How can I test using an analog multimeter if this 3 phase motor is shorted? It has a hard time starting( turn a little then hums) if there is a load connected, but if you remove the load (removing the belt from the pulley), it will start turning. My multimeter has X10k, x1K, X100, X10, & X1.
Fist make sure that the three-phase power voltages supplied to the motor are the proper voltage and balanced.
Check the resistance U1 to V1, V1 to W1 and W1 to U1. The resistances should be very close to being equal. Do the same for U2, V2 and W2. You should not find continuity between the groups (U1, V1 and W1) and (U2, V2 and W2).
There are two types of short circuits in a motor. One is called body short. It happens when the insulation between the winding and metal frame of the motor ruptures and the bare winding wire /wires come in contact with the metal frame. since the metal frame is kept earthed, heavy current flows, and trips the circuit breaker. This can be easily checked by the multimeter in the ohmic range by touching one probe of multimeter to the unpainted metal frame and the other probe to the winding terminals of the motor. In the case of short, the resistance will be very low. In the normal course, this insulation resistance is of the order of several mega ohms.
some times there is a short between the windings or among the turns of a winding coil. This happens due to the melting of the insulation of winding wires on account of the overheating of the motor. In case of a short between the windings heavy current will flow and the circuit breaker will trip. however, if a few turns of one winding gets shorted the resistance of that winding will become lower than its actual value and motor will not be able to take the load.
The humming sound comes in the motor when the air gap between the stator and the rotor is not uniform and the motor is therefore not able to pick up speed quickly. In your case the motor gives a humming sound when the belt is put on the pully, it can also be due to the belt being too tight.
Humming fault also occurs due to the non-replacement of worn-out bearings of the motor for a long period.