3 phase motor tripping

[fuzzytrifle]

Please could anyone give me some advice before I start searching for replacement motors - yesterday my saw bench ran for about 5 seconds before tripping both its breaker and the RCD on the distribution board. It's one of three small identical 2.2kW 3hp 3 phase motors fitted to an old combination woodworking machine.

The machine's roughly 25 years old and has been running virtually maintenance free in all that time.

I had the same thing happen to a planer motor about a year ago after I'd spent two days feeding 9" oak beams through it - I guessed that I'd just overworked the motor, but the saw table hasn't done any hard work for months.

Subsequent attempts to start the saw causes the breakers to trip immediately, the other two motors are working as normal.

The machine is run on a 3 phase supply - not through an inverter.

Many thanks in advance.

 

[Spark123]

It could be the motor yes, it could be a problem with the supply ie it is single phasing. Without testing all the motor coils end to end, insulation testing it and checking the supply it isn't possible to say it is definitely the motor.

 

[conny]

Simple method to narrow it down is to isolate the motor feeds by connecting into a junction box so all three ends do not terminate with anything. Switch on the power and if the breakers don't trip then the problem lies with the motor.

Unfortunately there are a number of things that could be wrong such as shorted coils, one or more phases shorted to earth or even seized bearings. Only remedy for this is to have it looked at professionally. A brand new standard 2.2kw foot mounted motor shouldn't cost much over £100, much cheaper than a repair, and should be available 'off the shelf'.

 

[crystal ball]

take the cover off the motor terminal box and have a sniff, burnt smell = bad news

 

[Bolty]

Morning,

As Crystal ball above said, isolate the supply to the motor, open the terminal box and if it smells burnt - chances are it's knackered.

If the connections look ok, are tight & secure and there is not a distinctive burnt smell, then you'll need to use a multimeter or a megger and set to ohms and check for a fault on the motor.

only fault find with the power OFF and ISOLATED.

1. measure the resistance between the windings - they should be balanced.
2. check for a earth fault between each winding and the earth on the frame of the motor. If you have a megger, do an insulation resistance test (500v) between each winding and motor frame, if you only have a multimeter - check for a low resistance between each winding and frame. high value = good, low value = bad.
3. is the motor mechanically jammed?.
4. is there a fuse gone at the starter? this will cause excessive startup current and should trip the overload but sometimes it's so high the mcb reacts first.

If the above is ok, then you'll need to investigate the starter next.

PS. if the motor is knackered and you can get a new 2.2kw 3 phase motor for just over £100 then grab it with both hands - even at trade, i cannot get a new motor at this price, even with a hefty discount!

Best of luck with the fault finding.

Bolty.

 

[fuzzytrifle]

Thank you for the replies - I won't have a chance to have a proper look until Monday now, but I don't remember a burning smell and the motor hasn't seized.

I phoned the company (who still make a version of the machine) and was quoted £260 for a replacement motor!

I had quick a look at the wiring diagram in the manual too, and just to make things even more confusing it's different to my installation.

Will see what I can find on Monday.

 

[Adam_151]

Not sure where abouts in lincolnshire you are but PM me if you want a recommendation for a motor sales / repair place in lincoln

 

[conny]

Maybe my estimate of just over £100 was a little optimistic, I haven't needed to buy one for a few years but this one looks suitable. If you need a 2 pole, (2900rpm), just search the site for a new price.

http://www.ebay.co.uk/itm/Electric-...d=100005&prg=1048&rk=3&rkt=4&sd=280542680206&