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Joined: Mar 2004
Posts: 947
twh Offline OP
Anyone interested in a guessing game?

I got a call from a maintenance man at a grain elevator. They are having trouble with two small motors blowing fuses, and I am going Tuesday morning to look at it. The information he gave me sounds like something I've seen before, but I'm interested in hearing if any of you have different ideas. Here are the facts, that I have:

The complex is about 10 years old and is generally in good condition and well maintained. The service is 600 volt, three phase. Most motors are fed from an MCC and controlled by a PLC. There are about 50 motors, ranging in size from a few 100 HP to many fractional HP. The larger motors have PF capacitors at the starters, and there is a bank of individually controlled capacitors for the entire service.

The problem motors are part of a cleaning system that includes a couple larger (40 HP?) motors on drives. They run all day long without any problem, but when they turn their system on at the beginning of the day, the PLC shows "overload error" and the fuses are blown. The overloads do not trip. Some times, the fuses blow when they start the motors.

The two motors are for blowers. They have checked for material build-up that may be dragging on the blowers. The motors start well (quickly). They are both FLC = 3.8 A. The running current of one - using a crappy clamp-on meter - is:
A = 1.1 Amp
B = .8 Amp
C = .4 Amp

I don't remember the numbers for the second motor, but they were really unbalanced, too.

The MCC modules feeding the motors are breakers with 6 amp fuses below them. The wires for the motors leave the fuses and "go into the drive module". There are two other wires on two of the fuses that "go somewhere else".

I didn't ask for voltage readings, because I don't like to ask mechanical maintenance people to work on live equipment. Perhaps, I should have, but it's about 2 hours away and it's a pleasant break from my normal work.

Two motors with similar readings sound like a common problem to me. However, they are Weg motors, so anything goes, I guess. To make this interesting, if anyone guesses correctly, I will buy them a beer, but they have to come here to get it.

What you you think?

Joined: Apr 2002
Posts: 2,527
twh — Logic and an understanding of electrical principles are your best tools when it comes to troubleshooting industrial controls. There seems to be a conflict in your description of the system at hand. Forty-horsepower motors are mentioned, but with current checks of around 1 ampere. For 600V 3ø motors, nameplate current for 40hp units should be around 40 amperes.

Are the problem motors on VFDs or plain-vanilla magnetic starters?

Comment on the cheesey meter—they are best used as one-shot wheel chocks for one truck rear tire. Suggested is a decent-quality DMM with a “clothespin-type” AC-current probe—like a Fluke i200 / Y8101; or AEMC MN 212 / MN185. If at all possible, obtain a good overrange-protected digital multimeter, “electrician’s” [TP238+AC220] lead kit and current probe with a minimum safety rating of “EN61010 CAT III 600V or CAT IV 600V”.

Low-range current-meter “slop” can be best limited by centering the lead under test as close to the window middle as possible. Consistent, repeatable poleface centering and absence of contamination on CT “jaws” is especially important on lower-range AC-current readings. Another trick is to make a 10-turn ‘donut jumper’ temporarily connected in the measured circuit to effectively increase meter sensitivity by a factor of 10.

Offline motor winding-resistance balance should be close—within a few percent. Resistive balance checks of stator windings are made with motor leads removed from controller terminals. Balanced overcurrent indicates a possible mechanical motor overload, or possibly a significant undervoltage at the motor terminals. [Investigate over 2-3% voltage imbalance, for the average of the three phase-to-phase readings, but 550-630V at motor terminals.]

Be aware that for each percent of line-voltage imbalance at the terminals of 3ø squirrel-cage induction motors can itself cause about 6% current imbalance.

Any chance the “two extra wires” serve a significant load like a magnetic break or solenoid coil—upsetting the current balance on the controller load-side terminals? If possible, trace and temporarily disconnect the extra pair of leads and make current checks to verify. Such a “piggyback” may cause undesirably added current on the motor side of the controller. It may pay to ask how long the fuse-blowing problem has persisted, and then the harder question, could there have been any changes external to the controllers and motors?

Another check is lead rolling — 1/2/3 to A/B/C — then 1/2/3 to B/C/A Does the current imbalance follow the motor or controller? Compare wide-open, full-tilt {100% speed} online V/I readings at the starter cubicle.

Fuse-in-holder rotation with finger pinching should be close to equal. Warmth to touch may signify loose clips. It is possible that fuses were used in a local motor-isolating disconnect switch. Only in that case, given the remote location if the malfunctioning gear, IF there is adequate short-circuit protection immediately upstream of the motor controller, and suitable running motor-overload protection integral to the controllers, and given the fuses electrically and mechanically located between the controllers and motors, slugs may be installed replacing fuses if the starter or drive is configured properly. {like Bussmann NTS }

[This message has been edited by Bjarney (edited 07-10-2004).]

Joined: Mar 2004
Posts: 947
twh Offline OP
The two problem motors are probably 3 HP. I mention the 40 HP motors because it may be relevant. I see the two motors with unbalanced legs as being unlikely that two motors failed at about the same time and in the same manner. It looks to me like incorrect voltage on one leg.

Where I saw these readings before was on a system with about 4 motors, one of which was 75 HP. One line fuse blew, and a small motor started tripping its overloads. It ran for a day like that.

If I was more certain that it was phase failure, I would have gone Friday. This seems different because the motors will start and they report no problems with the single phase loads. As it stands, I have a planned shut-down on Tuesday, unless they can't start up on Monday.

To Bjarney's comments:

The small motors are probably on magnetic starters.

I can't use the meter as a wheel chocks. It isn't mine, and I already use Weg motors for that.

My meter is a Fluke 87 III. Its pretty close between .5 and 1 amp. Below that or for better readings I loop the conductor through.

Winding resistance tests are a part of my normal practice, although it is usually next to meaningless above 40 or 50 HP.

The two extra wires are probably feeding a small control transformer. I'm not sure why when the PLC power used for everything else. I suspect it is the reason that a blown fuse shows as overload failure.

Lead rolling: Good idea.

Fuse rotation: If I suspect bad connections, I measure the voltage drop across the connection. I've been able to track problems down to a loose screw that way. I love my meter.

Slugs: That's two ways to get fired. First, bypassing fuses, and second, paying $40 for 6 inches of copper pipe. It reminds me of the guy who wanted to keep a pump jack running by wedging a piece of wood between the door and the contactor. I almost pissed myself.

Joined: Apr 2002
Posts: 2,527
Take a spare battery?

Joined: Mar 2004
Posts: 947
twh Offline OP
I started this so I'll wrap it up in case anyone is interested. Maybe I should have said you have to post your guess before you can win the beer. It is pretty specific to the situation, though.

It turns out, the two motors were running on VFDs. The problem was grain dust on the circuit boards. I heard rumors of this problem years ago, so the winner is an unknown electrician. I'm going to drink his beer.

The maintenance man's readings were correct, but they were from the line side of the VFD. The motors were fairly even at 1.6 amps.

I guess the spare batteries will come in handy some other time.

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