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2 equal sized motors on 1 VFD
4

2 equal sized motors on 1 VFD

2 equal sized motors on 1 VFD

(OP)
Is it possible or will one motor feedback onto the other and cause problems??

RE: 2 equal sized motors on 1 VFD

Quite possible.  Should be no problem.  The VFD just generates the waveform, the motors just do what they always do.   This would likely not be a good idea with a vector drive though.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: 2 equal sized motors on 1 VFD

This is done all the time.  As itsmoked says, this works ONLY with an open loop V/f drive.

RE: 2 equal sized motors on 1 VFD

This is only true with multiple motors on one vfd.

just remember to size the VFD hp x 1.33 (total hp of all motors) or you will undersize the vfd.

RE: 2 equal sized motors on 1 VFD

Also, each motor must have its own overload protection.  

RE: 2 equal sized motors on 1 VFD

and be mindful of cable length. The total cable length is the total between each motor from the VFD.
Other advice is to size the drive(VFD) on the current rating of each motor x 1.1 to provide a safety margin for overloads.

RE: 2 equal sized motors on 1 VFD

Realize that BOTH motors will run the same speed.

I know this sounds obvious, but you'd be surprised how many people don't think this through...

RE: 2 equal sized motors on 1 VFD

OK,

I've got to ask, why a safey factor for multiple motors?

RE: 2 equal sized motors on 1 VFD

Starting currents don't scale the same with HP?  Just a blind guess.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: 2 equal sized motors on 1 VFD

the 10% margin is used typically to cope with any minor overloads on the motors connected. It's a general rule and one that is there if costs permit to allow for but usually you don't have that luxury so you rely on the overload capability of the drive to cope with certain load changes in the motors.

RE: 2 equal sized motors on 1 VFD

I've never used more than a 10% de-rate, 33% seems a bit extreme. 10% allows a fudge factor for slight differences in the load that may make one motor accel and decel differently. If the motors are on dissimilar loads then maybe the 33% would make more sense, but I can't imagine off hand many applications where you would have 2 dissimilar loads that need to run at the same speed reduction.

RE: 2 equal sized motors on 1 VFD

(OP)
Thanks everyone for the feedback. FYI this is for a retrofit application on a ship where they want to change the shape of a cooling radiator and run two smaller fans instead of one. Otherwise I would have just replaced the one VFD with two, but in this case it would be a whole lot easier to run the one VFD.

RE: 2 equal sized motors on 1 VFD

An excellent application for your plan.  Have at it!

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: 2 equal sized motors on 1 VFD

Two independent drives will allow you to maintain some form of  operation, albeit at reduced cooling capacity, even if one fan is out of service for maintenance. As the original system used a single fan this might not be significant.
 

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If we learn from our mistakes I'm getting a great education!

RE: 2 equal sized motors on 1 VFD

Just put a small lockable disconnect on each motor and you can remove one while operating the other.

RE: 2 equal sized motors on 1 VFD

application where you have two disimilar loads would be if you have to bring up motors on conveyors at same speed from a bar code scan tunnel to a sorter.  Sorter usually has the biggest motor versus the tracking belts between the scan tunnel and the sorter.  If you just put all the motors on one vfd and use the 1.33x multiplier hp the vfd would not be undersized.  This way the boxes track consistantly and as long as the gear boxes on the conveyors do not have a big difference between them

I guess fans that are the same do not need this multiplier.

RE: 2 equal sized motors on 1 VFD

I guess I don't understand the whole horsepower multiplier issue.  I usually size VFDs based upon total amp requirements (even on single motors since I get a lot of unusually size motors with large compressors and centrifuges) instead of horsepower.  Is there any reason why adding up the total FLA (including service factor amps if they will be used) will not work?  I haven't had any problems as yet.

Another comment as controlsdude surmised is that centrifugal fans and pumps are variable torque loads where conveyors, positive displacement blowers and pumps, etc. are constant torque loads which will make a difference in loading at less than nominal speeds.

RE: 2 equal sized motors on 1 VFD

Quote:

Just put a small lockable disconnect on each motor and you can remove one while operating the other.

