Single phase electric motor problem

Although this is not strictly an engineering problem, I hope there i
someone who can point me in the right direction to solve a strang
problem I have with an electric motor.
For about 25 years I have had an Elektra Beckum table mounted 12 inc
circular saw which for several years has been used rarely, maybe onl
one job in a year. The motor is a 2.2 kw, 2,800 rpm capacitor star
single phase induction motor, which is rated at 11.5A.
Recently I needed to use the saw, but when I pressed the start butto
the motor started in reverse ? not what you want with a circular saw!
It has never done this before and there is no reverse option on th
no-volt switch.
The next time I attempted to start the saw it was obvious that i
didn?t ?know? which way to rotate and blew the 13amp fuse ? it ha
blown fuses ever since I?ve had the saw but not often enough to warran
any serious investigation.
At the next start up I spun the saw blade in right direction and the
pressed the start button, whereupon the saw ran perfectly.
I have used the saw frequently over the last ten days and have foun
that I have to spin the blade to ensure the motor starts in the righ
direction. It has been cutting long lengths of timber with no signs o
distress. If I don?t spin the blade the motor will mostly start in th
wrong direction, very occasionally in the right direction, and has als
blown another fuse.
Before I pull the motor to pieces does anyone have any ideas why thi
is happening and what to look for when I get inside the motor?
jlh45's Profile:
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I would check capacitor, internal starting switch, and start winding/connections in that order. I suspect a shorted capacitor might possibly cause reversed starting, all could cause failure to start and blowing of fuse.
Don Young, Alabama USA
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Don Young
On or around Fri, 22 Jun 2007 18:26:30 -0500, jlh45 enlightened us thusly:
it sounds like the capacitor's had it. Chances are, for that application, it's a "constant capacitor" (i.e. low torque) motor, unless you know otherwise. If so it hasn't got start windings nor an internal starting switch.
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Austin Shackles
Thanks for the advice Austin.
It is a 'constant capacitor' motor. I had a go at testing the capacito and then had it checked when it appeared to be duff.
Fitted a new capacitor and the motor now works perfectly.
-- jlh4 ----------------------------------------------------------------------- jlh45's Profile:
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May be the cap, but it's also worth checking all wiring to the motor, particularly the wiring to the start winding. If the motor has an internal centrifical switch (motor clicks on startup and slowdown), check the contacts and associated wiring for continuity. Use a battery and lamp for continuity checking and wiggle all the wires while testing.
Wiring and switch contacts left unused for a time can corrode and oxidise, especially in a damp environment...
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