13th Mar 2008, 05:04 PM #1
Capacitor replacement in bench grinder.
My Bench grinder has a faulty 4uf capacitor so it wont spin up without help. I tried a 10uf of the same 400 ac 50/60 hz rating and it appears to work fine.
However, working fine for 5 -10 seconds test is encouraging but is there likely to be any problem using the larger capacity? It will fit easily into the base and its diameter is the same as the original, so the mounting clip is fine also.
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17th Mar 2008, 10:41 PM #2
dont do it
Hi, the capacitor is there for a phase shift between the 2 windings in the motor. you can easily get a capacitor at any appliance repair place that is the same as the original. going too far from the standard one in the unit is a road for disaster to the motor.gyro
17th Mar 2008, 11:47 PM #3
they will have one
they cost cents per unitray c
dunno what's more fun, buyin' the tools or usin' em'
18th Mar 2008, 09:06 AM #4
What I needed to know..
Off to Jaycar...
18th Mar 2008, 09:18 AM #5
Gyro.. I just checked.. Jaycar only have 6uf upwards.. They are motor start.. would this be OK?
18th Mar 2008, 11:56 AM #6
IMHO, yes. There is a reasonable amount of tolerance. I would not be overly concerned even with the 10uf, methinks Gyro is being overcautious. But it is a very long time since I have looked at electric motors, better safe than sorry.
Are you sure that the 4uf is "faulty"? I am having the same problem as you at times, but worked out that the motor is just struggling when the power supply is low (can be a bit erratic here, especially during a heatwave.)
18th Mar 2008, 12:19 PM #7
I'd rather not if I can avoid it as they are about $15.
I'm sure its the Capacitor.. Mainly as it has a "dodgey" on/off switch and one morning I went into the shop to find the grinder pounding away.. and pretty warm at that! After that it would only start with a "spin" of the wheel.. I notice in one of the suppliers listings that these are "not for continuous use" which presumably is what happened to the original.
I wonder what the possible harm could be by staying with the 10 uf ??
18th Mar 2008, 07:35 PM #8
You need to install the exact same cap as pulled out imho.
I spent 12 months in an electric motor shop rebuilding 1 and 3 phase motors and power tools and car alts and starters etc during my apprenticeship.
If it is a permanent split capacitor motor installing a 10 uf cap instead of a 2 uf will cause enough current to flow through the aux winding to cook it,
Also if you a 10 uf that is designed for starting only running it continuously will cook the capacitor you just paid $20 for.
I didn't know jaycar sold electric motor type capacitors...
I thought they only sold electronic type capacitors but I could well be wrong.
If you have no starting torque without the capacitor that is normal.
My bench grinder has a centrifugal switch. I don't know if your does?
If it does you hear a click at startup at about 70-90% full speed.
If this is faulty it will cook your motor as well and may or may not still click!
A sure sign of a faulty capacitor is the leaked oil everywhere and a distinctly I am fubarred smell.
This could occur from the switch contacts welding closed....
And so for you to replace the cap and hope its fixed isnt really a sure thing.
You need to make sure the switch is operating also.
This can be done by listening to the motor start and also with a current meter in the circuit to see the drop in current when the centrif switch disconnects the start winding.
I hope this helps
18th Mar 2008, 09:03 PM #9
Need a bit of clarity here, permanent split and centrifugal switch are mutually exclusive. Have a look at http://www.iprocessmart.com/leeson/l...se_article.htm, it might help, it is written in reasonably simple terms.
18th Mar 2008, 09:53 PM #10
Yes thats correct
A permanent split motor doesnt have a start winding... well it does its just called an auxillary winding and is connected in the circuit all the time thus negating the need for a start switch.Where as a cap start motor has a centrifugal switch and operates to disconnect the start winding after the motor is at almost synchronous speed.
Single phase motors come in a hundred varieties
here are some off the top of my head
cap start eg bench grinder
cap start cap run sometimes called two value motor eg belt drive air comp
permanent split eg fantech exhaust fan
shaded pole eg record player small exhaust fan
universal motor/Series motor batt drill vacuum cleaner circ saw grinder etc
I did state here that my bench grinder is cap start.
Im not sure that all are though as there are some pretty cheap and nasty bench grinders round these days and the real cheapies may have just a permanent split motor as they are cheaper to make without the centrifugal switch.These machines lack the grunt of a cap start motor and will not go the distance of a lifetime of home handyman abuse...
The term split is used to define the family of motors split phase induction motors.
The basis of which is you cannot create a rotating magnetic field with 1 phase and so you must create a second phase.
To do this one must split the phase!
By using a capacitor the current in the second winding will peak at a different time and so a second phase is made to get the motor up to speed. Once up to speed the pulsing of the opposing fields in the main winding is enough to keep the motor running and so the capacitor may be disconnected except in high torque demanding fields.(air comps-cap start cap run)
The reverse of this is true with the permanent split motor as it is a small motor designed for low load applications and made to a price.
The second winding is connected all the time as start switches cost money and as long as the load isn't to great on the motor it will last a good while...
Loading this type of motor causes more current to run in both windings and will cause the capacitor to fail prematurely sometimes.
Malfie please tell me for my own piece of mind is there more than the original designed load on this grinder?
Im interested to know if Ive blundered my way into an explanation for us all!
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