Capacitors are pretty generic... there are a bunch for sale on places like Amazon for fairly cheap that should be a direct replacement.

BTW: It doesn't matter if you wire the motor for 120 or 240 volts... the coils all see the same voltage (120v) regardless. If it runs on 120v, I'd keep it that way so you don't have to use up a 240v outlet. You don't get any additional power from going to 240v.

Also - are you sure the capacitor is bad... did you test it?

Cheers,
Brad

 

Good to know it doesn’t matter, thank you for the help. States on the saw that’s 125v rated so didn’t want to mess too much with it. Going 220v following possibly a lore or fact that it will make it more energy efficient giving extra juice.
I haven’t test it. My saw has been doing this rougher sound and vibrating more on the start and causing the breaker to trip. So I’ve checked whatever I could, changed the bearings and nothing so I figured could be the capacitor. I don’t have a tester and the saw only stays one for some 5 seconds before it trips, so figured changing wouldn’t be too much of a loss.

 

Oops, Brad beat me to it...

I couldn’t find a replacement for the exhisting capacitor, but also the current is rated for 110v and I’m taking advantage of the fact that I’m having to do some work on it to switch it to 220v.
wonder if anyone knows what the specs of the capacitor I should get?

Stop.
Do not pass go.

No need to change capacitor when switching the supply power to higher supply voltage; unless the capacitor measurement is bad.
On a dual voltage motor, the start circuit voltage is the value of the lower voltage; regardless of the supply voltage.

IF THE CAPACITOR IS BAD, Here are my start capacitor rough rule(s) of thumb:

2650 times Full Load Amps (name plate FLA) divided by supply voltage equals MFD of start capacitor.

For dual voltage motor: the start circuit supply voltage and capacitor voltage rating is always the lower supply voltage.
I.E. use the lower voltage on the calculation.

If FLA is not listed, use the FLA value from NEC motor tables based on HP.
More starting capacitance is seldom an issue. Less capacitance will burn out the capacitor faster, and may fail to start motor rotation quickly enough; causing tripped overloads/breakers. Extreme oversized (150%) start capacitors cause more wear on centrifugal switch or current relay; contacts, and accelerate these parts wear out. Large electrolytic (start) capacitors have +/-20% tolerance. So most folks tend use next larger size when guide calculates a value just above a common capacitor value. This also why start capactor value, tends to be a range.

PS - I find many of the online start capacitor calculator(s) produce values that are too low. The above formula is commonly used by HVAC tech's on AC compressors and fan motors. It's calculated values closely match the OEM start capacitor values in WW tool motors I restore.

 

Oops, Brad beat me to it...



Stop.
Do not pass go.

No need to change capacitor when switching the supply power to higher supply voltage; unless the capacitor measurement is bad.
On a dual voltage motor, the start circuit voltage is the value of the lower voltage; regardless of the supply voltage.

IF THE CAPACITOR IS BAD, Here are my start capacitor rough rule(s) of thumb:

2650 times Full Load Amps (name plate FLA) divided by supply voltage equals MFD of start capacitor.

For dual voltage motor: the start circuit supply voltage and capacitor voltage rating is always the lower supply voltage.
I.E. use the lower voltage on the calculation.

If FLA is not listed, use the FLA value from NEC motor tables based on HP.
More starting capacitance is seldom an issue. Less capacitance will burn out the capacitor faster, and may fail to start motor rotation quickly enough; causing tripped overloads/breakers. Extreme oversized (150%) start capacitors cause more wear on centrifugal switch or current relay; contacts, and accelerate these parts wear out. Large electrolytic (start) capacitors have +/-20% tolerance. So most folks tend use next larger size when guide calculates a value just above a common capacitor value. This also why start capactor value, tends to be a range.

PS - I find many of the online start capacitor calculator(s) produce values that are too low. The above formula is commonly used by HVAC tech's on AC compressors and fan motors. It's calculated values closely match the OEM start capacitor values in WW tool motors I restore.

Yeah Capt, thanks for the thorough answer, was looking for an answer and got a lesson, appreciate that. I mentioned the reasoning for changing it above, hopefully the swap will fix it, if not I think I’ll just get a delta motor a friend offered. Im in SoCal and it’s hard to find people that fix these, unfortunately, because I love old Rockwell tools.

