3 phase motor disconnected - everything fried

[TwoBlocked]

From Wiki: https://en.wikipedia.org/wiki/Rotary_phase_converter ​

Types​

"A rotary phase converter (RPC) may be built as a motor-generator set. These completely isolate the load from the single-phase supply and produce balanced three-phase output. However, due to weight, cost, and efficiency concerns, most RPCs are not built this way.

Instead, they are built out of a three-phase induction motor or generator, called an idler, on which two of the terminals (the idler inputs) are powered from the single-phase line. The rotating flux in the motor produces a voltage on the third terminal. A voltage is induced in the third terminal that is phase shifted from the voltage between the first two terminals. In a three-winding motor, two of the windings are acting as a motor, and the third winding is acting as a generator. Because two of the outputs are the same as the single phase input, their phase relationship is 180°. This leaves the synthesized phase to be +/-90° from the input terminals. This non-ideal phase relationship requires a slight power de-rating of motors driven by this type of phase converter. Also, since the third, synthesized phase is driven differently from the other two, its response to load changes may be different causing this phase to sag more under load. Since induction motors are sensitive to voltage imbalance, this is another factor in de-rating of motors driven by this type of phase converter. For example, a small 5% imbalance in phase voltage requires a much larger 24% reduction of motor rated power.[1] Thus tuning a rotary phase converter circuit for equal phase voltages under maximum load may be quite important."

You know, I remember a motor on an agitator than must have been a beast. A fuse on just one leg blew, but it kept running because it had oversized overload heaters. It had tripped when the fuse blew, but somehow kept going when reset. The next day we went back to check things and got a weird voltage reading on the blown leg. The motor read fine. Replaced the fuse and overloads.

So, if the motor on this post was a single phasing, 3-phase motor, and the "produced" leg was the common used as a multi-wire circuit, and this leg went open, then it would be like losing a neutral in a house and all sorts of bad things can happen. Yeah, a lot of ifs.

 

[Open Neutral]

It may have been installed for power factor improvement.

My thoughts, too. Was it a synchronous motor?

 

[GoldDigger]

The device mentioned by @drcampbell in post 9 is called a synchronous condenser. An overexcited synchronous motor has a leading power factor and can be used to supply kVARs for power factor correction.

A rotophase can be used to develop the third leg to start three phase motors. But once those motors are running, if they are lightly loaded, they will also act to strengthen that third leg. On one of the electrical discussion boards, someone called ?hillbillysawmiller? described starting a small rotophase, which was then used to start the smallest saw. This was then left unused and spinning freely, while the next saw was started, working up to the largest saw they needed to use.

I could imagine one unloaded motor being shut down, thereby taking out other equipment...but I can't see how that would have survived random power hits over time.

Jon

The synchronous condenser is literally a replacement for a capacitor bank for power factor correction. But removing it would, at worst, result in POCO seeing a high power factor load and should have little effect on the voltage seen by connected equipment.
On the other hand, if there is no POCO supplied third phase, and RPC would be self starting and would provide a not particularly stable third phase. Complete loss of the third phase could produce effects similar to loss of a neutral and subject some connected equipment to abnormal voltages.
If in addition to the self-starting RPC there is a large motor set up as a synchronous idler to increase the available current on the third phase, On application of power, it would be started by the RPC as long as there were no other immediate heavy third phase loads. Removing that motor could overload the actual RPC with unpredictable results.