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Thread: Motor problems!
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19th Sep 2022, 10:50 PM #1Diamond Member
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Motor problems!
1) Commercial kitchen fabricator is annoyed. Had to replace an extractor fan motor. It was only 2 weeks old. I investigate the failed unit with a multimeter.
3Ø motor. About 20 ohms between 2 of the wires, around 20K ohms between another pair. Hmmm. That has died. Take the cover off, smell a burnt motor. Check wiring. The incoming cable is earth plus 3. The terminal block has cables to U1 V1 W1, and a plate joining U2 V2 W2. Motor was configured as star, but wired as Delta?
Meter shows 10 or 11 ohms between V1 & V2, and W1 & W2, but 20Kohms across U1 & U2. Yep, nothing I can do to fix that. The coils have had 400+volts instead of 240?
Begin to doubt my knowledge of 3Ø motors. Tell fabricator to advise client to call an electrician.
Wonder if fan+motor was pre-wired with 3C+E cable, and original installer/electrician just wired it the obvious way?
2) A 2HP motor from a large rotisserie:
IMG_3248.jpg
It looks nice and new and clean, but vibrates badly, rattling the whole machine it is mounted in. Bearings feel good, shaft runs true. Hmmm. Time to strip it down:
IMG_3218.jpgIMG_3219.jpgIMG_3220.jpgIMG_3221.jpg
A little surface rust on the cores and shaft, but nothing loose. Mount shaft in the lathe, investigate:
IMG_3228.jpgIMG_3229.jpgIMG_3230.jpgIMG_3231.jpgIMG_3232.jpg
Shaft to bearings is less than 10microns, outside surface of magnetic cores about 40 microns. No problems there.
Centre of shaft about 80 or 90 microns TRO. A bit out of balance. And the aluminium casting around the cores? Over 1mm TRO on one side. I consider turning the high spots off, to see if it reduces the vibration.
Factory neighbour proposes spinning it up in the lathe, to feel the vibration.
At this lathe's highest speed (1200RPM or 1400RPM, depending on the belt setting), we can feel a bit of shake on the tailstock. Neighbour proposes eliminating any chuck runout, by spinning up between centres.
I found a centre to put into the chuck:
IMG_3233.jpg
but it wasn't very centred, so refinish with the sharpest tool I can find - a thread turning insert:
IMG_3234.jpgIMG_3236.jpgIMG_3237.jpg
Less than 10 microns? That will do. Now, re-test TROs:
IMG_3238.jpgIMG_3239.jpgIMG_3240.jpg
Tailstock end still good, but centre shaft still 80 microns.
For fun, I try static balancing. Oil up the centres well, and spin the motor many times, marking the top when it stops:
IMG_3243.jpg
Fairly even around the circumference. I thought about finding a grinding wheel balancer, but gave up.
Instead, I drilled some holes in the high point of the aluminium casting, guessing it might sway the balance:
IMG_3246.jpg
It feels better when spun up in the lathe, but sadly no difference after re-assembling the motor.
I also thought about the electrics. This motor has no startup switch, just windings and a capacitor. Disconnecting the capacitor had no effect. Wiring it a different way:
IMG_3247.jpg
(which is power across both sets of windings) was the same - still serious vibration.
I could wait for access to a dynamic balancer, but it is a lot of hassle.
Unless anyone else has other suggestions, I'm about ready to throw this quality Italian made motor away
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19th Sep 2022, 11:51 PM #2Golden Member
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I'm confused.
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20th Sep 2022, 12:31 AM #3Most Valued Member
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Sounds like somethings not machined properly, a bit of swarf under the jaws, when manufactured possibly?
Is it possible for the centre armature? to be machined slightly off centre?
Could you mount the bearings into something like V blocks and then measure your run outs, to confirm everything is parallel?
KrynTo grow old is mandatory, growing up is optional.
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20th Sep 2022, 08:45 AM #4Member: Blue and white apron brigade
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I don't understand, why put so much time into investigating balance if one of the coils is not right?
When coils behave like this 9/10 times following a cursory inspection to see anything obvious they get chucked in the scrap bin.
The one time I chased this on a motor was for one I was going to convert from Y to ∆, and it tuned out to be a dud joint. Most common joints are copper/copper but this one was something else.
FWIW one of my Italian cousins owns and runs a custom electric motor factory about 70km from the stated location of that motor (Vicenza).
Just because it says made in Italy means little these days.
My cousin outsources most of his component manufacturing to eastern european countries and he mostly assembles and tests the motors he sells
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20th Sep 2022, 09:48 AM #5Member
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I'm also confused.
