# VFD on 1 of 2 interlocked motors?



## Hatrabbit (Nov 10, 2009)

I have 2 motors, high and low speeds in a cooling tower, which are controlled by mechanically interlocked contactors so only one can run at a time. There's a short delay between one motor dropping out and the other coming on. What I'd like to do is keep this configuration but have the low speed fan on a VFD. I'm a little stuck on how to do this electrically, though. If the supply side of the VFD comes from the low speed fan contactor, I'll be cutting power to the drive every time that motor shuts off and rebooting it every time it starts... seems wrong. But if I use an early break aux contact to shut off that motor, this doesn't provide the positive safety that comes with the mechanical interlock.
I'm sure someone smart has done this before... does anyone here know the method that I'm missing?


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## I&ETech (Mar 30, 2016)

I think you could wire the run command through a mechanically interlocked contact to control the VFD. The VFD could remain energized and waiting for the run command. You could also have a hardwired enable wired through the mechanical interlock on a seperate circuit from the run command. The enable circuits is usually your e-stop and possibly a vibration switch since it's for a cooling tower fan.


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## John Valdes (May 17, 2007)

Avoid using a contactor to feed the drive.
Use the control logic built into the drive to electrically isolate each motor.
You do not want to use the main power to stop and start any drive.

Get out the manual on the drive and see the control board connections and what they do. It is here you find a way to accomplish your goal.
Draw it out, then implement it.


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## I&ETech (Mar 30, 2016)

I agree...do not start and stop the drive with the main power.


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## mitch65 (Mar 26, 2015)

Put the interlocked contactor downstream of the drive. Then there is power to the drive all the time. They have a setup like this for water pumps at the town water treatment. My question would be why you wouldn't put a drive on the high speed motor and eliminate the other one altogether or keep it as a backup in case of failure?


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## JRaef (Mar 23, 2009)

mitch65 said:


> Put the interlocked contactor downstream of the drive. Then there is power to the drive all the time. They have a setup like this for water pumps at the town water treatment. My question would be why you wouldn't put a drive on the high speed motor and eliminate the other one altogether or keep it as a backup in case of failure?


I agree. Also, I take it there is one fan with two motors belted to the same shaft? Complete waste and sets up a dangerous possibility of the motor regenerating into the VFD when run on the high speed and the VFD is still connected to the low speed motor. Any residual magnetism in that low speed motor could cause it to put voltage on the VFD output terminals and might damage it. There would be no need for the low speed motor any more if you have the VFD, that's what they are for.

But if you do insist on going ahead with this, put the contactor down stream of the VFD, make sure you use an aux contact of the contactor to disable the VFD before the main contacts open and enable the VFD only after the main contacts are closed. That often means using a timer, because not all contactor mfrs make "Early-*Break */ Late-*Make*" aux contacts (and do NOT confuse them with EM/LB aux contacts, which virtually everyone offers, that is the exact OPPOSITE of what you need).

And yes, do NOT use the contactor ahead of the VFD, it will kill it eventually to be used that way.


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## Hatrabbit (Nov 10, 2009)

Your point about backfeeding the drive tears it... I'm going to find another way. There was no way I was going to kill the line side of the drive, but even with the aux relay for shutting off the small motor I was uncomfortable with the lack of positive safety (i.e. someone puts the drive in bypass and the EMS calls for hi speed... bzzt. I can't put the drive on the big motor and abandon the small one because the size discrepancy is too great - the big fan ramped all the way down to a minimum Hz would still move more air than the small motor.


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## JRaef (Mar 23, 2009)

Hatrabbit said:


> Your point about backfeeding the drive tears it... I'm going to find another way. There was no way I was going to kill the line side of the drive, but even with the aux relay for shutting off the small motor I was uncomfortable with the lack of positive safety (i.e. someone puts the drive in bypass and the EMS calls for hi speed... bzzt. I can't put the drive on the big motor and abandon the small one because the size discrepancy is too great - the big fan ramped all the way down to a minimum Hz would still move more air than the small motor.


So there are two fans then, not two motors connected to one fan? Odd setup...


