Switch wiring - Jet 8 X 18 mill

[yellow_cad]

I have a Chinese lathe/mill combo (Jet 8X18) made in the 1980s. It calls for capacitors in the start switch system for both of the single phase motors. For the lathe, it calls for capacitance of 25 mf for 110V and 12mf for 220V. The capacitor that was in the lathe circuit when I got it look like the original (due to the hook up) and was rated 20mf and W.V. 250 VAC. First of all, would a 250V capaitor work if the lathe is hooked up 110V? Secondly, this is more mf (20 vs 12) than specified. Is it OK to have more mf just not less? Thirdly, no where in the specifications does the mfg. state whether the capacitors are start, run, or start/run. Which type would be appropriate in this situation? Thanks for any input. Jim

 

[Tony Wells]

Re: Capacitor questions

Working voltage ratings on capacitors are just a safety margin that lets you know if the insulation they are built with will withstand the needed voltage. Within reasonable limits, you can exceed the minimum voltage requirements with no ill effects. Never go under on WV, however. Bad things can and probably will happen.

On the capacity, the mfd rating is related to a time constant that tells us how long the capacitor will take to "charge", similar to a battery. They discharge when the AC current reverses polarity at 60 cycles. The higher the capacity (mfd) the longer this cycle is. In general, the tolerances on this type of capacitor are fairly generous, so I wouldn't be afraid to sub the 20 for the 12 if I couldn't get a 12 easily. I probably would get a 12 mfd and swap as I could, but I wouldn't hesitate to run it for a while. Just keep check on how the motor sounds, and whether is seems hot. I think you'll be OK with the sub though.

 

[yellow_cad]

Re: Capacitor questions

Working voltage ratings on capacitors are just a safety margin that lets you know if the insulation they are built with will withstand the needed voltage. Within reasonable limits, you can exceed the minimum voltage requirements with no ill effects. Never go under on WV, however. Bad things can and probably will happen.

On the capacity, the mfd rating is related to a time constant that tells us how long the capacitor will take to "charge", similar to a battery. They discharge when the AC current reverses polarity at 60 cycles. The higher the capacity (mfd) the longer this cycle is. In general, the tolerances on this type of capacitor are fairly generous, so I wouldn't be afraid to sub the 20 for the 12 if I couldn't get a 12 easily. I probably would get a 12 mfd and swap as I could, but I wouldn't hesitate to run it for a while. Just keep check on how the motor sounds, and whether is seems hot. I think you'll be OK with the sub though.

Thanks for the response Tony. Do you have any thoughts on the type of capacitor (start, run, start/run)? My primary problem is that the lathe two speed motor will only run in the low speed range. Since higher speeds require more starting power, I thought that the capacitors might be my problem since I have replaced both and never seemed to get a clear indication of what type I should use. The originals were physically two or three times bigger than the replacements but I guess that doesn't matter.

 

[Tony Wells]

Re: Capacitor questions

There are differences in the two types. Start capacitors only function for a short time while the motor spins up. Capacitors conduct only while they are charging, then they "switch off" and cease to conduct, rendering whatever winding used to start the motor offline. The size (in mfd) determines how long the start winding stays energized. Usually just for a few seconds, then the motor is spinning the right direction and the cap shuts off the start winding. These are common non polarized electrolytic capacitors. Start capacitors you will see rated plainly at the operating voltage of the motor. And as mentioned, more voltage is OK. So, if you find a proper sized capacitor, but the voltage can be more. You have a 110 volt system, but you can use 240 or more rated capacitors.

Run capacitors, on the other hand, are in the circuit constantly (hence the "run" name), and are built to withstand a continuous charge/discharge cycle and are usually oil filled. Common is an aluminum can for these, as opposed to the common round Bakelite case of a start capacitor. Typical for a 240 volt motor you will see a 370 volt rating on a run capacitor.

So, they are not interchangeable at all. This may well be your problem, and is worth replacing the capacitors to eliminate the possibility. They aren't that expensive. Unless you have a way to test them. Some supply houses do claim to be able to test capacitors, but in my experience, the testers they have only check the value, and not even that at the operating voltage (which is critical), and not to check the leakage (electrical leakage). IMO, best to replace them and know for sure.


Hope this helps.

