Selection of VFD

Hello friends ,
I want to select an VFD for Two different condition :
1.Running Two motors from an single VFD the motor ratings are the same (suppose say 5hp motors) what should be vfd rating ? can i use 5hp VFD for both the motors and give an parallel connection or the motor rating should be added (i.e: 5+5=10hp VFD)?
2.In the same situation out of the two motors only 1 should be working using VFD and other by dual starter . (controlling is done by plc)
Is it good if i use and higher hp VFD For an low HP Motor and what could be difference in max range one can exceed ?

You can parallel connect several motors connected to one VFD. The power and current values should be multiplied accordingly. When parallel-connecting motors it is important is to use an output choke on the VFD to counter reflections.

The second question I do not understand.

thanks for the reply,
The same plc must be used for an condition where it has to operate only 1 motor .
my question was, can we use an higher hp VFD for an lower HP motor ?
and what is the safest value(VFd) that can be used ?

1. Each motor must have its own overload protection device. You CANNOT use the overload system of the drive. Wire the overloads to a drive input and configure it to shut down the drive if either overload trips.
2. If both motors will be started at the same time then the drive should be sized for the total amperage of the motors. However if the motors start at different times you must size the drive for the sum of current of the running motor PLUS the current inrush of the the motor [Generally 6 times nameplate current] that will be started. As the inrush when starting the second motor is of very short duration you size the drive based on its peak current overload rating [most commonly 1.5 x Drive output current].
Example 1:-
Motor 1 Nameplate current = 4 Amps.
Motor 2 Nameplate current = 5 Amps.
Motor 1 is running and motor 2 is started, total current = 4 + [5 x 6] = 34 Amps. 34/1.5 = 23 Amps = Drive Rated Current.

Example 2:-
Motor 1 Nameplate current = 4 Amps.
Motor 2 Nameplate current = 5 Amps.
Motor 2 is running and motor 1 is started, total current = [4 x 6] + 5 = 29 Amps. 29/1.5 = 19 Amps = Drive Rated Current.

Note that drive size is different between example 1 and 2 due to the order of starting the motors.

3. JesperMP is absolutely correct regarding drive output filters, particularly with long motor leads.

My 3 Cents

As to sizing drives with multiple motors, you cannot get optimum sizing by simply adding hps of the motors. The better way is to use amps. Take the FLA of both motors and add them together. Then add 10% for each motor over the first---in this case there is one extra motor. For example: A 10hp 17amp motor in parallel with a 25hp 32amp motor. Add 17 to 32 equals 49amps. Add 10% which is 4.9 which totals about 54amps. The VFD will need at least 54amps continuous rating to avoid short circuit faults when starting. And you must protect each motor with its own overload block as mentioned above.

As far as I know, the same rules apply for adding a motor lead reactor on multiple motors as exists on single motors. It's just that you have to add up all the motor leads as if they were in a single string.

The discussion above about starting the second motor after the first one is running and how to size the drive has stirred my curiousity a bit. I've always advised against starting a stationary motor into a running drive but the approach mentioned above seems reasonable. Has anyone actually done this and will it work ok even when the first motor is up to full speed?

DickDV said:

The discussion above about starting the second motor after the first one is running and how to size the drive has stirred my curiousity a bit. I've always advised against starting a stationary motor into a running drive but the approach mentioned above seems reasonable. Has anyone actually done this and will it work ok even when the first motor is up to full speed?

Click to expand...

We once considered this as a measure to increase the capacity of a conveyor plant.
The whole idea was dropped again when it was realised that the VFD would have to be sized to all motors running + 1 biggest motor starting up. That, and that all cables had to be exchanged made the VFD solution quite expensive.
And the power to all motor startes was tapped from a central busbar, which would have had to be replaced by a totally different solution (I didnt even want to think of if there would be any EMC problems with having a complete busbar on a VFD).
Instead we investigated carefully which conveyors were the most troublesome and had their gearmotors replaced for faster and bigger ones.

Edit: If we would ever consider a VFD solution again, I would suggest one small locally installed VFD per conveyor.

I think to clarify and summarize all that has been said
1. Yes you can run multiple motors off one VFD. VFD must be adequately sized to do this as Dick DV says use total motor current
2. Each motor must have individual overcurrent protection
3. You can disconnect one motor from VFD - it will not hurt it in the least to run at half load.
4. Starting one motor on VFD with other motor running has been not recommended by all that I have seen.

