vfd multiple motors long runs.

what is the correct method of driving multiple motors from a vfd with centralized control? example is 460v 3ph to vfd or xl contactor with multiple overloads(for each motor) to motors whose locations are up to 280 feet away from the control cabinet. 1 conduit with all of the leads or multiple conduits for each motor or other. I have 17 units in need of attention as we have smoked about 12 motors and 3 vfd's in our current configuration.

What size are motors?
I assume motor to VFD length varies.
I assume you are sizing VFD for total motor HP

How many motors are you running on same VFD?
I would reduce distance from VFD to motor if possible.
I would also NOT run multiple motor lines in same conduit. One conduit for each motor.

Dan Bentler

thanks Dan
most of the motors are .5hp 1150rpm there are several refrig units. some have 3 motors per unit some have only 2 and some have up to 4. motor failures on all systems.
yes the drives are sized for above total hp for the units they are driving. most of the damage has been cooked motors. but when i have a fan fail there is nothing to keep the fan from running freely. (ice, debris ,ect)
I cannot change the layout without a lot of $.

how do I correct this problem with the least cost.

motor overloads are in the vfd cabinet but were bypassed by the installing contractor. only 3 lines running to each fan unit and distributed to each motor at the end of the line.

The VFD should be able to protect the motor in an overload situation.

Make sure the VFD parameters for motor full load amps is correctly configured.

Make sure that there is a ground wire connected from each motor to its VFD.

The longer the motor lead length, the greater the problem caused by reflected waves. In those situations you may be able to reduce the carrier frequency of the VFD which might help.

It is advisable to use motor wire with 1000v insulation rating with VFDs. Keeping the motors leads separated from each other is also advantageous to avoid cross talk.

Use motors that are rated for use with VFDs. They'll have better insulation and usually better cooling capability. At least for your replacement motors, ensure that they are VFD ready.

Paul

Firstly, If you are running the motors without protection any fault will burn them out. To save on cost I would consider moving the overload (you already have) into a local juction box before the motor. This is no way an ideal situation but will save a fortune in motors.

I have instaled VFD's upto 100m or 330 feet with no issues (400 feet with chokes) although make sure you use screen cables and don't ground to a plastic isolator junction box. As for conduits single conduits are fine as long as proper shield grounding is used.

Hope this helps M.W (www.ac-sys.co.uk)

I assume your running Output Reactor(s)?

tntmachine said:

thanks Dan
most of the motors are .5hp 1150rpm there are several refrig units. some have 3 motors per unit some have only 2 and some have up to 4. motor failures on all systems.
yes the drives are sized for above total hp for the units they are driving. most of the damage has been cooked motors. but when i have a fan fail there is nothing to keep the fan from running freely. (ice, debris ,ect)
I cannot change the layout without a lot of $.

how do I correct this problem with the least cost.

motor overloads are in the vfd cabinet but were bypassed by the installing contractor. only 3 lines running to each fan unit and distributed to each motor at the end of the line.

Click to expand...

OK I assume
this is food processing or cold storage
these are chillers one or two to a room and keep room cold.
As complany expanded they would migrate into a room buy whatever chiller off E bay and stick it in and put the compressor wherever.
So what you have is a mismash of varying size motors types etc etc - nothing all the same.
That was the setup for outfit I used to work for - glad I am gone. PITA

QUESTIONS
You are NOT trying to run single phase fans on 3 phase?
You are running the 3 phase VFD output to room then connecting paralleled motors to the "feeder" by means of a breakout box (junction box).

THOUGHTS
The junction box is where your overloads should be to protect each motor.

I would also add lockable switches 3 pole for lockout purpose on this box.

I would NOT put the junction box in the room - too cold and they always got the damn product in the way.

I dont know why this would not work but am not sure how to deal with line reactors in this case. I am not even sure they are needed since the length from j box to each motor is 50' or so. For the 200 plus runs maybe there.

