Various ancient induction motors
 

hi folks

In the 'for sale' section you may have noticed that I have been donated a pile of elderley induction motors, to sell for charity. Quite a few have 'issues', which is interesting as it provides an opportunity to learn something - and being 60-70 years old in some cases, they are perhaps permissible for this forum.

Single-phase types with an internal centrifugal switch to the start windings and no capacitor, I am familiar with. But of the capacitor types, a few have capacitors either missing or so badly corroded that I don't know what they are. Some of these have a centrifugal switch (cap start) and some, have no switch - which suggests the cap is either switched out manually, or stays in for run.

My question is - can anyone point me to a reference to work out the capacitor value in such cases? I am almost interested enough in this to make a small band-brake, or maybe lash something up out of motorbike brake bits - I have a tacho and some weighing scales - and to plot some curves of torque and speed for various values of capacitor. But it would certainly be easier to read about it in a book!

cheers
Mark

Re: Various ancient induction motors
 

Most of them should run on as low as 50 volts if the bearings are not too stiff.
They will run well below full speed but the speed will most likely peak at close to the correct capacitance.

Re: Various ancient induction motors
 

Thanks, that's a good idea to reduce the torque (I just tried it - very slow starting but coming up to speed eventually at about 60v with the one I have on the bench, so far with a guestimated cap value). It suggests to me that the 'start' winding will just be helping out the main winding with a bit more torque as the speed falls on-load. I guess this won't be a problem if the thing is wound for it - which makes me wonder, what the difference is between a motor with start windings which can stay in, and one where they are switched out as it spins up.

For instance, I have a big motor on my saw bench with start cap and centrifugal switch, which I was given a long time ago because it smoked and stank - this was due to the switch being bunged up with (someone else's) sawdust. I guess in that case the start winding impedance must be rather low, so a big current for a short time is OK, but then it gets hot. Maybe there is some ratio between the impedance of the start, and run, winding that I need to think about, when wondering whether it is OK to leave the start winding connected all the time on an unknown and rather ancient machine.
cheers
Mark

Re: Various ancient induction motors
 

Alright, next time I'll read a bit more before posting. To make this post worth a little more than that apology, I see that the DC resistance of the main winding of my mystery motor is 15.3 ohms, and that of the start winding is 18.6 ohms. I guess that doesn't really tell me much about their respective reactive impedance when running.

Re: Various ancient induction motors
 

Just switch the capacitor out once the motor is close to speed and the bearings have begun to free up. If it stays running it needs a start button that is latching for the run winding and push to make for start.

Re: Various ancient induction motors
 

Mark:
The problem you face is that the precise nature of the "secondary" winding is in the lap of the motor designer.

Broadly speaking, capacitor-run motors will usually

a) not have a start capacitor/centifrugal switch, but will rely on the run capacitor to heave the thing up to speed. Hence cap run motors are generally only used in applications where the starting torque requirement is modest: circular saws, fans etc.

b) have a similar (but rarely identical) winding to the main winding, fed via the capacitor, which makes sense, as this is really a two-phase machine and the VA of both windings should be similar. The voltage rating of the run capacitor is often around 450V ac and they're always metalised polyprop nowadays.

So, if you have two windings and they are similar resistance and inductance, it's a safe bet it's a capacitor run motor.


With capacitor-start motors, the start winding is effectively parastitic when the motor is operating normally, i.e. it's occupying space but not being used. So, it will be wound with much thinner wire guage and relies on intrinsic thermal inertia to stop it overheating during the second or two starting period. The start capacitor is similarly intermittently-rated and is often a non-polarised electrolytic.

I've attached a Brook Motors guide to motor winding connections/configurations, if they are of any help.

John

Re: Various ancient induction motors
 

Thanks John - some holiday reading there :)

Re: Various ancient induction motors
 

I've just grabbed that Brook Motors document.... very useful! Thanks, John.

Brooks were major employers in my home town, Huddersfield, at the Folly Hall end of St Thomas' road. The land now seems to hold a casino, which is rather appropriate in Folly Hall.

David

Re: Various ancient induction motors
 

We had a centrifugal switch starter type motor (no cap) giving strange readings on the ammeter, it was also noted it would sometimes only turn one way by hand after the first & second test run.
Trying it again resulted in the circuit breaker tripping, turned out one of the springs had failed in the the centrifugal switch and the mechanism had damaged the stator windings, this was getting stuck & sometimes preventing it being turned one way.

Still got a couple more to test, including the one I posted about recently, just haven't had enough spare time.

David

Re: Various ancient induction motors
 

Do you have any pics? These motors sound cool

Re: Various ancient induction motors
 

I don't have pictures (I'm not very digitally inclined, though I can do it if I really have to!), but I have a list of specs and I am slowly going through them and seeing which need bearings, and if the windings insulation is OK (on a 500v Megger). If you have a specific requirement feel free to PM me, or indeed PM me (a general invite) for a list of those motors remaining.
Thanks
Mark

Re: Various ancient induction motors
 

Hello Mark I was reading your request regarding motor capacitors values and thought you might like a copy of an article I wrote for our Model Engineering club several years ago. Ted

Re: Various ancient induction motors
 

Having just sent the capacitor details I thought you might like some details of single phase motors and their winding connections. Ted

Re: Various ancient induction motors
 

Thanks Ted - they're nice articles, and I'm sure they will be useful to other people on here too. I don't half learn slowly these days, but I am making progress with the pile of motors! Can you tell me (simply) why a capacitor for 'run' on 240v needs to be 415v rated - is it something to do with exciting an LC series resonance with the 'clunk' as the thing is switched on, or off?
thanks
Mark

Re: Various ancient induction motors
 

Mark. 240 volt is the RMS value of the mains supply and the peak value of the sine wave is near enough 340 volts. So, as the run capacitor is in circuit all the time the motor is running it has to rated to cope, so a working voltage of 400 or more.
The LC circuit .is more applicable to the start winding and a fluttering centrifugal switch.
For those of interested in electric motors, the following books titles are informative reads,
Electric Motor Test & Repair, The repair of small of small electric motors, Rewinding Small Motors, Electric Motors by Jim Cox in the Workshop Practice series N0.16 and the most comprehensive is Electric Motors by Robert Rosenberg ( expensive to buy, but might be in the library) I'm sure there are more.Ted