How to Test the Starting Capacitor on dead Motor?
-N.
trying to fix it.
Been getting good advice on rec.woodworking newsgroup, but I need more
precise testing methods to check out the starting capacitor and was
hoping someone on the group could guide me through it.
The motor is 120VAC, 14 amp, 3750rpm. Motor wasn't overloaded when it
died, was used moderately over the past 11 years in a non-commercial
capacity.
When I turn the motor on, it doesn't spin but only 'growls': I can hear
it getting current, but it just will not turn over and get up to speed.
The motor has two capacitors, a plastic cased starting cap and a metal
cased run cap. I've disassembled the motor and have the rotor removed
and am in the process of checking out the centrifugal switch. Before I
disassemble the centrifugal switch, I want to thoroughly test the
capacitors to make certain they are not the problem.
I have no experience fixing motors, but I have a good Fluke multimeter
and a VERY basic electronics grounding (i.e., a willing novice).
I would deeply appreciate it if I could receive safe and detailed
guidance on how to test the capacitors, beginning with the start
capacitor.
Thanks a bunch.
Cheers,
Neal
To contact me very carefully remove the Firing Pin from my email
address.
Dave M
"-N." <n.w...@firingpinverizon.net> wrote in message
news:3DD6E617...@firingpinverizon.net...
Neal, from my experience with AC induction motors (I do a bit of woodworking
myself), especially those used in a dusty environment such as a wood shop,
I'd troubleshoot in this order:
1) First and foremost, take the time to look at the centrifugal switch.
It's the most likely thing to cause trouble in this type of motor. The
contacts can become pitted from switching inductive loads, and you may be
able to clean and burnish them to get back in operation. If they're heavily
pitted, get a replacement set from a tool repair shop. The growling sound
is typical of a bad centrifugal switch, because the starting capacitor isn't
switched into the circuit.
2) Next, check the overload breaker switch (overload protector). You can
check this with your multimeter... it should have zero ohms resistance (or
as close to zero as your meter can check). If it does not, (reads infinite
ohms (open circuit), that's your problem. This is likely not your problem,
because an open protector will not allow any power to the motor at all.
3) Now we worry about the capacitors. Of course, the easiest way to check
these is by replacement. If steps 1 and 2 don't turn up anything, and if
you can get the starting capacitor without a long ordering delay or extreme
expense, get a replacement and plug it in. There is no reliable way to check
a capacitor like this with only a multimeter. Best bet would be to take it
to an appliance parts store or motor repair shop. They will be able to test
it for you, probably at no charge.
4) If those steps don't fix the motor, you're probably looking at a bad
starting winding on the motor, which usually means either a replacement
motor, or taking it to a motor rewind shop.
Hope that gets you going.... Good luck
--
Dave M dgma...@atxxt.net
Remove the xx for my correct email address
-N.
Thanks for the advice.
I just removed the bell housings (nomenclature: bell housing is the the
term for the motor's end caps?) of the motor. The motor
arbor and rotor easily came out through the shaft end. I can examine the
mechanical part
of the centrifugal switch easily now.
The part of the switch on the motor shaft is made of simple stamped metal parts
with springs which in default position pushes a
plastic ring axially AWAY from the rotor (nomenclature: is the plastic ring in
the
class of THRUST rings?). Conversely, pulling this plastic ring axially towards
the rotor
compresses the springs, and upon releasing the ring, it automatically snaps
back to
the default position, extended away from the rotor. There do not appear to be
any
adjustment provisions on the cheap stamped metal parts/springs/plastic ring
assembly.
Inside the bell housing, on the opposite inner side of a plastic insert that
contains the
incoming 120VAC terminals, there are additional wires, terminals, soldered
connections. There is also a little cheap plastic type of lever, spring loaded,
that
is obviously moved by the outer edge of the thrust ring and makes or breaks the
electrical contacts. The lever is part of the plastic insert, and I can see
various
metal contacts in there, not too unlike battery contacts on a Sony Walkman.
I suppose the next step is initial examination of the contact assembly, then
disassembly to further examine and test it?
Question:
1) Basic Centrifugal Switch operation: the switch's contacts are normally
closed
when the motor is off, current is flowing to the starter capacitor? After
startup,
when the motor attains a certain RPM, the mechanical element of the switch
moves the
thrust ring inward towards the rotor, which caused the thrust ring to
eventually
lose contact with the little plastic lever/switch that controls the electrical
contact: this causes the switch's contacts to open, thereby turning off the
starting
capacitor's circuit?
