Auto transformer help?

[dieseldunce]

Hello all, I am an electrical contractor that does alot of control wiring and troubleshooting. I recently had to have a motor rebuilt due to power co. single phase problems during a storm. Upon reinstalling the new rebuilt we turned power off to the other motor. These are 60 hp 230v motors and are using auto transformer reduced voltage start. When we tried to restart the motor that had previously been running it sounded as though it was single phasing. All incoming voltages were good and during start up only experienced about 2 volt drop. My question is, is there any real good way to check voltage output on transformer? Or is there a proper way to troubleshoot the autotransformer?

 

[jraef]

Shouldn't be anything special. Just measure the phase to phase voltage on the output of the transformer during start. The voltage reduction should be proportional to the taps that you are connected to.

If it is single phasing, (and assuming you already checked fuses etc.) most likely you burned up a set of contacts on one of the contactors when you lost a phase from the utility.

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[waross]

Measure line to line voltages. The line to neutral or line to ground voltages from the autotransformer secondary will be offset on two phases.
respectfully

 

[dieseldunce]

Thanks for the replies. If I understand some of the other articles I read these type of set ups should only be started and stopped 2 times per hour, the operator on call said he heard the motor stopping and stopping repetedly. When we checked the output we thought we were doing it right. Phase to ground @ 80% tap was around 90 volts. This is actually on a 208v system. Line to ground prior to start up is 118 and drops to 116 at startup. The line to line was 212 prior and 210 at start. The phase to phase on auto secondary was about 175v. As a matter of proper troubleshooting I always check contacts on starters and fuses, I had to learn these only one time the hard way. Can someone also comment on the amount of time delay that should be present before going line to line or how I can determine this on site.

 

[waross]

I suggest searching Eng-Tips for reduced current starting and reduced voltage starting. (There may be discussions in other fora such as Electric Power Engineering.)
In a properly designed system the starter should be capable of as many starts per hour as the motor.
The time delay is usually as short as possible. Ideally the acceleration should be as smooth as possible. If the motor stops accelerating on the autotransformer and noticably "hangs" at an intermediate speed the time delay should be shortened.
respectfully

 

[jraef]

In a properly designed system the starter should be capable of as many starts per hour as the motor.

True of all but Autotransformer starters. The transformer itself is the limiting factor. They are typically designed for 2 or no more than 3 starts per hour. An Autotransformer that could withstand 6 starts per hour would be massive. Good ones have a thermal cutout switch embedded in or near the windings so that the motor starter is disabled if the transformer heats up too much, but relying on that is like relying on the "idiot light" on your car dashboard to tell you that the engine is overheated. By the time it kicks in, the damage is done.

Timing is going to vary by load. Officially, you want to transition at the moment the motor stops accelerating because to not do so is stressing the motor and unduly wasting thermal time on the OL relay. Typically though, any longer than 15 seconds and you are risking the transformer as well. If it is a pump, I rarely see the transition time setting be over 5 seconds.

Just out of curiosity, what makes you think it is single phasing?

 

[dieseldunce]

The motor that was delivered for rewind was reported to have single phased. While on site we pulled the end off of the other motor because it smelled hot. the windings directly opposite each other were equally blackened while the othre windings were still copper colored. I will search for the thermal cutout on the auto transformer today. if the cut out is bad does this mean that the transformer is bad or just that there was a problem with to many starts.

 

[edison123]

I would check the cable terminations and the cables continuity from the auto to the motor. Also, the contactors, if any, in this circuit.

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