Hopefully Dick DV will come galloping in here on his white stallion wearing white hat and shoot down questions mistakes with silver bullets.

For sure if you control fan speed proportional to temperature and you keep all running you will save power and should reduce breakdown since you are not running fans so fast. They did this in apple storage here in Wa and saved bunches. You are on the right track just need to get the rails a litle more parallel.

Dan Bentler

tntmachine said:

what is the correct method of driving multiple motors from a vfd with centralized control? example is 460v 3ph to vfd or xl contactor with multiple overloads(for each motor) to motors whose locations are up to 280 feet away from the control cabinet. 1 conduit with all of the leads or multiple conduits for each motor or other. I have 17 units in need of attention as we have smoked about 12 motors and 3 vfd's in our current configuration.

Click to expand...

tntmachine said:

thanks Dan
most of the motors are .5hp 1150rpm there are several refrig units. some have 3 motors per unit some have only 2 and some have up to 4. motor failures on all systems.
yes the drives are sized for above total hp for the units they are driving. most of the damage has been cooked motors. but when i have a fan fail there is nothing to keep the fan from running freely. (ice, debris ,ect)
I cannot change the layout without a lot of $.

how do I correct this problem with the least cost.

motor overloads are in the vfd cabinet but were bypassed by the installing contractor. only 3 lines running to each fan unit and distributed to each motor at the end of the line.

Click to expand...

tntmachine,

I'm sure our resident expert DickDV will come by soon. If you wouldn't mind can you clarify a few things? It will help him and us give you a good answer.

1. You have a centralized control cabinet with VFD's inside correct?

2. You have multiple motors on some or all of the VFD's?

3. On the motors that are ganged together on one drive
3a. They are 1/2 hp units
3B. Some are 280 feet from the VFD, correct?
3c. The control cabinet has individual overloads for each motor but the contractor has bypassed them and only installed one set of "T" leads to the group of motors, correct? Do you have any leverage on the contractor? Details on the overloads would be good also ie. pilot duty, IEC style w/phaseloss protection, ect. ect.
3d. Are you multiple motoring the fan motors only?
3e. What is the input voltage on your VFD's, and are your fan motors 240 / 480 volt units? And are they three phase units (those pesky refrigeration folks sometimes use single phase 480 fan motors)?

4a. In your first post you mentioned an "xl contactor with multiple overloads". Is this your present configuration?
4b. Can the "xl contactor" disconnect the motors from the VFD at any time when the VFD is running (details please)?

Edit: I see Dan has asked more questions, they need answers too.

I forgot -- what are you describing with term X 1 contactor?
Where is it in the VFD to motor wiring?

Dan Bentler

1. You have a centralized control cabinet with VFD's inside correct? yes one vfd per cabnet 1 cabnet per refrigeration coil.

2. You have multiple motors on some or all of the VFD's? yes 2 or more motors per vfd. up to 4 motors on some (4)refrig coils)

3. On the motors that are ganged together on one drive
3a. They are 1/2 hp units yes
3B. Some are 280 feet from the VFD, correct? yes
3c. The control cabinet has individual overloads for each motor but the contractor has bypassed them and only installed one set of "T" leads to the group of motors, correct? unfortunately yes
Do you have any leverage on the contractor? hopefully Details on the overloads would be good also ie. pilot duty, IEC style w/phaseloss protection, ect. ect. overloads were setup from panel manufacturer to 1 provide individual ol protection for each motor and give feedback to the computer as to a fan fail condition. that function has never worked unless the vfd trips on oc.
3d. Are you multiple motoring the fan motors only? vfd and overloads
3e. What is the input voltage on your VFD's, and are your fan motors 240 / 480 volt units?460v 3 ph

4a. In your first post you mentioned an "xl contactor with multiple overloads". Is this your present configuration? yes
4b. Can the "xl contactor" disconnect the motors from the VFD at any time when the VFD is running (details please)? Xl contactor is used to run fans full line voltage. config is hand off auto in either vfd or XL (full voltage) really nice setup if it was setup and working well.

tntmachine said:

Do you have any leverage on the contractor? hopefully Details on the overloads would be good also ie. pilot duty, IEC style w/phaseloss protection, ect. ect. overloads were setup from panel manufacturer to 1 provide individual ol protection for each motor and give feedback to the computer as to a fan fail condition. that function has never worked unless the vfd trips on oc.

