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B&D Grinder Broken

C

Chris

Jan 1, 1970
0
I have a small Black & Decker bench grinder that I picked up at a
second-hand store. When turned on, it just buzzes loudly and will not spin,
even when forced. I removed the rotor from the stator and sanded rust off of
both but to no avail. Any help would be greatly appreciated.

Chris
www.chrisbot.com
 
W

Wild Bill

Jan 1, 1970
0
Small bench grinders often have capacitor run motors in them (not the same
as capacitor start split phase). Typically, these models have oilite bronze
bearings in them.
If the bearings are in good enough condition that you can spin the shaft
with your finger tips, your model might have a bad capacitor. If the shaft
doesn't spin freely with no AC power applied, you'd need to correct the
bearing friction problem first.. polish the shaft in the areas of the
bearings and oil the bearings).

If you model has a capacitor, it will be an AC capacitor, probably a small
value (uF size) and a voltage rating of at least 120VAC (maybe as high as
370VAC).
A DC capacitor can't be used for an AC motor operation application.

So where can you find a suitable AC capacitor, you might be wondering. In
several of the home improvement retail stores (the big box stores), in the
electrical parts or fan section. The ceiling fan capacitors are the same
type used in AC motors as run capacitors.
You'll want to chosse a value that's the same as the original one. The
voltage would most likely be in the range explained above.

Larger bench grinders will have a split phase capacitor start motor with
ball bearings. The capacitor used in these motors will have a value size of
about 100uF per HP.
The voltage rating will be.. same as above. These capacitors aren't
generally stocked in retail stores, so you would probably have to get one
from an electric motor shop or a HVAC supply dealer.
In these fractional-HP motors there is an internal centrifugal switch used
for start/run switching. The contacts of these switches become pitted
(decreasing their performance) from years of usage. The contact's
reliability can be restored by cleaning and burnishing them (not just filing
or sanding them).
I'd service the contacts (or replace them if they're badly burned/pitted)
before replacing the capacitor in this type of motor.

Cheers
WB
.................
 
C

Chris

Jan 1, 1970
0
The grinder uses a shaded pole induction motor. This kind of motor has not
brushes or capacitor. The poles of the rotor are fabricated in such a manner
as to mechanically alter the magnetic field for startup. The grinder will
spin freely as long as no current is applied. When current is applied, the
motor buzzes loudly and movement by hand is opposed with great resistance.
The coil's DC impeadence is approximately 50 Ohms. I cannot detect a short
to frame so I do not suspect one.

Any ideas to resolve the problem are greatly appreciated.

Chris
Tampa, FL USA
 
W

Wild Bill

Jan 1, 1970
0
The start capacitor value for fractional HP split phase capacitor start
motors should read..
a value size of about 500uF per HP (100uF for 1/4 HP, 500uf for 1 HP).

Cheers
WB
 
S

Sam Goldwasser

Jan 1, 1970
0
Chris said:
The grinder uses a shaded pole induction motor. This kind of motor has not
brushes or capacitor. The poles of the rotor are fabricated in such a manner
as to mechanically alter the magnetic field for startup. The grinder will
spin freely as long as no current is applied. When current is applied, the
motor buzzes loudly and movement by hand is opposed with great resistance.
The coil's DC impeadence is approximately 50 Ohms. I cannot detect a short
to frame so I do not suspect one.

Any ideas to resolve the problem are greatly appreciated.

There isn't much else besides the windings at this point. Even if the
shading rings were damaged or open, the rotor would spin but not
start. You could do a ring test on the coil.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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Important: Anything sent to the email address in the message header is ignored.
To contact me, please use the feedback form on the S.E.R FAQ Web sites.
 
J

Jeff Wisnia

Jan 1, 1970
0
Chris said:
The grinder uses a shaded pole induction motor. This kind of motor has not
brushes or capacitor. The poles of the rotor are fabricated in such a manner
as to mechanically alter the magnetic field for startup. The grinder will
spin freely as long as no current is applied. When current is applied, the
motor buzzes loudly and movement by hand is opposed with great resistance.

When you rotate it by hand against that "great resistance", does it
"snap" from one pole to the next pole on the rotor, or does it feel like
it's bound up mechanically?

