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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.

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  • \$\begingroup\$ Revise your question to include all of the results of the tests you have done. Also add a picture of the nameplate. \$\endgroup\$
    – user80875
    Sep 22, 2020 at 13:09

3 Answers 3

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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).

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  • \$\begingroup\$ Hi charles. initial check seems to indicate there is nothing wrong with the motor. Is it possible that the problem is with the magnetic contactor? we see a some oil on it. Is it possible that the oil has prevented the contactor to make perfect contact thereby turning the 3 phase to single phase? \$\endgroup\$
    – norbss
    Sep 24, 2020 at 8:11
  • \$\begingroup\$ Yes. That is certainly possible. A check of the voltage balance after the contactor would give some indication of that. A clamp-on ammeter check of the current balance would give a better indication. There may be several points where there could be connection problems or other problems with the supply. \$\endgroup\$
    – user80875
    Sep 24, 2020 at 13:39
  • \$\begingroup\$ I did check with a clamp meter the wires leading to the motor before. 2 had readings but the last one had no reading so I thought the motor winding might have been shorted. I've replaced the contactor and will connect the motor again next week. I'll let you know the outcome. Thanks for the assistance. \$\endgroup\$
    – norbss
    Sep 24, 2020 at 23:51
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    \$\begingroup\$ after installing a new magnetic contactor, the 3 phase motor ran smoothly. So there was no problem with the motor after all, the problem was with the magnetic contactor. Thank you all to those that helped. \$\endgroup\$
    – norbss
    Oct 12, 2020 at 13:41
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With a normal multimeter you can only test for open windings.

To test for shorts you need an insulation tester (high voltage ohms meter).

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  • \$\begingroup\$ That's for testing shorts to frame / chassis. \$\endgroup\$
    – Transistor
    Sep 22, 2020 at 7:23
  • \$\begingroup\$ Even that won't test for an interwinding short that is not to ground. That is a hard one. \$\endgroup\$ Sep 22, 2020 at 7:43
  • \$\begingroup\$ how to test for open winding? Its better than nothing. \$\endgroup\$
    – norbss
    Sep 22, 2020 at 7:57
  • \$\begingroup\$ what reading should I get for the 2 wires that are wrapped together? they are connected to UVW power source. \$\endgroup\$
    – norbss
    Sep 22, 2020 at 7:59
  • \$\begingroup\$ I tested all wire to the body of the motor and it's not grounded. \$\endgroup\$
    – norbss
    Sep 22, 2020 at 9:33
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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.

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