Soft start for servo motor

[Edsbear]

I thought VFD's were to control 3 phase motors if only single phase was available 

[dikman]

Yep, that is their primary function, but the fact you can vary the speed is what makes them so useful. It is very difficult to make a single phase AC motor variable speed - grinders, lathes, drill presses etc - but by replacing the single phase motor with a 3-phase and adding a VFD you can adjust the speed on-the-fly with a simple potentiometer.  My belt grinder, lathe and drill press have all been so modified, much better than changing belts to adjust the speed.

I haven't, however, bothered with using such a setup on any of my sewing machines as the ubiquitous servo is perfectly adequate (and a lot simpler to set up).

[friquant]

On 1/9/2026 at 3:30 AM, beefy said:

Something I've only heard mentioned once is the use of a VFD (variable frequency drive) for a 3 phase synchronous motor.

See my post here: https://leatherworker.net/forum/topic/131403-my-first-vfd-w-3-phase-ac-gearmotor/

and blog entry here: https://leatherworker.net/forum/blogs/entry/518-option-d-3-phase-ac-gearmotor-with-vfd-variable-frequency-drive/

In the blog you'll see reference from other folks too. 

(I'm actually using an asynchronous (induction) motor, if I've got my terminology right)

 

On 1/9/2026 at 3:30 AM, beefy said:

External braking resistor option to reduce stopping time.

It's not actually a resistor...but yes the braking is quite effective. The braking settings on mine are confusing but I did get it to brake less than it used to 😃

 

On 1/9/2026 at 3:30 AM, beefy said:

This is a guess but I bet they also have a ramp up option which will be adjustable.

Yes mine has maybe five options for ramp up speed. Would be better if the ramp-up was exponential instead of linear, but I'm being picky.

 

On 1/9/2026 at 3:30 AM, beefy said:

Something I should add is that I've heard running at very low rpm can cause motor heating as the internal fan of the motor is not doing anything useful. I'll leave anyone interested to do further research on that one. However, if the current limit is turned down and thus the current is low relative to the normal operating current of the motor, perhaps this heating effect will not be of any concern.

Sewing at very slow speeds does not add any more energy to the coils than sewing at high speed. Yes the fan will spin slower, but there is also less heat going into the bearings because they are moving slowly. To even get the motor warm you will have to run it both at a considerable duty cycle and at considerable load. (At low load the motor doesn't slip much, meaning not much inductance, meaning not much heat generation.) When I'm sewing it's typically one minute or less of slow stitching followed by lots of minutes of figuring. My motor never feels warm...always feels room temperature.

Adoption
The biggest hurdle at the moment for the adoption of VFD is finding an inexpensive source for an AC induction gearmotor. 

 

[friquant]

On 1/9/2026 at 6:20 PM, Edsbear said:

I thought VFD's were to control 3 phase motors if only single phase was available 

You can do that too. In a different setup I have a 3-phase clutch motor being driven by VFD. Instead of running the motor at full speed (60Hz) I usually run it between 17-25 Hz. This means it's quieter and the pace is easier to manage.