Motors keep burning out

[stray spark]

400 volts?

400 volts?

Man, you guys in California get all the neat stuff, first no stray voltage from your utility, then a 400 volt system. I have never seen one in Wisconsin. I am going to assume it is a 50 hz system (odd to me) and a machine from overseas?:-?

 

[bwyllie]

have the new motors furnished with shaft grounding rings to see if this solves the problem. These rings are not too expensive so it maybe worth a shot.

 

[John Valdes]

Has your friend actually measured the motor current and voltage at full load. I have seen instances where a high resistance connection is not visible or readable until it is checked under max load. Good examples are, circuit breakers, bus duct stabs and contactors. A high resistance connection on a motor could very well be a single phase condition as you lose one leg of the 3 phase. Very common. And yes, just because someone did not change the overload setting on the OLR would not be the direct cause of motor failure.

 

[brian john]

John:

There is a simple way to check for high resistance connections, with the motor (or other load) on line, measure the voltage drop from the line connection to the load connection, balanced loads make this easier. Commonly referred to as the Fall of Potential test method. On would expect the Voltage Drop to be similar phase to phase as each lines impedance should be the same and current is the same.

Depending on distance a DLRO/micro ohm meter (digital low resistance ohm meter) with source voltage off can be utilized.

Or an IR camera.

 

[elvis_931]

Low Voltage

Low Voltage

It is possible that the overloads are set too high, but if this is a manufactured machien bought from a machine company, then the overloads should be set correctly from the manufacturer(still need to check them). It is unlikely that all of the motors are actually under a larger load than what they are installed and designed for for this particular machine. In my experience, the biggest threat to the life of a motor is undervoltage. Low voltage supply puts an incredible strain on a motor; especially at start-up. It is possible that you have (for some reason) a lower voltage at this particular machine that keeps burning motors up. Maybe a high resistance short that is leaking enough voltage to be a problem, or a long feed that has a significant voltage drop on the line. I would check the voltage at both machines and compare the two, then start looking for why the voltage is so low.