Ask The Trades - Print Page

Ask The Trades
https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Electrical Questions >> 3 phase motor https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1311614422 Message started by supra59 on Jul 25^th, 2011, 6:20pm

Title: 3 phase motor
Post by supra59 on Jul 25^th, 2011, 6:20pm

Hi guys got a problem with a 3 phase machine and im baffled, this machine is for cutting marble it as 4x 3 phase motors just wired it up to the main and one of the motors goes the wrong way so i thought simple swap the phase at the motor, and all should be well, but it made no difference still goes the wrong way no mattter what phases i use, checked the phase rotation with my meter and the machine is correct and incoming phases are correct, if anyone can shine some light on this would be a great help.

thanks Paul

Title: Re: 3 phase motor
Post by Lectrician on Jul 26^th, 2011, 7:30am

Although supply in TP, are you sure this one motor is also TP? Perhaps stating the obvious!

Title: Re: 3 phase motor
Post by CWatters on Jul 26^th, 2011, 10:53am

Was it really going the wrong way or did it just look like it due to the strobe effect of the lighting?

Title: Re: 3 phase motor
Post by thescruff on Jul 26^th, 2011, 11:09am

It will if the Phases are crossed CW and or not wired up right.

Title: Re: 3 phase motor
Post by CWatters on Jul 26^th, 2011, 4:21pm

I know that. What I meant was it's easy to fool yourself into thinking a motor is spinning the wrong way then swap the phases and low and behold it's still spinning the wrong way (because it was spinning the right way to start with). Don't ask how I know :-[ but fortunately the motor didn't have a propeller attached.

Title: Re: 3 phase motor
Post by thescruff on Jul 26^th, 2011, 11:10pm

I know nothing about 3 phase, but Stoday was on about star and delta a couple of days ago.

Title: Re: 3 phase motor
Post by supra59 on Jul 26^th, 2011, 11:59pm

Hi guys thanks for the quick replies i'm hundred percent this motor is 3 phase and its defo going the wrong way watched the fan when turned off, its a real strange one just cant think of any way to correct it other than swapping the phases which i did, further more there's no neutral to the machine so i'm sure all the motors are 3 phase my only thoughts could it be the control gear that controls the motor? still don't understand even if it was
so if any other ideas would be welcomed

Title: Re: 3 phase motor
Post by thescruff on Jul 27^th, 2011, 6:11am

Why not post a pic showing the data plate.

Title: Re: 3 phase motor
Post by Lectrician on Jul 27^th, 2011, 6:45am

If it is a standard three phase motor, swapping any two phases (only) will swap the direction of rotation. Difficult to help further :-/

Is it a direct on line motor (controlled directly by a single contactor/overload)? Not inverter driven for speed control or anything?

You actually swapped the two phases in the motor terminal housing? Not in the control panel or anything?

Definitely swapped two phases, and not confused any possible thermal cutout terminals?

Star/Delta would have 6 terminals and 6 cables for the motor (the ends of all 3 windings), this allows you to connect the motor in star of delta config, designed for use with a timed star/delta starter unit (starts at low speed, high torque, and then switches to high speed, low torque).

Title: Re: 3 phase motor
Post by CWatters on Jul 27^th, 2011, 11:14am

I'm on shakey ground here but... what happens if you have a missing phase in the motor? I half remember that a three phase motor might still run but in a random direction?

Title: Re: 3 phase motor
Post by Lectrician on Jul 27^th, 2011, 11:51am

It would just hum, draw a hug current on the two connected phases, and hopefully operate the overload before the windings burnt out!

I believe, although never tried, that if you spin the rotor of a motor with only two phases connected that it may spin judderally (if thats a word!) and then stop.

We do quite alot of work with supplies for refrigerated vehicles - the TPN 32amp plugs/couplers often do not get pushed fully onto the socket/inlet, and arc and burn the brass pins shorter and shorter until one phase drops, often burning a motor out.

Title: Re: 3 phase motor
Post by sparxxxx on Jul 28^th, 2011, 2:59pm

Lectrician wrote on Jul 27^th, 2011, 11:51am:

I believe, although never tried, that if you spin the rotor of a motor with only two phases connected that it may spin judderally (if thats a word!) and then stop.

It will spin as normal and if on low load it will run forever, on full load the windings will burn out. I'm pretty certain that it will spin in either direction.

Title: Re: 3 phase motor
Post by supra59 on Jul 28^th, 2011, 10:00pm

hi again thanks to all the peeps who as replied and tryed to sort this problem out with me, it is a star delta motor 6 terminals, three terminals at the top and three at the bottom, the bottom three is where the three phases connect to the motor, the top three terminals are linked together which im pretty sure is wired as star, i think this motor is going to have to run the wrong way as im out of ideas as for correcting this problem, as lectrician said swapping any of the two phases should have corrected this but its not weird !! on the note of a phase going down the motor goes much slower and hums or lets say when i have had this in the past it did lol, and as for where i changed the connections it was at the motor not at the control board maybe its something in the control panal thats causing the problem ? but if it is im not going to get involved with that ,the thing is when i do swap the phases, it shows up on my meter its wrong at the motor when i do a phase rotation test, the only bonus is the pump still seems to work ok but it just bothered me as to why this was happening, i hate leaving something when i know its not right if theres anymore info i could give you guys just ask, anyway once again thankyou for all your help with this matter