Good way of killing drives unless they are specifically designed for this. I know some are supposedly able to withstand opening of an output contactor or load-break switch but not all drives will put up with this. Of course shutting down the drive for a few seconds would work too, and on a fin-fan cooler this would probably have little consequence.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

RE: 2 equal sized motors on 1 VFD

Here is my two cents on shutting off the disconnect

I believe you would get an overvoltage fault on the drive if you switched one of the disconnects off.  You should as a general pratice to have an aux contact on the disconnect that pulls the enable on the vfd to prevent this type of situation either through the plc or hardwired.

I had a job where their was a backup motor and used contactors to swapped between motors with a keyswitch.  Their was some interlocking between the disconnect aux and the contactors, and the enable on drive to prevent opening the output between the motor and vfd.   

If your going lock out one, then you need a written procedure posted at vfd that you need to shut both off before switching over to just one.

RE: 2 equal sized motors on 1 VFD

A minor comment about two fans in parallel.  Should one stop, often air from the working fan will blow a significant amount of air backwards throught the non-working fan appature.  This can cause the cooling downstream to be greatly reduced.

RE: 2 equal sized motors on 1 VFD

(OP)
The shroud will be designed with a center baffle so that if one fan dies the other will not be affected. But it is unlikely they will ever service this unit without shutting the whole system down, so it's not really a concern. I will however, run an interlock on the individual overloads to prevent damaging the drive.

RE: 2 equal sized motors on 1 VFD

Might be worth putting a fuse in each phase of each motor, unless the two motors are always at the same load. The drive wont know if one motor is unloaded and the other overloaded

RE: 2 equal sized motors on 1 VFD

I for one would not put fuses downstream of drives running one motor. Aside from the issues of single-phasing the drive output, a fuse blowing has the same effect of opening a contactor (or disconnect) downstream of the drive; potential loss of the transistor. But there are some NEC installation issues to consider for those of us in the US (and Canada, who's CEC is virtually the same) which usually end up in requiring new SCPD devices downstream of the VFD. Read section 450.53 of the NEC carefully when considering this; there are a lot of what-ifs and issues involved, too complex to go in to here.

But here's the upshot: when you have multiple motors running downstream of a VFD and one trips off line, the dv/dt issues with regards to damaging the output transistors are somewhat mitigated by the fact that there is still an inductive load running in parallel, absorbing a lot of the voltage spike issues created by the contacts opening. I have used Motor Starter protectors (IEC manual motor starters) as downstream protective devices for years, never had a problem. They take care of the individual OL protection as well as the short circuit protection, provide the means of disconnect that LionelHutz mentioned, and open all three phases, which prevent single phasing one of the motors.

By the way, to LionelHutz's defense (not that he needs it), I don't believe he meant using the downstream disconnect to open one motor circuit while running. His comment came after ScottyUK's concern about running one fan after the other had failed. So if one motor fails, a lockable disconnect allows the user to RESTART the remaining fan while the failed one is being serviced.

RE: 2 equal sized motors on 1 VFD

xJAyx
another point to ponder.
You mention that the VFD is going on a ship.
VFD's on ships will generally require some sort of certification from Lloyds Register or ABS otherwise the ship owner might get a little upset you putting it on there. They generally have to be carefully selected with regards leakage current and control isolation. Make sure any sort of RFI filter is disconnected or specially designed for ship applicatiuons otherwise the leakage current will be a problem.
 

RE: 2 equal sized motors on 1 VFD

Yes, Jraef you are correct. Put the disconnects inside the enclosure or something similar so someone can't randomly switch them off while running. You really just want it for the ability to apply a lock for service reasons while still being able to restart and run the other motor at the same time.

RE: 2 equal sized motors on 1 VFD

(OP)
Thanks again everyone, It's all in and works smoothly.

RE: 2 equal sized motors on 1 VFD

Re the disconnect: Since it is shipboard, ABS (and other Reg Bodies/IEE Std 45) a required disconnect may be at the controller, but it must be externally operable and isolates the controller as well. So if single disconnects are put inside the controller to isolate motors another may be required externally that disconnects the entire circuit. Unless the feeder breaker is withing the same compartment as the controller.

The same entities require individual motor overload protection (they don't all agree on the settings, but ABS is 100-125% FLA), but you can have the OL relay isolate the drive input.

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