AC motor speed control

[Johny B Good]

It's an asynchronous 50Hz 2 pole induction motor. That 5% slip looks like the max torque loaded speed. Unloaded (spinning freely) it'll probably run at something like 2990 rpm (2998 rpm seems to ring a bell in my mind though).

It's the slip frequency that induces the necessary magnetism in the rotor required to make it run. Often the rotor has thick copper or aluminium bars in the rotor slots all joined to end rings of equally thick copper or aluminium which suggest a squirrel cage entirely filled with an iron core, hence the name "Squirrel Cage Motor".

A synchronous 50Hz 2 pole induction motor would have had a speed of

3000rpm (permanent magnet rotor or dc excited via slip rings).

[Johny B Good]

Which is why you'll find them used as washing machine pump motors as well as in fans. They don't lend themselves very well to speed control unless you use a variable frequency converter with an output voltage that varies in proportion to the frequency.

[harryagain]

Drivel. Few single phase motors these days have centrifugal switches, most are "split phase", the "start winding" is permanently connected.

The centrifugal switch on the ones that do have it operates at far below the running speed of the motor.

You are confusing the issue with some single phase inverters outputing three phase variable frequency. This is because a three phase motor has slightly better starting torque and is more compact and cheaper. Also, it is easier to reverse the motor. This is often what you find on these cheap lathes.

[harryagain]

That is a separate gearbox. The one where you have a boxfull of gears to fit yourself in cheap lathes.

There's one here.

[harryagain]

What drivel you talk. With a brush motor the speed would vary with load conditions, ie every time you made a cut the speed would fall away. You need the near constant speed output from an induction motor for a lathe.

[The Natural Philosopher]

I notice you don't answer the question. So: please cite an example of an electric motor designed never to be switched off.

[The Natural Philosopher]

Not at all. The point I was trying to make is that 'continuous operation could be anything from 5 minutes to 50 years without stopping, depending on the context.

I can only think of one motor in THIS domestic context that runs truly continuously and that's my shit tank aerator motor.

Real heavy duty motors are not induction motors - they are 3 phase synchronous usually.

[Dave Plowman (News)]

But you wouldn't use a universal motor for a fan which is designed for long periods of operation.

Look at things where they are both used - saw benches, pillar drills, hobby lathes etc. Generally, at the more expensive end it's an induction motor.

[Dave Plowman (News)]

You are showing your stupidity even more. But no surprise there.

I'd guess everyone else here knows the difference between something rated for 'continuous' use as opposed to 'intermittent'.

I'll give you a hint. Continuous doesn't mean it runs 24/7 for a thousand years.

[Dave Plowman (News)]

So we can add 'duty' to the terms you don't understand.

[The Natural Philosopher]

Dear Dave.

There isn't a a bookshelf big enough yo hold the things you dont understand, let alone the opinions you have about them based on zero real experience or evidence. So be a nice boy and just f*ck off will you?

[Dave Plowman (News)]

Right. So every heavy duty motor is designed for continuous use, in your view.

Thought even you might see the flaw in that.

You might just try and find out what those words actually mean in the context of electric motors.

[Gazz]

My chinese mini lathe has a 90 volt DC brushed motor, toothed belt drive to a 2 speed gearbox in the headstock (plus the usual changewheel setup for driving the lead screw)

The speed controller has a tacho, and also senses load, and tries to keep the motor speed constant, mine needs a little tweaking as it overshoots the compensation a little, as the more load i put on it, the faster it goes... but only by about 100 or so rpms,

[charles]

In my student days, I worked at the a firm which made heavy electrical machinery. Their "small" motors were over 1ft in diameter.

[Bob Minchin]

Yes that is the type I'm familiar with mainly from people asking me to repair the control boards.

[harryagain]

What drivel you talk. Any motor can be deemed continous or non-continuous, all dependes on the power it is rated at. If that power is exceeded, it will only be safe to run for a stated period.

Continuous rated motors are dumping ecess heat as fast as it is generated, ie a state of equilibrium has been reached.

[harryagain]

In this case he's right.

[Cursitor Doom]

A lot of Myford lathes (made in England past or present) still require changewheels and they don't come cheap. In fact they're going up in value at a rate greater than the officially stated UK inflation figures.

>

[Johny B Good]

Not forgetting the ones used to drive HDD spindles at 5400 or 7200 rpm 24/7 year in year out if we exclude certain models of Seagate consumer grade drives.

[The Natural Philosopher]

No. I didnt say that.

stop lying.