Form Wound Vs Random Wound Coils for Motors

[W@ttson]

If you can peek into the housing and see the coils, Form Wound would have square or rectangular conductors pressed into precise neat shapes and often vacuum impregnated insulation around them. If you see small round wires wrapped in what looks like sting or tape it's random wound.

Form wound:

Random wound (with some damage):

200HP would be a generous lower limit of size to find form wound. It's usually motors that are "above NEMA frame", generally equating to over 250HP at 460V, and even then it's not common until you hit maybe 500HP and up.

Unfortunately, most of the motors I find in the field ware TENV, getting a look inside is often difficult.

 

[paulengr]

Unfortunately, most of the motors I find in the field ware TENV, getting a look inside is often difficult.

Look at PdMA. Designed for testing when you don’t have access. It runs the “standard” battery of tests plus a few more. It checks insulation resistance, PI, graphical PI, and capacitance. It tests coil resistance and inductance. If you need it you can do a rotor influence test that measures inductance every few degrees. This picks up on eccentricity, rotor bar issues, and shorted turns. It does online power quality too which picks up on other issues and finally you can look for eccentricity and rotor bars based on harmonics in the current. Motor shops typically use a Baker machine which does the first few offline tests and does surge testing instead of inductive tests. Areva has one that is mostly online tests but mostly only known around power plants.

However you can get a decent Megger for around $200 these days and a milliohm meter for a few bucks more. You also need to pay attention to the Megger readings as it goes for ten minutes especially in the first minute. Between these two offline testers you can do all the tests that the PdMA tester that costs over $50,000 does. Motor shops usually have ONE of these type of testers for the whole shop. The whole idea is to determine if the motor is good or not without opening it. Those two tools cover 90% of the failures. Only time say you need surge or inductive unbalance is in the rare case of surges damaging the first couple turns.