Motor speed control on AC motor?

I know little about this subject so please forgive my ignorance, I ran a 240 ac drill by accident on 110 ac the drill just ran slower with no apparent damage to the drill ( yes I Know !!!). Using a dc power pack I can vary speed on dc equip by adjusting the voltage, I don't understand why this is not apparently ok. Have attached a google search which indicates that among other things a variac can be used as a speed controller,

Mick

Muzzer has explained it well, starting is the problem. Blacksmith forges had repulsion type motors.They had a wound rotor/armature with a commutator much like a DC motor. Some had brush lifting arrangement others had brush shorting arrangement. They would also start against a load, so some times they would to be used on garage compressors, maybe before unloaded valves ?. Often the blacksmith shop had the speed controller mounted on the chimney. The speed controller was much like a Face plate starter a series of studs with resistance wire behind. Very similar to a face plate starter used to start a shunt wound DC motor. Brings back memories.John

Apologies for double post

Mick

Why was that convenient when I agreed with you by saying "Of course they are"

regards Martin

Thanks for putting the link up Michael ! I really must learn how to create hyperlinks on this forum, i'm sure there used to be a little icon on the top menu bar for this ? Shame they don't let you read the last 3 pages of the article where it explains how to set the motor up and adjus the start up ramp so it doesn't overload the drive unit .

Ian.

Edited By XD 351 on 24/01/2018 12:42:34

XD351. Thanks for pointing us at a source of very interesting information.

It is good to see the response of "here is how to do it" after all the can't be done statements.

Comment on the info from our electronic whizzes would be very welcome.

paul

.

My pleasure, Ian

The icon you seek is this one:

MichaelG.

I have a home made forge blower. It started life as a domestic vacuum cleaner some years ago. This has speed control via a knob on a box, this is as it came off the old cleaner. I never looked at how it worked, but have just found it again and will have a look when I find time.

Vacuum cleaners (unless Dyson battery types) use series wound brushed "universal" motors that can operate on AC or DC, but usually used on AC. Speed control is usually done through a thyristor circuit that switches on at some point in each half-cycle of the AC waveform, and you control the speed by controlling the switching point. It's a sort of voltage control, or you could look at it as an "electronic rheostat". You could also use a variac to control the voltage and hence the speed - including turning the voltage down to 110V. No reason at all why this would damage the motor.

This won't work on an induction motor, generally. There's one category of induction motor that you can control by varying its supply voltage, typically by a series resistor (though not I suspect a thyristor) - this is a "torque motor", typically used for tensioning magnetic tape or thread or anything which is being wound up or unwound and you want to exert a tension on it but the speed is controlled by something else. Torque motors though are very inefficient and you wouldn't want to use one in a machine tool since by definition they generate maximum torque at zero speed rather than where you want it which is at near synchronous speed. Sometimes they might be used on a small fan and there was a thread a while back by someone trying to fix a cooker hood.

The motor on my Quorn is a capacitor run type which must have symmetrical windings - when I'm back home and have some time I'll do some measurements to try to figure out how amenable such a motor is to using a VFD.

BTW in answer to a question above - battery drills use permanent magnet fields in their motors - the standard way to control either these or shunt wound field DC motors is to control the voltage to their armature.

Just to put a cat among the pigeons:

www.justfans.co.uk/electronic-speed-controller-efsc-p-369.html

"A range of Electronic Fan Speed Controllers designed for speed controllable single phase motors up to 10 amps.
The EFSC3,5,6 & 10 all have an illuminated on/off rocker switch and a stepless rotary knob for speed adjustment."

Bear in mind fans are a low starting torque application with a pretty much constant speed/torque relationship.

A Variac is OK provided the right kind of motor is connected to it, but many motors are unsuitable. Several types of electric motor available with big differences between their internal workings, what they do, and how they are wired up.

A DC motor has a kind of rotating switch, the commutator, that shifts the magnetic field inside the motor as necessary to keep the rotor turning. Many small AC motors have a similar arrangement: as the rotor turns, a commutator switches the current so that the magnetic field turns with it. Called a Universal Motor they are often marked 'AC/DC'. Varying the voltage of this type controls speed with no problem. Most mains powered DIY tools, sewing machines and vacuum cleaners etc use Universal motors - but not all.

