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Thread: 3 phase : rotation of motor.

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    3 phase : rotation of motor.

    Donkey's years ago if you wanted to change which direction a 3 phase electric motor ran you just swapped two of the live wires over (after switching everything off obviously !) Can anyone tell me if that is still the case please ? TIA for any advice/help.

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    Re: 3 phase : rotation of motor.

    Yes, just swap over any two of the three live conductors (usually done in the motor's terminal box) which will reverse the rotation.
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by Footsfitter View Post
    Yes, just swap over any two of the three live conductors (usually done in the motor's terminal box) which will reverse the rotation.
    Thanks Peter. Was hoping you would come up with a reply.

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    Re: 3 phase : rotation of motor.

    From York Machinery auctions we have acquired a used - probably 1970s - metal cutting band saw *thing* it is very industrial and heavy duty from some foundry most likely.
    It has a three phase motor, but only have single phase here.
    We knew this and a relation is knowledgeable and qualified in electrickery - and speaks of a converter box(?)
    Or would it be a better option to fit a single phase motor ...
    Having had a quick look under guards etc there's a lot of still bright filings accumulated it must have been in regular use until very few weeks/months ago.

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    Re: 3 phase : rotation of motor.

    Not played with a phase converter, but IIRC they came in 2 styles, old way was through a motor type converter I think. More common way with the advent of sheeps-guts or rather electronics are modern phase converters and all I know is that you have to be very aware of matching up the right size motor to converter?

    Another IIRC- doesn't going single phase bring the penalty of a much larger motor to get the power/torque?
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    Re: 3 phase : rotation of motor.

    I think it's only rated at one horsepower and the 3 phase fitted does look surprisingly small, I think it is primarily driving a hydraulic pump.
    I'm guessing something similar to what's fitted on our 6" grain augers would be overkill - they are very heavy mind.
    It probably isn't especially demanding in torque but must say at the moment I am somewhat hazy about how it works, it can saw through RSJs up to about 18" by the look of it.

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    Re: 3 phase : rotation of motor.

    A converter box has been obtained but the plug on the saw has four pins but the box has five.
    I wondered if it would still fit initially but no, the saw plug can simply be changed though.
    A few things have been freed off such as adjustment to how fast it descends while cutting.
    Once it can run, there are further mysteries to reveal such as how/if the cutting fluid stuff is pumping round properly.

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    Re: 3 phase : rotation of motor.

    If it has an auto cutoff switch, it will be a very useful tool. I fitted a limit switch which operates when the blade cuts through and drops a few millimetres. Insert the material to be cut, press the start button & walk away to do other work. Return at a convenient time to do it all over again (if there are multiple cuts). Great machine!

    JV
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by john maddock View Post
    If it has an auto cutoff switch, it will be a very useful tool. I fitted a limit switch which operates when the blade cuts through and drops a few millimetres. Insert the material to be cut, press the start button & walk away to do other work. Return at a convenient time to do it all over again (if there are multiple cuts). Great machine!

    JV
    I have one of the above on my saw but very rarely have the courage to walk away and go and do something else !
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by 4wd View Post
    A converter box has been obtained but the plug on the saw has four pins but the box has five.
    I wondered if it would still fit initially but no, the saw plug can simply be changed though.
    A few things have been freed off such as adjustment to how fast it descends while cutting.
    Once it can run, there are further mysteries to reveal such as how/if the cutting fluid stuff is pumping round properly.
    Saw plug has 1 x earth & 3 x live 240v pins

    The converter socket should be as above but with the addition of a neutral so you can have a 240v supply ie- controlgear, lighting etc.

    In your case wire up the lead on the saw into a 5 pin plug obviously leaving the N pin empty and if it runs backwards swap any 2 of the 3 live wires around to reverse rotation.
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by zaza View Post
    I have one of the above on my saw but very rarely have the courage to walk away and go and do something else !
    Thats the thing with power hacksaws, the idea is to be able to leave then while you bugger off and do something else! Usually attendants are frightened like yourself to leave them or they are mesmerised and just stand there watching
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    Re: 3 phase : rotation of motor.

    I'll get a couple of photos of the beast tomorrow, if it runs with new plug on maybe a short video.
    Something I find perplexing you have to manually lift the main saw carrying part and it seems quite a fight against the hydraulic damper.
    A few times is OK but one planned use was making dozens of substantial gutter brackets out of half a round pipe - that will soon get tedious manually lifting every time.
    At the opposite end we have fitted an assistor spring from something else to preinstalled eyes, that does help a bit.

