Electric Motors in the Home Workshop parts 1&2

[Tony Spear]

Bob, only just had a chance to have a thorough read through and I must say I'm very impressed with your progress so far.

Rather than clutter up the forum with comments, I've sent you a PM - more to follow.

Are you going to cover variable speed (variable frequency drives)?

PS: The coverage of motor protection reminds me of a survey I did on a large pumping station in Nigeria. Part of my remit was to establish why they kept burning out the pump drives (1200 kW slip ring machines with rotor resistance starters). I said that the problem was nothing to do with the motors themselves, but was due to the crap design of the pumping station and that for most of the time the motors were way overloaded. Sure enough, when I had my hydraulic test gear fitted up and I knew that the Consultant, Water Works staff etc. would be watching what I was doing, I sent our electrical engineer to have a sneaky look at the overloads in the main control panel and he dicovered that the overloads had been wound off to 150% FLC! :shock: Obviously what had happened is that due to the poor design the motors were being badly overloaded, so the Waterworks staff had wound off the overloads to stop the motors tripping thereby allowing them to overheat and burn out the windings!

Moral: a little knowledge is a dangerous thing. :roll:

 

[9fingers]

Hi Tony,

Yes I will be covering VFDs in the section that will be the core of the work.
This will address the main problem expressed in this forum of how to run an ageing lump of cast iron machinery designed for 3 phase from a home workshop supply.
So far the stuff I have written is background that I expect to need to refer back to in this core section.

It also serves to an extent to keep the wolves from the door by showing that I have at least made a start.

I am going on holiday to France for most of September to stay with friends as well as
Installing a oak door I have made for them,
Installing a new oil tank for their boiler,
Building a block wall and fit the flue for a wood burner in his garage and
assisting with the renovation of a 2CV,
I will take the laptop and aim to write the main section of this work.

Naturally I will also aim to reduce the wine lake and make an assault on the cheese and pate mountains.

Wish me Luck!

Bob

 

[studders]

Bob, my 1949 Harrison Lathe has a motor (a very big, cast Iron one that looks quite old) that behaves as you've described for an induction motor (whiring and click on start up and shut down etc) but I can see what look like brushes. There's no boxes or cylinders attached. I'm confoosed as to what it is. Will try to take a pic later if it helps.

 

[9fingers]

Hi Studders,

Your description has me confoooooosed too!
Pictures would be interesting please.
Ideally one of the rating plate if still readable, one of the overall beast and a close up of this 'brushgear'.
Who knows I might have to edit my writings?
Does the motor look original to the lathe?

Harrison made both wood and engineering lathes and around that era I would expect a non TEFC motor with ventilation holes through to the windings and just two windings and starter switch. In those days mains voltage rated capacitors were both bulky and expensive to make and manufacturers used the resistance and inductance of the start winding to provide the starting phase shift where at all possible.

Bob

 

[studders]

OK Bob, soon as I get back from the shops i'll go take some snaps.

 

[CHJ]

Look up "Repulsion motor" or "Repulsion-Induction motor" might be what you have there.

Quick ident would be a Pair of shorted brushes running on slip rings.

 

[studders]

Thanks for that Chas. Looks like you are spot on.

Hi Bob.
OK, after clearing a ton of 'stuff' off the lathe I could finally get close enough to take some pics.

Here's the actual Lathe which, according to the serial No, was made in 1949.

Here's a pic of the Motor which measures approx 16" x 11" I'm fairly sure it's not the original as it was mounted in the wrong place and had no belt adjustment. I've since fixed the adjustment issue.

And a close up of the plate on the motor. Full marks to CHJ, it is indeed as he thought it might be (though the plate means naff all to me still).

And finally a pic of what look, to me, like brushes?

Will have to read up on 'repulsive induction motors' and see if I can understand any of it. :?

 

[CHJ]

OOOOO! Nice Lathe :mrgreen: :mrgreen:

 

[studders]

OOOOO! Nice Lathe :mrgreen: :mrgreen:

Just a shame about the colour. 'The Gay Lathe' as it was once referred to as. It seems to be the original colour, wonder if it was ex GPO?

 

[9fingers]

The repulsion start motor has improved starting characteristics over the capacitor start motor for the range of practical capacitors available in its day.

The rotor of this type of motor has a winding on it which is brought out to a commutator and brushes. During starting, the winding is shorted out creating a magnetic field that repels the field created by the stator (the frame windings). When nearly up to speed a centrifugal switch removes the short circuit and the motor runs like an standard induction motor.
In high class examples of repulsion motors, the centrifugal mechanism lifts the brushes off the commutator to reduce wear. If these motors refuse to start, the first suspect is the brush mechanism.

These motors are pretty rare and so I don't propose to add a section to my write-up to cover these.

There is a further interesting twist to the repulsion motor (no pun intended). Some have a third brush which can be moved in an arc around the commutator to effect variable speed running. I've only seen one operational which was possibly about 1/6 hp powering a watchmakers lathe in the workshops when I started work.

Thanks for posting the pictures Studders. If that motor every gives up the ghost, it will be quite a collectors item so don't skip it straight away.

Bob

 

[studders]

Thanks for posting the pictures Studders. If that motor every gives up the ghost, it will be quite a collectors item so don't skip it straight away.

Bob

Thanks for the tip and the info Bob. I thought the motor was old but have no idea just how old, very little info on the web about old Century motors.