View Full Version : 3 phase motor

Dave Smith
06-21-2009, 06:39 PM
if I were to change my 3 phase 1hp motor on my bridgrport to single phase what size shpould I use?

Ohio Mike
06-21-2009, 07:13 PM
H&W in Fort Wayne, Indiana sells a 1.5HP Baldor conversion for the 1HP factory motor. They run about $450 if I recall.

06-21-2009, 07:41 PM
Think about:
1. Variable Frequency Drive
2. Rotary phase converter
3. Static phase converter
Going from three phase to single phase is the wrong direction. IMO

I ran my mill on a static phase converter with no problems for several years. When I moved the shop I changed to the RPC. Actually I piggy backed off the one on my lathe. I changed mills from a step pulley to a Variable Speed this change was to upgrade from a tired and somewhat abused machine to one in allmost like new condition. In hindsight I liked the step pulley better. A VFD with a step pulley would IMO give the best of both worlds. Think about it a little before you commit

06-21-2009, 08:03 PM
I have always used an inverter. SquareD/Telemechanique Altivar 16 - picked it up on Ebay for a bit over $100 several years back. A cheaper and (IMHO) more versatile option. Takes single phase 220 and produces variable speed (frequency and amplitude control) up to 208 3 phase.


Jim Shaper
06-21-2009, 08:05 PM
Another vote for the vfd.

06-21-2009, 08:16 PM
Dave, if you've got a step pulley machine, a 3-phase motor on a Variable Frequency Drive ("VFD", or "inverter") is the cat's meow -- you'll have electronic varispeed, which is much smoother and quieter than the Reeve's Drive on the Bridgeport Varispeed head.

An induction motor driven by a VFD loses power according to the RPM, so if it's a 1725 RPM motor running at 863 RPM, for example, the VFD/motor would give you half of the power you'd have at full speed. At 431 RPM, you'd have 1/4 power.

So what a lot of folks do is double the motor size -- i.e., go for a 2 - 3 HP motor, and with the back gear, you'll have full power down to 100 RPM without ever having to change belts.

06-21-2009, 08:21 PM
I run my vertical mill9step pulleys) with a vfd , i welded a bracket up and attached the vfd to the mill head.

As others have said, this is the way to go, rather than messing with a single phase conversion.

06-21-2009, 09:30 PM
...and... if 230 is your problem, up to 1hp, for less then $150 you can get a VFD that will take 115 in and put 230 three phase out.

06-21-2009, 09:44 PM
Another vote for a VFD. I have a Bridgeport J head mill that has a 240V 3 phase motor but no 3 phase power close to my shop. I powered the Bridgeport with a small FM-50VFD a few years ago.
Now I have them on six other machines but only two are the 50's

J Tiers
06-22-2009, 12:05 AM
hey...... let me put this bluntly and in the most crude and blatant terms......

Why would you consider paying MORE money to install an inferior drive system on your mill, when you could have a proper drive system using the same motor, for less money than a new replacement motor will cost?

Your options are a static converter, which will cost around $130 and give same performance as a single phase replacement motor.

An RPC, which you can build, or buy, likely for a similar amount of money, plus cost of contactor and overload, and which will at least give you 3 phase power.

A VFD, which will give you very nice 3 phase, AND provide variable speed, not to mention replacing/eliminating the motor starter and overload relay. Cost for that will be probably about $150 plus a box and a $30 disconnect (the box and disconnect you'd need anyway).

Dave Smith
06-23-2009, 01:31 AM
thanks I did'nt know there were so many options it appears a vfd is the answer this my first foray into 3 ph machines.

Jim Shaper
06-23-2009, 02:19 AM
It's been about 6 months since I sold my RPC and have been running vfd's on the machines ever since. I don't miss the RPC at all.

Without a braking resistor, you won't have the instantanious reversing you do on hard 3ph power, but you also don't risk breaking taps when plug reversing, and you can dial down the machine from the previous bottom speed to a fraction of it if you like.

06-23-2009, 08:30 AM
Even though i have 3 phase available, I use vfd's because of the simplicity to achieve speed change. The vfd on a surface grinder allows for keeping the wheel at constant sfm, allowing you to use up the wheel.