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Need advice on sizing a motor capacitor for a Jet horizontal band saw

Randalthor

Cast Iron
Joined
Mar 7, 2018
Location
Kansas City
I picked up a Jet Bandsaw like the one linked below in case anyone needs to know the model., except mine is used, not new.


I know Jet isn't the best equipment, but the price was right. It was from a plant closure (not a machining plant). It apparently wasn't working and one of the employees had taken the switch cover off to inspect the various switches.

I haven't tried to hook it up to power yet, but I noticed the starter capacitor cover was dented, so I removed the capacitor cover and the capacitor was also dented. I plan to get a new capacitor, which brings me to my question.

The motor is single phase 2hp, 230v (not 115/230V, but straight 230V on the name plate.) However, the capacitor is rated 400UF and 125VAC.

Everything I've read says that the capacitor needs to match, or be higher, than the voltage rated for the motor, but I thought I'd get a second opinion from the experienced forum members. I know it's possible someone could have put in the wrong capacitor, but this capacitor looks like it's been in there a long time and fits the cover exactly. I'm guessing it's original because I can't even find it on the internet after a lengthy search.

I found some off-brand 400UF capacitor 250vac, which will fit inside the cover (see link below).

https://jcmelectronics.com/products/25-am400ufx250v?_pos=1&_sid=135f7dbe6&_ss=r

I just wanted to check to make sure it's OK to use the 250V capacitor?

I have read that one needs to be careful of the capacitor's Effective Series Resistance (ESR) and the capacitor's resonant frequency, when switching capacitors. Is that something I need to be worried about?

Below are pics of the capacitor found in the motor, and the motor nameplate.

PXL_20240117_213509967[1].jpg

PXL_20240117_215619751[1].jpg
 
First thing I'd do is make sure it doesn't work. I knew of a horizontal saw where they said it didn't work. That's because they were trying to start the saw before lifting it off the stop. Tipped it up, hit the button and it worked fine. Also verify the E-stop is released. Assume nothing.
 
remove the cap from circuit and test it . most modern multimeters have a capacitance
setting . a dedicated L-c meter is great , but i prefer one of these:


for under $10 they test any component you can throw at them , including cap mfd
and esr value.

for a start cap, which has a tremendous task, i'd go w/ a 370v job . 250v is marginal
and a 125 would certainly let the smoke out at 240vac.
as i said, those start caps take a beating , but also keep in mind they can blow from
other factors like a bad centrifugal switch or burnt windings.
i'd try the motor for a few seconds first . it just may be ok as is .
 
There is a saying that, you can't know what you don't know. And I get that. Hence the nature of your question. So with that in mind, let me offer the thought process of, "What is the worst that can happen, if I just try it ?"
Well the cap could blow up. Fair as fair.
If it were me, and I tried to match up a cap as close as was available, I would just try it. Make sure you put the cover on, in case the cap explodes. But really, if the cap does explode, and the cover is on the motor, nothing bad can happen. Maybe an exploding cap seems bad to you, but if you take pre cautions, why would that be bad? Caps are cheap. Downtime can be expensive. I would just try it instead of analyzing it to death. Do what you feel to the extent you can control the variables. If you don't feel good about just trying it, maybe confront your fears and try to attain a higher perspective. See the big picture, so to say. Maybe you can't control the unforeseen variables, and that is ok too. You can always pay money to solve this a different way. You could buy a new motor, as it is a package solution that eliminates some of the variables, or has a different set of variables that you can feel more comfortable with the risk associated with them.

-Doozer
 
Yep. So much more likely that one of the switches is either off (because they didn't know how to operate it) or because it's sticky from coolant. If the down-switch is stuck or sticking, it won't start. It spends most of it's life parked on that switch.
 
Thanks for all the replies. Really appreciate it.

I probably ought to clarify a couple more facts. I haven't hooked the machine up to power yet because somebody had unhooked some wires in the switch panel. I have to figure out where those wires go before I juice it.

