Wiring 2 speed motor to VFD

[milemaker13@att.]

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]

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.

 

[milemaker13@att.]

Ok, thats what i figured. Single speed or wire thru the switch.

 

[Greg Menke]

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.

 

[milemaker13@att.]

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

 

[jim rozen]

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]

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]

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]

Perhaps rare but it was the stock 2-speed motor on my mill (Taiwanese BP clone)

 

[milemaker13@att.]

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]

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.

 

[milemaker13@att.]

Very good info. Keep it coming, I'm listening.

 

[jim rozen]

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.

 

[sunshine.fab]

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.

I'm performing my first installation of a VFD and I'd like to understand why this is the case.

I have a Hardinge TM with the original motor that runs on 3 phase power. From my understanding, the motor is of the consequent pole (Dahlander pole changing) type, so it has constant torque at full (high) and half (low) speeds, but full power (3/4hp) at full (high) speed and half power (3/8hp) at half (low) speed.

With this motor and the 4 stepped pulleys the Hardinge TM has 8 speeds, which is plenty for the light work I do. As such, I wasn't intending to use the VFD for speed control. Rather, I was hoping to set the VFD output to a constant 60hz and wire the power output of the VFD to the line side of the built-in, lever-operated barrel switches for speed and direction. My plan was to use the built in levers to select a speed and direction before turning the output of the VFD on, and wire a toggle switch to the logic input of the VFD to switch VFD power output on (fwd/stop). I.e. I would only operate the speed and direction switches down stream of the VFD before turning on power to the motor, and never operate them when the VFD is sending out power.

Will this work? Or is there some reason other than "well the VFD can handle all of your speed control" that I should wire the VFD to just the high speed configuration? I'm new to this but I'd like to learn. Thanks

 

[Greg Menke]

Thats how I run the vfd on my ATW lathe- 12 speed headstock, which offers quick and convenient spindle speed changes- not continuous obviously but given the 12 speeds the coverage has been enough for me. I did wire for fwd or rev though, only have used reverse once that I can recall. The ATW headstock has a clutch, so the fwd/rev switch starts the motor and then its clutch and shifting. If it were a 2 speed motor, then I'd set the motor speed then turn on the vfd output.

I suppose if I had an ultra-low rpm job then I'd be in low gear and fiddling with the vfd to also slow the motor. But I've been using the machine for about 10 years now and haven't needed to adjust the vfd frequency.

 

[mksj]

If it is constant torque, then there would be no difference between using the low/high speed windings vs. turning the VFD down to 30 Hz, as most motors will maintain constant torque down to that point. Also, you VFD is setup for one set of motor operating parameters, that changes when you switch to the low speed winding. If running sensorless vector, which gives you better low speed control, it is also tuned for a particular set of motor settings. I have setup VFD's with two sets of motor parameters for 2-speed motors, in most cases the results were disappointing and not worth the trouble. Wire the motor directly to the VFD using the high speed connections. Vary the motor speed to something like 30-75 Hz, use the step pulleys for the larger speed steps and/or if more power is needed. If a wider speed range is needed, and if there is space, switch out the motor to a 2Hp inverter/vector rated and run it from 20-120 Hz.

 

[jim rozen]

"Will this work? Or is there some reason other than "well the VFD can handle all of your speed control" that I should wire the VFD to just the high speed configuration? I'm new to this but I'd like to learn. "

It will work Ok that way. It's not clear the VFD would smoke even if you made one or two mistakes and flipped the control sticks while the output was on. Once you get it working you can think about expanding your knowledge base and using the variable speed option. You can mimic the Hi/Low effect by having two preset speeds, which you could control with the existing drum switch contacts, and use the fwd/off/rev stick to again operate logic controls on the VFD. If you go that route, best to wire the motor set to the higher HP speed.

 

[sunshine.fab]

Thanks to all three of you.

Once you get it working you can think about expanding your knowledge base

I like the idea that follows this, but as an influential mentor told me: "the most permanent solution is a temporary solution." To abide by that lesson, I'll wire the motor to the higher HP speed and learn how to repurpose the barrel switches to perform fwd/stop/rev and set high/low preset speeds from the get-go. This sounds like the best way to retain the cool factor of the built in switches and employ the sophistication of the VFD.

 

[Jraef]

Thanks to all three of you.



I like the idea that follows this, but as an influential mentor told me: "the most permanent solution is a temporary solution." To abide by that lesson, I'll wire the motor to the higher HP speed and learn how to repurpose the barrel switches to perform fwd/stop/rev and set high/low preset speeds from the get-go. This sounds like the best way to retain the cool factor of the built in switches and employ the sophistication of the VFD.

But again, don’t miss the IMPORTANT fact that with the VFD in the circuit, you can NEVER switch speeds using the barrel switches while running, you will damage the transistors on the VFD output.

 

[JST]

But again, don’t miss the IMPORTANT fact that with the VFD in the circuit, you can NEVER switch speeds using the barrel switches while running, you will damage the transistors on the VFD output.

That is no longer a guarantee at all. We made VFDs up to several HP and some had switching on the output. Never had a failure, and would not expect one. No special parts in the units, none needed.

Older tech VFDs were very sensitive to such things. The reputation has stuck.

The real reason not to do it is that a VFD that will "run" a given motor, will not "start" that motor if it is simply "switched". You must have a VFD at least 5 or so times larger in current rating to start a given motor, due to inrush (that counts the typical short term "150% overload" spec). "Matched" VFDs must slow-start motors.

Many run several machines with one large VFD as a converter, and no issues. The "Phase Perfect" is pretty much a VFD in terms of circuit, and runs well with switched loads.