HOWEVER DicDV said something that stirred my brain cells
"The discussion above about starting the second motor after the first one is running and how to size the drive has stirred my curiousity a bit. I've always advised against starting a stationary motor into a running drive but the approach mentioned above seems reasonable. Has anyone actually done this and will it work ok even when the first motor is up to full speed?"

VFDs have what is called flying start ie VFD can start a spinning idle motor eg a HVAC fan. VFD will match output to motor speed and then energize motor. So in case of stopped motor could flying start take speed down to zero and at that point allow second idle motor to be connected (relays etc) and then speed both back up.
IF second motor is spinning but not driving the VFD would match to that motor enegize it and bring both back to desired freq (RPM)

Dan Bentler

There are some drives (Omron 3G3MX2 for example) that have provisions for two different sets of motor parameters. These are designed to operate two different motors individually, but I suppose the could be used to control one or two motors if the drive is sized correctly.
Switching the load between two motors while the drive is running is always BAD! Remember also that the drives have built in Accel and Decel ramps to limit starting current. Switching drives with an external contactor while running would defeat these ramps and probably cause drive failures.

Seems to me it would just be easier to add a second drive and about as cost effective.

Also think about the poor bast&rd troubledhooting all this at 3:00 am.

Just my 2 cents.

I concur with The PLC Kid.

I have worked on Lines that had VFDs with multiple Motors on them. They had to be oversized quite a bit to allow for Motor Starts. We experienced a higher than normal failure rate on the two VFDs used for the multiple motors.

At the cost of AB PF40 VFDs, and VFDs in general. It just doesn't seem worth all the hassle. Overload Blocks and all the extra wiring etc costs money too.

Stu....

The way Flying Start (also called Dynamic Start or Catch a Spinning Load) works is that the VFD looks out at the motor to check on its speed (actually frequency) and then starts at that same frequency to avoid inrush or breaking currents. If there were two motors out there, one running and one stationary, I don't see how the VFD could see the stationary motor. The rotating motor would cover it. And since Flying Start is only active during a start command and there is none because the first motor is already running, I don't think we have any kind of workable solution.

In my years of experience with VFD's, about the most radical multi-motor system encountered was on a roll mill in a steel mill. There were 47 rollers on the roller table feeding red-hot ingots into a roll mill. Each roller was driven by a 1.5hp motor thru a gearbox. We put all 47 motors on one 125hp drive with each motor fed thru a fused disconnect and overload block.

When a roller would lock up(not uncommon), the operator would pull the disconnect and keep running. The mill could keep functioning until five rollers were dead. Then it was time for a maintenance downtime.

This worked good until the mill closed!

Hello ,
sorry for the late reply,Thank's for the reply,was not well .
ok any ways, i have be guided by some 1 that i can use an single V.F.D for an parallel connected motor's of same rating .
Eg: I have 3 motor's which works alternately in series of two at a Time
(i.e-1&2,2&3,3&4).
say i have 5 hp motor's can i use 5hp VFd or shu'd i go for an 10hp Vfd.

Add up the motor FLA's for proper sizing... HP is irrelevant! Different motor designs and speeds have differing FLA's for the same HP.

Always use an output reactor if it's possible to disconnect one motor while running.

If a motor is to be 'line started' from a vfd output, use an output reactor, and a rating as stated above (enough o/l current capability to handle LRA on the starting motor.

For example, we routinely use vfd's for single to 3 phase conversion. On a center-pivot irrigation leg, for exmple, there is a large pump (say 75hp) that is running whenever you are active, and a bunch of small (say 1hp) crawler motors which switch on and off. Using a 100a (480v) rated vfd (after single phase derate), there is plenty of capacity to start a 1hp motor at full voltage.

The output reactors protect the vfd from:
voltage spikes when disconnecting the motor while running
current spike from line starting a motor with a running vfd
long leads and reflected voltage

gibson said:

Hello ,
sorry for the late reply,Thank's for the reply,was not well .
ok any ways, i have be guided by some 1 that i can use an single V.F.D for an parallel connected motor's of same rating .
Eg: I have 3 motor's which works alternately in series of two at a Time
(i.e-1&2,2&3,3&4).
say i have 5 hp motor's can i use 5hp VFd or shu'd i go for an 10hp Vfd.

Click to expand...

You are asking the same question again. Please re read all posts especially 5 and 7. You will find answers there.

Now then if you do not like these answers then go ahead and do what ever you want. It is your equipment not ours. You have to fix the results of your mistakes - we dont.

Dan Bentler

Sorry sir ,
leitmotif thanks for your reply i was a bit confused with that post. I read it again and again, my concepts are almost clear and the project is on the processing stage .
Thanks again ..