 

[yellow_cad]

Re: Capacitor questions

Is there such a thing as a start and run capacitor? The type of capacitor that looked like it was original was like a larger tin can. From that would I know that it was a run only capacitor?

 

[Tony Wells]

Re: Capacitor questions

There are multi-section can capacitors, so there is no real reason they couldn't be housed in the same container, but I can't say for certain without reading the markings and having a look. Usually because of the performance requirements, the cost would probably be prohibitive. I'd say manufacturer's discretion.

 

[Rbeckett]

Re: Capacitor questions

Is there such a thing as a start and run capacitor? The type of capacitor that looked like it was original was like a larger tin can. From that would I know that it was a run only capacitor?

Start and run caps on electric motors are commonly housed in the same can. The only difference is the number of connections to the components inside. I have seen common ground and 4 caps all housed in an oil filled can on some old military electronics so anything is possible. But start/run caps are commonly housed together on AC and refidgeration gear all the time.
Bob

 

[yellow_cad]

Re: Capacitor questions

I'm back from Vacation! Usually a single phase dual capicator motor will have 2 caps. (I have never seen this type motor with a dual cap. but it's possible) Why? I assume it's due to size. I guess they want to keep the size envelope as small as possible. Bob is right, when I worked for the Fedders Corp we used one dual cap. for start and run. Again it was done because the cap was never mounted on the compressor, it was attached to the frame and wired to the compressor. Same for the fan motor in applications that used a cap. So I guess the answer to the question about using one start and run cap. is yes however the one dual purpose cap. is 2 caps. in one can so the correct answer is no. I remember having caps connected to fan motors at Fedders, I don't remember them having the disconnect switch to disconnect the cap. after the motor starts so I assume it's possible to have a single start/run cap. in very low torque fan type motors but can't be used in machine tool applications.
Frank

This is a Taiwanese built lathe/mill combo. The lathe has a 1 hp 2 speed motor and the mill has a1/4 hp single speed motor. Both motors are 110 volt single phase. The lathe uses a 25 mf capacitor and the mill uses a 10 mf. Both of these capicitors are the tin can type. I am trying to determine what the function of these capicitors is in these systems so I can understand if they are working as intended. My only problem right now is that the 2 speed motor only wants to work on the low speed. The capiciator for the lathe has been tested and it is working according to a motor shop using a small tester.

 

[Benji]

Re: Capacitor questions

Capacitors conduct only while they are charging, then they "switch off" and cease to conduct, rendering whatever winding used to start the motor offline. The size (in mfd) determines how long the start winding stays energized.

While what Tony says is correct the function of a start capacitor is different. There is an AC (as opposed to DC) current flowing through the combination of the capacitor which is in series with the start winding and the start winding. This current is shifted in phase, by the capacitor, with the current flowing through the run winding. Shifted in phase means that the current reaches it maximum (and minimum) current points at a different time than the current in the run winding. Ideally (for maximum starting force) this time shift would be 1/4 cycle (90 degrees) The two currents with different phases cause a rotating field, The rotor follows the rotating field to get up to speed. Once it reaches some minimum RPM a switch cuts off the current flow in the capacitor/start winding circuit. The field is then generated only by the run winding. It does not rotate but alternates. The rotor has enough inertia to follow the alternating field and not get stuck at one point. It (almost) synchronizes with the field. If you tried to start a motor of this type without the capacitor the rotor would go to the nearest pole on the run winding and get stuck their.

The start cap has an AC current flowing through it It charges and discharges at line frequency. The electrical size of the cap, in combination with the inductance of the start winding determines a phase shift. You can never have more than 90 degrees shift from a single LC circuit (several in series will give you more). For motor starting the shift range is quite broad so a range of caps will work

A smaller cap will give more shift, but if the cap is resonant with the inductance (Xl = Xc) the net reactance would be zero and the current through the combination would be limited only by the start winding resistance. This is not a good thing from the stand point of circuit design so there is a limit to how small to make the cap. If its too big you dont get enough phase shift and the motor may not start.


Hope I haven't muddied the waters too much.

 

[Tony Wells]

Re: Capacitor questions

Agreed. Benji. I just didn't want to get into the total explanation of phase shift and its effects. I stopped short of the full explanation.
Thank you for explaining it in plain language.