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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by Footsfitter View Post
    Thats the thing with power hacksaws, the idea is to be able to leave then while you bugger off and do something else! Usually attendants are frightened like yourself to leave them or they are mesmerised and just stand there watching
    I think that my main reason for just standing there is not that I am "frightened" it's because I need to make sure that the suds pipe is in the right position all the time and not, because the blade has worked itself lower, that the suds are missing the saw blade entirely and are being shot over the blade and onto the floor.

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    Re: 3 phase : rotation of motor.

    We have had the big saw running early afternoon and cutting fluid pumps round too.
    It's running through the converter box to a normal 3 pin plug but rather oddly after initially running for a couple of minutes OK it now trips the fusebox, we are going to try it in the larger amps welder plug thing but needed to get a longer cable to reach that.
    Inevitably it ran backwards at first so needed wires swapping.

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    Re: 3 phase : rotation of motor.

    I have just re-read through most of this topic and I have recently discovered that there are motors being produced that can be either single or 3 phase. It depends on how they are wired up as to what voltage they will run on. If you wire them Star then that's 3 phase, if you wire them Delta that's single phase. I bought a new motor a week ago and it could be either single or 3 phase and I wired it up correctly for 3 phase but it ran for about 30 seconds and then blew the trip. I tried it again but it wouldn't even start, it blew the trip immediately. I thought I had done something wrong but after checking the cabling, all the connections I couldn't find anything wrong so took it back and it transpired that there was a fault with the motor !

    IF I HAVE GOT ANYTHING WRONG IN THE ABOVE THEN CAN SOMEONE CORRECT ME PLEASE.

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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by zaza View Post
    I have just re-read through most of this topic and I have recently discovered that there are motors being produced that can be either single or 3 phase. It depends on how they are wired up as to what voltage they will run on. If you wire them Star then that's 3 phase, if you wire them Delta that's single phase. I bought a new motor a week ago and it could be either single or 3 phase and I wired it up correctly for 3 phase but it ran for about 30 seconds and then blew the trip. I tried it again but it wouldn't even start, it blew the trip immediately. I thought I had done something wrong but after checking the cabling, all the connections I couldn't find anything wrong so took it back and it transpired that there was a fault with the motor !

    IF I HAVE GOT ANYTHING WRONG IN THE ABOVE THEN CAN SOMEONE CORRECT ME PLEASE.

    Pretty sure Star-Delta is the old way of getting large-ish motors to start by connecting the motors internal wiring in a star to start up - ie 3 lines with one end of each being connected together and the other 3 ends wired to the 3 phase wires and then when its spinning you connect the wires in Delta, ie all 3 internal wires are connected into a triangle

    Remember the old starters back in the day when you had a handle on the side of the huge starter that you invariably would pull down for Star and then when its spinning you would simply shift the handle upwards 180 degrees where the wiring inside the starter would change the motors supply from Star to Delta and the handle would lock/be held all the time until there was a fault or you pulled the handle to stop/switch off the motor or it may have stop button to hit? After the old mechanical starters came the modern push button style where just pressing the start button will initiate star start sequence, then an electrical timer runs for its set period which will then cut off the star contactor and engage the delta contactor thus changing the speed/torque of the motor.

    Of course then there was Split-phase before 3 phase? the former used 440volts with 2 x 240v lives as opposed to 3phase's 415v coming from 3 x 240v lives

    Our old sparky who is pretty hot on dryers and motors got caught out with a motor we had for the lime quarry where it was common to wire them up with 380v - he'd missed that on the motors plate, wired it up for 3phase which made it growl a lot and not do anything much!!
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    Re: 3 phase : rotation of motor.

    Mmmm. I had forgotten all about the lever that you had to pull down on the starter for the grinding/roller mill and then push up smartly when she had started. These kids of today don't know they're born.

    This is the wiring diagram and the image for a 3phase/single phase motor that I have been involved with recently. I needed to wire it 3 phase but it looks like if you change the position of the brass coloured slotted couplers under terminals A1, B1, and C1 as per the wiring diagram, which can also be seen in the image of the connection box, then it becomes a single phase motor.

    DO NOT TAKE MY WORD FOR THE ABOVE. ANYONE CONTEMPLATING THIS MUST SEEK PROFESSIONAL HELP.
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    Re: 3 phase : rotation of motor.

