Is my motor especially weak , or is it something else?

[RJSakowski]

All motors experience slip under load. A 2 hp motor running at 2500 rpm will theoretically develop 4.2 lb-ft. of torque. The available torque is further reduced by frictional losses in the bearings and pulley system. Contrast that with running at 500 rpm where the theoretical torque is 21-ft. Also, a 1725 rpm motor speed is the speed under load. The no load speed is actually closer to 1800 rpm.

Don't discount the effect of voltage drop due to inadequate supply wiring. My RF30 clone has a 2 hp motor and draws 20 amps at 120 volts under full load.

If it were me, I would get a better idea of the amount of slip experienced. Your motor may be functioning normally. Compare mottor speed with no belts attached, then with the belts but no load, and finally, under load.

 

[Canus]

Google "How to test capacitors". If you have a volt/ohm meter the test is pretty simple.

 

[pontiac428]

Google "How to test capacitors". If you have a volt/ohm meter the test is pretty simple.

While I understand and respect the steps of troubleshooting, I've arrived at an even simpler test- buy a new cap and install it. If it runs, it's fixed! How did I cut all those steps? Well, I've had motors before, and if it ain't running, smoking, or seized, it's about 80/20 on capacitors vs. centrifugal switch. Assuming the wiring is right (visual check). I know full well that motor caps can last for 50 years, but somehow new caps are only "good" for 24 months, after that you're on what the manufacturer calls "borrowed time" in their new and improved maintenance schedule.

The bad caps in the 1990s were supposedly from some lots of bad adhesive traced back to a specific tank in Southeast Asia, or so the story goes. Once that happened, manufacturers started treating caps like disposable, unreliable items that they can't be held responsible for. Like incandescent bulbs.

 

[markba633csi]

I like John's approach

 

[pontiac428]

I like John's approach

Hey, when a light bulb burns out, I don't grab my $400 Fluke meter and take apart the wall switch. I grab another bulb and try it. It's called The Principle of Parsimony, or simply Occam's Razor: The most obvious solution is the correct solution. You can navigate 80% of life's problems using that principle, and only pull out the big bag of tricks for the real stuff.

 

[DavidR8]

Well said John

 

[bulgie]

Google "How to test capacitors". If you have a volt/ohm meter the test is pretty simple.

Hmm, not seeing it. All the hits I'm seeing say I need a multimeter that measures capacitance. My meter doesn't have that function.

The other test I'm seeing described by some self-appointed experts is to charge it by connecting briefly to mains, then discharging with a screwdriver to see if it sparks. That seems both dangerous and not very useful; it doesn't tell you a value for the capacitance. It only shows that it can hold "some" charge.

If I take the advice of some other posters in this thread and just replace the Run cap, do I try to match the rating printed on my cap? Any advantage to using a different value, like "hot-rodding" it a bit, or do we assume the maker put the best capacitance in there? I haven't tried taking my Run cap out yet, so I hope the writing on it is clear enough to read.

For buying a new one, are there quality levels? Know of a good source for reliable ones?

Thanks to everyone who has commented.
-Mark

 

[tq60]

Do you have a basic ohm meter?

Remove cap from motor, then test.

Put it in ohms mode and use it to check the capacitor

No, it does not tell you capacitance or good/bad, it shows how yours acts.

Better with analog meter.

When you first connect the meter, the needle will move a bit then go back to open.

Now reverse the leads, the needle should go backwards then forwards hen back to open.

This shows the capacitor will take a charge.

Test 2, find some battery or dc power output power adaptor.

Use your voltmeter to confirm the voltage.

Now touch the capacitor leads to the power supply.

Remove, now measure the voltage on the capacitor leads, it should be same as supply.

That it the capacitor taking and holding a charge.

Sent from my SM-G781V using Tapatalk

 

[bulgie]

Do you have a basic ohm meter?

Remove cap from motor, then test.

Put it in ohms mode and use it to check the capacitor

No, it does not tell you capacitance or good/bad, it shows how yours acts.

Better with analog meter.

When you first connect the meter, the needle will move a bit then go back to open.

Now reverse the leads, the needle should go backwards then forwards hen back to open.

This shows the capacitor will take a charge.

Test 2, find some battery or dc power output power adaptor.

Use your voltmeter to confirm the voltage.

Now touch the capacitor leads to the power supply.

Remove, now measure the voltage on the capacitor leads, it should be same as supply.

That it the capacitor taking and holding a charge.

Sent from my SM-G781V using Tapatalk

Just a sanity check before I do all that — will it really tell me anything I don't already know? I mean, my motor seems to run pretty well, in everything I've used it for except the highest spindle speed. Are those tests that merely show that it takes "some" charge useful in this case? Would my motor run OK if I had no run cap at all or a blown one? Seems to me (total newb non-expert) that I should be looking for a cap that's out of spec, not one that's totally blown.

I've lived my whole life so far without ever wanting a capacitance tester, so I probably won't buy one. Just buying a new Run cap and swapping it in seems like the easiest test.

Oh yeah, one more dumb Q: how do I know which one is the Run, and which one is the Start cap?

Thanks all!

 

[RJSakowski]

Just a sanity check before I do all that — will it really tell me anything I don't already know? I mean, my motor seems to run pretty well, in everything I've used it for except the highest spindle speed. Are those tests that merely show that it takes "some" charge useful in this case? Would my motor run OK if I had no run cap at all or a blown one? Seems to me (total newb non-expert) that I should be looking for a cap that's out of spec, not one that's totally blown.

I've lived my whole life so far without ever wanting a capacitance tester, so I probably won't buy one. Just buying a new Run cap and swapping it in seems like the easiest test.

Oh yeah, one more dumb Q: how do I know which one is the Run, and which one is the Start cap?

Thanks all!

The start capacitor is the highest capacitance and is usually a non polarized electrolytic. The run capacitor is usually an oil filled.

You can get a fairly good idea of the capacitance value using a digital voltmeter by charging the capacitor to a particular voltage and discharging through a high value resistor. Connect a multimeter across the capacitor and measure the time it takes for the voltage to drop to 37% of its initial voltage. That time is equal to the product of the capacitance in microfarads and the resistance in megohms.

The internal resistance of the multimeter has to be taken onto consideration for this measurement. This can be determined in several ways. One is to measure the resistance in the voltage range to be used with another multimeter. Another is to place a known resistance in series with the voltmeter and observe the voltage drop across the voltmeter. The ratio of the voltage across the meter to the applied voltage will equal the ratio of the meter resistance to the total resistance. For most accurate results, the resistance of the known resistor should be close to that of the meter resistance.

Charge the capacitor to a starting voltage and connect the meter, starting a stopwatch at the same time. When the voltage reaches 37% of the starting voltage, stop the stopwatch. The time in seconds divided by the resistance in megohms is the capacitance in microfarads. To make the measurement less hectic, I charge the capacitor to a voltage exceeding my start voltage and start the stopwatch when the voltage drops to my starting voltage. For exampl, I migh select a start voltage of 10 volts. I charge the capacitor to 12 volts and connect the meter. When the voltage drops to 10 volts, I start the stopwatch. I stop the stopwatch when the voltage drops to 3.7 volts for my discharge time.