Speed Controls

I don't believe you can get a VFD for a single phase motor (I've not heard of one and you'd imagine there would be a market). I suspect therefore you need to be on the lookout for a small three phase motor of resonably modern construction (for wiring reasons). 'VFD' is a different (mainly US) name for what we tend to call inverters.

Try this page for some more info :-

Steve,

The speed controllers are usually designed for use with a 3-phase motor. They take a single phase mains input and convert it to 3-phase and change the frequency / phasing in the process. You will therefore need a VFD and a

3-phase motor. Some have an adjustable control range that lets you set the output frequency (along with several other parameters such as ramp-up speed or breaking etc) but these often require a separate control head which adds to the price. Some VFD's allow you run from about 10% to over 200% although IIRC it is unwise to run 50 Hz motors near 100 Hz.

The down side with VFD's is that although they try to maintain torque as the revs decrease, they are seldom totally successful. If you consider a lathe with a back-gear, it may have a 4:1 reduction in the gearing which means that when the back-gear is working, you have 4 times (approx) more torque at the spindle than without it. Even if a VFD was totally effective and produced 100% torque at minimum revs you'd still only have 1 x the maximum motor torque - believe me you'll notice the difference if you're machining a large flywheel!

My electronics is iffy at the best of times but IIRC using a bridge rectifier does sort of double the frequency (albeit with a horrible waveform) but would effectively halve the voltage as it takes the negative cycle and inverts it into the positive cycle so you get 100 Hz at 120 V peak to peak - (please correct me if I'm writing rubbish here!) Suffice it to say, I don't think it will work. (About 100 years ago I got a degree in electronics, never used it and forgot 99% it - shameful I know!)

I hope it helps

MARK

I have a single-to-three phase speed controller fitted to a Chinese lathe/mill. It does a very good job. I can screw cut at 20 or 30 rpm without problems and to within 1/10th of a turn. This is especially useful when screw cutting up to a shoulder.

For general turning, especially of 'difficult' materials, the ability to vary speed while cutting is a revelation. I haven't shifted the belts on this machine in 5 years!

The message from "Mark Howard" contains these words:

I don't have speed control, but is there anthing to stop you using back-gear when it would be advantages to do so?

snip

Frequency doubling by a bridge rectifier will not work and if tried will burnup the motor.

The only related scheme that does work is frequency tripling but this needs a threephase supply and saturable reactors.

Jim

Suppose you put a 470mF capacitor capable of handling 2 amps in series and the motor start and run capacitor values were adjusted for a 100Hz supply?

Steve

No, not at all except that most of the smaller Chinese lathes seem to come without a back-gear which IMHO is a serious omission.

Mark

from "Steve" contains these words:

I may be missing something here as my knowledge of electronics is negligible but I recently bought a 'gadget' on eBay that gives me speed control of any single-phase electric motor.

It's made by United Automation Ltd., Southport and will handle up to 15 amps when it has a 150mm sq x 5mm thick heat sink attached (2 amps without). They call it a 103-852 AC Regulator.

I have set it up in a box with mains wired to one side and the output going to a twin 13amp Socket. I can plug any power tool (Pillar Drill, Grinding Wheel, Angle Grinder, Lathe) into the sockets and adjust the maximum speed with a dial on the front which simply turns the spindle on the Regulator.

My initial idea was to reduce the speed of an Angle Grinder so that I could use cup wheels where their maximum safe speed is lower than the fixed speed of the grinder - thus potentially giving me a cheap cutter grinder (the drawings for the support mechanism [for the Angle Grinder] are at an advanced stage !!) - but the thought of plugging my Myford in, setting the belt to top speed and having fully variable speed is a bonus. Setting the back-gear is just so fiddly!

JG

Sounds like it's a phase angle triggered controller, basically a high power dimmer switch; it will work fine with non-synchronous motors like an angle grinder but it will not work with a motor without brushes such as the motor on a lathe. Martin.

from "Martin Whybrow" <

Thanks for the explanation Martin - No bonus for me this week then !

JG

The 470uF capacitor would simply block the DC output of the bridge rectifier and no current would flow.

Jim

Jim

There really is no excuse for me not realising that. Gutted. I used to teach this stuff!

Partially right. The capacitor will prevent any direct current flowing but will allow an alternating current to flow which will be predominantly 100 Hz. Unfortunately the current is likely to be too low to get any useful torque out of the motor.

Russell.

Not true. The capacitor would simply charge up to the peak value of the supply waveform and because all four diodes are now reverse biased no further current can flow - DC OR AC.

Jim

Whoops! It's nearly 40 years since I gained my elec eng degree and the brain cells are getting a bit rusty. Of course, if I were to be pedantic I could point out that I was (accidentally) right because real diodes have leakage current which will allow an AC current to flow! Of course, for motor driving purposes this current is useless.

Russell.