The name plate says it is a single phase motor? It also has a capacitor, which is not required for 3 phase motors?
My guess is that the motor was likely wired incorrectly, now you have a burnt out motor/capacitor.
It is probably not worth the trouble doing all this work, a new motor will save a lot of time and effort.
Small generic induction motors are not worth the time and expense to get them rewound.
When a new motor is installed, it is a good idea to check the electrical run current using a clamp meter.
Hope this helps.
Paul
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20th Sep 2022, 10:33 AM #6Member: Blue and white apron brigade
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OK - re-read the post and see there are 2 motors involved - a 3P and an SP and now I understand what you are doing.
That's the second motor
It is probably not worth the trouble doing all this work, a new motor will save a lot of time and effort.
Small generic induction motors are not worth the time and expense to get them rewound.
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20th Sep 2022, 10:47 AM #7I break stuff...
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There are two motors being discussed. The 3 phase one has been condemned (as Bob has now seen).
As for the single phase one - it's rated at 1380rpm, and you spun it at 1200 or 1400 rpm in (from memory) a pretty light lathe without noticing THAT much vibration, tells me it's not mechanical.
Given that it has a single capacitor, no centrifugal switch, and can be reversed (see motor plate for connections), suggests to me it's whats known as a 'capacitor run' motor. Although in theory disconnecting the capacitor should have stopped it running altogether. I would suspect the most likely suspect to be the capacitor, it seems the correct capacitor is also specified at the bottom of the motor plate. Check the windings first though, in case one is burnt out, and confirm that the original incoming connections match the motor plate (not sure on the logic of swapping them to where you did?).
Unbalanced or fluctuating electromagnetic fields can create horrendous vibrations...
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20th Sep 2022, 06:55 PM #8Most Valued Member
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20th Sep 2022, 06:59 PM #9
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22nd Sep 2022, 11:05 PM #10Diamond Member
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Good suggestions.
• One winding is 3.7ohms, the other ~1.6ohms.
• I don't have a spare 50µF cap handy. Will try testing it tomorrow, or maybe replacing with two 25µF caps in parallel.
• My wiring hack was to try and make the magnetic field more unbalanced?
I haven't tried reversing rotation yet. It also shouldn't have any effect, but you never know?
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23rd Sep 2022, 07:38 PM #11
Hello Nigel, guys,
That single phase induction motor has what is called a "Squirrel cage rotor". Basically an aluminium cage cast into an iron frame. Remnant magnetism will cause the rotor to turn when power is applied to either winding. There won't be much rotational power. Single phase motors run quite roughly compared to a three phase motor, which is why you often see them on a rubber or flexible mounting, done to reduce the transmitted vibration. Notice that the rotor bars are set at an angle, this is done in order to try and smooth out the rotation as the rotor bars pass through the change in magnetic field.
The balance could be perfect, but the motor will still vibrate.Best Regards:
Baron J.
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23rd Sep 2022, 10:18 PM #12Diamond Member
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Checked the motor run cap. 51µF. Also tried replacing with two 25µF in parallel. Same vibration. If I lift the motor off the bench, my arms mostly dampen the vibration.
John, this was/is mounted on large rubber mounts, but as the output is mechanically coupled to a large resonator (worm gear to worm wheel to a shaft mounted on pillow block bearings on the side of a 4m3 sheet metal case), it doesn't do much to quieten the squirrels
I agree that all single phase motors will vibrate a little (compared to 3Ø, brushless DC, or general DC), but plenty of other machines with 240V motors seem 1000% better?
e.g. the other motor and gearbox I installed in the same machine:
IMG_3091.jpg
This vibrating Italian motor is larger than most, so I'm wondering if that is the issue?
(24mm output shaft, 2HP, rated 9.5A at 240V)
... but many other machines like lathes or mills have similar sized motors???
Thinking further, their output is usually through a rubber belt, and their shafts are usually 16mm?
Anyway, still confused by this motor.
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23rd Sep 2022, 11:54 PM #13
Hi Nigel,
I agree that some single phase motors are better than others in respect of vibration. I've not done an analysis of the effect that varying the capacitor value has on the motor vibration, but have noticed that capacitorless motors seem to vibrate much more than centrifugal switched capacitor motors.
Both the value of start winding inductance and associated capacitor value do affect the phase angle of the current through the start windings, but this should only affect capacitor start and run motors, without centrifugal switching, since the winding currents ideally should have a constant 90 degree phase difference.Best Regards:
Baron J.
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