To avoid these ads, REGISTER NOW!

Need a replacement starter capacitor for a grinder..

L

larryq

Well-known member
Joined
Jul 12, 2011
Messages
2,421
I have an older bench grinder with a cylindrical starter capacitor. I want to redo the wiring, including the capacitor, but have no idea what to get as a replacement, what specs I need. The existing capacitor has no specs on it.

The grinder itself is 1/3 HP, 3.5 amps and 115v. The existing cap is about 3.5" long and has a 1.5" diameter.

WRT to connecting to the two posts on the replacement, does it matter which wire goes to which post?

Any suggestions on what to get from, say, McMaster-Carr?:

http://www.mcmaster.com/#ac-electric-motor-capacitors/=10ca1rw
 
Last edited:
To avoid these ads, REGISTER NOW!
M

mattblast

Well-known member
Joined
Jan 30, 2013
Messages
784
Location
Bridgewater, NJ
I buy motor capacitors from Home Depot. They have all the most common sizes available and ship free to your local store. Cheaper than eBay or Amazon. Takes 3 or 4 days before they are ready for pickup.

http://m.homedepot.com/s/capacitor

If old one is soldered on I suggest to crimp terminals to wires to allow for easy change out in the future.

Be sure to match voltage and capacity (mfd) to original. If unknown try looking up in parts manual from motor model number.
 
D

dogdog

Well-known member
Joined
Nov 15, 2011
Messages
12,711
Jim Pelosi said:
Oops! 10,000 uf. Been a while.

Sent from my SM-G360P using Tapatalk
Click to expand...

Not sure if there are any 10,000 uf AC capacitors around if they exist probably not cheap or an unusual item. 350V - 400 DC yes and it's common. Besides that motor would have to be monstrous to needed 10,000 uf start capacitor and definitely won't be 3"x1.5" diminsions.

Those cap might have marked with mfd/MFD which usually is uf.

Easiest is for OP to post make model and pic of the capacitor with markings.
 
Last edited:
E

exmaxima1

Well-known member
Joined
Jun 25, 2011
Messages
6,341
Location
Midwest
larryq said:
I have an older bench grinder with a cylindrical starter capacitor. I want to redo the wiring, including the capacitor, but have no idea what to get as a replacement, what specs I need. The existing capacitor has no specs on it.

The grinder itself is 1/3 HP, 3.5 amps and 115v. The existing cap is about 3.5" long and has a 1.5" diameter.

WRT to connecting to the two posts on the replacement, does it matter which wire goes to which post?

Any suggestions on what to get from, say, McMaster-Carr?:

http://www.mcmaster.com/#ac-electric-motor-capacitors/=10ca1rw
Click to expand...

What is the Brand of the grinder? Approx age? Pics would really help.
 
P

pepi

Well-known member
Joined
Mar 27, 2013
Messages
2,883
Location
Woodstock, GA
Does the grinder work, start and run? Makes no sense to take a working machine apart rewire and randomly replace parts. Clean it, paint it and use it a more logical approach.
 
D

dogdog

Well-known member
Joined
Nov 15, 2011
Messages
12,711
Jim Pelosi said:
http://www.amazon.com/gp/aw/d/B00NT..._SY170_QL70&keywords=10000+uf+capacitor&psc=1

Sent from my SM-G360P using Tapatalk
Click to expand...

yea those marking are MFD usually means uF . Not MF as in 10,000 uF in some reporting convention.

the one in your link is 10uf capacitor not 10,000 uf.


as far as the op's scenario, best is to post a pic of that capacitor to determine the type.
either beg borrow or steal a multi meter that would measure capacitance.. most meter will be capable or measure 10uf ranges. probably not 10000uf ranges.
 
Last edited:
To avoid these ads, REGISTER NOW!
W

wvrailroader

Well-known member
Joined
Jan 20, 2014
Messages
951
Location
West Virginia
Take it to your local electrical supply store. They can help you determine what size it is and get you a replacement. The local one I use has always been very helpful in situations like this.