    I'm keeping out of the way of it today there are two retired electricians and the owner of it (not actually me) playing about in there.
    So far they have ruled out a leak to earth making it trip, and tried a more powerful converter box with no benefit. Running it on the higher rated welder line didn't help either, but since that is almost never used for welding now it could go on there semi-permanently once sorted..
    They have various testing gear giving them confusing and unexpected readings.

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    Re: 3 phase : rotation of motor.

    They have discovered failed insulation and shorting where wires go into main motor under a cover so at least a real fault has been identified.
    It sounds like the motor might need taking for repairs somewhere rather than just have those wires replaced.
    It still seems odd it seemed to run OK for a minute or two after it's period of rest before the auction.

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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by zaza View Post
    Mmmm. I had forgotten all about the lever that you had to pull down on the starter for the grinding/roller mill and then push up smartly when she had started. These kids of today don't know they're born.

    This is the wiring diagram and the image for a 3phase/single phase motor that I have been involved with recently. I needed to wire it 3 phase but it looks like if you change the position of the brass coloured slotted couplers under terminals A1, B1, and C1 as per the wiring diagram, which can also be seen in the image of the connection box, then it becomes a single phase motor.

    DO NOT TAKE MY WORD FOR THE ABOVE. ANYONE CONTEMPLATING THIS MUST SEEK PROFESSIONAL HELP.

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    How I read that terminal box and diagram, they are both 3phase.

    You are wiring the LH diagram as Star connected because each of the 3 windings has one end that goes to the linking plates, its other end id connected to the supply (240v)

    You are wiring the RH diagram as Delta connected because the link plates have been separated and placed to link two windings and also you connect a 240v power supply to each link plate

    So in effect both are 3phase as you need 3 x 240v supplies but Star connected will have about 30% less torque than delta connected. Using a Star-Delta changeover switch enables you to have the best of both worlds in old kit and supply networks as they start more "softly" and then once you have the motor and what its driving revolving the change to delta allows the higher torque to take over. The Modern way with motors are direct 3phase motors which is direct start delta connected- push start and bang zero to rated speed like that, ok if the supply, driveline and motor can poke up with it

    I think I have that right, I'm not a sparky but as you do in this job its one of the skills you come across and need to deal with now and then


    This sort of explains it reasonably well- https://www.electrical4u.com/star-delta-starter/
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by 4wd View Post
    They have discovered failed insulation and shorting where wires go into main motor under a cover so at least a real fault has been identified.
    It sounds like the motor might need taking for repairs somewhere rather than just have those wires replaced.
    It still seems odd it seemed to run OK for a minute or two after it's period of rest before the auction.
    That'll be about right, could be damp/humidty related too if its throwing a trip/earth leakage. Our big old colchester lathe had a moment a few yrs ago that turned out to be mickey mouse had crawled into the starter and died. When he "leaked" it kept throwing the trip out and then once dried out he did it again when it rained and made him conductive!!
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by zaza View Post
    Mmmm. I had forgotten all about the lever that you had to pull down on the starter for the grinding/roller mill and then push up smartly when she had started. These kids of today don't know they're born.
    they'd be stuck with one of these then?

    Last edited by Footsfitter; 22-05-21 at 08:09 AM.
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by Footsfitter View Post
    How I read that terminal box and diagram, they are both 3phase.

    You are wiring the LH diagram as Star connected because each of the 3 windings has one end that goes to the linking plates, its other end id connected to the supply (240v)

    You are wiring the RH diagram as Delta connected because the link plates have been separated and placed to link two windings and also you connect a 240v power supply to each link plate

    So in effect both are 3phase as you need 3 x 240v supplies but Star connected will have about 30% less torque than delta connected. Using a Star-Delta changeover switch enables you to have the best of both worlds in old kit and supply networks as they start more "softly" and then once you have the motor and what its driving revolving the change to delta allows the higher torque to take over. The Modern way with motors are direct 3phase motors which is direct start delta connected- push start and bang zero to rated speed like that, ok if the supply, driveline and motor can poke up with it

    I think I have that right, I'm not a sparky but as you do in this job its one of the skills you come across and need to deal with now and then


    This sort of explains it reasonably well- https://www.electrical4u.com/star-delta-starter/
    I connected the three live wires of a 3 phase supply to A2, B2, and C2 and the motor ran fine. It was after a while that the motor tripped the lecky and wouldn't start again. The motor was returned to the suppliers who have investigated it and admit to the motor having a fault and have told me that they are going to re-imburse me. Frankly I thought it was a really crappy motor anyway, just look at the earth terminal. The silver screw outside of the connection box. I enquired about how I was supposed to wire that up and all they could say was that they normally put the earth wire under the head of the screw. So you put 3 live wires inside the box, screw the lid back on, and the somehow connect the earth outside of the box !!!

