Troubleshooting Split-Phase Motors

 

Troubleshooting Split-Phase Motors

 

If a split-phase motor fails to start, the trouble may be due to one or more of the following faults:

o   Tight or “frozen” bearings

o   Worn bearings, allowing the rotor to drag on the stator

o   Bent rotor shaft

o   One or both bearings out of alignment

o   Open circuit in either starting or running windings

o   Defective centrifugal switch

o   Improper connections in either winding

o   Grounds in either winding or both

o   Shorts between the two windings

 

Tight or worn bearings

Tight or worn bearings may be due to the lubricating system failing, or when new bearings are installed, they may run hot if the shaft is not kept well oiled. If the bearings are worn to such an extent that they allow the rotor to drag on the stator, this will usually prevent the rotor from starting. The inside of the stator laminations will be worn bright where they are rubbed by the rotor. When this condition exists, it can generally be easily detected by close observation of the stator field and rotor surface when the rotor is removed.

Bent shaft and bearings out of alignment

A bent rotor shaft will usually cause the rotor to bind in a certain position but then run freely until it comes back to the same position again. Test for a bent shaft by placing the rotor between centers on a lathe and turning the rotor slowly while a tool or marker is held in the tool post close to the surface of the rotor. If the rotor wobbles, it is an indication of a bent shaft. Bearings out of alignment are usually caused by uneven tightening of the end-shield plates. When placing end shields or brackets on a motor, tighten the bolts alternately, first drawing up two bolts, which are diametrically opposite.

 

 

 

 

Open circuits and defective centrifugal switches

Open circuits in either the starting or running winding will prevent the motor from starting. This fault can be detected by testing in series with the start and finish of each winding with a test lamp or ohmmeter.

A defective centrifugal switch is generally caused by dirt, grit, or some other foreign matter getting into the switch. The switch should be thoroughly cleaned with a degreasing solution and then inspected for weak or broken springs. If the winding is on the rotor, the brushes sometimes stick in the holders and fail to make good contact with the slip rings. This causes sparking at the brushes. There will probably also be a certain place where the rotor will not start until it is moved far enough for the brush to make contact on the ring. The brush holders should be cleaned and the brushes carefully fitted so they move more freely with a minimum of friction between the brush and the holders.

Reversed connections and grounds

Reversed connections are caused by improperly connecting a coil or group of coils. The wrong connections can be found and corrected by making a careful check on the connections and reconnecting those that are found at fault. The compass test with a DC power source can also be used for locating reversed coils. Test the starting and running windings separately, exciting only one winding at a time, with direct current. The compass should show alternate poles around the winding.

The operation of a motor that has a ground in the winding will depend on where the ground is and whether or not the frame is grounded. If the frame is grounded, then when the ground occurs in the winding, it will usually blow a fuse or trip the overcurrent protective device.

A test for grounds can be made with a test lamp or continuity tester. One test lead should be placed on the frame and the other on a lead to the winding. If there is no ground, the lamp will not light, nor will any deflection be present when a meter is used. If the lamp does light or the meter shows continuity, it indicates a ground is present—due to a defect somewhere in the motor’s insulation.

Short circuits

Short circuits between any two windings can be detected by the use of a test lamp or continuity tester. Place one of the test leads on one wire of the starting winding and the other test lead on the wire of the running winding. If these windings are properly insulated from each other, the lamp should not light. If it does, it is a certain indication that a short or ground fault exists between the windings. Such a condition will usually cause part of the starting winding to burn out. The starting winding is always wound on top of the running winding, so a defective starting winding can be conveniently removed and replaced without disturbing the running winding.