Sent from my SM-G900R4 using Tapatalk
 
B

bonneyman

Well-known member
Joined
Apr 22, 2010
Messages
8,795
Location
Desert SW
The only way to know for sure is to contact the motor manufacturer. They designed and built the motor, and they will know what capacitance the windings need. Since it's an old unit, that's probably out of the question.
So we're back to square one.

The voltage rating can be estimated somewhat, as AC motors run a back emf voltage that's higher than the supply voltage. 120VAC supply would require around a 220v or 250V cap. Going over on voltage rating won't hurt anything. Over is OK, under is a no-no. All that limits you is the physical size of the cap - will it fit? If a 250v will fit, you should be OK.

mf is another guessing game entirely. Best chance - still a long one - is take the OEM cap into a reputable capacitor wholesaler and let them try and determine what size it is. Not great, but looks like all you've got - unless someone else with that exact same grinder can open theirs up and tell you what the cap is.
 
P

pepi

Well-known member
Joined
Mar 27, 2013
Messages
2,883
Location
Woodstock, GA
You can check the cap it with a meter, see it charge and discharge that's what it does. It will be one of three readings, shorted, open or working charging and discharging.
 
D

Davefr

Well-known member
Joined
Jan 7, 2010
Messages
11,825
Location
OR
It's common to find old motors with bad starting caps and no specs. I'd start by searching the internet for similar motors and see if you can find a parts list with cap values. Similar motor designs will likely have pretty similar caps.

Plan B is the trial and error method. It's easy if you have a Turbo Cap.

85809.jpg


If not, just get a handful of caps with a 220V rating and start increasing the capacitance until the motor starts and comes up to speed quickly. (you increase capacitance by wiring caps in parallel). You won't hurt the motor as long as you make quick trials. Once you find the right value get a cap with that value or one notch higher. Capacitance is generally not a precise spec. Just make sure you get a voltage spec higher then line voltage. (ex: 180-220v for a 120 V motor).

I used that method on my Milwaukie project and 50 MFD turned out to be the sweet spot.

grinder%20005.JPG


Sometime you can also narrow down the cap's value based on it's physical size. It would at least get you in the ballpark.

Around here, Ace H/W sells motor caps.

At one time there were electric motor repair shops. I haven't seen one in years. The few that are left only do the big stuff.
 
Last edited:
OP
L

larryq

Well-known member
Joined
Jul 12, 2011
Messages
2,421
Thanks everyone. The grinder itself is an old Black and Decker, probably from the 1950s-- the capacitor has no markings on it regarding capacitance or other specs unfortunately.

Definitely the original capacitor in the grinder, which is why I want to replace it-- instead of a hard plastic cover it's a thick cardboard that looks 50 years old.
 
M

mikebaker1129

Well-known member
Joined
Oct 16, 2014
Messages
1,576
Location
Huffman,TX
You can also have the capacitor measured by some one with a meter that measures capacitance. This will work if the cap is good,you will still have the issue guessing with the voltage rating.
 
4

404

Well-known member
Joined
Aug 23, 2014
Messages
3,463
Location
Mass
larryq said:
Thanks everyone. The grinder itself is an old Black and Decker, probably from the 1950s-- the capacitor has no markings on it regarding capacitance or other specs unfortunately.

Definitely the original capacitor in the grinder, which is why I want to replace it-- instead of a hard plastic cover it's a thick cardboard that looks 50 years old.
Click to expand...

Might be good for another 50 years:thumbup:
 
E

exmaxima1

Well-known member
Joined
Jun 25, 2011
Messages
6,341
Location
Midwest
Davefr said:
If not, just get a handful of caps with a 220V rating and start increasing the capacitance until the motor starts and comes up to speed quickly. (you increase capacitance by wiring caps in parallel). You won't hurt the motor as long as you make quick trials. Once you find the right value get a cap with that value or one notch higher. Capacitance is generally not a precise spec. Just make sure you get a voltage spec higher then line voltage. (ex: 180-220v for a 120 V motor).

I used that method on my Milwaukie project and 50 MFD turned out to be the sweet spot.

grinder%20005.JPG


Sometime you can also narrow down the cap's value based on it's physical size. It would at least get you in the ballpark.

Around here, Ace H/W sells motor caps.

