Multi motor VFD drive applications

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vincecharles

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Bonner Springs, KS USA
- I would appreciate your help with a energy reduction program being implemented by Wal-Mart Stores Inc. Proto Design Engineering Department and the Technical Service Departments. Vendors, suppliers, manufactures and third party consultants are providing Wal-Mart information from their prospective and are conflicting. I'm looking for a non-biased opinion on what would be the correct business decision.

Subject: Retrofitting existing refrigeration condensing units with VFD drives to regulate air flow. Ambient operations control, Delta T control across condenser, compressor discharge pressure control, and most important energy savings for the entire refrigeration system. A single VFD drive controls the RPM for anywhere from 4 to 10 condenser fan motors.

Existing motors are installed on a refrigeration medium and low temperature application air cooled condensers. The condensers have anywhere from 4 to 10 motors being staged using individual contactors for each motor. The condenser fan motor contactors are being controlled by a Energy Management System.

Presently we are installing the VFD drives on condensers equipped with 3 phase 460 volt 575, 850, 1140 RPM non-rated VFD motors equipped with internal thermal overloads. NOTE: the VFD drives are equipped with a LC filter to eliminate any spike voltages that could be developed in the feeder to the motors. The existing wiring is single conductor #14 THWN str.wire installed in a C channel bundled together. The wiring varies in length from 8ft - 35ft depending on the motor location within the condensing unit deck plate. If any existing motors fail we are replacing them with VFD rated condenser fan motors equipped with shielded cabling, externally wired thermal protection interconnected with a starter assembly. The external wired motor thermal overload and the started current sensing overload are wired in series with the starter coil control circuit. The starter assembly current overload is set for manual reset.

QUESTION: Replacing existing 3 phase 460 volt 575, 850, 1140 RPM non-rated VFD motors equipped with internal thermal overloads refrigeration condenser motors with equivalent specification high efficiency VFD rated motors with externally wired thermal overloads when the VFD drives are installed the correct decision?

1/2 HP, 575 RPM, CONDENSER MOTOR, W/ BASE
1 HP, 850 RPM, CONDENSER MOTOR, W/ BASE
3/4 - 1 HP, 850 RPM, CONDENSER MOTOR, W/O BASE
1-1/2 HP, 850 RPM, CONDENSER MOTOR, W/ BASE
1-1/2 HP, 850 RPM, CONDENSER MOTOR, W/O BASE
1-1/2 HP, 1140 RPM, CONDENSER MOTOR, W/ BASE
1-1/2 HP, 1140 RPM, CONDENSER MOTOR, W/O BASE

Here are my thoughts

Multi-motor applications on a single drive typically requires motor thermal overloads to be externally wired. Typically, a internal motor thermal overload doesn't always disconnect all three legs simultaneously. VFD drives need a balanced load to work properly.
The LC filter wouldn't be needed if we used VFD rated electric motors feed with shielded cable.
VFD rated motors are better equipped to handle the low air flow conditions associated with VFD applications
Wal-Mart is cost driven and a return on investment must exist to drive this decision (Safety and Reliability are a given)
The replacement of sunlight/exposed THWN with rated cable will reduce maintenance

Other questions

If we eliminate the LC filter on the drive then the shielded cable will need to be rated for what voltage? Presently the cable we are using is rated for 600 volts. The drain leg on the shielded cable is wired to the condenser frame and in connected with the equipment ground terminal lug.

Thank you,

vincecharles
 
Assuming you already know that the application is valid (I would have my doubts), here are my thoughts on the installation.

  • I'm not a fan (no pun intended) of "internal thermal overloads" even if it were not a VFD application. They are not accurate as a rule and they may or may not open all 3 legs. An external bi metal OL relay is best for this task.
  • You not only need individual overload protection for each motor, you also need short circuit protection. For that reason, I would use IEC style Motor Starter Protectors (MSP) which combine a disconnect, bi metal overload and short circuit protection in one unit. That way you also have a way to turn off and lock out an individual motor for service without shutting down the entire system.
  • I think your wiring issues leave a little to be desired, but I can't tell by your description exactly what you have. You need to make sure you run separate conduits for each motor, if you parallel them you will get induced voltages and probable motor and drive damage. Your conductors must be in metallic conduit as well, PVC will allow radiated interference. Shielded "VFD hookup cable" would be a better choice and less expensive in the long run.
 
What you are describing as condenser motor, is that driving the compressor or is it the cooling fan. If this were the compressors, i think it is too big of can of worms to be worthwhile. The loads are cyclical and would be inconsistant for the VFD to handle. The fans, no problem.

you WOULD need to use external IEC overlaods for each of the motors.

Looking again, I am unsure how the different pole motors would react to being on the same drive. Every multi-motor application I have designed and started-up, all of the motors were the same syncronous speed.

What else have you heard from some of the VFD reps/manufacturers?
 
I am curious are we talking cooler condensers? Freezer condensers? the reason I am curious is because the cycle times are different. I agree with Jraef about the extenal IEC msp. the external wiring is a diferent story. order the new units with the wiring built in. Ten fan units can be costly to repair if you get to much junk on them. Then all savings are negated. You also need to be aware of the increased level of frost which could cause a moisture problem to your products. Freezing sections of your condenser up will also cause you to do whole unit defrosts which could cause you unnecessary labor costs due to unscheduled unit down time. I guess the costs of this in states with higher average outdoor temps and high open door activity could be a savings. Yet I shall never know..:smile:
 
As far as using shielded cable, we just had a response to an RFI that said shielded cable for motor leads on small drives can create capacitive coupling that the drive has to supply power and that they do not recommend the use of such cable. This response came from a major drive manufacturer. I have never heard that before and many drive manuals recommend shield cables.
Don
 
1./ Are all the condensers are on the same header?
2./ Otherwise you can not control them with a single drive.

If 1./ is correct then use only one drive and drive only one motor with it. Switch the other motors ON-OFF as demand varies to provide step change and use the ASD driven motor to provide variable control within the steps. Some ASD manufacturers have this as a built in macro program and provide relay out put to control the constant speed motors.

If the objective is energy efficieny then get rid of all the multispeed motors as they are NOTORIOUSLY inefficent and use single speed high efficiency ASD rated motors, IEEE-841 or similar.

Do not drive dissimilar motors off of one drive, neither in size nor in base speed.

As I said in another post NO ASD application should be done on the back of a paper napkin. Consult an engineer who is versed in ASD applications.
 
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