1 vfd 5 motors needs same line distance?

I have an application that uses 5 motors off the same VFD. We have been experiencing a lot of problems with blown vfd's. One electrical engineer informed us that the distance to each motor from the VFD must be the same? According to him this is due to EMF effects? Is this true or is he merely guessing?

Applying communications theory, the lines should be approximately the same length to reduce the harmonics and reflected power. In real life however, it depends a lot on how long your lines are, how much different the longest lead is compared to the shortest lead and a few other things as well.

Reflected waves will eventually do a drive in, same as with radio equipment. The drive usually doesn't just go poof, but degrades over time to the point of finally failing.

I'm sure DickDV can expound on this a lot farther than I can. I am but a grasshopper in the drive world.

well as far as overall distance the seperation is 50 feet from the nearest motor to the farthest motor on a 3 phase 3 wire 460 volt configuration.

Ok, what is the total distance to the nearest motor?

Ok, what is the total distance to the farthest motor?

Do you have line reactors? If so, where are they located, on each motor, one on output for all motors, or on input only?

How are the drives failing? Is it on the input, the link, or the output? If in the inverter section, then the motor line lengths may be the cause, as your EE suggests. If it's on the input rectifier, then I'd suggest input reactance to quell the problem. If the failure is on the link, then input reactance, again, may settle things down.

You probably already have an isolation transformer of some sort; this is input reactance. If you don't, then common mode voltages could be to blame for the drive failures. Where is your system grounded, if at all? Common mode voltages can account for approximately V L-N worth of increased insulation stresses. If your system is ungrounded and the motor frame is grounded, then you'll often see insulation failures in the motor or the input line reactors when common mode is the cause. Less often, common mode will kill the drive itself.

In addition, in rush currents can kill diodes over time, as well as the input fuses. Additional reactance will mitigate this effect.

How long is your longest line? Fifty foot delta on a 1000 foot (to 1050 foot) line is much less severe than the same on a 10 foot (to sixty foot) line. I'm assuming the line lengths to each motor are the same, for that motor. Then again, 1000 foot lines will cause increased VSW on the motor, which can be reflected back to the drive. It's really a matter of "weakest link" to see which will fail first in that case.

You're probably going to need to add some reactance or more precise filtering to the system, when the actual cause of the failures is determined. Try contacting the drive manufacturer and see what they say.

Multi-motors on one VFD?

Okay, what if the drive cables to all these motors were the same length? IE. extra cable lengths are coiled up in the drive cabinet.

Would that work? I've never seen such a thing.

Rod (The CNC dude)

The cable distance doesn't have to be the same, but you should make certain of a few things.

One, it would be an extremely good idea to put individual output reactors between the drive and each motor, or at least a sine-output filter at the drive.

Another, is keep in mind, that you do NOT want to individually start and stop motors (as with a contactor or disconnect) while the drive is running.

Regarding the cables, you might want to invest in some actual VFD Power cables for the motors, instead of individual wires, if possible.

You didn't describe your application, but if all the motors are hard-coupled to a common element (conveyor belt, for example), you can run into problems using standard induction motors. In cases like that, you might need to look at using synchronous motors (PM Type). Otherwise, there is a good chance that currents can go through the roof.

You also need individual overload protection for each motor. We generally have any overload trip shut down the drive, but allow the individual motor OL's to by bypassed by a contact off of a physical disconnect for the individual motor.

EDIT - Should have mentioned, while the above are guidelines, it may be best for you to directly contact the support department of your drive manufacturer, and see if they have any usage restrictions on your application.

The line distance to the nearest motor is 150 feet to the furthest motor 200 feet. They were wired with no reactors on each motor. The suggestion of VFD power cable is one to consider. The application is 5 overhead fan units used for cooling. Each one is 5 hp the vfd is 30 hp. Each one has its own thermal contact. The electricians have primarily been working on this problem so I'm still gathering a lot of the information

i don't want to appear critical, but you should really consider just using individual drives for this application. If they need to all run at the same speed (frequency), you can use a plc with analog inputs and outputs to provide the reference signals for all. It would be better to shorten the distance from motor to invertor as much as practical and use line reactors as others have indicated.

Most drive manufacturer suggest/require a "constant amount" of inductance on the VFD output when connecting to multiple motors. When in V/F mode the drive acts like a current source so it just wants to see a maximum load with specific impedance range, it doesn't care what the load is.

This type of muliple motors on one drive is very common in ventilating systems especially for wood drying kilns. All of the motor/drive problems one of my customer has seen, (5) 15HP motors 100HP drive, have been resolved by the additon of an output dv/dt filter or line reactor (sized to the drive output) as close to the motors as possible. What they did was to mount the filter at the kiln and then tap off to each motor.