Universal motors aren't brilliant. Suitable for some applications, bad for most. I wouldn't put one on a lathe unless it was a small high-speed model. Larger electric motors use methods other than a commutator to spin the rotor. A variac won't speed control them, though it will reduce torque with risk of magic smoke.

The other problem with a Variac is the cost, especially if you need a big one. Expect to pay between £500 and £1000 for a new 10A Variac.

Dave

Thanks Michael 👍

One factor that hasn't been discussed is cooling , if you slow down a fan cooled induction motor it can overheat as the fan is mounted on the end of the armature .

The origional induction motor speed controller i mentioned in my  first post was designed to control swimming pool pumps - supposedly to save electricity and because converting to 3ph could be difficult if not impossible without replacing the entire pump . I never really considered using it on a lathe or mill  mainly because of the cost of the kit as i can get a VFD for less money and  a 3ph motor runs smoother than single phase . The latheboy unit to me is more of a gimmick as you have to input material type  , surface speed required ,belt or gear ratios  and a whole pile of parameters which you have to go and look up first then enter them in so the micro can work out the  rpm  required . It then tweaks the motor rpm to get the exact rpm you require (which  you have told it ) it then pretty much just displays what you have told it anyway! The controller kit retails fro $275 au  and the display kit (if they release i kit ) somewhere between $100 & 200 au i would guess .

I fitted a vfd  & 1 hp 3ph motor to my mini lathe for iirc around $300 - $350 au  and the tacho  is a cheap unit from China so around the same cost as the latheboy set up and i didn't have to build the damned thing ! 

The magazine i mentioned regularly gets requests for a speed controller that works with mains powered permanent magnet motors  like the ones on mini lathes , mills and treadmills but they keep saying that there is not really a market for one and considering you can buy these cheap enough out of China they may be right .

Ian 

Edited By XD 351 on 25/01/2018 21:19:05

As suspected my old forge blower is a cummutator type motor, can't get into the controler as it's rusted up. It has been said that this system is only for low-torque motors, how does a mains drill control speed, most have good torque at low speed ?

They use a thyristor to give a variable DC pulse width to the motor. The better ones, during the off period, use the back emf generated by the motor to monitor the speed and give feedback to the pulse width control.

Russell

At 3kW, this is almost certainly an induction motor, whether single or three phase, in which case the only way to change the speed is to change the frequency of the supply voltage. You have to reduce the voltage at the same time as you reduce the frequency otherwise the magnetising flux will become excessive and lead to saturation. For a basic VFD you would traditionally maintain a constant V/f to control that, whereas flux vector control achieves it inherently.

It's perfectly possible to maintain full torque at zero rpm, although obviously the cooling become a problem if there isn't an independent fan. Quite simply, the torque generated by an induction motor is proportional to the slip frequency eg the difference between the speed of rotation of the magnetic field and the speed of rotation of the rotor. That applies pretty much at any speed in either direction and both positive and negative torque (ie motoring and generating).

If the rated power is developed at 1440rpm with a no load speed of 1500rpm, the rated slip frequency is 60rpm. So if you applied a 60rpm rotating field to that motor with the rotor stalled, you'd generate the same torque as you would at rated speed. And if you applied a stationary field and spun the rotor at 60rpm, you'd generate a similar torque.

Murray

Interesting to see variacs up for discussion. In my youth we had lots of them in labs, mainly controlling heaters and occasionally motors. They were banned from our labs many years ago on safety grounds following electric shock accidents. Basically a variac is a tapped autotransformer, so one end of the coil is common to input and output. That's fine if the input is correctly wired so that the common side is at 0V, but trouble if the input is accidentally wired backwards. In the proper case, an output voltage setting of say 40V would give 0V and 40V on the output terminals and you might think it safe. If the input is wired backwards you get 240V on one terminal and 200V on the other, so you still have 40V across the load but safety is greatly compromised. I've still got a variac in my workshop collection but I'm always very careful to double check the wiring before switch-on - mistakes are easy to make.

I have a 13A socket permanently wired to the output and a flying input lead with plug which addresses the safety aspect.

cheaper than you think: **LINK**

Not my choice for motor control though.

as long as the supplying socket polarity is ok.

I always run mine via an isolating transformer but that is more so I can safety connect up test equipment to the device under test.

All my supplying sockets are correctly wired!