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).]
