I have a single phase ac fan motor (from a regular household stand fan) which operates on 240V 50Hz power source. It has a start capacitor.

The problem I am facing is the motor is not spinning or jammed. If I spin the motor by hand, the motor spin freely (not yet turned on), meaning that it is not mechanically jammed (bearings etc), but rather possibly electrically. When I let the motor spins fast by hand, then I switched on the power, the motor immediately stopped. It looks like the magnetic field is causing the rotor to be attracted to stator and get locked, preventing rotation. This is not suppose to happen. The magnetic field is supposed to be rotating. I have removed the capacitor and try to start by hand with power switched on, but still jammed.

What caused this phenomenon to occur? How to solve it?

  • \$\begingroup\$ Open or shorted coil winding might be expected. \$\endgroup\$ Feb 7 '21 at 4:24
  • \$\begingroup\$ Motor use is hard on a capacitor, so inspect it thoroughly. The point of the capacitor in a single phase motor is to put the currents in the coils out of sync so they can push against each other in a useful direction. If the cap fails closed, the coils will synchronize, and if it fails open one coil won't get current. \$\endgroup\$
    – K H
    Feb 7 '21 at 4:39
  • 1
    \$\begingroup\$ Make sure the bearings or bushings are OK. You should not be able to move the shaft of the motor side to side and cause the rotor to touch the stator. \$\endgroup\$
    – brhans
    Feb 7 '21 at 4:59
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    \$\begingroup\$ This does sound like float in the bearings that allow the fan to mechanically jam when the magnetic field is present. Overheated plastic enclosure, or shock (did it fall and hit something hard) or simply wear if it's over (say) 30 years old \$\endgroup\$ Feb 7 '21 at 14:05
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 I am expecting that too. If that was the case, then it is beyond economic repair. But what is the explanation for open or shorted winding be the root cause for this behaviour? \$\endgroup\$ Feb 10 '21 at 14:22

Could be that the thrust bearings, that keep the motor from moving forward and backward along it's axis, have failed. So as soon as you energize it, the magnetic field makes the motor move forward and jams against the housing. There might not be an economical way to fix that, meaning the fix may cost more than a replacement fan.

  • \$\begingroup\$ I have opened one end of the motor, removing front cover and it's bearing, exposing the rotor and stator, while keeping the other end (back cover) still attached. I then switched on the motor. From what I observed, there is no forward or backward movement visible. The rotor just stick hardly to stator, even I cannot rotate the rotor by hand, unless I cut off the power. \$\endgroup\$ Feb 10 '21 at 14:16

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