Click to expand...

Sounds like the contractor thing needs to get ironed out first. Also sounds like the panel builder did the right things. Nick B asked if there were any output reactors. The reactors might be the only thing (if omitted) that the panel builder might have done wrong.

Output reactors are needed when the wire length between the motor and the VFD become too long. DickDV gives some guidelines in this post.
http://www.plctalk.net/qanda/showpost.php?p=308673&postcount=11

(DickDV stumbles in rubbing eyes and being careful to put one foot in front of the other) Note no white stallion or goofy hat.

Anyway, I have no problem putting all of the motor leads into one conduit as long as the are all fed from the same drive. There can be no crosstalk or other bad interaction since they all have the same pulses at the same time. The individual motors' leads must contain an overload block for thermal protection. Otherwise there is none......the drive can't do it because it thinks there is one big motor out there when, in fact, there are multiple small motors.

The overload blocks should be the type that have a control contact. These control contacts should be normally closed and the blocks should all be wired in a series loop back to the drive. The drive needs to be configured so the overload loop forms a closed fault loop. That way, when any block opens, the drive faults out and the motors are protected. Overload blocks that open the three-phase motor leads when overloaded are not acceptable.

The drives should be sized according to the following rule. Add up the motor FLA's, then add 10% extra for each additional motor beyond the first one. So, for example, if you have 4 motors on one drive and each motor FLA is 1 amp, adding the four up gives you 4 amps. You have three extra motors on the drive so add 10% x 3 to the 4 amp total. That would be 4 x 1.3 = 5.2amps. The drive output amp rating should be no less than that. The purpose for this oversizing is not to protect the drive but to avoid overcurrent faults when starting.

The motor failures are due, no doubt, to the lead lengths and the resulting high spike voltages. You need at least a dv/dt filter at the drive output to strip off the highest frequency components. It's ok and cheapest to put a single filter on the drive output rated for the full output amps and then split off the individual motor leads. I would give the reactor manufacturer the full details of your drive, motors, and lead lengths so they can figure the correct filter. And, for sure, use only motors that have an MG1 Part 31 endorsement.

One additional concern that I have not seen mentioned much has to do with situations where you have more than three motors on a single VFD. The concern is that the drive is such a large source of current that branch circuit fusing ought to be added. I would suggest at least time delay fusing as well as overload blocks in each of the motor leads on those drives with 4 or more motors just to keep from burning up a motor or wiring and not having the drive even see it.

When you have multiple motors on a drive, it is generally considered that the motor leads all add up at least as far as reflected wave and resonance issues are concerned. So, if you have four motors each 280 feet from the drive, that would behave similar to a single motor with 280 x 4 = 1120 feet of lead length. That would be big trouble indeed and probably explains the motor failures. What doesn't help is the small size of the motors. If these were all 100hp, trouble would be much less likely, but, 1/2 hp! Protection is definitely required.

I spotted a white stallion and a goofy hat!!!

Nice post

not sure if this has been mentioned by anyone yet, but make sure you get the OK from the VFD designer/supplier about whatever your final cable configuration is. The problem with multiple parallel cable run is high capacitance to earth and line to line. This can cause the following..

Cooked output stage of the inverter, including current measuring CTs

Nuisance triggering of earth fault circuits

They may recommend oversizing the drive to allow for these higher than normal capacitive current spikes

Paul B said:

I spotted a white stallion and a goofy hat!!!

Nice post

Click to expand...

I could not agree more! I believe this is exactly the information that the OP needs.