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## mitch65 (Mar 26, 2015)

Add pitch controlled fan blades. No need for any speed control then...... and lots of future maintenance for the millwrongs and or instrument mechanics.


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## Nom Deplume (Jul 21, 2013)

Feed the VFD from the contactor and program the drive to start on power up.
I have done many drives this way for retrofits on HVAC units.

If you install the contactor on the load side of the VFD you will need an early break set of aux contacts to shut down the drive before the motor looses power or you can damage the drive. You are just looking for trouble using this method.

If you start on power up, then there is no control wiring changes that need to be made other than the speed control, unless you are going with a preset speed.


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## JRaef (Mar 23, 2009)

Nom Deplume said:


> Feed the VFD from the contactor and program the drive to start on power up.
> I have done many drives this way for retrofits on HVAC units.
> 
> If you install the contactor on the load side of the VFD you will need an early break set of aux contacts to shut down the drive before the motor looses power or you can damage the drive. You are just looking for trouble using this method.
> ...


Every VFD has what's called a "pre-charge" circuit that prevents the capacitors and diodes from being damaged by the charging current of the caps when power is first applied. that circuit usually involves a small current limiting resistor that is in the circuit for only a second or two when power first comes on, then it is switched out. Putting a contactor ahead of the VFD stresses those components and hastens their demise. Almost all VFD mfrs will tell you NOT to use a contactor ahead of the VFD for this reason, in fact some will void their warranty if they find out that's what you are using.


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## micromind (Aug 11, 2007)

If there are 2 fans, I'd be a bit hesitant to leave one of them connected to a VFD. When one is running, the other one is very likely spinning in reverse direction. Any residual magnetism will cause the motor to become a generator and most likely wreck the VFD.


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## Nom Deplume (Jul 21, 2013)

JRaef said:


> Every VFD has what's called a "pre-charge" circuit that prevents the capacitors and diodes from being damaged by the charging current of the caps when power is first applied. that circuit usually involves a small current limiting resistor that is in the circuit for only a second or two when power first comes on, then it is switched out. Putting a contactor ahead of the VFD stresses those components and hastens their demise. Almost all VFD mfrs will tell you NOT to use a contactor ahead of the VFD for this reason, in fact some will void their warranty if they find out that's what you are using.


I have never had a problem with the dozens of AB Powerflex 400 drives that were installed that were to start on power up.
The AB manual also has instructions for this procedure.


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## Peewee0413 (Oct 18, 2012)

Vfd constant power.. Load side on slow speed contactor. Use aux. To give start command. Not sure about switching back to high..but you could use a relay on the drive to fire the high speed coil. I know AB has a relay option for running/not running. I didn't read all the posts ...I'm sure I missed something..


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## glen1971 (Oct 10, 2012)

Nom Deplume said:


> I have never had a problem with the dozens of AB Powerflex 400 drives that were installed that were to start on power up.
> The AB manual also has instructions for this procedure.


Are you refering to Parameter: A094 for your set-up? 

Pretty much every Powerflex 700 I deal with is set to restart on a power-up, if the PLC is telling them to run. The OP is asking about cycling the power to the drive for process control to shut the fan off and to restart it. This is not a desirable method of control as JRaef mentioned before, causing stress upon the components in the drive.


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## JRaef (Mar 23, 2009)

PF 400s and 7 series above 10HP and below 400HP have an SCR front end that ramps into the DC Bus instead of a resistor precharge. That overcomes the risk, but is not usual for all drives do I don't recommend it generically.


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## Tsmil (Jul 17, 2011)

JRaef said:


> PF 400s and 7 series above 10HP and below 400HP have an SCR front end that ramps into the DC Bus instead of a resistor precharge. That overcomes the risk, but is not usual for all drives do I don't recommend it generically.


If you have any discrepancies between the manual and JR, toss the manual.


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## Peewee0413 (Oct 18, 2012)

Use 1 motor.. 1 fan.. 1 VFD.


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## Nom Deplume (Jul 21, 2013)

Peewee0413 said:


> Use 1 motor.. 1 fan.. 1 VFD.


That is the best solution. No point of two motors if you have a VFD. :thumbup:


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