New information is that they are currently getting a quote on the various options. I've given them my suggestions and they are considering them. Thanks for the info its been a great help

In my experience, I have never seen any requirement that the motor leads be the same length on multiple motor systems. In fact, I have never seen any requirement that the motors even be the same.

Good thing too because I have not followed these rules and have not had problems. In fact, I have one system with one motor ten times larger than the smallest one. Another system has 47 motors on one drive.

In my view, we really cannot deal with the original poster until we have information on how the drive failed. Any failure other than output transistors would certainly not be related to multiple motors. Failed output transistors would be questionable but a long-shot possibility.

Reflected waves rarely damage drive outputs. The damage is primarily to motor insulation and the reason for that should be fairly clear. The drive end is the controlled end while, with long leads, the motor end or ends are, at high frequencies, basically uncontrolled. This lack of control or actually high impedance permits the reflected waves the freedom to form and pile up on top of the drive pulses generating the excessive voltages that cause motor insulation damage.

From a multiple motor standpoint, I have always added up all the motor lead lengths and treated them the same as one long lead. That usually means adding one common reactor at the drive output. If there are more than three motors on the system, in addition to motor overload blocks for each motor, you really should add a fused disconnect for each motor, especially the fusing.

Let's hear how the drive failed. That would be the primary piece of data.

Good points DickDV. Like I said, I am but a grasshopper. I learn more from your posts than from just about any other drive guy I have known.

I hadn't really thought about the additive effect of the wiring, but it does make sense.

Dickdv according to all the writeups I've read on multiple motor operations you are correct on size of motors not needing to be the same. Also none of those write ups even mention any concern for line distance. This is part of the reason why I asked the question here.

DickDV,

You obviously have a lot of experience and knowledge about drive systems. You also have a lot of well-deserved respect on this forum. I've read many of your other posts and know you are more knowledgeable than I am, when it comes to VFD's.

I'm just a guy who's seen too many things that are "not possible" cause systems to shutdown and drives to fail.

I don't want to sound like an argumentative upstart, but I object to your blanket statement that unequal output line lengths cannot be the cause of this failure (admittedly overly paraphrased, but essentially your point).

I agree that we need more information, but the EE who suggested the line lengths has been on site and knows more than I know about the system, and presumably has taken some measurements or performed some calculations to support his statement concerning line lengths.

Things we don't know about this system, for example:

Is it a voltage source or a current source drive? Is there any dv/dt or di/dt protection? What is the drive operating mode? Have there been any custom changes done to apply this drive to this load?

What kind of cables are on the drive? How are they run to the motors? What are the ambient conditions where the motors are operating? What are the motors' load characteristics? Is there any additional impedance anywhere in the system?

What horsepower rating (now we know, but didn't originally) is the overall system?

Is there a non-drive-related system fault before the drive failure? Is there a dynamic brake or other sink to absorb any excess energy caused by the failure? Are the load conditions changing, and, if so, is there any relationship between these changing load conditions and the coincident drive failures?

What type of motors are on the drive? Are there output contactors between the drive and the motors? Is there any data record to see the sequence of failures or shutdowns? What other protections are provided for on this system? Assuming there are other protective devices: Where are they monitoring the signals, what signals are they monitoring, and what actions do they take when they detect an abnormality?

I tend to initially believe what I hear from the field until the evidence disputes the suspicion. As Reagan said, "Trust but verify."

I agree with everything you said, if this is a modern PWM voltage-source drive system, if there are no filters, if there are no contactors opening at the wrong time in the wrong place, if there is no additional energy being supplied to the drive because of the shutdown, if... You understand my point: There are just too many "if's."

I'd also like to hear the engineer's EXACT statement before we condemn it out of hand. Perhaps Mordred has not accurately reported the EE's statement. I am not suggesting that Mordred is lying or obfuscating. I am suggesting that maybe the EE didn't say exactly what Mordred thought he heard.

The symptom reported here is repeated drive failures. The presumably qualified man on site thinks unequal motor line lengths may be the cause. Until I see evidence to the contrary, I will treat his suspicions with due respect.

Again, I'm not trying to be argumentative. I just think we can't dismiss the on-site suggestion without knowing more than we currently know.

Having said everything above, I find it hard to believe that unequal line lengths are the cause of these drive failures. Much of my previous response's conjecture was done trying to justify the EE's reported statement. Before I'd spend a lot of time and money changing cables, I'd like to try to pinpoint the cause of the failure, and then correct the situation.

Mordred,

"...none of those write ups even mention any concern for line distance."

I completely disagree with this. You'll kill a motor quicker than you can spit if your leads are too long on a PWM system.

http://www.electram.com/Rex_reactor_dvdt_filters.htm

That is a sale's pitch for their dv/dt filters, but the first link that I found with my google search on this topic. Long leads can kill motors or drives, unless adequate precautions are taken, such as the filter in the link above.

I have no connection with electram. It was just the first link that came up that seems to cover this situation.