Switched Two or Three Speed Three Phase Motor

Does anyone know the internal connections for a three speed, three
phase motor? I do not mean a motor driven by an inverter.
I'm guessing that it's done by effectively changing the number of
poles the motor 'sees', but I don't _know_ and haven't been able to
find any reliable information.
TIA
Richard
Reply to
Richard
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Yes, I have a Holbrook C10 with a 3 speed motor - 3 separate star connections. Motor speeds are 925, 1430 & 2850 RPM
Dave
Reply to
NoSpam
This is what I've seen too, but I believe there are also motors with tapped windings rather than separate windings.
If you can provide a picture of the terminal box, that might help us to know what type of motor you've got.
Best wishes,
Chris
Reply to
Christopher Tidy
Thanks for the response. There are 10 wires from teh motor and I have now measured the various continuities and resistances and concluded that there are two sets of windings. Low speed (940rpm) is simple star. Medium (1400) is delta on tapped windings High (2800) is star-star on the tapped windings
I've had the motor apart and it looks just possible I may be able to access the star and other interconnection points. However I've remembered I still have a pre-historic solid? state inverter/converter thing. It's a solid slab of transformers and capacitors about 12"x9" and 30" tall weighing about 1cwt.
It's got a heap of screwed links and tappings that have to be selected for a given motor, so it's not very user friendly. But as this is the only machine I have with this sort of motor I'll give it a try first as it's probably better than risking wrecking a good motor picking it apart.
Cheers Richard
Reply to
Richard
Sounds to me like is wired with different numbers of poles, which dictate that the speeds are roughly multiples of each other.
Rob.
Reply to
Robert Wilson
Yes indeed Rob, you're quite right. There are physically 36 poles and it's configured to run as a 2, 4 or 6 pole motor at high, medium or low RPM. The switch has zillions of terminals and interconnections to to re-configure the windings to run at the different speeds.
I have two concerns with trying to use my VFD:
Firstly potential damage to the motor during extraction of the inteconnection tappings.
Secondly, the middle speed is delta connected to 415V. Each winding is two windings in series which are switched into parallel with the line to the centre tap and both ends being the star points to become the star-star for high speed. Assuming I can get the interconnections out, I could connect the YY as DD and run it from the VFD for high speed, but I can't quite get my head round whether I could do the same for the mid speed having swapped the polarity of one winding of each pair(phase).
I think I probably can, but high power electrical machines was only a one unit course nearly 30 years ago......
Richard
Reply to
Richard
Hi, If it is a fairly intelligent drive then you should be able to put into the drive the number of poles via a menu parameter.
What suppy are you running the drive from? Not sure I can really comment with any confdence without seeing a diagram.
Most people are trying to run the drive from a asingle phase supply with a VFD, but then wonder why things are unpredictable when you try and run a 415 volt motor from it. You know you can turn a 400v machine into a 200v one by re-wiring the motor for delta. Don't forget the current will go up, but the torque will stay the same, just the speed will reduce.
Is that of any help or am I waffling?
Rob.
Reply to
Robert Wilson
The Ward 2A at home has a two-speed motor with a mechanically switched arrangement:
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It's all been connected since the picture was taken, but it seems reliable enough, and I can get details of connections of it, if it would be of any use.
Peter -- Peter A Forbes Prepair Ltd, Rushden, UK snipped-for-privacy@prepair.co.uk
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Reply to
Peter A Forbes
Thanks for the replies chaps. I haddn't considered the 'cleaverness' of the inverter, but I vaguely recall some mention of poles in the 200 odd parameters that can be set - I'll look into that.
I have also dragged the old inverter contraption from the back of the barn and will have a play with that. It's a Westinghouse Brake & Signal Co 'auto-converter'. An auto transformer of sorts with a considerable slab of capacitors to create a bastard phase - weighs about 80~100kg!. The data sheet has pre STD phone no's so it's pretty vintage gear, but if it does work it's easier and less risky than trying to disconnect the motor windings (to convert from star to delta) without damaging anything.
I'll try to get so pics on photobucket or somewhere for collective amusement. Rgds Richard
Reply to
Richard
snip
Sounds like a 240V to 415V auto transformer plus phase shift capacitor to provide the phantom third phase.
These can work quite well but need the capacitor value (or capacitor voltage tapping) chosen to match the motor characteristcs.
This means you have a good chance of success on ONE of the motor speeds (probably the 4 or 6 pole connection) but you are very unlikely to preserve the 3 speed capability.
Jim
Reply to
pentagrid
You're exactly right Jim. In the event it took a couple of hours to get the voltage tappings near enough to the suggested figures as they are very inter-dependent and the screw&nut terminals are tedious to adjust. Nevertheless I did manage it and the motor initally ran at low and medium speed as you predicted. In high speed it turned at about 300RPM and was quite noisy with very low phase voltages (120V ish). However after re-tapping the capacitors to a larger value it would start from standstill in high speed as well as the low and medium speeds. I've checked the phase-phase voltages in all speeds and they're adequate - not as even as a 'real' 3 phase supply obviously but I think it'll do.
Glad I've finally found a use for the converter and its antiquity amuses me... For those interested
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Richard
Reply to
Richard

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