Amusing electrical anecdote

I was asked today to inspect a large semiconductor production tool today, at work.

They had replaced the motor with a brand-new 7 hp baldor and replaced the VFD
that drove the motor as well.

However it was obvious (from outside the lab!) that something was badly wrong.
The motor seemed to be hunting for speed, and was growling and groaning. Yet
it was driving the spindle of the machine at the required rate.

The spindle speed was sensed by a tach, which provides feedback via the control
computer, to the VFD. Basically a closed-loop servo system.

My first question was, was the motor wired up correctly?
The next was, could we run the spindle open loop?

The answer to both was yes, so they reconfigured the system to run open loop
and the problem was still there. I asked them to boost the carrier frequency on
the drive, and while that reduced the VFD whine the base problem was still
there.

I was on the floor staring at the motor, wondering if the belts might be too
tight or something. Absently mindedly I asked again, are you sure the electrician
wired it right?

My buddy (the person who called me in on this) said "yes but."

"There were those three other wires."

Ah.

Three other wires? "Yes, he didn't know what to do with the three other wires
so he just insulated them and put them back in the motor junction box. "

Ah.

Yep, the motor was wired for 208, but the center point on the star connection
was full open. The three wires that were supposed to be tied together, weren't.
The nine wire motor had pairs of windings going to the three lines, and three
individually insulated wires tucked into the peckerhead.

For the life of me I cannot figure out how this motor was producing ANY torque
at all, much less holding speed in a closed-loop control system! I'm also amazed
that the VFD did not fault out with the motor configured this way. Fortunately
there was no damage to the motor or the VFD, when the star point got
connected it ran up to speed with zero vibration at all. Amazing.

Jim

Hee hee... I think I can tell 'ya...

Hi Jim!

I THINK I can tell 'ya how...

A while back... mebbie two years or so, I did some experimenting... I was curious whether I could use a 230/460 three phase motor as a combination RPC idler AND step-up transformer.

My thinking was, that if I set up the wiring in series (for 460v), but instead of hooking up the three windings at the tail end, drive the motor from the MIDDLE of the series, leaving the far ends hooked up...

This puts 230v on the star-wound half of the windings... and the other set of windings (being in series, and in phase with the 230-powered side) SHOULD generate 460...

Of course, the motor ran just fine on 230... with oh... about half the rated torque or less, but I wasn't even loading it, just using it as a spinning transformer.

In this case, the guy who hooked up the motor, simply got lucky- he grabbed the three wires on the star-side, and put his 200v three phase into a 200v three-phase Y, and left the other two windings unused, like using only one half of a center-tapped transformer.

I'm not surprised it ran, and with reduced torque, but somewhat intrigued that it 'hunted'... perhaps the VFD was somewhat confused by the difference in reactance in comparison to the applied current and voltage?

And an added note, since I see I didn't totally clarify- when you're looking at the motor's diagram, it shows connections for the wires, but it usually DOESN'T show you one connection- the center-star... that's wired-by-default inside the motor. You have two windings for each phase... a total of 6 windings, each with two ends... that's 12 wires, right? ONE set of windings is open at both ends, and those wires appear at the junction box. Looking at the common diagram, there's only nine wires. That's 'cause the other set of windings... is already connected in star configuration inside... you can't change it. The motor was obviously running on just this connection, and the other half (which was supposed to be all wired together) was open, and carrying no current.

A few years ago I sold a 12 inch thickness planer ( for wood) to a guy I worked with. The planer had a 2 hp. motor, which could be wired for 120 or 240 volts , and I took great pains to explain to the buyer how to switch those connections. I had been using it on 240 volts, and had a plug on the cord typically used only for 240 volt circuits. The buyer got the planer home, and phoned me a couple of hours later. He complained that the motor only growled and hummed, but wouldn't run the machine. After I asked him a couple of questions, he advised me that the plug on the machine cord wouldn't fit his receptacles, so he "converted" the machine to run on 120 volts........ by just installing a 120 volt plug !
( urge to strangle)