Re: [CAD_CAM_EDM_DRO] VFD drive problems with older 3 phase motor?

On Thursday 25 December 2003 21:10, Ted Inoue wrote:

> For testing, I bypassed all the controls, contactors, etc. and hooked
> the VFD directly to the motor. The motor has 6 wires which I've tested
> with an ohm-meter and concluded that it's wired in a delta format with
> 1-2-3 being higher resistance, 4-5-6 being lower.

I'll take an educated guess and say that what you have there is a '2 speed'
wound motor.

it had 2 speed settings, low (which you already tried) is power to 1,2,3 ..
4,5,6 are left floating. This is a 'delta' configuration, the points 4,5,6
are centre taps on each coil,

so the three coils are 1(4)2, 2(6)3 and 3(5)1

asci art follows :)

1
/ \
4 5
/ \
2---6---3

and ... high speed is still power to 1,2,3, but 4,5,6 tied together to make a
'star' configuration ... try that .. you'll find it does at least spin, but
you will still be down on power. (you can also see why connecting power to
4,5,6 effectively cancelled itself out and the motor just whines ...)

to actually get your motor to work properly with your VFD you will need to do
some re-wiring to parallel the windings that are currently in series on each
leg of the delta.

take the leg 1-2 that has its centre-tap as 4 ... lets call it 1-4-2 ok? ..
4 is actually 2 connections .. so it goes 1-4a,4b-2 ... ok? you need to wire
it to take the 1-4a coil in parallel to the 4b-2 coil, rather than in series
as they are now. If you are lucky, you'll find that the 4 terminal in the
motor block has 2 wires under it ... its then just a question of labelling
everything up, confirming with a meter and doing the same for the other two
legs. What you should end up with is 3 terminals that hav 1/4 the impedance
of 1,2,3 at the moment ... and full power from the motor ...

I've done this on 2 machines now and both run without a hitch ...


Without actually seeing the motor its hard to say, but it was a fairly common
motor configuration, if its labelled 415 volts the modification I described
will make it work fullpower on a VFD from a 240V supply .. scale those
voltages to your local mains voltage.
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