View Full Version : yet another 3 phase question.

Jim Wilson
10-13-2002, 11:11 PM
I have read the other threads on this BBS and armed with that got 3 phase idler motor wired everything as suggested, wrapped a cord around a pulley on the idler, fired everything up and works just fine. Now the question. It is stated in several places that the idler supplies the third of the 3 phases. I have 220 1 phase into the idler. Are the two 110's out of phase with each other or is the idler doing the phase shifting and generating all three phases at output?


10-14-2002, 12:39 AM
The idler generates the third phase, crudely.

I instrumented one once, and found that the phase varied a lot, but it does work.

VFD would be better, and considerably more versatile

Forrest Addy
10-14-2002, 02:35 AM
Amen to Oso's post

Jim Hubbell
10-14-2002, 03:20 AM
I know a “ rope start “ for idler motor is commonly done but could be dangerous if rope was snagged by turning shaft.
I was wondering if a start capacitor connected across second phase with momentary push-button switch might start idler “ two-phase “. Maybe Oso could comment on this or mention a working method.

10-14-2002, 01:45 PM
You bet it works.

Stats like a capacitor start single phase.

Size depends on motor a bit, there are plans all over the net. Check archives on the Chaski site for links.

You want to disconnect it after starting, there are plans including that feature, drop-out on power fail, etc. All stuff you want.

And then there is a VFD. Benefits outweigh cost, I think.

Jim Wilson
10-14-2002, 05:18 PM
I seen 1 HP VFD for around $200. What does it do? I am new to this so be simple.


10-14-2002, 09:41 PM
Phase converters (or shifters) do a good job at what they do. VFD'S give pretty much the same plus variable speed, braking and other niceties as well.

10-14-2002, 09:50 PM
When I made my phase converter using a 2 horse idler motor for the third phase, my voltages checked within 10 to 15 volts of each other. I did quite a bit of playing with different cap values before I got it that close.
To start it, initially I used a 110 single phase motor out of an old clothes dryer to start my idler motor. I would plug in the dryer motor and then throw the switch for the idler motor. Then unplug the 110 motor.
One day I decided to try a rope to pull start the idler and it worked just fine. Then about a month later I wanted to see just how fast the pulley had to be going before the idler did not "catch". So I started flipping the pulley by hand.
You would be surprised how slow it can be going and still "catch" and take off. So I no longer use the rope, I just give it a flip of the wrist and then push the start button. I have about a 4" pulley on the idler motor. Then I built a doghouse for the motor and caps with just the shaft and pulley sticking out the front of the house. It has a steep sloped roof to keep chip off and it sets near a wall. The entire back of the house is open for air circulation. I have had no problems.
Hope this helps.

Treven Baker
10-15-2002, 09:07 PM
I have a five horse rotary converter with a capaceter start button. I needed one of my motors to go the other way so instead of using the capacater to start my converter I just kicked the pully with my foot and turned on the main switch and away it went in the opposite direction. I always have reverse if I need it.

10-15-2002, 10:12 PM
Jim, a VFD varies the voltage and frequency to the motor so that the speed of the motor can be varied. If you take a standard motor and run it straight off the incoming line, the motor will run at the line frequency of 60hz, or whatever your line frequency is. If you put a VFD inbetween, you can now take the 60 hz incoming frequency and vary it to either slowly ramp up the speed of the motor to full speed, gradually slow down the motor, or to run it at any speed you desire. The VFD also has many bells and whistles to allow you to monitor the motor, program different speed ranges for different applications, etc.
Adding a VFD is a great way to easily vary the speed of your lathe or mill. It does have some limitations, the HP is proportional to the speed, so if you slow it down too much, you may not have enough HP for what you want to do and you need a 3 phase motor. You can take single phase input to the VFD and drive a 3 phase motor, but the VFD usually must be derated by about 25-50%.