies somewhat with the size, bigger motors have somewhat better efficiency than smaller motors, but for this topic it is irrelevant since we talk about FLA, not rated kW.
Based on that I conclude that the starting current of 12 small motors, is about the same as one motor 12 times bigger (in FLA) than the small motors.
But I might be wrong. Just don't understand yet.

Like I said before. It is like 20 minutes stop, 5 seconds accel, 50 seconds run, 5 seconds decel, repeat..

This is 12 motors, in the past I have had at most 10 motors in parallel. And I usually set the current limit to between 150-200% of the total motor FLA. Has worked for me until now.

DickDV said:

Putting 20 rotors in parallel is even closer to zero impedance, I would think.

Click to expand...

JesperMP said:

Its all about current or isnt it ?

Based on that I conclude that the starting current of 12 small motors, is about the same as one motor 12 times bigger (in FLA) than the small motors.

Click to expand...

i would agree Jesper here. although impedance in parallel of 20 small motor is small, so is the impedance of big motor, and the starting condition are almost the same. and VFD can control starting current
for starting simultaneously 20 motor in parallel it is ok to sum up all FLA's for VFD cause situation is like one big motor. if however sequential start is in question it would be needed to calculate worst case for oversizing the VFD.

Since I've never had more than 6 in parallel, I won't argue Dick's advice. Mine have been 25% or so over the total FLA.

I will advise, since it hasn't been mentioned, that each motor needs overload protection.

If you use overload relays, you can wire the n/c contacts in series, and then program one of the inputs of the drive for a N/C fault input. This will stop the drive if any 1 motor faults.

In some applications, such as fans, folks use Manual Motor Starters (Motor Circuit Protectors, etc.), which will disconnect the motor which is overloaded. Caution with this one, due to the high voltage spike when disconnecting a running motor.

If the total of the motor leads are longer than 200' or so, or the motors might be disconnected while running (like with the MCP above), use an output reactor from the drive to the overload devices.

Try not to design a system where the motors are switched on with a running drive (line starting) PLEASE! The 6-12xFLA doesn't do anyone or anything any good!

I dont understand that.
How is it that many small motors looks like a dead short, in relative to a single motor with the same FLA ?
REPLY Use Ohm Law. Assume each motor resistance is one ohm then calculate the circuit resistance with 5 10 and 20 in parallel. One ohm is chosen to keep math simple - not an estimate of actual motor resistance.

I would like to learn. Right now I am sizing a project with 12 motors in parallel.
REPLY Dick DV I think is a good source of info.

According to the manufacturers specifications I can easily overload the VFD 100% during the accel and decel time which I figure will be around 5 seconds.
What you write makes me nervous. Is the above not enough ?
REPLY Using overload capacity I think is bad business. Overload does heat things up and they need time to cool. The main trick with all machinery especially so electrical is to keep it cool. Staying at or below 100 % duty cycle is a good way to keep stuff cool. Oversizing I admit looks expensive at the time of writing the check but is cheap over a 10 or 20 year life span.

With a 20 minute time between starts I think I would want to ensure that VFD has plenty of cooling. I would keep fan(s ??) running between starts.

Has it been mentioned that with parallel motors and/or long cables, an output filter should be mandatory ?
REPLY with longer cable from what I have read more than likely. Multiple motors I am not sure. Good question for Dick DV or the factory.

Dont forget to increase conductor size for long runs to keep voltage drop to a minimum 3% has been a good value for me.

Dan Bentler

leitmotif said:

I dont understand that.
How is it that many small motors looks like a dead short, in relative to a single motor with the same FLA ?

REPLY Use Ohm Law. Assume each motor resistance is one ohm then calculate the circuit resistance with 5 10 and 20 in parallel. One ohm is chosen to keep math simple - not an estimate of actual motor resistance.

Click to expand...

well ohm's law works in both ways. if you have 20 motor in parallel that is small resistance, BUT on same voltage higher current motor will have small resistance from scratch so where is the difference then? i think that's what Jesper refer to and where i tend to agree with him.

Only referring to filters - I have never had motors in parallel off a VFD - I believe filters and DC bus chokes should be mandatory anyway. I NEVER use a drive without them.
Currently doing a job with 79 VFDs ranging from 4kW to 75kW - most in the 22-55kW range.
They all have DC bus chokes, full filtering, low loss filters for both the EMI and RFI sides and they are all running on standard type AC RCDs set @ 30ma (people in water) - normal off the shelf drives cannot generally do this - they trip the RCDs. The manufacturers generally want to remove the filter connection to ground and then they do not comply with international standards with respect to noise. We have them built for us but they are now generally a standard drive in the smaller sizes for the manufacturer.
They are also all rated @ IP66 so we can mount them only feet away from the motors and use very little expensive shielded cable. Water (swimming pool or water park) plant room and the operators have a habit of hosing everything down weekly!
The current project is a huge water park in Sydney with all water features imported from the US - slides - tornado 60 - tornado alley - you name it.