 

You can test the capacitor with a regular multimeter in resistance mode. Google will give you several ways to go about it. It might be a good idea to check the centrifugal switch as well, as a failed one can cause premature failure of the capacitor. And you said you replaced the bearings... did you do the motor also? Might not be a bad idea, and easy to do since you need to open it up to check the centrifugal switch anyway.

Cheers,
Brad

 

Just for your edification, you have current and voltage mixed up in your original post. Voltage is the 110v you list, although most of the US is now considered 120 volts. Voltage is the potential, think of it as the pressure pushing the water in the hose. Second, current/amperage is the circuit's ability to flow, think quantity, or how fast you'd fill a 5 gallon bucket of water from a hose. A typical household circuit is rated for 15 amps as an example, but your voltage could be 120 v or 240 volts both with a 15 amp rating. Last, resistance is like your thumb over the end of the hose restricting how much water can come out of the hose.

You can purchase a very inexpensive multimeter that will allow you to test capacitors, voltages, resistance, continuity (part of a resistance measurement), and in some cases current (this takes a little more work and skill and some knowledge of the flow of electricity). The cheap meters may not be very accurate but for some basic measurements they should suffice.

Keep in mind that if you decide to embark on learning more about how electricity works and how things like switches, motors, and circuits work you could be dealing with lethal levels of electricity so don't "wing it", make sure you know what you are doing or ask someone who has the knowledge and experience to help you. Further, if the insulation or any part of your test equipment is faulty don't continue to use it and either replace it or properly repair it before proceeding.

That said, testing a capacitor in this case shouldn't expose you to dangerous levels of electricity in your electric motor as long as the power is disconnected, but keep in mind that in some circuits a capacitor stores electricity and the capacitor can give you a lethal shock if you don't know what you are doing.

 

I meant current like in the exhisting capacitor not the eletric current. Thanks for the comment!

 

For a 1.5hp 3450rpm Rockwell, you’re usually looking at a start cap around 250–300 uF at 250V AC. Best bet is to pull the old one and match rating. Please update when you find one.

 

A search of the details from the capacitor you showed in the picture led me to this one from Granger:
In case the link does not work it is a Dayton capacitor, model 2MDU2

Whoops, we couldn't find that.

The capacitance of the item in the picture unfortunately has the first number damaged. But, the capacitance of the one in the link is 460-552 uF, matching the values you can see in the picture, and is a 115v-125v motor start capacitor. They don't often fail, but if you don't have a way to test capacitance and leakage then at $10 it is cheap enough to swap anyway.

The breaker tripping may also be from the motor starting at just the wrong phase if your supply breaker is at the limit. I have this issue with an 18" 5hp bandsaw. The breaker really wanted to be larger for the starting surge current that can happen if the motor happens to be at the wrong rotation point. But, that was going to need rewiring too many things. For me it only happens once in a while, maybe 1/50 starts. I live with it.

In your case, if it is every time, then yeah, could be the capacitor has lost a lot and the motor is now trying to draw too much.

 

The voltage rating of a "start capacitor" needs to be at least 2X the supply voltage. The capacitance is in series with the inductance of the motor. That combination produces a voltage across the capacitor about 2X the line voltage. This phenomenon is sometimes referred to as "resonant voltage rise. The opposition of the capacitor, capacitive reactance, is instantaneously opposite to the opposition of the motor windings, inductive reactance. They essentially cancel each other out and the resistance of the start winding is all that is limiting start current so start current is large. High current produces high torque which is good for starting. An incorrectly sized capacitor, microfarads (uF or MFD) will not increase starting current. It may produce too little current resulting in low starting torque. The centrifugal switch stays closed too long because of the slow acceleration and the breaker trips because of prolonged current flow. Starting current will be greater than running current in all cases.

 

I'm going to play devil's advocate for a second here:

So, you say the voltage for the cap must be twice the line voltage in your post. Please then explain why the rating for the capacitor in the original poster's picture is 115 volts? Was this done by the manufacturer to cause a premature failure? Was the cap replaced at some point? Was there some other circuit analysis that should be considered? I'm not sure I've ever seen a start cap that had twice the line voltage rating.