DISCONNECT BETWEEN MCC w/VFD AND MOTOR

[kaveenkw123]

Im working on a renovation project for all mechanical equipment in the building. This is my first MCC project.There are three MCCs in the building and all three are federal pacific. I suggested replacement of said MCC given that FS is no longer in business and some of the pumps are being increased from 5HP to 20HP. Client wants to add VFDs to all the new HVAC equipment.
I was looking around and found that some MCCs offer built in VFDs to each bucket. Any concerns regarding using such MCCs?
Additionally, I read in some articles that it is not advised to add a disconnect between VFD and the motor; in this case since the MCC w/VFD will not be in the visinity of the equipment, im guessing I would still need a means of disconnect as per NEC; any thoughts or concerns with adding a disconnect between VFD and motor?

Thankyou!

 

[petersonra]

We use vfds in mcc buckets quite regularly. Very cost effective because ab does most of the work including getting all the Ethernet cabling and switches in place.

If you decide you need to have a disconnect between the motor and VFD it is a real good idea to add an early break aux switch to your disconnect that you can wire into the enable signal to the VFD. Personally, I would look closely at the exceptions to the local disconnect rules and see if your situation applies. Hint - I think one of the fpns says something about using an adjustable speed drive might be a good reason not to have a local disconnect.

I would not get real excited about the mcc manufacturer being out of business. Mccs last a long time if taken care of and usually you can get parts for old ones. You might want to poke around on the internet and see what is available as far as spare parts.

I avoid putting larger vfds in mccs. They just eat up too much space and generate lots of heat but vfd maybe 30 hp and smaller tend to work out pretty well.

Please don't cheap out and leave out the him modules and communications. They make an enormous amount of difference in keeping the vfds running and debugging problems. Make sure the comm links and hims are accessible without having to open a bucket up.

 

[TwoBlocked]

I agree with Bob about the aux in the local disconnect. I think it should be there for ALL motors. Imagine an operator being in a rush to clear a jamb-up and stopping the equipment with the local disconnect. He clears the jamb, and has other operations to perform before starting back up. But somehow the disconnect gets closed before things are ready, which would not be a problem if the equipment wouldn't restart without hitting the start button. But if it could, might have paint spraying when it shouldn't, or something.

 

[petersonra]

I agree with Bob about the aux in the local disconnect. I think it should be there for ALL motors. Imagine an operator being in a rush to clear a jamb-up and stopping the equipment with the local disconnect. He clears the jamb, and has other operations to perform before starting back up. But somehow the disconnect gets closed before things are ready, which would not be a problem if the equipment wouldn't restart without hitting the start button. But if it could, might have paint spraying when it shouldn't, or something.

The operator should not be using the disconnect as an estop. Personally I would avoid having local disconnects. If you need some way to stop the drive best answer is a stop pb.

 

[kwired]

The reason not a good idea to have switches between the drive and the motor unless using an aux contact on the controls like Bob mentioned is that the drives don't particularly like having a load switched in or out of circuit when drive output is active. Worst case might be closing the switch while drive is at a rather high output rate and essentially drawing or attempting to draw high current just like across the line starting of the motor would draw. That kind of current may very well damage drive components, as they are intended to ramp the motor up to speed and never would put out that kind of current in normal operation or startup. Worse yet would probably be closing the switch into a fault on active drive output. If starting a faulted motor with the drive in normal operation it is only putting out a few volts at a few hertz initially so fault current would be low and not damaging to the drive yet it detects it is higher than it should be and goes into a controlled fault shut down process.

 

[kwired]

The operator should not be using the disconnect as an estop. Personally I would avoid having local disconnects. If you need some way to stop the drive best answer is a stop pb.

I agree even with early close aux contact on controls, you cut power to motor before it is done decelerating in many applications. Maybe ok if it is a setup that is programmed to coast to a stop instead of decelerating.

