VFD AC Inverter and Induction Motor

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tishjoh

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I am new to this so if this has been discussed before, please point me to it.

I have a customer that wants to use a small AC Inverter/VFD for:
1. takeing 230v 1 ph and running a 230V 3 ph 5 hp motor. (which the specs say it can do it.

2. use the VFD to control the motor speed for a wood lathe in a wood shop

Question #1 The documentation for the VFD says the power wireing must be shielded. It does not specify if this is between the VFD and the motor and/or the power input to the VFD. Any thoughts?

Question #2. I have seen in a couple of other discussions here, mentions of the motor having to be inverter rated if being used on a VFD, for clarification this is a seperate rating from the duty (continous, intermitant) correct?
 
tish,
because the drive is going to apply a fluxuating d.c. voltage of about 850 volts to the motor the drive manufacturer usually recommends a motor designed for use with drives. these motors have a better insulation rating than a standard induction motor. the induction issue again is a factor of this higher applied voltage that could induce voltages into surrounding conductors that the original voltage didn't.

i have a three phase --- 480 volt motor that i can run on a 120 volt receptacle outlet. i used to use it to explain the advantage of vfd aplications.

and when installing a vfd on an existing motor that is not vfd rated, i always explained to my customer that the motor may(?) fail due to this insulation problem. i can remember four or five times that this occured. once was a very large job and i was bidding against a different drive manufacturer (GE) and we installed the first drive at no charge just to show the customer what a savings in power, maintainance, and noise reduction they could acheive. they didn't want to change the motors on their system. the drive failed about two weeks later and my competitor convinced them that my drive caused the problem! i lost the job and they went with GE drives. of course they, GE had the same problem, and lost about half the motors in the first six months. there is not a large difference in motor costs between those rated for vfd and the standard motor....................

on the shielding --- consider an existing system that has other motor feeders in the same raceway. the increased voltage on the set of conductors could cause increased induction into these other wires or even control wires within a cabinet where the vfd motor conductors are installed close by....................
 
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Depending on the drive being used, with 230V single phase going in, the 'DC rail voltage' may be only about 300-350V. Others will boost the DC rail voltage up to 850V.

The output of the VFD is switching at very high frequency (2-10KHz), and rich in high frequency components that can cause electromagnetic interference. Shielding the load side is probably suggested for this reason.

The output of the VFD has some amount of harmonic content, which will require a small % derating of motor output. Charlie's comments about insulation rating are far more important.

With a VFD, the meaning of 'full load' changes. Power output is the product of speed and torque, and the limiting factor in a motor is mostly the heat caused by _torque_ production. If the motor is a 5hp 1725RPM machine, and you operate it at 350RPM, the power output will be limited to about 1hp. This is an important consideration if the VFD is being used to replace a system with gearboxes; when you reduce speed with a gearbox, you get more torque.

-Jon
 
tishjoh said:
I am new to this so if this has been discussed before, please point me to it.

I have a customer that wants to use a small AC Inverter/VFD for:
1. takeing 230v 1 ph and running a 230V 3 ph 5 hp motor. (which the specs say it can do it.

2. use the VFD to control the motor speed for a wood lathe in a wood shop

Question #1 The documentation for the VFD says the power wiring must be shielded. It does not specify if this is between the VFD and the motor and/or the power input to the VFD. Any thoughts?
Click to expand...

Its a good idea to use twisted conductors going to the motor from the VFD. I am unaware of any VFDs that suggest using shielded cables. I generally like to see SJO or similar cord used. But, of course whatever the manufacturer recommends is what you should use. Just curious what brand of VFD wants shielded cable used?

tishjoh said:
Question #2. I have seen in a couple of other discussions here, mentions of the motor having to be invertor rated if being used on a VFD, for clarification this is a seperate rating from the duty (continous, intermitant) correct?
Click to expand...
Inverter duty motors have a better class of insulation. The VFD does not actually put out a 230V sine wave. It puts out a series of square wave pulses that approximates the voltage and power content of a sine wave, but it is really a series of fairly high frequency, higher voltage DC pulses. The motor does not care much about not getting real AC, but the insulation may.
 
petersonra said:
Many have been pushing a new cable made just for this purpose, but I am not aware of any that claim it is required.
Click to expand...

