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capacitor question

slateef

slateef

Active Member
Hi all,

Asking this here since you guys are very knowledgeable and helpful. Not dealing with a stereo, but rather a general capacitor issue.

I bought a vintage bandsaw for woodworking and it seems that the start capacitor is bad (motor just whines/hums when turned on and actually starts running if you give the belt/pulley a nice manual pull). When I opened up the compartment that holds the capacitor, there is only one wire running to the capacitor from the inside of the motor! Plus, there is no marking on the capacitor marking positive or negative.

Have you heard of only one wire being used with a capacitor? I was skeptical and thought that there should be another wire somewhere, so I opened up the motor completely....no loose or hiding wire anywhere.

Is it even possible to use a capacitor as a motor starting capacitor with only one wire connected?

By the way, its a cylindrical capacitor, with two terminals on the top, but no markings to indicate positive or negative. Its 150 mfd, 125 vac.
 
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slateef said:
By the way, its a cylindrical capacitor, with two terminals on the top, but no markings to indicate positive or negative. Its 150 mfd, 125 vac.
Click to expand...

Not all capacitors are DC especially motor run capacitors as the one you have. You said it yourself that it is a 150mfd @ 125 Volts AC so no polarity. Its possible that the chassis or metal cover acts like a common connection. Do you have any pics?
 
I believe there are motor start, motor run or both on some motors that are / were made. I’m thinking it’s single phase use but I could be totally wrong on that point.
 
slateef said:
Is it even possible to use a capacitor as a motor starting capacitor with only one wire connected?
Click to expand...
Is the original capacitors case grounded to the motors case? If so..thats the second connection.
 
That's a motor start cap for sure. A run cap would be in the 5-10 uf range.

I was thinking the same thing as Avionic about the case being the other side. But, the case should be grounded and that's not where a start cap would be connected.

Typical circuit:



Yours may not have a Run cap. There is usually a centrifugal switch that shuts off the start winding after it gets up to speed. That winding is not meant for continuous duty [Edit: or, maybe it is, but the big cap is not in the circuit all the time]. Anyway you can see that the start cap has to have two connections: one to the hot side of the line, and the other to the motor via the switch.

Are you sure that single wire doesn't have two conductors in it?

Or, is there another wire that's not hooked up? Is it possible someone either disconnected a wire, or tried to replace the cap and didn't hook it up right?

They are usually nonpolar electrolytics since it's an AC circuit, so no positive or negative as I recall. In fact, some people like to use oil filled motor caps for crossovers.
 
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The metal mounting tab welded to the body of the cap is the other connection as stated. These caps are available from even more than the usual sources, Grainger is where I got the motor run cap for my Crown RTR.
 
slateef said:
Hi all,

Asking this here since you guys are very knowledgeable and helpful. Not dealing with a stereo, but rather a general capacitor issue.

I bought a vintage bandsaw for woodworking and it seems that the start capacitor is bad (motor just whines/hums when turned on and actually starts running if you give the belt/pulley a nice manual pull). When I opened up the compartment that holds the capacitor, there is only one wire running to the capacitor from the inside of the motor! Plus, there is no marking on the capacitor marking positive or negative.

Have you heard of only one wire being used with a capacitor? I was skeptical and thought that there should be another wire somewhere, so I opened up the motor completely....no loose or hiding wire anywhere.

Is it even possible to use a capacitor as a motor starting capacitor with only one wire connected?

By the way, its a cylindrical capacitor, with two terminals on the top, but no markings to indicate positive or negative. Its 150 mfd, 125 vac.
Click to expand...
Got photos? But that is a start capacitor, and most were dry paper caps w/ plastic case (some were sealed oil w/ metal case) so I am sure by just replacing it with something close, 150 - 175 MFD @125vac you'll be fine. Though I suspect yours is a metal can, and the can itself (non-insulated) is the other connection to the motor. In which case, the one terminal should be marked that the wire connects to, but wouldn't really matter. It's an AC motor so there is no actual "positive or negative"
 
But as you can see from the schematic I posted, neither side of the start cap connects to ground (or the motor case which should be the same thing).

Maybe the schematic does not have the same layout as OP's bandsaw. :dunno:

I guess, replace the capacitor and see if it fires up.
 
Could the two terminals both be the same...one for another optional connection but on this only one is used...and the case is essentially the other connection? Check continuity of the terminals and terminal to case? :dunno:
 
toxcrusadr said:
But as you can see from the schematic I posted, neither side of the start cap connects to ground (or the motor case which should be the same thing).

Maybe the schematic does not have the same layout as OP's bandsaw. :dunno:

I guess, replace the capacitor and see if it fires up.
Click to expand...
But the schematic isn't for his unit. ;) Plus his motor may be isolated on rubber mounts. But yeah I agree, replace the cap and see what happens!
 
Wow, go away for a few days and come back and see all these nice replies! Thank you all!

Here's a pic:

upload_2019-12-23_11-54-56.png

upload_2019-12-23_11-55-17.png

Sorry, not sure why the orientation changed....the original is right side up!

Anyway, there is only one wire coming out of the motor into the capacitor housing. I opened up the motor housing to be sure and there was no other loose wire. I went to Grainger the other day and got a replacement capacitor, but the top has 4 tabs (not sure what you call it). This capacitor housing is plastic. I hooked up one side and still the motor wouldn't start (just the same whining sound). I then hooked up the other side and same thing happened. Should I try to connect a separate wire from one tab to the motor housing (to act as a ground)?
 
slateef said:
Wow, go away for a few days and come back and see all these nice replies! Thank you all!

Here's a pic:

View attachment 1707583

View attachment 1707584

Sorry, not sure why the orientation changed....the original is right side up!

Anyway, there is only one wire coming out of the motor into the capacitor housing. I opened up the motor housing to be sure and there was no other loose wire. I went to Grainger the other day and got a replacement capacitor, but the top has 4 tabs (not sure what you call it). This capacitor housing is plastic. I hooked up one side and still the motor wouldn't start (just the same whining sound). I then hooked up the other side and same thing happened. Should I try to connect a separate wire from one tab to the motor housing (to act as a ground)?
Click to expand...
Well if that's the original cap, then there is a wire missing somewhere for the other terminal. Since you bought it this way, it's hard to tell where it is missing from. The make and model of the saw might help, along with some photos of the motor, it's tag, and maybe the end where the wires enter the motor, and switch wiring. You need both terminals connected for the cap to work and give the motor a momentary boost to start. Some induction motors don't employ a centrifugal switch, but instead use the loading of the motor to draw from the cap. When the current drops, the cap is basically just sitting there under no load.
 
You need the motors wiring diagram at this point...see if you can Google it. Might be a good idea to test the motor first to see if the switching circuit the cap is on is actually disconnecting after the motor is running..otherwise it will burn that cap and/or motor out as well.
 
No idea what the model of the bandsaw/motor is. Bandsaw is a generic (non-branded) unit from Taiwan. The motor's label doesn't indicate a manufacturer or model number. Just a generic 1 hp motor. The motor/bandsaw works perfectly once I give the belt a jumpstart by hand (just a gentle tug downwards and the motor starts right up and stays on perfectly as long as I need for it to). I may just have to live with it.
 
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