2a) What am I looking for during this part of the inspection? I have my meter,
do I
test for continuity? If, so, am I looking for the centrifugal switch's
electrical
contacts to be closed when it is depressed, that is to say, closed when in it's
default state, no current to the motor, with the motor switched off, which
closes
the start capacitor's circuit?
2b) Then, next step, remove pressure from the lever (simulating the thrust ring
being pulled towards the rotor during high RPMs) and thus allowing the little
lever
to return by spring action ... and observe if the if the switch's electrical
contacts become open, breaking electrical current flow to the start capacitor?
2c) Inspect all metal contacts for wear, contamination, mechanical soundness
and
electrical continuity. Inspect all wiring related to the switch circuit and to
the
start capacitor circuit for continuity?
Thanks again for the help.
Cheers,
Neal
Dave M wrote:
--
Sam Goldwasser
you should be able to spin the motor by hand and get it started. You
may have to remove the blade to get enough speed by hand. If it then
runs quiet without overheating, you've localized the problem to the starting
components or winding.
--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Site Info: http://www.repairfaq.org/REPAIR/F_mirror.html
Sam Goldwasser
of full speed (you can't test for that but if they are closed when stopped,
probably OK). The starting capacitor, switch, and starting winding are
in series with the AC line.
Joe Leikhim
Sam Goldwasser wrote:
--
Joe Leikhim K4SAT
Jlei...@nettally.com
"Jazz is not dead. It just smells funny." -F.Z.
-N.
The motor is now functioning smooth and strong once again. I believe the
problem must have been with the centrifugal switch, possibly dirt. I did
reassemble once after a quick general cleaning of the motor with a blow gun run
off the compressor. However, I later completely removed the plastic terminal
block in the motor's endbell to gain access to both sides of the centrifugal
switch. I blew the hell out of it with a fine nozzle on the blowgun, then was
going to clean with contact cleaner, alcohol, and Q-tips, but discovered I was
out of all of these (shopping time). I decided to reassemble anyway and test.
It now works!
I put the belt back on and connected it to the TS blade and it runs well: I
couldn't run it under load cutting wood because it's 2:00 am in the morning
here.
Thanks a bunch to you all for contributing to the solution and thankfully, to a
successful repair!
P.S: Sam, I may need to pick your brain again. You and others had helped me on
this newsgroup (about 6 months ago) with I was attempting a repair of a Kodak
Ecktagraph slide projector motor. I abandoned the repair when I hit a wall and
at the very same time I began a MAJOR shop fixture/furniture building
operation. I'm nearing the end of the project building and one piece I made is
a dedicated mobile electronics workstation. So, when I get setup again and
organized, I'll post back within the next few weeks and bug ya', then hopefully
resume the repair and gain a bit more electronics knowledge in the process.
Cheers,
Neal
To contact me very carefully remove the Firing Pin from my email address.
Sam Goldwasser wrote:
--
Dave M
> Neal
>
> To contact me very carefully remove the Firing Pin from my email address.
>
>
There are no adjustments. The switch is normally closed when the motor is
stopped, placing the starting capacitor in the circuit. When the motor
speeds up, the centrifugal switch opens, taking the starting capacitor out
of the circuit, since it's done its job; getting the motor up to speed.
If the switch doesn't close properly when the motor is stopped, the starting
capacitor isn't in the circuit, and the motor won't start unless helped
along, as Sam indicated.
The springs are there to make the switch snap closed when the motor stops,
ready for another startup cycle.If they've lost their strength or broken,
the switch may not close properly. There are a couple of weights on the
mechanism that, as the motor gains speed, by centrifugal action cause the
switch to open (hence, its name). You should make yourself a drawing of the
connections and which wires go where. If you go by memory, you risk
damaging the motor beyond repair.
If the switch contacts are pitted or burned, a bit of cleanup with a small
flat jeweler's file may restore them well enough to last till you can get a
suitable replacement. I've been able to keep all my motors going this way
for years.... haven't replaced a switch yet. You should be able to see the
condition of the contacts immediately when you get the switch apart. If
they don't make a good, low resistance contact (use your ohmmeter on its
lowest scale, and properly zeroed) when closed, clean them up with some
contact cleaner or non-flammable solvent (WD-40 works OK in a pinch) and
file down the pits or burned areas so that a good low-resistance contact is
made.
-N.
Thanks for the review and for the added info on cleaning and checking contacts.
It's stored in my memory for the next repair job.
Cheers,
Neal
To contact me very carefully remove the Firing Pin from my email address.
Dave M wrote:s, beginning with the start