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Hooking up a LIM to a VFD

Hooking up a LIM to a VFD

(OP)
Hi all!

New to this forum. I am an electrical engineer by education but now I primarily write embedded software -- I don't usually work with industrial controls and motors. I am helping a friend (who is also a grad student at the local university) set up experiments in his lab.

The university purchased a custom-built linear induction motor to generate a variable frequency and strength magnetic field to study its effect on certain materials. To be clear, they are not using the LIM to move anything. Just to generate a varying linear magnetic field.

They supplied me with the linear induction motor and a PowerFlex 753 VFD. The drive plugs into a nearby 480V, 3-phase outlet with a 20A rating. The LIM's sticker states it's rated for 200VAC, 120A.

I have gotten to the point where I have everything hooked up. The PowerFlex has an "EtherNet" option board that I am using to access the drive remotely. I'm using the ConnectedComponents Workbench software from Rockwell Automation to interact with the drive over a LAN.

I am now trying to set up the drive and do some testing. I went through the wizard to set up the motor. I wasn't sure about some of the parameters so I left them default (like RPM -- it's not a rotating motor).

When I try to start the motor, after a short while an internal alarm goes off:

CODE -->

Alarm 169 [PWM Freq Reduced]
The PWM Frequency has been reduced from the value set in P38 [PWM Frequency] due to excessive IGBT junction temperatures. 

The alarm occurs a few times until a fault occurs:

CODE -->

Fault 64 [Drive OverLoad]

This stops the VFD output until I clear the fault. I am not sure why this is occurring, but I believe it has something to do with me misconfiguring the drive to run the LIM.

I configured the VFD to operate in V/Hz mode. The fault occurs at a relatively low output power (25A, 8V or so).

Any tips on how I should configure the VFD to drive this LIM? Can you point me to some resources? Any help is appreciated!

RE: Hooking up a LIM to a VFD

Likely, the drive expects the motor to be doing something, but in this case it is missing its "rotor" and the drive never sees the back EMF it is looking for.

RE: Hooking up a LIM to a VFD

That 25A at 8V is made up very short pulse of a much higher current. A short pulse is arguably the worst condition for a semiconductor switch because the conduction period is small and the switching losses dominate. The fault reported is possibly correct, although without knowing the VFD rating it's not possible to say for sure.

RE: Hooking up a LIM to a VFD

(OP)
Since there is no real load on the motor, perhaps I should try driving the motor at a lower peak power?

I did a bit more research and found this on another forum:

Quote (JRaef)

Newer generations of VFDs, like the PF750, still use the current feedback loop for better motor performance even if in V/Hz mode, so they will trip off too now.

What could be done to account for the lack of a feedback current/EMF?

RE: Hooking up a LIM to a VFD

The PowerFlex 755 has a Linear Motor control option, but I don’t think the 753 does.

What is happening is that the drive is seeing the current go too high too fast and is interpreting that as a stall. The default response to that is for it to lower the frequency to limit the current, but in this case that’s not going to work. Also, although you CAN program the drive to limit the output RMS voltage to 230V, the PWM pulses are still going to be based on the 650V+ DC bus, not at all good for the windings.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Hooking up a LIM to a VFD

This is a lot like powering a standard induction motor with no rotor installed.
Once you get it running, if you do, the parameters will change when you introduce magnetic material, conductive material or both into the LIM field for testing.
Any thoughts on how pulling the rotor out of an induction motor will change the setup of a VFD, Jeff?
That may be a good starting point.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Hooking up a LIM to a VFD

I am baffled by why you're using 480 to run a 200V load. I see this causing lots of issues (possibly even failure of the LIMs insulation system). Other problems include MUCH higher current spikes and very twitchy triggering of error alarms. Likely, little useful adjustability of many of the drive parameters. The drives while amazing and spectacular are still constrained by physics and what more than twice the voltage brings as baggage. You should try it on 240 or 208V first to see if the whole thing gains some stability and adjustability. If it doesn't then as Jeff pointed out you may have the wrong VFD.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Hooking up a LIM to a VFD

(OP)
Well, I called the manufacturer of the LIM and they said they pretty much "put a sticker on it" because they had to. Their engineer didn't seem to be bothered by the 480V voltage source.

When I was given the VFD it was already wired for 480V. I was told the University had tested the LIM with this VFD before so I went along with it.

I understand this is a very unusual application.

Can anyone point me to some resources on the theory of operation of VFDs? Any tips on running a motor with no load/rotor?

RE: Hooking up a LIM to a VFD

A LIM motor is a combination of a set of coils and a LIM rail.
If you are using the coils without a LIM rail that would be similar to running an induction motor without a rotor.
You may try energizing the coils with a varying voltage. As the voltage is increased the current will increase proportionally until the iron saturates. The the current will increase dramatically.
Vary the voltage while monitoring the current to find the saturation point.
Don't keep the coil in saturation for more than a second or so.
Back the voltage down about 10% or 15%. That will be your safe voltage at 60 Hz (in the US).
Calculate the Volts per Hertz ratio and use this to set up the VFD and to set the operating voltage of the LIM coils.
If you do not have three phase available as a variable voltage, the single phase V/Hz can be used as a starting point.
Set the VFD output to a lower voltage and raise the voltage while watching the current until the current starts to jump.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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