 

[Jraef]

I’ll chime in on the disconnect issue. It’s fine so long as you add that aux contact in the disconnect that gets wired back to the VFD to disable the output before the main contacts open. On all knife switch type disconnects, that is always the case. On rotary type, you have to check.

VFDs in MCCs is fine up to a point. Personally I like being able to pull out and swap an entire bucket with a spare, keeps down time to a minimum. But once it becomes fixed mounted, which for most is above 60HP, it’s kind of a toss up.

As to FPE MCC, I wouldn’t add anything to it. With no data, you can’t verify the SCCR of anything you do now. They went out of business before that was a thing.

 

[TwoBlocked]

The operator should not be using the disconnect as an estop. Personally I would avoid having local disconnects. If you need some way to stop the drive best answer is a stop pb.

I agree, they shouldn't, but they do. And consider the alternative to local disconnects: locking out at the MCC. Much larger chance of locking out the wrong thing along with the temptation of not bothering to lock out at all. If you want operators to do the right thing, make it easy for them to do so.

The best setup I have seen is "line stops," local disconnects, and ZES (Zero Energy State) buttons. Line stops go to the PLC, local disconnects have aux switches to the PLC, and ZES buttons are hardwired to the starter or VFD, and also to the PLC.

 

[petersonra]

The best setup I have seen is "line stops," local disconnects, and ZES (Zero Energy State) buttons. Line stops go to the PLC, local disconnects have aux switches to the PLC, and ZES buttons are hardwired to the starter or VFD, and also to the PLC.

Too complicated. Operators should probably not be using a disconnect anyway. It is there to allow motor servicing. What operator services motors?

I would want to look at the whole of an individual situation before deciding what might be most appropriate but the more choices you give people the more likely they are in a bad situation to made a bad choice.

 

[kaveenkw123]

Thankyou everyone for your thoughts! Much Appreciated

 

[kaveenkw123]

We use vfds in mcc buckets quite regularly. Very cost effective because ab does most of the work including getting all the Ethernet cabling and switches in place.

If you decide you need to have a disconnect between the motor and VFD it is a real good idea to add an early break aux switch to your disconnect that you can wire into the enable signal to the VFD. Personally, I would look closely at the exceptions to the local disconnect rules and see if your situation applies. Hint - I think one of the fpns says something about using an adjustable speed drive might be a good reason not to have a local disconnect.

I would not get real excited about the mcc manufacturer being out of business. Mccs last a long time if taken care of and usually you can get parts for old ones. You might want to poke around on the internet and see what is available as far as spare parts.

I avoid putting larger vfds in mccs. They just eat up too much space and generate lots of heat but vfd maybe 30 hp and smaller tend to work out pretty well.

Please don't cheap out and leave out the him modules and communications. They make an enormous amount of difference in keeping the vfds running and debugging problems. Make sure the comm links and hims are accessible without having to open a bucket up.

I do have a few 40hp motors. So in that case, would I install the VFD between the MCC and the motor? Could there be any conflicts by pairing both a MCC and a VFD to a motor?

 

[TwoBlocked]

Too complicated. Operators should probably not be using a disconnect anyway. It is there to allow motor servicing. What operator services motors?

I would want to look at the whole of an individual situation before deciding what might be most appropriate but the more choices you give people the more likely they are in a bad situation to made a bad choice.

A local disconnect is mostly for operators to do maintenance. For an electrician, it is an additional fail point. Better not to have it at all and just lock out at the MCC.

 

[garbo]

Im working on a renovation project for all mechanical equipment in the building. This is my first MCC project.There are three MCCs in the building and all three are federal pacific. I suggested replacement of said MCC given that FS is no longer in business and some of the pumps are being increased from 5HP to 20HP. Client wants to add VFDs to all the new HVAC equipment.
I was looking around and found that some MCCs offer built in VFDs to each bucket. Any concerns regarding using such MCCs?
Additionally, I read in some articles that it is not advised to add a disconnect between VFD and the motor; in this case since the MCC w/VFD will not be in the visinity of the equipment, im guessing I would still need a means of disconnect as per NEC; any thoughts or concerns with adding a disconnect between VFD and motor?

