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Electric Motor

Electric Motor

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Paul Scholey03/04/2019 19:02:43
87 forum posts

Further to a previous post of mine, can anyone tell me how running a motor "Not" under load can damage it ? I've been told this before but have just read on the Lathe site under Colchester Student, it says " frequent start ups not under load will prematurely shorten the life the motor, " referring to single phase. What damage does it actually do to the motor running it not under load ? it does also mention capacitors failing.

Edited By Paul Scholey on 03/04/2019 19:04:21

Phil Whitley03/04/2019 20:33:36
929 forum posts
130 photos

I find that a bit of an odd statement, almost like saying "it will last longer if you don't use it". Starting a single phase motor under load is NOT a good idea as the starting charicteristics are such that not a lot of torque is developed at first, unles you are using the double capacitor type which are a bit better, but on load starting single phase is generally not a good idea. Having said that, most clutchless lathes will be on load of sorts, as the drive train and chuck will start with the motor. I think the reason that it say it for the Colchester is that the single phase motor was a bit underpowered on the roundhead students, which I believe had the matrix clutch fitted, but again, this does not make much sense, surely it should be "frequent start ups UNDER LOAD, will shorten the life of the motor" I know of no situation where running a motor unloaded or lightly loaded shortens the life. . Because of the way the rotor in a motor is always "chasing" ie, behind the magnetic field trying to catch it, little torque is developed under light loads, but the motor will draw correspondingly little current. When more load is applied, the rotor "slips" further behind the field, and torque rises as the magnetic field is stretched. I would drop tony@lathes.co.uk an email and ask about this. Interesting!

Edited By Phil Whitley on 03/04/2019 20:35:22

Brian Sweeting03/04/2019 20:53:25
385 forum posts
1 photos

Having read the writeup on lathes.co I think it is refering to early electric motors.

It should be remembered that every motor start generates heat and the motor needs to run for a reasonable time to dissipate that heat. Frequent stop/starts will kill a motor over time.

Also a motor running without a load will often run at above full load current which is not good for the motor.

SillyOldDuffer03/04/2019 21:05:59
4838 forum posts
1017 photos

I've been musing on this too, and came to the same conclusion as Phil - lathe.co.uk is wrong, probably the 'not' is a typo.

Starting single-phase motor requires a way of providing a second magnetic field shifted 90 degrees behind the main winding. Hence start windings, capacitors and centrifugal switches.

When power is first applied, the motor has a hard time - struggling to turn with low torque, the capacitor is stressed, high current flows briefly in the windings, two windings get hot, and the cooling fan isn't working. Once the motor is up to speed, the run winding and capacitor can be disconnected, the fan works properly, and the motor is electrically efficient.

I'd expect firing up under load to prolong the uncomfortable start period causing more damage, not less.

A clutch in the system is helpful to this type of motor because it can be quickly run up to speed before any load is applied. Then the clutch allows the motor to be run continuously whatever the lathe is doing, avoiding the need to start and stop it continually.

Dave

Paul Scholey03/04/2019 21:24:02
87 forum posts

Thanks Phil, and Brian what you just said about running without a load is right then ! It's to do with heat , the other person who told me not to let it run on idle without a load was an electric motor re winder, but I never asked for an explanation. it beats me how a motor can run at above full load current when not loaded but that's why I am asking because my knowledge of electric motors is very limited to you people who help on here.

Paul Scholey03/04/2019 21:25:45
87 forum posts

and thank SillyOldDuffer

Paul Scholey03/04/2019 21:27:18
87 forum posts

You replied while I was typing

Frances IoM03/04/2019 23:06:32
657 forum posts
24 photos
possibly the reference was not to synchronous motors but commutated motors common in days of dc supplies that would need some load to limit speed - eg try removing the air moving fan from a vacuum cleaner motor and see how fast it can get before some bearing decides it can't cope.
Paul Scholey04/04/2019 11:11:30
87 forum posts

Thanks Frances

SillyOldDuffer04/04/2019 11:43:56
4838 forum posts
1017 photos
Posted by Frances IoM on 03/04/2019 23:06:32:
possibly the reference was not to synchronous motors but commutated motors common in days of dc supplies that would need some load to limit speed - eg try removing the air moving fan from a vacuum cleaner motor and see how fast it can get before some bearing decides it can't cope.

