Speed Control for Clutch Motor

[AlZilla]

Here's a search on practical machinist about this very subject and it's a rabbit trail of complications with no clear answer. Single phase motors are harder than 3 phase and lose torque when you slow them down. I coughed up the money for a purpose designed servo. I wanted to spend my time sewing, not reinventing the wheel.

https://www.practicalmachinist.com/search-entire-site/#gsc.tab=0&gsc.q=speed control single phase motor&gsc.sort=

Years ago I wanted to slow down a drill press and used a tread mill motor to do it. I just mounted the entire control panel on the wall behind the press and used the speed control knob. It's working great to this day, it just lacks finesse.

[Tigweldor]

In my up to date experience, most industrial (sewing) machines use 3-phase motors and single phase motors are usually found on the "home" models for private citizens.

Simply because in the industry, 3-phase wiring is standard procedure to power motor driven implements - not so in every home or apartment/condo or garage workshop.

Also a 3-phase motor is more efficient in converting electrical into mechanical energy - due to the construction of its´ magnetic fields.

You can regulate the speed of most single phase motors with a potentiometer (dimmer switch) - but current not needed/used will be transformed into heat.

This is why most speed controls have a max. wattage rating that they can handle - the same goes for a VFD - you buy one built for max. 1,5kW and try to run a 4kW motor on that - it will not last long due to overload and overheating.

Plus you have to take into consideration : starting current is most often manyfold of the actual current needed , once running rpm has been reached.

 

The nice thing about a VFD though is, that it can convert single phase house hold current into 3-phase current needed to run any 3-phase motor.

With the added benefit of being able to control output frequency = motor speed and the ability to severely cut down on run on time after the motor is shut of = motor brake.

On some VFDs you can also increase starting current to give it a boost to 125% or 150% of current needed - a "starting ramp" so to speak. 

I will say it again - they are a pretty bullet proof apparatus once set up and as you can tell by the dates on my pics - I have been using them for years with success, even long before the pics were taken.

And I am talking real hands-on experience - not some theoretical mumbo-jumbo why they can´t and will not work on a three phase clutch motor.

As to the motor overheating - in the industry, motor speed is controlled with VFDs - they would not use such units if it meant that the motor would burn out in short time. They do such calculations with a very sharp pencil.

Another benefit : as more and more people are switching to servo motors - the availability of used 3-phase clutch motors in good condition has increased - while at the same time prices for them have dropped. Since I use them, I see that as a win-win situation (for me at least).

 

Greetings

Hans

Edited May 7 by Tigweldor

[Digit]

53 minutes ago, Tigweldor said:

in the industry, motor speed is controlled with VFDs - they would not use such units if it meant that the motor would burn out in short time

Slight nuance: in the industry motors are built to specifications. If a motor needs to be able to run between a range of rpms, the cooling system is calculated to those rpms. Clutch motors are not designed to provide a range of rpms at the axle; that's what the clutch is for: transform a fixed rpm to a range of rpms (plus heat dissipation on the clutch plate).

I'm not saying that a clutch motor can't be speed-regulated; they can, just like any motor; but that's taking them out of their spec and into the realm of safety margins. As long as you don't venture into extremes you're safe. The older the motor, the more likely it has been overdesigned and can take some thrashing.

[Tigweldor]

Jup - you got that one right.

Some of my 3-phase clutch motors weigh (almost) as much as the sewing machine that it powers.

And if I do go way below 20Hz - they do get very warm.

I have tested them with an infra red thermometer - at the speeds that I run them at, they do not exceed 60° to 65°C.

And that is a temp, that most all industrial motors can cope with.

[GerryR]

Many of the three phase motors produced today are "inverter" rated and are spec'd to run from 10 to 90 Hz (175-2295 rpm) and are usually attached to a gearbox.  In my particular case, a 5:1 gearbox giving 35 to459 rpm final output.  Further reduction from the motor pulley to the sewing machine pulley keeps you well above the 10 Hz minimum with plenty of low speed control.  Couple that with the VFD setup which allows the top speed to be programmed so that the full range of the pot, in this case foot pedal, is from 0 to programmed top rpm and you have very fine speed control.  (The DCservos operate the same way as far as programmed top speed is concerned.)

[MarlinDave]

Interesting but my machine has a single phase motor. I have single phase shop power and some 3 phase shop machines (lathe,mill, welder) running on converters. I would convert all these to single phase if not so expensive. I got a free 10 hp 3 phase compressor and swapped motors to single phase to dodge the converter hassle. 

[Tigweldor]

Now that is an interesting philosophy - as most people upgrade their shop wiring to 3-phase sooner or later and never look back to regret it.

Nice thing about 3-phase wiring - you can also run single phase motors on that - you just use/hook up one phase and omit the other two.

The wiring does not care - neither does the motor.

Greetings

Hans

[MarlinDave]

Where I live I would likely have to pay the utility $100,000 to get three phase to do projects  give away. 

[AlZilla]

3 phase is not in homes in the US, generally. I buy electric motors when I see them at yard sales and such and never see a 3 phase motor.

[dikman]

13 hours ago, Tigweldor said:

In my up to date experience, most industrial (sewing) machines use 3-phase motors and single phase motors are usually found on the "home" models for private citizens.

 

You can regulate the speed of most single phase motors with a potentiometer (dimmer switch) - but current not needed/used will be transformed into heat.

 

Things must be different on your side of the world. Every industrial sewing machine I've bought came with a single-phase clutch motor, 240v/50 Hz, all of which I either sold or scrapped and replaced with servos as the most cost-effective and practical solution.

The reason I did that is because those clutch motors CANNOT be controlled with a simple dimmer control/potentiometer device - the controller will immediately blow up if you try. As I mentioned before, those controllers are intended for universal-type motors as used in power tools, hair dryers, fans etc, not single phase induction motors. I wonder how many unsuspecting people buy those cheap controllers on ebay thinking they can slow down their grinder or drill press only to have them go poof when they try?

Trying to make a single-phase motor variable speed is a complex issue and not worth the trouble/cost, as you say three-phase motors are the best option in that case (if you don't want a servo).