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.
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
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
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......
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?
The Ward 2A at home has a two-speed motor with a mechanically switched
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 A Forbes
Prepair Ltd, Rushden, UK
On Mon, 14 Apr 2008 12:23:46 +0100, 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
On Tue, 15 Apr 2008 09:58:54 +0100, Richard <sharkface-pilot at
toucansurf dot com> wrote:
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
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.
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
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
For those interested
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