I thought maybe I'd order a new start cap partly because this one looks damaged from the outside, and partly because it's rated at 125V, when the motor is rated at 240V. I just didn't know if this was the prudent thing to do or not. I don't currently have a voltmeter which tests capacitors. Thanks especially for that idea.

Thanks also for the comment about the limit switch. I plan to check that.

I have an old Fluke meter. It's a nice digital meter I've owned for 30 years, but it doesn't have the capacitance feature. I did test the capacitor for resistance. I've read that a good capacitor will start out with basically zero resistance, then slowly climb to infinity. I don't know if that's B.S. or not? This capacitor pretty much reads about 3M ohms and doesn't seem to change much.

If I can figure out the wiring, I'll probably hook up power and see what happens with the old capacitor. As everyone points out, I suppose it can't hurt.
 
You capacitor is not going to be stressed long enough to burst.
Not a good excuse to replace it, but I would. You don't know what accumulative effects will do.
 
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Thanks again for the comments.

Today I was able to wire everything back up that had been disconnected and put power to the saw. Took me a while to track down everything and draw a sort of sketch of the circuits. Once I got it back together I pushed the start button and got nothing. I couldn't even hear the relays clicking. So I have to start going through switches, relays, and transformer with a voltmeter to find out where the power break is.

But I have an issue with this machine that I know nothing about. Allow me to explain.

The circuits seem fairly straightforward. There are two relays in the panel. One runs the oil pump, one runs the motor for the blade. The coil for the motor relay is started by the momentary start switch, then the coil is backfed by the said closed relay, which of course is a pretty standard way of doing it. Once the first relay is energized, it also feeds the coil to the second relay, which runs the oil pump.

The relay coil circuits can be broken by the stop switch or limit switch or an overload switch located at the bottom of the motor relay. The oil pump switch can break the circuit of the oil pump relay only. It has no effect on the motor relay.

My question regards the overload switch. It has a dial on it labeled RC Amp. I've no idea what the correct setting of that dial is supposed to be. Do I set it at the rated amps of the motor (15amps)? Also the overload switch has a reset button. You can see the reset button by the dial. I pushed it in and out, and still got nothing. Is it supposed to be pushed in or out to close the circuit?

Here are is a pic of the overload switch at the bottom of the motor relay, and pic of the panel. You'll notice a small broken piece of plastic below the panel. That's a piece somebody had broken off the end of the busbar. It has no effect on anything, other than to hold the end of the busbar in place.

PXL_20240119_032511369[1].jpg

PXL_20240119_032609957[1].jpg
 
My question regards the overload switch. It has a dial on it labeled RC Amp. I've no idea what the correct setting of that dial is supposed to be. Do I set it at the rated amps of the motor (15amps)?
Yes
Also the overload switch has a reset button. You can see the reset button by the dial. I pushed it in and out, and still got nothing. Is it supposed to be pushed in or out to close the circuit?
Pushed in once to reset a power break due to a over-current condition.

Pushing the reddish-brown part on the block (far right) will connect power to the motor.
If it works then your control stuff isn't correct. I wold check the fuse first.
Push at your own risk.
 
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Just wanted to follow up, in case anyone is interested. Got the saw running. It has a few main issues as far as I can tell.

Transformer, contactors and all manual switches were ok. However, as Donkey mentioned, the limit switch works inconsistently. Also the motor seems to have trouble starting. On the first start up, the overload tripped before it could start running. Maybe it's the capacitor, and/or the overload dial isn't set correctly. I have ordered a new capacitor.

The motor has been overloaded or overheated before. I noticed the busbar/terminal strip had some melted plastic where the motor connections occur (pic below). It remains to be seen if there is damage to the motor.

On the second try the motor fought it's way to starting. Probably took about 3/4 to 1 second of struggling before it actually got going. After that it seemed to start more normally, albeit maybe a small fraction of a second slower than what I would expect. Once it starts, everything sounds smooth.