Thankyou!

Place I retired from had over 500 VFD'S. None were in buckets. We had to PM everyone from 1 to 4 times a year depending how critical the load were. Just about every drive was within 50' of motor. This made troubleshooting easier. They had four 75HP cooling tower fans located in third level basement and over 80 steps to walk up so contractor who cleaned the towers and performed mechanical repairs used the outdoor safety switches to turn running fans on & off. These switches did not have an early break NC switch on disconnect handle so twice he blew out a drive most likely when drive was running and motor at dead stop during restart. I P touched labels on all four switches telling him that he must go down to the basement to start & stop these fans. On maybe 16 40 to 75 HP lab exhaust fans they installed the drives on the roof in drive cabinets that besides having a fan on cabinet had a heater to heard reduce moisture in cold weather. While MCC could last 50 years seems like most drive manufacturers come out with new models every 8 to 10 years. We had several 50 to 125 HP drives that were only 8 years old and could no longer secure new boards or replacement parts ( except for LCD = local control panel and the 220 volt) cooling fans so we had to replace them. I rather have drives out in the open and very close to the motor. Company was smart enough when they had drives installed on 20 year old boilers they installed them in mechanical room that never got over 80 degrees. I kinda of remember that drives are approximately 97% efficient so even a 50 HP VFD might produce 1,700 BTU'S of heat.

 

[petersonra]

A local disconnect is mostly for operators to do maintenance. For an electrician, it is an additional fail point. Better not to have it at all and just lock out at the MCC.

When is an operator typically allowed to do the type of maintenance that would require locking out a single motor?

 

[petersonra]

I do have a few 40hp motors. So in that case, would I install the VFD between the MCC and the motor? Could there be any conflicts by pairing both a MCC and a VFD to a motor?

You have to look at your individual situation but mostly I would put a couple feeder breakers in the mcc and put the larger drives outside the mcc.

It is not an absolute thing though.

 

[TwoBlocked]

When is an operator typically allowed to do the type of maintenance that would require locking out a single motor?

On a production line cleaning a conveyor, or replacing a pump filter. And why just a single motor. On a down day a whole series of motors are locked out, keys put into a single lockout box, and operators put their locks on the box. Yes, this can and is done on breakers in an MCC, but then the operators need to remember just which motors are being locked out, rather than just look at the local disconnect to make sure.

Bob, this is a very common practice.

 

[TwoBlocked]

I do have a few 40hp motors. So in that case, would I install the VFD between the MCC and the motor? Could there be any conflicts by pairing both a MCC and a VFD to a motor?

I have seen this done for critical pumps that must run even if the VFD dies. An across the line starter is installed parallel to the VFD with addition contactors so full voltage is NEVER applied to the output of the VFD. Doing so can destroy the VFD.

 

[TwoBlocked]

I've seen all varieties of VFD installations including ones I've designed and installed. First consideration is heat dissipation. Sticking a bunch of small VFDs in an existing MCC in an area without a/c can cause them to trip out on heat. Then there is the power feed. If the VFD is within 100 ft of the motor, output filtering is usually not required. The general location of the VFD needs to be fairly clean, dry, and dust free. But if the situation is where the operator controls the equipment directly with the VFD, models that are NEMA 4 (washdown) might be available.

Heat, location, and environment are the biggest three that come to mind. Getting with your VFD supplier is a very GOOD idea.

 

[petersonra]

On a production line cleaning a conveyor, or replacing a pump filter. And why just a single motor. On a down day a whole series of motors are locked out, keys put into a single lockout box, and operators put their locks on the box. Yes, this can and is done on breakers in an MCC, but then the operators need to remember just which motors are being locked out, rather than just look at the local disconnect to make sure.

Bob, this is a very common practice.

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I don't recall being in plants where operators perform these kind of tasks.

 

[TwoBlocked]

I don't recall being in plants where operators perform these kind of tasks.

Then they have no need for lockouts ?????