Yes VFD cable, I have only used it once.

But in the little I have read they do want shielding, a metal raceway usually takes care of this.

The one time I used VFD cable was in a very large control cabinet, it was my understanding at the time the engineer was concerned with the VFDs un shielded output would create problems with the other electronic devices in the same cabinet.

Soon I will actually be shielding some VFD conductors for testing purposes.
 
we were called to troubleshoot some vfd problems at an injection plant that pumped explanding foam into molds to create marine products. their motors were static with static drives. that is the first time we came across shielded cables. the customer bought the cable from the drive manufacturer. problem was they bought just enough cable to go between the motor and the control cabinet. the load side of the vfd was not shielded as it ran thru the computerized control cabinet. as the controller actually measured the amount of foam injected into the mold by the number of turns the motor turned the positive displacment pump. as it reached the stopping point is when the controls would go crazy! it took us some time to determine the problem -- we installed this portion of the load side in emt and that solved the problem. it was very expensive for the customer -- because each time they injected too much foam --- most of the time it would cause the mold to explode. some of these molds had 1/2 nuts and bolts holding them together!! scarry for sure! it was a big plant and i don't think this system was engineered --- just seemed as though it was "trail and error"! but they were working three shifts --- and these boat parts were very expensive. thats the only time i have come across shielded cable -- i have always kept the load side in metallic raceways and separtate from other wires...
 
We had to deal with just this issue on our experimental systems. They would run fine while _running_, but as the synthesized output dropped to 0V, we'd start to see transient failures on control systems and sensors.

The issue is the way the pulses of DC are produced. When all the phases are switched to the positive supply, there is no voltage difference on the phases. Similarly when all the phases are switched to the negative supply, there is no voltage difference on the phases. Voltage is actually applied across the motor windings when some are positive and some are negative.

The way most VFDs work, all the phases are being switched positive-negative-positive-negative some high frequency (2KHz - 20KHz) square wave, and then to synthesize the desired low frequency output the duty cycle of this square wave is modulated.

When you are actually producing low frequency output, the phases are all switching at different times. But when you are 'synthesizing' zero output, all the phases are switching at the same time, leading to a much sharper high frequency switching transient. Our experimental systems were 18 phase systems, so the switching energy in a single phase was much less than the combined system.

Combine this with _multiple_ inverters all operating at slightly different switching frequencies, and sometimes the switching transient of one inverter would coincide with the switching transient of another inverter. Bang, lots of noise, enough to screw up insufficiently shielded digital electronics in the area.

-Jon
 
Q 1/2 Welcome to the forum.

Q1. As Winnie correctly pointed out, with possibly massive emissions. If the existing motor uses metallic flex, EMT, etc., that will work fine for shielding here. You will need to assure that the motor is grounded/bonded; not the same thing as shielding the VFD UVW outputs and the low voltage control connections, as shielding will terminate only at the VFD ground terminal end. This will be inside a cabinet, right?

Q2. I've always just set parameters to 200V motors, never had a burned motor. This is a holdover from before there were 'vfd rated' motors.
 
I'm looking to add a AFD to a 220v, 3ph 24" wood surface planer. It is a neuman and is quite old with the original starters, one for the direct drive cutterhead and the second for the 1-1/2hp 4p feed motor.
The feed motor drives the rollers through a variable sheave pulley that is too fast when set at the slowest speed.
I'm considering an ABB ACS140, ACS143-4K1-1-U. I don't need a lot of bells and whistles but simple the ability to reduce the feed speed while providing the appropriate motor protection.
I have a couple of concerns in that the motor itself is a basic Delta name brand 3ph motor. being that it is not designed specifically for drive duty would there be a tendency for a motor failure?
The drive will be located and mounted on the planer with leads probably not more than 6' long. It would be controlled by simply On/Off PB devices. Because of the dusty environment it would need to be mounted in a NEMA12 enclosure which then may present a drive cooling issue.
I also am planning to replace the PB station also which replaces all control.
Should any of this old stuff fail renewal parts would be difficult if not impossible to find anyway.
Any thoughts on this matter would be appreciated.