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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by zaza View Post
    I connected the three live wires of a 3 phase supply to A2, B2, and C2 and the motor ran fine. It was after a while that the motor tripped the lecky and wouldn't start again. The motor was returned to the suppliers who have investigated it and admit to the motor having a fault and have told me that they are going to re-imburse me. Frankly I thought it was a really crappy motor anyway, just look at the earth terminal. The silver screw outside of the connection box. I enquired about how I was supposed to wire that up and all they could say was that they normally put the earth wire under the head of the screw. So you put 3 live wires inside the box, screw the lid back on, and the somehow connect the earth outside of the box !!!
    Although it looks like its probably not a waterproof motor, if it were wired with a 3 core SWA cable, the compression 20mm gland with a brass earthing tag would screw in there and then it would probably just need a hole drilled to enable the earth screw to fit as you can bet the tags pre-punched hole wouldn't align up !!

    Your right, the motor does look a bit "old school" There's a lot of WEG motors about which are Brazilian manufactured IIRC, not bad basic design but tend to be pretty heavy old things still!
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by Footsfitter View Post
    Although it looks like its probably not a waterproof motor, if it were wired with a 3 core SWA cable, the compression 20mm gland with a brass earthing tag would screw in there and then it would probably just need a hole drilled to enable the earth screw to fit as you can bet the tags pre-punched hole wouldn't align up !!

    Your right, the motor does look a bit "old school" There's a lot of WEG motors about which are Brazilian manufactured IIRC, not bad basic design but tend to be pretty heavy old things still!
    "the compression 20mm gland with a brass earthing tag" No, it was just a horrible cheapo plastic job ! I am told that the motor is British Made, and it did have that on a decal on the side of the motor but that's all there was, no plate giving rpm etc. Attached is an image of the decals with the model No. and manufacturer deleted.
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    Re: 3 phase : rotation of motor.

    Rusch powered hacksaw/band saw?
    It was about £500 at the auction



    The stop start box goes here and was under suspicion but can be put back it was probably fine



    The motor connection box is open, that's where it has shorted.
    The motor needs to be removed from where it connects but so far we have seen only a few mm of movement.



    Three phase converter box


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    Re: 3 phase : rotation of motor.

    This saw is finally up and running, it had burnt out wires where they go into the motor you might expect the oily liquids and such splashing about for at least 30 years to have some bad effect.
    A further problem has arisen in that the three phase converter donated is only 1.5kw which can run the 1hp saw motor and the smaller motor running the cutting fluid pump but starting them both at the same time really hits it.
    We borrowed a 3.5kw one which plays with it.
    Those transwave sets start at about £500 new for the smaller one and a new 3.5kw one is about twice that.
    It might help to have it so you can start the fluid pump separately but we did notice in a quick trial that as it starts to bite into metal the boost light is flickering like it does when you start the motor so things are stressed for longer term use.
    There is a chance someone else local has a 2.2kw one they aren't using any more that would be a good solution if bought cheaply.

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    Re: 3 phase : rotation of motor.

    With todays steel prices you better do the irish saying- measure twice- cut once!


    Its a fair old beast thats for sure. It was good times for buyers but sad for those who went bust back in the 90's when the crash came and loads of tools/machinery flooded the auctions on orders of banks and finance lenders with no reserve. There were some real bargains while it lasted.
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    Re: 3 phase : rotation of motor.

    Oh yes, I remember that well. I bought an artic. load of tools out of a GKN factory in Birmingham at that time. 2 x Herbert No. 4. Senior lathes as shown. 4 x No. 3MT pedestal drills (still using one), 4 power hacksaws and numerous bench grinders. I paid £15 each for the lathes and I think the drills were a tenner each.
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    Re: 3 phase : rotation of motor.

    Quote Originally Posted by zaza View Post
    Oh yes, I remember that well. I bought an artic. load of tools out of a GKN factory in Birmingham at that time. 2 x Herbert No. 4. Senior lathes as shown. 4 x No. 3MT pedestal drills (still using one), 4 power hacksaws and numerous bench grinders. I paid £15 each for the lathes and I think the drills were a tenner each.
    That looks old, were you bidding in £sd or even guineas?
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