If the later, then *something* is worn or shifted enough mechanically to
let the rotor get pulled against the stator under the attraction of the
magnetic fields. I'd open it up again and see if you can determine by
eye or with feeler gages whether the alignment of the rotor to the
stator has gone out of concentricity, especially when you push or pull
the shaft(s) "sideways".

Last year my own little Craftsman bench grinder,which I've owned for
maybe 45 years started "losing power". It uses a universal motor.
(Brushes and a commutator, with a small timing belt drive to reduce the
motor speed.)

I opened it up and saw signs of scraping on the armature. Turned out the
pot metal housing onto which the stator and motor bearings were mounted
had warped over the years and moved things out of alignment. A couple of
shims placed in the right spots put the rotor and stator concentric
again and it's good as new again.
The coil's DC impeadence is approximately 50 Ohms. I cannot detect a short
to frame so I do not suspect one.

Any ideas to resolve the problem are greatly appreciated.

Chris
Tampa, FL USA


Good Luck, let us know what it turns out to be.

Jeff
--

Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can smile when things are going wrong, you've thought of someone
to blame it on."
 
H

H. R. Bob Hofmann

Jan 1, 1970
0
Chris said:
The grinder uses a shaded pole induction motor. This kind of motor has not
brushes or capacitor. The poles of the rotor are fabricated in such a manner
as to mechanically alter the magnetic field for startup. The grinder will
spin freely as long as no current is applied. When current is applied, the
motor buzzes loudly and movement by hand is opposed with great resistance.
The coil's DC impeadence is approximately 50 Ohms. I cannot detect a short
to frame so I do not suspect one.

Any ideas to resolve the problem are greatly appreciated.

Chris
Tampa, FL USA

My experience is that there may be enough play in the bearings to
allow the armature to come in contact with the stator. Also, even if
the bearings are not worn, they frequently are mounted in a metal
spring-loaded bearing holder, and if the mounting spring loses its
tension, it will allow the bearing to shift a tiny amount, which is
all that is needed to let the armature touch the stator. If you can
move the armature radially in any direction even a fraction of a
millimeter, it is either the bearings or the bearings holder. As much
as I hate to throw anything out, if it is the bearing holder, I don't
have any suggestions on how to fix the motor, as the bearing holder is
not replaceable as far as I can tell. Let us know what you find.

H. R. (Bob) Hofmann
 
J

Jim Adney

Jan 1, 1970
0
Chris wrote:

When you rotate it by hand against that "great resistance", does it
"snap" from one pole to the next pole on the rotor, or does it feel like
it's bound up mechanically?

If the later, then *something* is worn or shifted enough mechanically to
let the rotor get pulled against the stator under the attraction of the
magnetic fields. I'd open it up again and see if you can determine by
eye or with feeler gages whether the alignment of the rotor to the
stator has gone out of concentricity, especially when you push or pull
the shaft(s) "sideways".

Check also to see if any foreign matter might have gotten in there.
There might be a piece of steel which is not obvious, but which might
stand up when the magnetic field comes on, and cause a problem.

-
 
W

Wild Bill

Jan 1, 1970
0
One of my bench grinders is the same type you describe, 2 poles with 2
shading rings. The stator has 2 windings connected in series and the
resistance measures 11 ohms total, at the AC plug blades (unplugged from the
receptacle with the switch on, of course).
Your resistance reading is likely to be different due to differences in
meters and the gauge of wire in the winding(s). The wire looked to be about
22 ga in this unit.
If your stator has 2 windings, you might be able to determine which one is
faulty. I don't know if a growler would work for this test.. if you have
other test instruments, you might be able to verify (and possibly isolate)
the fault. A milliohm meter probably won't provide enough resolution.

As Sam has said, the only fault it could have is the coil windings. I'm not
able to describe the ringing test to you since I don't do that test with
laminated steel cores, but I think it involves using an audio generator and
a 'scope.
That would only verify that the windings have shorted turns, and you'd most
likely need to rewind the stator (or half of it).

An alternative would be to omit the stator and use the housing as an arbor
to be driven with a belt and an external motor (cutting a hole in the back
or bottom of the case for a belt to connect the other motor).

The grinder I checked is a working K-Mart 5" model, probably from the 70s.
Model/code 30-10A T1
115 VAC 3.2A 3500 RPM
Rotor size about 2" x 1-1/2" diameter

Cheers
WB
.............
 
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