What's new
What's new

Wiring 2 speed motor to VFD

Joined
Mar 29, 2022
As Im learning more about running a milling machine on a VFD I have a question on how to wire a 2 speed motor.
The switch on the mill has HIGH FWD, HIGH REV, LO FWD, LO REV. I understand how to wire the FWD/REV... but how do I deal with the second motor winding?
 

jwmelvin

Aluminum
Joined
Sep 5, 2018
Location
northern Virginia
Probably best to wire directly to motor as a 4-pole (~1800 rpm) configuration. It’s also possible to wire through the switch and have multiple motor parameters set up in the VFD. Just don’t switch the config while running. I do that with my mill and rarely use it but it was handy the other day when I wanted high speed for a 1/16” end mill.
 

Greg Menke

Diamond
Joined
Feb 22, 2004
Location
Baltimore, MD, USA
I had a Sheldon horizontal with a 2 speed motor- I wired the vfd thru the winding selector switch. Concur with set the switch and don't change it while running. The vfd ran the motor just fine in both settings though presumably the # of poles setting only matched one of them. I did wire the fwd/stop/rev switch to the vfd inputs though, so the front panel didn't need changing.

Didn't end up using vfd speed control since the mill had a CV belt transmission in it- very cool- so I just cranked away on that instead.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
I think i will likely wire thru the selector switch. Then add a new fwd/stop/rev switch or buttons that will control the vfd

The advantage to that configuration is, you retain all the original information and setup for the manual switches, if you ever want to go back to regular three phase power.

If you were certain you never wanted to go back to the original setup, you could simply decode the wiring for high speed running and hard-wire that to the VFD, and eliminate
all the manual switchgear on the machine.
 

Jraef

Titanium
Joined
Aug 10, 2004
Location
San Francisco Bay Area
There is no reason to have the low speed available any more, the VFD will do that for you. With a 2 speed motor, the motor has the same OR LESS torque at the lower speed. With a VFD it will always be capable of the SAME torque, so it is universally capable of whatever the two speed arrangement was set up for.

Just wire the motor permanently for high speed and directly to the VFD. Switches on the output side of a VFD are a bad idea.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
There is no reason to have the low speed available any more, the VFD will do that for you. With a 2 speed motor, the motor has the same OR LESS torque at the lower speed. ....

For consequent pole motors, yes. There are rare 2 speed motors with the same HP rating on high and low speed, separate windings. But in general, your statements are spot on.
 

jwmelvin

Aluminum
Joined
Sep 5, 2018
Location
northern Virginia
0d399e61307c0e209d001a2174788895.jpg

Perhaps rare but it was the stock 2-speed motor on my mill (Taiwanese BP clone)
 
Joined
Mar 29, 2022
Ok. I'm listening... only wire high speed.

What are the limits an older motor can handle from a VFD? How much faster and slower can it run without issue? I'll have more info tomorrow after the machine arrives.
 

mksj

Cast Iron
Joined
Aug 10, 2010
Location
Tucson, AZ
Disagree with wiring the motor for high speed, it depends on the motor and if it is constant Hp or not. In your case the motor is a constant Hp, you would wire it 4P/1700RPM with a base frequency of 60 Hz. You would run the motor to 120Hz which will give you the same top speed as 2P motor winding and maintain constant Hp. Torque decreases above the base speed but you gain mechanical advantage. I have installed quite a few VFD's on 2 speed mills that use constant Hp 4P/2P motors, and I run all of them in the 4P setting. Mechanical limit speed of 4P motors is much higher than 2X their base speed, providing it it is not a really old motor.
 

jim rozen

Diamond
Joined
Feb 26, 2004
Location
peekskill, NY
Consequent pole = constant torque, hp value scales inversely with motor speed. Constant HP are typically two winding motors.

For consequent pole motors, wire up the higher power (read, high speed) configuration. For constand hp motors, this is not necessarily true.
 








 
Top