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Motor wont start

When I plug in my saw the motor just hums

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Assuming the motor shaft is not frozen and the rotor turns freely by hand
All single phase induction (No brushes) motors require energizing a "starting" winding for a second or so to get the thing turning.
Once the armature in turning the "run" windings take over and the start winding is turned off. If power is left on more than a few seconds without the motor starting, the starting winding will be permanently damaged. It can tolerate power for only a few seconds.
The momentary connection to the start circuit is controlled by any of several different methods. The most common system is a switch located at the opposite end of the motor from the output shaft. The switch is in the end cap and there is a flyweight on the armature that opens the switch when the motor gets up to speed. If the switch fails to make contact when stopped the motor won't turn. The contacts can be worn to the point of not contacting or it may have sawdust or other matter between the contacts. Whatever the reason the contact is not making connection to the start winding.
Another method use is an "Inrush" relay. This may be located in the motor or anywhere else like the manual switch box or another box exclusively for the relay. Radial are saws frequently use this system. When the motor is powered up the current is several times the current that it takes to keep it running. This high current (inrush) is detected by the relay and snaps closed until the current falls to the lower running condition. In this design the relay is open at rest and closes for a small amount of time at power up.. In the first design the contact is closed anytime the motor is at rest and opens when up to speed.
Look for a fault in these areas.
There are many other ways to start single phase induction motors, but these are by far the most common.

Posted on May 12, 2013

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