Re: [CAD_CAM_EDM_DRO] Re: VFD drive problems with older 3-phase motors (corre...

In a message dated 12/25/2003 7:58:11 PM Eastern Standard Time,
ted@... writes:
Ok, I thought I understood how these wirings were different, but it
sounds like I don't.

I measured the resistance between all permutations of the wires,
generating a table. From this and a little logic, I figured out which
wires were separated from others by one winding (~0.8 ohms). This gave
me all 'adjacent' wires. Then adding in the other measurements, I came
up with a complete diagram.

Now, my understanding of wye connections is that it has a common
connection in the middle and the windings radiate out from that
whereas the delta configuration have each winding only connected to
one other on each side, forming a loop, or a triangle shaped
configuration. I can see how an ohmmeter could make the same
measurements for either configuration as in the wye configuration
you'd just be measuring across pairs of windings.

With the dual windings, each leg of the triangle is composed of a pair
of windings. Taking every other connection (i.e. 1-2-3 or 4-5-6) you
get, I thought, a standard dual speed delta configuration.

As for phasing, I thought with three wires you can't get phasing
wrong. 1-2-3 will drive the motor one way. 1-3-2 will drive it the
other direction. There's no other possible arrangements that I can get
out of this that aren't identical.
1-2-3 = 2-3-1 = 3-1-2
1-3-2 = 3-2-1 = 2-1-3

If you charge the coils in the following order
1-2 phase 1
2-3 phase 2
3-1 phase 3
you drive the motor in one direction.
If you get the phases out of order:
1-2 phase 1
3-1 phase 2
2-3 phase 3
you drive the motor in the other direction

So what am I missing? Or do I totally misunderstand how the three
phase motors are wound/operate?

Thanks for the insights...
-Ted
Ted:

You are correct in that the Delta configuration is indeed a triangle when you
draw out the connections and the ends of each coil are connected to each
other whereas a wye connection does have a common connection and the 3 outside
legs of each winding are open for connection to the line phases. You are also
correct regarding the phasing, at least as long as the "internal" connections of
the motor have not been changed. All one needs to do is swap any two of the
incoming power leads to change the rotation of the motor. If the connections
of the the internal windings have been changed it is possible for the motor to
pull far more current than it is supposed to, causing the overloads or other
protections to trip. I ran into this problem on a 40 hp 3 phase motor from
Japan a couple of months ago and had to use the battery method to get the
windings connected correctly.

When I said you could tell the difference between a wye and delta
configuration using an ohmmeter, I was referring to motors that have multiple windings
and designed for multiple voltages. To change the speed of a three phase motor,
or pretty much any AC motor, you have to change the number of poles in the
motor or, the frequency of the AC power supplied.


Randy Abernathy
4626 Old Stilesboro Road NW
Acworth, GA 30101, U.S.A.
Phone / Fax: 770-974-5295
Cell: 678-772-4113
E-mail: cnc002@...

I furnish technical support, repair, and other related services for your
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extensive background in electronics, mechanics, pneumatics, electrical and CNC
machinery to offer you needed support for your machinery.


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