# Two motors on one drive



## GrislyZero (Jan 17, 2011)

Are there any side effects to putting two motors on one drive? I think its weird but that is what is spec'd. It isn't my problem but "I" want to know.


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## BBQ (Nov 16, 2010)

Code wise it is fine as lone as there are separate OL devices on each motor.

I can't really answer about the drives themselves.


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## Netree (Sep 3, 2011)

Why should you want to do such thing?


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## Wireman191 (Aug 28, 2011)

We have done it with fans for cooling, a bank of three.
A filter and reactor were needed for this set up. I tried to hook two motors up without the filter and reactor and it kept blowing the OL on one of the motors. It ended up smoking one of them, don't know if it was bad to begin with or if I messed it up.


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## Jlarson (Jun 28, 2009)

It's fairly common in some applications. Large, multiple fan installations for one. Like BBQ said as long as you have the proper OL for each one it's all good.


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## erics37 (May 7, 2009)

Jlarson said:


> It's fairly common in some applications. Large, multiple fan installations for one. Like BBQ said as long as you have the proper OL for each one it's all good.


I've heard of that but haven't seen it in person.

Do you just get any old sort of separate overload block, or do the drive manufacturers make expansion things tailored to their products that you can use?


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## BBQ (Nov 16, 2010)

erics37 said:


> I've heard of that but haven't seen it in person.
> 
> Do you just get any old sort of separate overload block, or do the drive manufacturers make expansion things tailored to their products that you can use?


You can use any OLs and just wire them in series with the VFD stop or estop input etc.


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## Jlarson (Jun 28, 2009)

Or you use a manual MS and it just trips and you lose only the offending motor.


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## BBQ (Nov 16, 2010)

Jlarson said:


> Or you use a manual MS and it just trips and you lose only the offending motor.


I have always been told that would be bad for the VFD.


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## Jlarson (Jun 28, 2009)

It's never been a problem for me. You've got other motors going and hopefully a proper load reactor too. Otherwise it be the same as opening a load side disco on a single motor install.


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## gesparky221 (Nov 30, 2007)

In our lamp coating machines we run as many as 10 motors on one drive. All the motors have to run at the same speed, and start and stop together. The ac drive/ac motor has worked well on this application for over 15 years. This is in a high heat area and since the ac motors are fairly inexpensive versus servos it works well for us. . The drive must be sized large enough to operate all the motors and we have an OL block for each motor. Reactors and filters are a must.


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## McClary’s Electrical (Feb 21, 2009)

I'l post pics when I get home of two drives we installed with 32 motors on each drive.


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## John Valdes (May 17, 2007)

BBQ said:


> You can use any OLs and just wire them in series with the VFD stop or estop input etc.


Only if you want to stop all the motors when one trips out.



BBQ said:


> I have always been told that would be bad for the VFD.


Theoretically you are correct. It is not a good idea to have any type of switching on the load side of the control. But sometimes the application makes this impossible. The manual motor starter with OL could be an example.
There are parameter adjustments that can be addressed for multiple motor applications.


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## JRaef (Mar 23, 2009)

BBQ said:


> I have always been told that would be bad for the VFD.


It's bad for one switching device on a VFD with one motor, because the opening of the device creates a voltage transient that has nowhere to go but the transistors in the VFD. If you have multiple motors running off of one drive, the transient is partially attenuated by the other motor(s), so it's not as bad. Done all the time. I like using the IEC style MMS units because they provide both the thermal and short circuit protection for the motors. Technically, the VFD becomes the SCPD for the motor (the upstream device is isolated from the motor now), but the settings are typically not adjustable and if you have a lot of small motors, may not qualify because the trip point is too high. Adding the separate IEC MMS units adds back in the individual SC protection for each motor.

What is bad regqardless of the number of motors however is CLOSING a switching device on the load side of a running VFD, because the motor inrush and starting current can damage the transistors. So the rule, if you use MMS units downstream, is to never reset them without shutting down the VFD first. That is difficult to enforce, but it can be done by providing no thru-door operating mechanism for the MMS, forcing you to open the cabinet to reset it, in which case (if you have done it right), you have to turn off the disconnect for the VFD.

Side note, I ALWAYS add a load reactor to the VFD output ahead of the motor OL devices, motor shorts are bad for VFDs and when you have multiple motors, you increase the chances of that happening. Reactors are cheap insurance.


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## BBQ (Nov 16, 2010)

JRaef said:


> It's bad for one switching device on a VFD with one motor, because the opening of the device creates a voltage transient that has nowhere to go but the transistors in the VFD. If you have multiple motors running off of one drive, the transient is partially attenuated by the other motor(s), so it's not as bad. Done all the time.


Seems like you are counting on only one fault and it being repaired quickly before another.


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## BBQ (Nov 16, 2010)

John Valdes said:


> Only if you want to stop all the motors when one trips out.


The OLs I had in mind do not open the motor conductors they only open a control circuit. 

Or were you going to install complete individual motor starters for each motor? 





> Theoretically you are correct. It is not a good idea to have any type of switching on the load side of the control. But sometimes the application makes this impossible.


Auxiliary switches to dump the VFD. We have a had a quite a few projects that required aux contacts on manual diaconecting means to dump VFDs.


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## mattsilkwood (Sep 21, 2008)

BBQ said:


> The OLs I had in mind do not open the motor conductors they only open a control circuit.


Right. So if you have any one trips out, it breaks the circuit and tells the drive to stop all the motors. 

A better way to do it would be to use individual breaker style O/Ls. This will drop out an individual motor and you can take an input back to the plc to tell you you have a problem.


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## lefleuron (May 22, 2010)

This is done all the time.

Your problems come in because of how the drive has to be run.

Generally there are three modes to run a drive in. Volts/Hertz, Sensorless Vector, and Flux Vector.

These are in order of the drive to closely control the motor, V/H being the least accurate or the lowest ability to keep to a set point.



And by using more then one motor on a drive leaves you with one choise, and that Volts/Hertz. This is simply a straight ramping of the drive.

for instance, you have 480 volts AC running a motor with 1600 max RPMs. So at 240 volts you get 800 RPMs. Or at 120 volts you get 400 RPMs, etc.

Now not only do you not have great control of speed or torque, but as was stated already you dont have singular control. If one motor has a problem, they are all down.

I tried to explain this to our then engineer 10 years ago, and it did not sink in. His idea was to run a machine with multiple motors off of one drive, almost like an electronic line shaft. These motors all had to run to an almost exact speed, not just close, but super close.

Because each motor will act slightly differently, and not even have an exact same RPM at the same Hertz, this cannot be done.

This would be fine for non-critical applications, like fans or pumps. But if you need better speed/torque control you will need seperate motors each on its own drive, each running in Sensorless Vector or even Flux Vector mode. Both of these require feedback such as an encoder, and require both more money and also more knowledge to set them up.


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## JRaef (Mar 23, 2009)

BBQ said:


> Seems like you are counting on only one fault and it being repaired quickly before another.


Yep. But as was pointed out later, the only legit application for this is pumps and fans, so usually if one trips, it was either a problem in the motor or pump, or a problem in the drive. If it was already a problem in the drive, both units tripping is now irrelevant, if it was a pump or motor problem, it's not likely to cause another unit to trip. So yes, it can happen. But not likely.


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## John Valdes (May 17, 2007)

BBQ said:


> The OLs I had in mind do not open the motor conductors they only open a control circuit.
> Or were you going to install complete individual motor starters for each motor?
> 
> 
> ...


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