Yes indeed, Universal motors as used on sewing machines & vacuum cleaners are liable to run away and self-destruct, as are series-wound DC motors. Both types need a small load to keep them sane. I can't imagine Colchester using a Universal or series-wound DC motor in a lathe though, and the entry in lathes.co.uk definitely refers to single-phase:

'Although a 3-phase motor is very robust, and will put up with a good deal of abuse, its 1-phase cousin is a relatively delicate thing and best run near its rated capacity all the time (i.e. worked nearly flat out); if such a motor is switched on and off frequently against "no load" the windings will be damaged and, if run through a cycle where it is started, worked briefly, stopped and started again, the capacitor will fail prematurely.'

I'd agree entirely with that wording if 'against "no load"' was struck-out. I'm no expert though!

In theory I think single-phase motors are a bodge on most machine tools, not smooth running, and comparatively unreliable. Only used because 3-phase was difficult to get into a domestic workshop. Since electronics could do DC, VFD, and Brushless, I don't know of any current lathes sold with a single-phase motor. In practice, years of good work have been done on Myfords and similar, though it is true their centrifugal switches and capacitors may have been replaced a few times.

Dave

Paul Scholey05/04/2019 22:34:47
87 forum posts

Are you saying the windings will heat up if used under " no load " ?

Robert Atkinson 206/04/2019 07:06:53
avatar
398 forum posts
21 photos

I think the problem they are refering to is insulation breakdown due to high back EMFs generated under low load starting. This is less of a problem with modern winding insulation materials.

Robert G8RPI.

Mike Poole06/04/2019 09:58:13
avatar
2186 forum posts
52 photos

Running a motor with no load is an unlikely scenario unless testing, even a machine with a clutch is going to be spinning up the drive train as far as the clutch which is going to offer some inertia and belt or gearbox losses. As Andrew says a modern motor is a very robust device and failures are very rare these days, they can run so hot that the bearing lube is more likely to fail than the windings. A correctly specified motor can comfortably take 4000 starts a day year in year out.

Mike

not done it yet06/04/2019 10:56:55
3547 forum posts
15 photos

Posted by Mike Poole on 06/04/2019 09:58:13:

... A correctly specified motor can comfortably take 4000 starts a day year in year out.

Mike

Ha ha. Some of the motors I used to work with took between three and five minutes to go through the starting sequence, so no chance of those properly specified motors starting anywhere near that number of times in a day!

They were not hobby sized motors, of course (Up to 3000HP), and would boil the starting electrolyte (caustic soda solution) if started too often over a short period.

Mike Poole06/04/2019 12:25:07
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2186 forum posts
52 photos

You have a point there NDIY, I had in mind a car production line making 1000 cars a day with multiple start per car, biggest motors on the line are about 100hp but they would not normally operate every cycle the hardest working ones are less than 20hp.

Mike

not done it yet06/04/2019 14:14:19
3547 forum posts
15 photos

Mike,

Yeah - a row of robots that replaced a line of Roberts?smiley Roberts were apparently not hard-enough workers at BL back in the 80s(?)!

Edited By not done it yet on 06/04/2019 14:14:38

Paul Scholey07/04/2019 09:24:12
87 forum posts

Thanks for all your comments, really appreciated

Mike Poole07/04/2019 10:13:42
avatar
2186 forum posts
52 photos
Posted by not done it yet on 06/04/2019 14:14:19:

Mike,

Yeah - a row of robots that replaced a line of Roberts?smiley Roberts were apparently not hard-enough workers at BL back in the 80s(?)!

Edited By not done it yet on 06/04/2019 14:14:38

Legend has it that the first robot I worked on is now in the science museum a Unimate . 28,000 people worked there when I started in 1972 down to about 4000 now. 1100 robots now produce the body shell and things must be getting close to no direct operators required in the body in white facility. Car manufacturing for a production worker is relentless boredom and locally sourced labour has always been a problem, in the 1930s people walked from Wales to get a job in the factory, a large housing development in Cowley was called Cowley Welsh, today a significant part of the workforce come from all over Europe. Oxford is the most unaffordable place in the country to buy a house so if it wasn’t for robots I doubt there would be a factory still there. People still commute from Birmingham rather than relocate. Some lads still live in Portugal and house share in Oxford then fly home as often as possible.

Mike

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