3 motors controlled by a single VFD 5

[fabiosm]

Hello,

Has anyone experienced this kind of application? What should I be careful of?
In my case, I have a single VFD for three 200cv motors. I will use separated thermal overload relays, retarded fuses for each motor and a ultra-fast fuse in the VFD mains side.
The rated current and power of the drive is higher then the sum of the motor's rated values.

Thanks,

 

[Skogsgurra]

Not so sure what cv means? Cheveaux Vapeur? Cavallo?

If that is so, you are talking about 150 kW motors. It is usually not a good idea to run three such motors off one inverter. You do not gain much financially. But you lose availability and flexibility.

I wouldn't do it it there are not very strong reasons.

And, if you do it, you can not use vector control on parallel motors.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[fabiosm]

Thanks Gunnar,

Sorry, I forgot to mention, its "cavalo vapor", I think this term is only used in Brazil. The motor's rated power is 132kW.

Actually, this is an already installed application and I was hired for the comissioning. The motor will always run together and on the same rpm, that's why they're only using one inverter. If one of the motors stops for any reason, they'll have to stop the other two.

The motors doesnt have individual thermal overload protections, and that's one of the changes they'll make on the circuit. My biggest doubt is if I need individual short circuit protection, or the mains fuses are enough. Is there any other problem in this case?

The motors are for pumps and I'll set scalar control.

 

[Skogsgurra]

OK. "Steam Horses" - like the French use to say. But French steam horses are not the same. I think a typical French Steam Horse is around 7 HP. So the Citroën 2CV actually has around 14 HP.

I understand that there is no possibility to put an inverter on each motor. So just go ahead. The motor cables need to be protected and the inverter fuse will probably not be enough protection for the cables. Nor will the VFD current limit.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[controlsdude]

NEC in usa you need individual overload protectors for each motor. Not sure what they do in brazil.

 

[waross]

Steam horses. Interesting.
Boiler horses are about 13 HP.
I guess the French had hotter boilers than the Brits back in the day.
And now I understand the 2CV in 2CV4. Is the 4 the number of cylinders?
Thanks for the interesting trivia Gunnar.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

You are right that you need short circuit protection for each individual motor circuit.Most VFDs provide that, but it is adjusted in programming when you set the motor current, so to run 3 motors your short circuit current setting in the VFD will be 3X the motor circuits require.

I would not use fuses though, because they can create a single phasing problem and a severe current imbalance in the VFD output. What I would do instead is to use adjustable trip circuit breakers on the circuits for each motor, they provide short circuit, overload and disconnect capabilities. You don't want to use them to turn on a motor after the VFD is already running, but using them allows you to isolate the overloaded motor and restart the VFD to run the others. Make sure to use Tripping Indicator aux contacts however, not the type that change state with the switch, otherwise you lose that capability.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[Drivesrock]

I believe you should have a thermal overload relay with contactor on the output of the drive for each motor and for sure not fuses as said above.
In the event of an overload of any one of the motors the appropriate relay will operate. Use the auxiliary contacts of the relays in series to the drive trip input. Then when one motor stops they all will. Keep that wiring segregated from the motor/drive cables as it will probably be 24Vdc.
I would also recommend an output reactor to filter any voltage spikes when a contactor opens.
The drive should be programmed to the sum of the full load currents of the motors for overload purposes and any tuning of the drive even for scalar or voltage vector control (different from flux vector control). Instantaneous overcurrent (or short-circuit) protection will possibly not be programmable but the drive will have this built in the H/W.

What are you pumping - Just water? Iron ore slurries in Brazil are notoriously difficult and quite often the drives need to be 150% overload rating so that the starting conditions of a pump full of solidifying slurry can be met. Your output overload relays should have enough adjustment too.

 

[DickDV]

jraef, first, thanks for the CB recommendations on the individual motor leads. I've always recommended a fused disconnect with overload relay but yours sound better and with only one device.

I'm curious why the dread about single phasing one of the motors.

 

[Skogsgurra]

One of the problems is that it leads to excessive heat. And if there is no thermal protection, you will usually lose the motor.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

I was also concerned for the current imbalance coming from the VFD. Most likely the VFD would shut down on a fault but I'm not sure if everyone monitors individual VFD output phase currents, many of them only measure current on the DC bus. If so the VFD would only know the total current going through the DC bus but would/could end up being unaware that 2 of the transistor sets would be carrying more of the total load.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

Drivesrock,
A contactor and overload relay would not provide short circuit protection for the individual motor circuits, only thermal protection. The contactor in that scenario serves no purpose other than isolation of the tripped motor, which would be provided by the motor protector in my example.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

A question jraef: Normally if I was using one device to provide short curcuit protection to a group of motors I would use the same short circuit protection for three motors as for one motor. This would be based on the short circuit current being dependent on the source impedance and the conductor impedances. Is there a reason to do it differently for multiple motors on a VFD or was that a slip of the finger?
Thanks
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[fabiosm]

Drivesrock, you scared me, it really is an iron ore slurries' pump!

It looks like there are a lot of options for this situation.
So, I'll tell you what I'm going to do:

To protect the drive: ultra rapid fuses.
For each motor/cable protection: retarded fuses. I'll use existent TC's that are installed on the motor's cables to send the current measure to an overload relay. (sorry, I don't know if it was clear enough).
The VFD that I'm working measures individual output current jraef, so I think the inverter would shut down on a fault.

I'll keep you informed if anything happens.

Thank you all, you really helped.

 

[jraef]

Bill (waross),
The SC protection inside the VFD is electronic. It's very good and most mfrs have them UL listed to protect whatever motor is connected to the VFD, but the protection level is not user adjustable, it's tied to the VFD motor amp setting. So for example if you have 3 x 50HP motors and you buy a 150HP VFD to run them, you will have to program the VFD current level to handle all 3 motors. When you do, the SC protection will be 10X (or whatever it is) the 150HP FLC level, but that would make it 30X for any one of the individual 50HP motors.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[waross]

Thanks Jeff. I suspected that I was going to get some further education when I asked that question. It's clear now.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Drivesrock]

Fabiosm

What is the manufacturer and model of the drive you are using?

 

[fabiosm]

Drivesrock

It's an ABB ACS800

 

[jraef]

Just FYI, from the ACS800 manual:

Protection against short-circuit in the motor cable
The drive protects the motor cable and motor in a short-circuit situation when the motor cable is dimensioned according to the nominal current of the drive. No additional protection devices are needed.

So what that confirms is what I said, the ACS800 has SC protection for the motor cables and windings but it is based on the rated output current of the VFD and will not protect smaller cables or motor windings. There is no programming function for changing the value of the trip setting (nor do they specifically say what it is).

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[ozmosis]

fabiosm
what is the current rating of each motor connected to the ACS800?
what is the cable length between each motor? (usually these slurry pumps are a long way from the VFD and if you have three into one VFD, the actually cable length is each one x 3)
what is the current rating of the ACS800?

I know you have already purchased and installed but worth checking these before you spend a lot of time commissioning something that might not be correct.