Power dimming-out

[kjm4422]

Dear Sir or Ma’am: We recently had an upgrade done on a 3-floor hydraulic elevator. This included installing a new motor with a soft-start motor starter on the main hydraulic pump. Since this has happened we have had some “dimming” problems in the rest of the building; to the point where computer screens actually “wink-out” whenever anyone starts the elevator.

The Elevator Company says it’s the Power Company’s problem and the Power Company says it’s the Elevator Company’s problem. The maintenance boss suggested raising the taps on the main high voltage transformer serving the building since he says they were on the lowest setting.

Since I had no other suggestion we tried it and it did work, no more dimming or winking-out when the elevator is called. I am curious to know what the real problem could have been since it can’t be the transformer tap settings because the elevator worked for 25 years on those tap settings before the upgrade. What do you think it could be? Thanks

 

[ausphil]

Just keep an eye on the number of blown globes that now occur because of the higher voltage all the time (due to the changed tap). Also your Occupatinal Health and Safety Officer may see an increase in the number of complaints about glare in the offices.

Not all soft-starters are created equal, some are not as soft as others (sounds like a cross between Animal Farm and a marshmallow advertisement). Voltage drop from motor starts are due to the current pulled by the motor through the impedance of the system. I would tend to agree (due to the number of complaints that we have investigated, and the number that turn out to be the customer's problem) that the Power Company would be correct in saying that it is the Elevator Company's (and hence the owner's) problem. Any excessive loads that are put on by the owner must be setup correctly so not to pull the system around too much. The main voltage drop would probably be in the cabling from the main board to the motor, which probably goes through a number of sub-boards, all of which will see the voltage drop on starting of the motors.

By increasing the transformer output voltage, you have not removed the problem, you simply have moved the voltage high enough to eliminate the computer screens from "winking". You will still have roughly the same voltage drop, but you do not see the symptoms of the voltage drop.

Best to install some equipment to measure the voltage drop caused by the motors, and if necessary, change the starter to one that eliminates the hardness of the start.

The only time you would really go accusing the power company is if the power system was not very strong at the point of supply (ie any load will pull the system supply volts down, rather than just the volts at the motor supply). This can be shown by measuring the volts at the point of supply and at te motor terminals during a motor start, and seeing if the supply volts stay within the tolerances.

 

[busbar]

In ANSI regions, standard C84.1 ...Electric Power Systems and Equipment Voltage Ratings (60 Hz) is a consensus document involving utilities and utilization-equipment manufacturers. It and some on-site measurements may help resolve the responsibility issue.

 

[advidana]

you need a power study performed on your system . Contact a engineering firm to do this study . They will define the problem. Then you can take the correct action.

 

[jbartos]

Suggestion: Nature of the visible side effects of the new elevator motor and soft starter suggests that the motor size might have been increased. The existing power supply and associated power distribution are not sufficiently rated to support such load. The next reasonable step is the one suggested in the previous posting, namely, to have engineering expertise to look into the matter, and present a report supported by computer simulation results. There may be more than one solution available, which is typical in engineering.

 

[rbulsara]

While a study by a professinal may be helpful, but we forget the basics before jumping to fancy solutions.

I feel that your boss may be right. Speaking of basics, did you measure normal voltage before the tap change and after when elevator is not running or starting. What is your normal voltage now? If it is within reasons, your boss is right.

As jbartos pointed out you did not mention if HP changed due to the upgrade. Also the soft start starter may have not been set right to ramp up the voltage correctly and it may be starting the motor almost at full voltage. Please check all of above before taking next step.

 

[RAMConsult]

KJM, I'm curious if the motor horsepower is significantly higher than the one that was replaced or if the were changes in the control logic and/or sequencing on the hydraulic side. It's possible that the original system had a bypass or unloading scheme that allowed the motor to accelerate unloaded before pressure was built up on the hydraulic side. The new soft-start system was probably put in to mitigate the changes in motor size or control logic.

Having said that, I believe that the simple act of exercising the breakers and changing the tap probably broke a high resistance connection that went unnoticed until the new higher initial current motor system was installed. If you're still curious you can set the tap back to the original setting and see if the problem comes back; but why bother if everything is working for you and you're not seeing abnormally high voltages at your outlets.