Thanks,
Dave
 
Dave,

It sounds 'like' a conveyor, constant torque application. See whether this is mentioned in the manual. You're absolutely right it must be enclosed in a woodworking environment. One of the other members had a valid concern with a VFD cooling at high altitude. It gets over 100 here and I've never seen one have fits from overheating in an enclosure.

This is a fairly recent design. If they follow japanese practice there will be a thermal overload parameter which can be set to allow general-purpose, inverter duty or custom V/F loading. This should be available, and able to deal with the motor concern.

You mentioned replacing pushbuttons at the control station. Again, my experience is with japanese equipment which is fairly standard. Are you going to use the 0-10V three wire with a (say, 10K) pot for speed adjustment or use multispeed/ hardwired presets? Probably will be necessary to rig something up for 22/30mm 0-10V and it all depends on what your machine is doing, whether this is desirable...

Your new mushroom should be the kind with multiple sets of switches, so you can hardwire a set into the unit for either coast or "just stop asap." If you do the latter be careful with regenerative braking, because if the drive tries to stop too fast it will then go into alarm and the conveyor will simply coast. However you approach this will take some common sense. Just in case, it's probably worthwhile to bring at least a pilot light out front to indicate 'drive failure,' which can drive the operator crazy if the machine 'just doesn't work.' If the drive gets an internal or external fault, it's usually necessary to provide a 'reset' which can be quite aggravating if not perfectly clear to whomever is using the machine.

edited... '0-10V three wire'
 
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I went and looked at the manual.

It does indeed say both the incoming and motor power should be shielded, along with the control wiring.

Interestingly, their wiring diagrams show the shield for the control wiring, but not for the power or motor wiring.
 
Allen Bradley has publications out there about vfd installations. For power wiring, they recommend installing shielded cables with drain wires connected at both ends and installed in PVC conduit when possible. The new generation drives have much quicker swithing of transistors that create new kinds of electrical noise and RF problems.
 
I came across this thread while looking for a vfd related part. Maybe somone can point me in the right direction. I'm looking for a manufacturer of a set point controller that would send a 4-20 ma signal to a vfd that doesn't have an internal PID controller. This signal would be regulated by a pressure tranducer. Application is for constant pressure. Thnx for any help.
 
thunder15j said:
I came across this thread while looking for a vfd related part. Maybe somone can point me in the right direction. I'm looking for a manufacturer of a set point controller that would send a 4-20 ma signal to a vfd that doesn't have an internal PID controller. This signal would be regulated by a pressure tranducer. Application is for constant pressure. Thnx for any help.
Click to expand...

there are many manufacturers that make single loop PID controllers. they are almost commodity items these days. honeywell is probably as common as any of them.

buy one with autotune. it will simplify your life and add only a few bucks to the cost.
 
thunder15j said:
I came across this thread while looking for a vfd related part. Maybe somone can point me in the right direction. I'm looking for a manufacturer of a set point controller that would send a 4-20 ma signal to a vfd that doesn't have an internal PID controller. This signal would be regulated by a pressure tranducer. Application is for constant pressure. Thnx for any help.
Click to expand...


It sounds like you want a signal conditioner. The signal from the pressure transducer (0-10 v or whatever) would be directly converted to a 4-20 mA signal for the VFD. Just google "signal conditioner". A good source is AutomationDirect.
 
ed_man said:
It sounds like you want a signal conditioner. The signal from the pressure transducer (0-10 v or whatever) would be directly converted to a 4-20 mA signal for the VFD. Just google "signal conditioner". A good source is AutomationDirect.
Click to expand...

No, a plain signal conditioner would not do it, he needs a PID controller.

thunder15j,
Look in catalogs for process heating equipment, i.e. Watlow, Chromalux etc. They always have good ones. I use one made by Omron. Not the cheapest but not as expensive as Honeywell (the Cadillac of PID controllers), and very easy to set up.

If you already know your way around a PID loop and just want something to execute it inexpensively, this is the cheapest one I have ever found.
http://store.omniinstruments.net/n1100_controller.htm
 
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