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Bench mill problem

Motor will not run up to speed in top gear

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Phillip Priestley 108/11/2021 14:25:54
6 forum posts

Hi, hopefully someone can help with this baffling ( to me) problem. I have a Chinese vertical mill BMD 45 with a 6 speed geared head, it starts and runs in ok in five of the six gears but upon trying to start it in sixth gear the spindle can only manage about 100 rpm. I have read that capacitors can be problematic but why on only one speed if that is what it is?

Thanks in anticipation

Phillip

Clive Foster08/11/2021 14:46:53
3104 forum posts
107 photos

Phillip

A single phase motor cannot intrinsically self start because it is stable stationary when power is applied so not turning torque is to get it moving is produce.

The starter winding, and capacitor if its that sort of motor, are electrically offset relative to the main winding so an unbalanced torque is developed to get the motor going. Once its running fast enough the motor can continue accelerating on its own so the starter winding (and capacitor) are switched out.

The amount of offset torque depends on the winding configuration an capacitor size. If there isn't enough offset torque to overcome the load the motor will be unable to accelerate up to speed so will try and run at the fastest speed it can reach. A failing capacitor has less capacitance than it should so starting and run up torque is also lower.

Higher gear ratios impose more starting loads on the motor so when a capacitor begins to fail the effects on starting will be noticed first in the highest speed gear.

Exactly your symptoms which is why capacitor problems are the common suggestions for things not working.

Clive

Dave Halford08/11/2021 15:01:21
2007 forum posts
23 photos

If you run it first in 4th gear for five minutes and then start it in 6th is it better?

If it's OK like that, the oil just needs to warm up.

Bountyboy08/11/2021 18:19:13
48 forum posts
7 photos

Hi,

I had a similar problem with my EMCO mill and that also needed a new capacitor.

To quote Kiwi Bloke who helped me.......

"Just in case you don't know, make sure you get a 'motor run' capacitor, not a 'motor start' capacitor. The latter are electrolytic and will fail after very little running time. The former are usually metallised polypropylene. They are cheap, and available from electric motor suppliers, re-winders, electrical wholesalers, RS Components, Farnell (now called Element 14 - I think), etc."

I wasn't aware of this and I managed to get one from RS Components.

Hope this is of use smiley

Phillip Priestley 108/11/2021 20:19:14
6 forum posts

Hi, Thanks for the replies, I took the easy option and ran it in fourth gear for ten minutes, naturally it started straight away in top gear, then I left it a while and it would only start after I ran it in fourth again. I have just been back to remove the capacitor covers to obtain the details on them, replaced the covers and lo and behold it started first time in top gear! I am now off to sit in a darkened room for a bit!!
Thanks

Phillip

Phillip Priestley 108/11/2021 20:27:35
6 forum posts

Just as an afterthought I have two capacitors on the motor, one 30 if 450v ac

the other one is 130 if 250 v ac which is start and which is run?

Thanks

Phillip

Phillip Priestley 108/11/2021 20:29:35
6 forum posts

Sorry the above should read uf and not if

SillyOldDuffer09/11/2021 10:01:14
Moderator
8496 forum posts
1900 photos
Posted by Phillip Priestley 1 on 08/11/2021 20:27:35:

Just as an afterthought I have two capacitors on the motor, one 30 if 450v ac

the other one is 130 if 250 v ac which is start and which is run?

Thanks

Phillip

I always recommend getting a copy of Tubal Cain's 'Model Engineer's Handbook', which covers electric motors and much else. The book shows many variations but the low value capacitor (in uF) is usually the Run capacitor.

I'd change both capacitors: the poor things take a beating and don't last forever. If one has failed, the other is likely to be on the way out too. Sourcing them is easy because single-phase motors are common as muck and their capacitors often fail. The capacitors are of particular construction for this service so make sure the product description includes the words 'START' and 'RUN'. Most electronic component and motor spares places sell them, example link to CPC/Farnell, .

I'm suspicious your 130uF capacitor is only rated at 250VAC: I'd rate both of them at 450V. Can one of the experts comment please?

Dave

John Haine09/11/2021 10:15:15
4632 forum posts
273 photos

Interestingly CPC only list "run" caps and only up to 80uF. RS do list them but only available through affiliated suppliers. CPC list 30, 70, and 80uF, you could always replace the 130 with a 70 and 80 in parallel.

mgnbuk09/11/2021 11:08:48
1177 forum posts
71 photos

Capacitor supplier

Not used them, but came across them when looking for a replacement induction motor fan. They also do start capacitors.

Nigel B.

not done it yet09/11/2021 11:31:59
6736 forum posts
20 photos

Changing both capacitors (an 80 & 50 for the 130 plus the other one) could be getting close to the realms of changing the motor for a 3 phase one (second hand?) and buying in a VFD to drive it. Almost certain to solve the problem along with lots of programmable options.

that larger capacitor may well have been swapped in by a previous owner? Something he had, tried and it ‘worked’?

Edited By not done it yet on 09/11/2021 11:32:32

Dave Halford09/11/2021 11:58:25
2007 forum posts
23 photos

Run caps are in the 400V range

Start caps are 250V

The construction is different.

Dave Halford09/11/2021 12:04:49
2007 forum posts
23 photos
Posted by Phillip Priestley 1 on 08/11/2021 20:19:14:

Hi, Thanks for the replies, I took the easy option and ran it in fourth gear for ten minutes, naturally it started straight away in top gear, then I left it a while and it would only start after I ran it in fourth again. I have just been back to remove the capacitor covers to obtain the details on them, replaced the covers and lo and behold it started first time in top gear! I am now off to sit in a darkened room for a bit!!
Thanks

Phillip

Did you move the wiring, twist the caps round to read them etc. If so just check the wiring isn't bad.

Phillip Priestley 109/11/2021 12:22:49
6 forum posts

Yes I had to twist them round to obtain the details from them, it now starts every time, I think I should be sourcing a new pair of capacitors for my peace of mind. To answer another question I bought it new, so the capacitors are the ones it left China with!

Thanks

Phillip

Dave Halford09/11/2021 12:34:48
2007 forum posts
23 photos
Posted by Phillip Priestley 1 on 09/11/2021 12:22:49:

Yes I had to twist them round to obtain the details from them, it now starts every time, I think I should be sourcing a new pair of capacitors for my peace of mind. To answer another question I bought it new, so the capacitors are the ones it left China with!

Thanks

Phillip

By 'check the wiring isn't bad' I mean check the wires aren't loose in the spade connector crimp, that bare wire is actually touching the spade connector (and hopefully welded to it) and that the spade terminals are not loose on the cap body.

Roger Best09/11/2021 13:04:38
avatar
369 forum posts
56 photos

Moved wires now it works sounds like a bad connection.

New bits won't hurt, if you can avoid electrolytic capacitors, they dissolve themselves after 8 or 10 years, and anything they dribble acid onto.

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