# Multiple motors with one VFD.



## tates1882 (Sep 3, 2010)

Whats everyones thoughts about running 4-5hp 480v motors with a single VFD? Two per side of a belt conveyor for autos working in unison, both belts need to run exactly the same speed. Would a 30Kw Vfd work? TIA.


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## JRaef (Mar 23, 2009)

General rules are as follows:

Total drive output current rating should be 10% higher than the collective motor FLA ratings. This is because of the increased number of wires and the added capacitive charging current in them. Ignoring this runs the risk of the drive tripping and shutting everything down. 
Each motor must have separate OL Protection and may need added SC protection if the drive size exceeds the article 430 rules for OCPDs. I like to use the IEC style Manual Motor Starters because they provide both, but some of them are not rated to be used behind a VFD. 
An output reactor is a good idea, just one sized for the VFD, because you have a multiplied risk of circuit problems that could damage the VFD transistors.


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## just the cowboy (Sep 4, 2013)

Just watch when you say exactly the same speed. You may find a difference between motors and gearboxes, this might be only small but can add up over time. flattop belts don't usually have a problem due to product slip but cleats on a belt may drift position.

Cowboy


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## JRaef (Mar 23, 2009)

just the cowboy said:


> Just watch when you say exactly the same speed. You may find a difference between motors and gearboxes, this might be only small but can add up over time. flattop belts don't usually have a problem due to product slip but cleats on a belt may drift position.
> 
> Cowboy


Good point. Running multiple motors off of one VFD will give them all the same FREQUENCY, but that does not necessarily mean they will all run the same SPEED. However if they are all mechanically linked, the slight differences don't make too much of a difference.


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## gpop (May 14, 2018)

JRaef said:


> Good point. Running multiple motors off of one VFD will give them all the same FREQUENCY, but that does not necessarily mean they will all run the same SPEED. However if they are all mechanically linked, the slight differences don't make too much of a difference.


Ive always wondered if 2 motors mechanically linked will get into a fight loading and unloading or if they would work together. E.g if one that's only a tiny amount faster would overload while the other would act as a brake.


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## tates1882 (Sep 3, 2010)

I’m a bit skeptical.


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## splatz (May 23, 2015)

tates1882 said:


> I’m a bit skeptical.


That doesn't look to me like it would tolerate much variance at all, like none. 

I'd think you'd really want a differential between each motor and the shaft, then it seems very doable. 

Of course it might turn out to be better to just get a bigger motor.


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## micromind (Aug 11, 2007)

gpop said:


> Ive always wondered if 2 motors mechanically linked will get into a fight loading and unloading or if they would work together. E.g if one that's only a tiny amount faster would overload while the other would act as a brake.


I've seen this on high HP machinery, like rock crushers that have 2 motors driving a single shaft. 

Usually, one motor will draw more current than the other, as load increases, this difference often gets smaller. This is true even if both motors are the same brand and model. 

One trick I've used somewhat successfully is to 'roll the phases'. This is where you have both motors connected the same way, L1 = T1, L2 = T2 and L3 = T3. If you leave one motor this way and change the other one to L1 = T2, L2 = T3 and L3 = T1, the rotation is the same but the 'roll' is different. This will often balance the currents better and sometimes worse. 

Odd as it may sound, this can also work on a single motor to more closely balance the 3 currents, especially a 'high-strung' motor, like a refrigeration compressor or a submersible well pump.


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## paulengr (Oct 8, 2017)

On DC motors load sharing is easy. Wire fields in parallel. Wire armatures in series. Armature amps is torque so they naturally balance. Just need external field loss detection. Beyond 2 you are best off using 2 or more drives if for no other reason than maintenance. On Marion 8740 dragline excavators which are almost the largest in service they use 8 roughly 1200 HP motors fanned up to 1600 HP fed from 4 drives with 1 master and 3 slaves using armature (current) feedback on +/-10 V feedback signals.

On AC motors best to use a power transmission with some give and use motors with plenty of slip with starters only or one VFD. On just two motors you can adjust belt tension to balance them. On a common gear box get plenty of slip and use two identical motors (match the RPM exactly). Even then current differences of 25-50% are common. With more motors some adjustment is necessary. I’ve seen a machine recently with 12 identical DC motors from a single common drive adjusted with trim resistors. It’s a nightmare to tune. I’ve also maintained a 10,000 foot conveyor with 2 motors per drive station up to 4 stations on one belt at 1009 HP each. It used wound rotor motors so we could adjust the taps of the last resistor and always had some resistance to increase slip. High slip motors are much easier to tune. Think NEMA design C but at least 3% slip and ideally closer to 5-10% works great. You will have big control problems with an IEC 1% slip high efficiency motor. Get a bunch of old clunkers and rewind them.

Beyond this the best approach which is what paper machines use is called virtual line shafting. You use encoders and VFDs that are digitally linked in one of two ways. Master/slave is the most obvious. But another way that is smoother is to use a “ virtual axis” (simulated servo) that is the master and all drives are slaved to the virtual one. Unlimited motors driving either independently or into a common gear box with this design. Only thing to watch out for is gear lash. A common trick is to start all motors with a very low torque limit so that they run but stall as they pick up the load. This takes the gear lash out but nothing turns. Then switch to master/slave operation and run. Go back to independent when changing directions.

I’ve done this many times. It’s no big deal with experience and some planning. But with micro drives so cheap and since you can set one to generate a current output signal and a second to follow an input signal I don’t see much value in running multiple motors on one VFD anymore. On larger motors maybe but on larger motors if I have multiple VFDs my repair and replacement costs are less and quite often you can run reduced rates and limo along on one while fixing the other.


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