Induction Motor? sorry kinda long


  #1  
Old 09-17-03, 09:05 PM
djhurt1
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Induction Motor? sorry kinda long

I hope this is the proper place to ask this question. I had an electric motor I was putting on an old table saw. The motor worked fine when tested, after installing on the saw it would run but only at half speed and sounded very rough. I could actually stop the motor with my hand, twist it the opposite direction and it would run that direction. Now I put a new cord on it and thought I possibly screwed up one of the wires. There several going to white and only two going to black. I tried several combinations but nothing would work. What I did find was that if I took one of the wires and left it off, turned on the switch, let the motor start spinning, then momentarily touch this wire to the neutral bunch, the motor would kick up to full speed and run like a dream. If I touched the wire again to the same bunch, the motor would slow way down like I stated above. Does this sound like a problem I caused or maybe the motor went bad?
Thanks for any ideas, I have since thrown the motor out in disgust but I am really curious on what the problem was. I do not understand the induction motors, but after reading some stuff on the net, that's what this motor sounds like it is.
djhurt1
 
  #2  
Old 09-18-03, 09:54 AM
brickeyee
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What voltage are you trying to run on? It sounds like you have a 120/240 motor. For 120 V operation the stator windings are in parallel. For 240 V operation the windings are in series. If you hook up the winding for 120 V with one backwards, or for 240 V but use 120 V yo can produce very low torque. Look carfully at the wires. There should be either 4 or 6. They are actually in pairs. There are 2 pairs for the stator windings (4 wires), and one pair for the start winding.
 
  #3  
Old 09-18-03, 01:21 PM
P
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Are you positive that this was an induction-type motor? An induction motor has a rotating magnetic field that is constant in one direction until the connections to the windings are reversed.

"Several (leads) going to White)"???? For an induction motor, this would result in short-circuited coils.

Do you ever actualy operated this motor under a load-condition?
 
  #4  
Old 09-19-03, 07:15 AM
djhurt1
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Yes I did operate the motor. It was actually of an old air compressor. Ran quiet and strong. The only thing I did was change the power cord as the one that was on it was in pretty bad shape. I believe it is an induction motor because there is no electrical connection to the center(axle) magnet and it is not a real magnet, must be an electromagnet. The above post could be right because I must have gotten the wires messed up. I just don't see how but the motor did run fine prior to that.
djhurt1
 
  #5  
Old 09-19-03, 08:45 AM
brickeyee
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Induction motors actally operate in a very similar manner to a transformer. No electrical connections are mode to the rotor windings themselves. A single phase motor will need a centrifugal start switch and the switch often uses a pair of slip rings to make electrical contact with the switch. Around 3/4 horsepower many inductions can be connected to run on 120 V or 240 V. The stator windings are interconnected in series for 240 and in parallel for 120. At startup the motor is operating at stall and pulls very large currents (no back EMF is generated). As the rotor comes up to speed it generates a back EMF that reduces the current flow in the stator. If you load the motor it slows slightly (an increasing slip angle), the back EMF decreases, and the stator current increases. Interconnecting the stator windings incorrectly can indeed produce some funny behavoir.
 
  #6  
Old 09-19-03, 09:23 AM
djhurt1
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Well I must of had something funny in the wiring. I tried every possible combination. There is a centrifigul switch on it. I thought perhaps that was malfunctioning but it appears to be all well. Thanks guys for your input. I find these motors fascinating.
djhurt1
 
 

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