At one time there were electric motor repair shops. I haven't seen one in years. The few that are left only do the big stuff.
Click to expand...

The orange cap in that pic does not look like a non-polarized type, as it has a polarity mark. For motor use, you need non-polarized or "ac" capacitor.
 
P

pendragon1998

Well-known member
Joined
Mar 24, 2012
Messages
3,733
Location
NE Georgia
larryq said:
Thanks everyone. The grinder itself is an old Black and Decker, probably from the 1950s-- the capacitor has no markings on it regarding capacitance or other specs unfortunately.

Definitely the original capacitor in the grinder, which is why I want to replace it-- instead of a hard plastic cover it's a thick cardboard that looks 50 years old.
Click to expand...


My block grinder had one of those cardboard wrapped capacitors. I peeled off the cardboard and the specs were written on the capacitor body under the cardboard.


Read post 33 in the grinder link in my signature.
 
Last edited:
OP
L

larryq

Well-known member
Joined
Jul 12, 2011
Messages
2,421
pendragon1998 said:
My block grinder had one of those cardboard wrapped capacitors. I peeled off the cardboard and the specs were written on the capacitor body under the cardboard.


Read post 33 in the grinder link in my signature.
Click to expand...

I'll try that tonight, thanks!
 
D

Davefr

Well-known member
Joined
Jan 7, 2010
Messages
11,825
Location
OR
exmaxima1 said:
The orange cap in that pic does not look like a non-polarized type, as it has a polarity mark. For motor use, you need non-polarized or "ac" capacitor.
Click to expand...

Ideally yes. Especially for a run (ie high duty cycle) capacitor.

For a starting capacitor with a duty cycle measured in just a few seconds it really doesn't matter.
 
OP
L

larryq

Well-known member
Joined
Jul 12, 2011
Messages
2,421
Ok, as pendragon suggested, I removed the cardboard wrapping and sure enough, the specs were on there. 110 VAC and 72-87 microfarads.

Time to go shopping...I assume I should aim a little high on the replacement, once again using the mcmaster page as a guide:

http://www.mcmaster.com/#ac-electric-motor-capacitors/=10dqc07

...looks like the one listed at 125 AC and 88-105.6 would do the job?
 
Last edited:
M

mattblast

Well-known member
Joined
Jan 30, 2013
Messages
784
Location
Bridgewater, NJ
I would get the 72-86.4 rated one. With rounding that is identical to what you have.

It is rated for a higher voltage but that's ok. Just don't go with lower voltage.
 
4

404

Well-known member
Joined
Aug 23, 2014
Messages
3,463
Location
Mass
A higher value of capacitance is not "better". A higher voltage rating is fine.

A capacitor is like a spring on a car. A too soft spring is as bad as a too stiff spring. The inductance of the motor is like a mass. The capacitor is tuned to the inductance as a spring is tuned to the weight/mass of a car.
 
OP
L

larryq

Well-known member
Joined
Jul 12, 2011
Messages
2,421
mattblast said:
I would get the 72-86.4 rated one. With rounding that is identical to what you have.

It is rated for a higher voltage but that's ok. Just don't go with lower voltage.
Click to expand...

Ok, thanks. (and to 404 too.)
 
E

exmaxima1

Well-known member
Joined
Jun 25, 2011
Messages
6,341
Location
Midwest
Davefr said:
Ideally yes. Especially for a run (ie high duty cycle) capacitor.

For a starting capacitor with a duty cycle measured in just a few seconds it really doesn't matter.
Click to expand...

You might do that for your own DIY projects, but I would not advocate that in practice. It doesn't cost much to use the proper (AC) device. Do it correctly and it will last another 20-50 years.
 
4

404

Well-known member
Joined
Aug 23, 2014
Messages
3,463
Location
Mass
exmaxima1 said:
You might do that for your own DIY projects, but I would not advocate that in practice. It doesn't cost much to use the proper (AC) device. Do it correctly and it will last another 20-50 years.
Click to expand...

Agreed.:rocker::thumbup::beer:
 
You must log in or register to reply here.
To avoid these ads, REGISTER NOW!
Top Bottom