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#180755 09/08/08 06:35 PM
Joined: May 2008
Posts: 13
D
dura101 Offline OP
Member
Hi, I don't know much about these controllers but I have a question with regard to them.

Today I noticed something wierd, well wierd to me at least.

Here is the situation, please explain why I get voltage if you can.

Motor controller is soft start. Motor has a disconnect switch right by the motor. Soft start controller gets supply from a breaker, controller and breaker are housed in a MCC. Motor operates at 600 volt 3 phase.

With the main breaker on and I don't hit the run button. The motor is not operating. Ok thats right. But when I go to the disconnect and open it, I get 600 volts across all three phases.

Why do I get voltage at the disconnect when the run button is not pressed?

When I close the disconnect the motor does not run. When the run button is pressed the motor operates correctly.

There is nothing wrong with this motor or controller. It works good.

Just seems wierd to me why I get voltage when its not running. I figure its because of the controller but it would be nice if someone could explain it to me please.

Can anybody help me out?






Joined: Jul 2007
Posts: 265
W
Member
soft starters vary motor speed by varing hertz not voltage, even though you have 600 volts it is at 1 or 2 hertz, not enough to turn motor. When run button is pushed the hertz is ramped up to accelerate motor. this is my take on it any way, could be wrong, maybe some of the smarter guys will chime in and correct me.


Jimmy

Life is tough, Life is tougher when you are stupid
Joined: Nov 2005
Posts: 826
J
Member
I would expect that you are using a meter with a very high input impedance, probably at least a million times higher than your motor windings. The leakage currents of your output devices and any rf snubbers would present a relatively small voltage drop in series with your DMM. I wouldn't expect you to read full voltage though.
Joe

Joined: Oct 2000
Posts: 2,722
Broom Pusher and
Member
I was thinking the same as Joe - High input Z Meter reading leakage from the VFD.

Try connecting a "Real Wiggy" to the open Disconnect Switch, and see if it Buzzes (the Wiggy).

No Buzz = leakage from VFD.

Yes Buzz: see what the Voltage reading is shown on the Wiggy, as described below:

  1. Nice and solid pull-in, with indicator steady at apx. 600V would mean a large level of current may flow;
  2. Sluggish, or extremely light pull-in with very low Voltage shown would mean the leakage is quite large;
  3. Rapidly decaying pull-in, with nothing shown steady would mean a "Normal" level of leakage.


Just my 2ยข

BTW "Real Wiggy" = an actual Solenoid-Type Wiggy.
Equivalent would be a low input Z Voltmeter with a 1000VAC scale.

Be sure to use CAT III rated equipment for these tests.

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: May 2008
Posts: 13
D
dura101 Offline OP
Member
Thanks for the info, today I put my meter on it to see what frequency was on it. I got 60hz.

I will put a wiggy on it but I already know that there isn't capable of supplying much current because the motor doesn't turn when I close the disconnect. It only starts to turn when I hit the run button.

Anyway is it normal to read a voltage in this situation like I described?

Joined: Nov 2005
Posts: 826
J
Member
Dura,
Just for kicks and giggles, put your Wiggy & your meter across the line side of your open disconnect, at the same time. Or clamp on with your DMM while you watch with and without your Wiggy. If you were to get a buck for every volt, you probably wouldn't have enough for your morning cup of coffee. Let us know what you find.
Joe

Then we'll talk about my old saying, "All the world's a voltage divider!"

Joined: Nov 2005
Posts: 826
J
Member
I just want to add that while a VFD will provide a soft-start, a soft-starter does not have to be a VFD. I have seen them shown with inverse parallel SCR symbols. You wouldn't see high voltage AC across a VFD output to an open disconnect because output devices are connected to a DC bus. You would see a large % of line voltage via snubbers on thyristors used to limit motor current and voltage. I didn't chime in before because I have never worked on one or seen a detailed schematic. An AB website shows various voltage and current profiles, not frequency. It also shows separate, in-line modules for each phase with a control unit sitting atop. That points to the likelyhood of series thyristors.
Joe

Joined: Jun 2007
Posts: 301
J
Member
You cannot have a true 600 volts at the disco or at the soft start output terminals unless the soft start had ramped to full speed and the motor is running.
I think you are reading millivolts. Are you using a digital true RMS meter?
I bet if you use an analog meter set to the correct voltage range you will read less the 1 volt.
It is theoretically and physically impossible to apply 600 volts to a 600 volt motor and for it not to run. This is definitely a operator error regarding the meter.

To verify, check the current. There will be none.

Last edited by JValdes; 09/12/08 02:15 PM.
Joined: May 2008
Posts: 13
D
dura101 Offline OP
Member
Hi, I haven't had a chance yet to use a wiggy on it. Oh and yes it does says volts when I am reading the 600. 600volts phase to phase and 347 volts phase to ground. No operator error going on here. I know that there wouldn't be much if any current there but I just found it wierd that I was reading voltage. My volt tick was beeping so I put my meter on it.

This is my meter-

[Linked Image from ca.fluke.com]


Joined: Jun 2007
Posts: 301
J
Member
Nice meter. In the top right hand corner, are you sure it did not show "MV". Your meter auto ranges so when it see's millivolts or volts it will display them.
Also make sure you do not have the meter set to AC amps.
I'm am not saying you don't know your meter. But is very common for this mistake.

Have you clamped the motors leads yet? If there is no current then double check your meter settings one more time. The meter looks pretty new to me. New gloves too. How long you been doing this work?

Have we verified he has a soft start and not a VFD? Some VFD's that must hold the shaft in place will send out just enough current to lock it.

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