I'm plan to replace the busbar, starting cap, and gear oil before I try cutting anything. Gear oil doesn't look pretty. Probably never been changed.

Thanks to everyone for the help.

PXL_20240119_032539535[1].jpg
 
I wonder if the factory lubricant in the motor has dried out, gummed up, etc. Do the motor end caps have an oil or grease port? I have seen motors with a little rubber plug axially straight out from the bearings to the edge. You were supposed to periodically remove the plug and shoot a little oil in there and replace the plug. One motor was not free spinning at all and it took some working at it to get that lubed, purged of tarred up lubricant and whatever.
 
I wonder if the factory lubricant in the motor has dried out, gummed up, etc. Do the motor end caps have an oil or grease port? I have seen motors with a little rubber plug axially straight out from the bearings to the edge. You were supposed to periodically remove the plug and shoot a little oil in there and replace the plug. One motor was not free spinning at all and it took some working at it to get that lubed, purged of tarred up lubricant and whatever.
That's a thought. I've also seen some motors with an oil port. I checked this one and it's completely sealed. No ports anywhere.
 
Start capacitor only see's 1/2 the line voltage, reason why a 125VAC rated capacitor does not smoke at 230VAC, but doesn't hurt to put in a higher rated voltage providing you have space. That being said, a slow starting motor can quickly kill the start capacitor. I would pull the motor and see if it is difficult to turn/how it starts w/o any load. See how easily the bandsaw wheels turn, and check the blade tension and alignment. Also could have increase contact resistance at the terminals, so replace the burn one and check all the wiring. Motor bearings are usually greased and sealed, they may have dried out or seized.
 
yep, saw coolant can chowder motor bearings somethin' fierce . 25+ years ago, the
makers of import motors still insisted on shielded bearings (.50 cents more ?) in wet
environment . unless the motor insides are rusted up, it takes little effort to swap
motor bearings . 6203, 6204... 2RS can be bought for less than $2 apiece.
easier than trying to revive a gunked up drive .
 
Those are good thoughts. I will try those things. Thank you.

The machine has been significantly neglected. They used cutting oil for coolant. The cutting oil was so thick and gunky in the bottom of the tank, that it blocked all the oil to the oil inlet on the pump impeller. Instead of changing the coolant oil and cleaning the bottom of the reservoir, they just unhooked the pump.

After a good cleaning, the pump works fine.

I'll check to see if anything feels bound. The factory where the saw came from is a place which produces a lot of fine dust. Not an abrasive dust, but a lot of dust nonetheless. It may be the motor bearings are gunked up. I'll check to see how easy it spins.
 
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Start capacitor only see's 1/2 the line voltage, reason why a 125VAC rated capacitor does not smoke at 230VAC, but doesn't hurt to put in a higher rated voltage providing you have space. That being said, a slow starting motor can quickly kill the start capacitor. I would pull the motor and see if it is difficult to turn/how it starts w/o any load. See how easily the bandsaw wheels turn, and check the blade tension and alignment. Also could have increase contact resistance at the terminals, so replace the burn one and check all the wiring. Motor bearings are usually greased and sealed, they may have dried out or seized.
Start capacitors can see line voltage or even higher. They form a resonant tank with the inductance in the start winding.

On some dual-voltage motors, the start winding/capacitor combination only sees 120V regardless of whether the motor is wired for 120 or 240V. That's unlikely to be the case here.
 
The capacitor voltage is what is specified by Jet, so maybe they know something you do not. Motor overload is usually set to the motor FLA for the operating voltage.

1705849109550.png
 
A 125V capacitor does not fail at 127VAC....

The usage is known, and the voltage rating is given as a "use with" voltage, not an absolute limit on actual voltage failure. It should be fine with 125VAC applications.

A dual voltage motor has a 125V start winding which is is tapped-in across half the seriesed windings for 240V operation, so it sees nominal 125VAC.
 








 
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