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Old Jun 22, 2007, 07:26 PM
Registered User
Joined Oct 2006
35 Posts
Single phase electric motor problem

Although this is not strictly an engineering problem, I hope there is someone who can point me in the right direction to solve a strange problem I have with an electric motor.

For about 25 years I have had an Elektra Beckum table mounted 12 inch circular saw which for several years has been used rarely, maybe only one job in a year. The motor is a 2.2 kw, 2,800 rpm capacitor start single phase induction motor, which is rated at 11.5A.

Recently I needed to use the saw, but when I pressed the start button 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 the no-volt switch.

The next time I attempted to start the saw it was obvious that it didn’t ‘know’ which way to rotate and blew the 13amp fuse – it has blown fuses ever since I’ve had the saw but not often enough to warrant any serious investigation.

At the next start up I spun the saw blade in right direction and then pressed the start button, whereupon the saw ran perfectly.

I have used the saw frequently over the last ten days and have found that I have to spin the blade to ensure the motor starts in the right direction. It has been cutting long lengths of timber with no signs of distress. If I don’t spin the blade the motor will mostly start in the wrong direction, very occasionally in the right direction, and has also blown another fuse.

Before I pull the motor to pieces does anyone have any ideas why this is happening and what to look for when I get inside the motor?

Thanks.

John
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Old Jun 22, 2007, 10:58 PM
Don Young
Guest
n/a Posts
Re: Single phase electric motor problem


"jlh45" <jlh45.2sls4n@rcgroups.com> wrote in message
news:jlh45.2sls4n@rcgroups.com...
>
> Although this is not strictly an engineering problem, I hope there is
> someone who can point me in the right direction to solve a strange
> problem I have with an electric motor.
>
> For about 25 years I have had an Elektra Beckum table mounted 12 inch
> circular saw which for several years has been used rarely, maybe only
> one job in a year. The motor is a 2.2 kw, 2,800 rpm capacitor start
> single phase induction motor, which is rated at 11.5A.
>
> Recently I needed to use the saw, but when I pressed the start button
> 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 the
> no-volt switch.
>
> The next time I attempted to start the saw it was obvious that it
> didn't 'know' which way to rotate and blew the 13amp fuse - it has
> blown fuses ever since I've had the saw but not often enough to warrant
> any serious investigation.
>
> At the next start up I spun the saw blade in right direction and then
> pressed the start button, whereupon the saw ran perfectly.
>
> I have used the saw frequently over the last ten days and have found
> that I have to spin the blade to ensure the motor starts in the right
> direction. It has been cutting long lengths of timber with no signs of
> distress. If I don't spin the blade the motor will mostly start in the
> wrong direction, very occasionally in the right direction, and has also
> blown another fuse.
>
> Before I pull the motor to pieces does anyone have any ideas why this
> is happening and what to look for when I get inside the motor?
>
> Thanks.
>
> John
>
>
> --
> jlh45
> ------------------------------------------------------------------------
> jlh45's Profile: http://www.rcgroups.com/forums/member.php?u=119557
> View this thread: http://www.rcgroups.com/forums/showthread.php?t=703339
>

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


Old Jun 23, 2007, 04:07 AM
Austin Shackles
Guest
n/a Posts
Re: Single phase electric motor problem

On or around Fri, 22 Jun 2007 18:26:30 -0500, jlh45
<jlh45.2sls4n@rcgroups.com> enlightened us thusly:

>For about 25 years I have had an Elektra Beckum table mounted 12 inch
>circular saw which for several years has been used rarely, maybe only
>one job in a year. The motor is a 2.2 kw, 2,800 rpm capacitor start
>single phase induction motor, which is rated at 11.5A.


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.

--
Austin Shackles. www.ddol-las.net my opinions are just that
Travel The Galaxy! Meet Fascinating Life Forms...
------------------------------------------------\
>> http://www.schlockmercenary.com/ << \ ...and Kill them.

a webcartoon by Howard Tayler; I like it, maybe you will too!
Old Jun 26, 2007, 06:25 PM
Registered User
Joined Oct 2006
35 Posts
Quote:
Originally Posted by Austin Shackles
On or around Fri, 22 Jun 2007 18:26:30 -0500, jlh45
<jlh45.2sls4n@rcgroups.com> enlightened us thusly:

>For about 25 years I have had an Elektra Beckum table mounted 12 inch
>circular saw which for several years has been used rarely, maybe only
>one job in a year. The motor is a 2.2 kw, 2,800 rpm capacitor start
>single phase induction motor, which is rated at 11.5A.


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.

--
Austin Shackles. www.ddol-las.net my opinions are just that
Travel The Galaxy! Meet Fascinating Life Forms...
------------------------------------------------\
>> http://www.schlockmercenary.com/ << \ ...and Kill them.

a webcartoon by Howard Tayler; I like it, maybe you will too!


Thanks for the advice Austin.

It is a 'constant capacitor' motor. I had a go at testing the capacitor and then had it checked when it appeared to be duff.

Fitted a new capacitor and the motor now works perfectly.

John
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Old Jun 27, 2007, 06:53 AM
ChrisQuayle
Guest
n/a Posts
Re: Single phase electric motor problem

jlh45 wrote:
> Austin Shackles Wrote:
>
>>On or around Fri, 22 Jun 2007 18:26:30 -0500, jlh45
>><jlh45.2sls4n@rcgroups.com> enlightened us thusly:
>>
>>
>>>For about 25 years I have had an Elektra Beckum table mounted 12 inch
>>>circular saw which for several years has been used rarely, maybe only
>>>one job in a year. The motor is a 2.2 kw, 2,800 rpm capacitor start
>>>single phase induction motor, which is rated at 11.5A.

>>
>>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.
>>
>>--
>>Austin Shackles. www.ddol-las.net my opinions are just that
>>Travel The Galaxy! Meet Fascinating Life Forms...
>>------------------------------------------------\
>>
>>>> http://www.schlockmercenary.com/ << \ ...and Kill them.

>>
>>a webcartoon by Howard Tayler; I like it, maybe you will too!

>
>
>
>
> Thanks for the advice Austin.
>
> It is a 'constant capacitor' motor. I had a go at testing the capacitor
> and then had it checked when it appeared to be duff.
>
> Fitted a new capacitor and the motor now works perfectly.
>
> John
>
>


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...

Chris
 


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