Motor too small for static phase converter?

[Shootymacshootface]

I am putting an ancient Bridgeport power feed on my new mill. I was thinking of running it on the phase converter that I got with it, so I wouldn't have to fire up my rotary converter just to move the table. The motor on the power feed is 1/8hp 3ph, and the static phase converter says that it will power 1 to 3 hp motors. I seem to remember something about not using a motor smaller than recommended.
Thsnks,
John

 

[markba633csi]

It may work with the smaller motor, you might notice the motor getting hotter than you would like. Static converters use capacitors to phase shift one of the input power legs to generate the third leg, and the amount of capacitance required varies depending on the motor it runs. If your converter uses multiple capacitors you could disconnect one or more to match it more closely to the smaller motor.
Try it and see if it works but monitor the motor temperature. If you have a diagram for your converter post it here
-Mark

 

[Ulma Doctor]

for a 1/8 hp motor the static converter is superfluous.
if you were so inclined, you could easily install a 10Uf capacitor across Phase B and Phase C and send single phase power to it on phase A and B


the problem with using a phase converter on a motor smaller than intended is the capacitance for starting is too great
the motor may receive as much as 300Uf capacitance (dependent on manufacturer)
(i have used as little as 30Uf to start an unloaded 1 hp 3 phase motor- on single phase)
the starting capacitance overkill combined with the potential relay enclosed (in all but a few brands), may be very sensitive to voltage fluctuations in the primary circuit, the potential relay will open and close a set of contacts that will dump the start capacitor's charge into the start circuit
generally speaking, the motor will attempt to speed up to about 80% of the rated RPM before the potential relay opens again.
if the intended small motor sees moderate to heavy operational demand, the potential relay will most likely be called to operation many times in an hour (sometimes in a minute for very heavy loads) the constant start and stop will surely destroy the small motor and/or the static converter due to lack of heat dissipation

the work around would be to put a smaller start capacitor in the static converter (around 20 to 30Uf)
or
install a momentary switch on the 3rd leg between the static converter and the motor
the potential relay will not be able to dump its charge without the momentary switch being closed, the static converter is effectively no longer in the start circuit. the static converter lives longer and the motor is happy on light to moderate loads
heavy loads may require other means of supplemental momentary capacitance

 

[MrWhoopee]

Is your spindle motor single phase?

 

[ttabbal]

I just use a cheap VFD. It's overkill, but it works well.

 

[Shootymacshootface]

Is your spindle motor single phase?

No, it's all 3 phase. If the static converter won't work then I might wire it into where the forward/reverse switch is.

 

[MrWhoopee]

No, it's all 3 phase. If the static converter won't work then I might wire it into where the forward/reverse switch is.

Do what I do, use the spindle motor as an RPC. Wire the power feed to the spindle motor power lugs. When you want to use the power feed, be sure the spindle motor is turning.

 

[Shootymacshootface]

for a 1/8 hp motor the static converter is superfluous.
if you were so inclined, you could easily install a 10Uf capacitor across Phase B and Phase C and send single phase power to it on phase A and B


the problem with using a phase converter on a motor smaller than intended is the capacitance for starting is too great
the motor may receive as much as 300Uf capacitance (dependent on manufacturer)
(i have used as little as 30Uf to start an unloaded 1 hp 3 phase motor- on single phase)
the starting capacitance overkill combined with the potential relay enclosed (in all but a few brands), may be very sensitive to voltage fluctuations in the primary circuit, the potential relay will open and close a set of contacts that will dump the start capacitor's charge into the start circuit
generally speaking, the motor will attempt to speed up to about 80% of the rated RPM before the potential relay opens again.
if the intended small motor sees moderate to heavy operational demand, the potential relay will most likely be called to operation many times in an hour (sometimes in a minute for very heavy loads) the constant start and stop will surely destroy the small motor and/or the static converter due to lack of heat dissipation

the work around would be to put a smaller start capacitor in the static converter (around 20 to 30Uf)
or
install a momentary switch on the 3rd leg between the static converter and the motor
the potential relay will not be able to dump its charge without the momentary switch being closed, the static converter is effectively no longer in the start circuit. the static converter lives longer and the motor is happy on light to moderate loads
heavy loads may require other means of supplemental momentary capacitance

Thank you for explaining that to me.
I actually understood what you were saying!

 

[projectnut]

You might want to check with North America Phase Converter Company. They do make static converters for fractional HP motors. I have an SPC-3/4 on a 1/3HP drill grinder.

Home

WE'LL GET YOU POWERED... Prior to today, you may have never thought about a phase converter or why you would ever need one. But now, you need one. Figuring out what type and size you need can be confusing; we get it. We’ve helped thousands of business owners and would be honored to help

www.northamericaphaseconverters.com

 

[macardoso]

I just use a cheap VFD. It's overkill, but it works well.

I agree. A cheap 1/2HP VFD is $115 from Automation direct. Ebay is even cheaper.