Modifying a control system without proper research can result in repeated system failure.
By Myron J. Boyajian, Contributing Editor to Design News
Last spring in a Chicagoland suburb, a lightning strike resulted in damage to a pole-mounted, 3-phase, step-down transformer. The lightning strike didn’t stop at the pole pig, but continued into the three-story office building where it damaged the power distribution panel for the building’s single freight/passenger elevator. The single-phase power lines were not damaged, so lighting and HVAC were not interrupted.
The utility company replaced the 3-phase transformer and cables. A local electrical contractor was summoned to repair the elevator breaker panel and wiring and get the elevator running again. The contractor pulled the charred panel, replaced the breaker and wires, and the elevator seemed to function alright for the most part. The problem was that whenever there was a lot of elevator usage, power would cut out, stopping the elevator. This resulted in frantic calls via the elevator’s emergency telephone to the building’s custodian to hustle down to the basement to reset the breaker.
The electrical contractor was recalled to find the cause of this malfunction and restore proper operation. In this case, aside from any technical issues, the contractor was reluctant to do any more work saying that his work was correct and that the problem lay with the elevator system. The building owner refused to pay for work done, at first saying the “insurance claim wasn’t paid,” then saying that pay would be forthcoming only when reliable operation was restored. To break this deadlock, the contractor retained me to inspect the elevator, its controls and the contractor’s work.
I ran the elevator from the first to the third floor and back down. All the elevator pushbuttons functioned correctly so I went to the basement vault to observe the power unit in operation. During lift, a 3-phase 220V ac motor drove a hydraulic pump. Pressurized fluid passed through a solenoid check valve into a telescoping cylinder that raised the elevator. To lower the elevator, the solenoid valve was energized, releasing the fluid from the cylinder to return to the reservoir. While operating the elevator I measured 210V phase-to-phase voltage and 50A line current in each phase. Motor startup inrush current was 85-100A. Voltage and currents seemed nicely balanced and were within the motor nameplate rating. The motor operation sounded smooth without any sound of fluid blowing past the relief valve. There was no pump cavitation, especially with full cylinder extension. So just what was causing the breakers to trip, stop the elevator, and trap passengers?
I asked both the building manager and the contractor to walk me through the steps they took to restore power to the elevator when there was a stoppage. Expecting them to go to the elevator control panel located in the vault containing the motor and pump, I was somewhat taken aback when they both went to the new panel that replaced the lightning-damaged panel. It was located in the main vault where the utility cables entered. I asked them if they ever had to reset the 3-phase starter/breaker that was controlled by the elevator pushbuttons when commanding a lift to a desired floor. They both said that only the new 3-phase breaker in the main electrical vault ever tripped.
I checked the breaker manufacturer’s characteristics for the 100A, 3-phase unit and saw that there was a significant current-time de-rating. A call to the breaker manufacturer’s tech line confirmed that the inrush current from multiple motor starts could internally heat and de-rate the breaker and result in nuisance breaker tripping. At my request, we dug through a junk pile in the basement and pulled out the lightning damaged breaker. The old breaker was 200A! Seeing the difference, I asked the contractor why he replaced the 200A breaker with a 100A unit. He said he thought the original breaker was incorrectly applied and wanted to provide lift motor protection with a breaker that was rated closer to the motor nameplate ratings.
Mystery solved - the contractor, with good intentions, had second-guessed the design by the elevator system engineer. But, this resulted in installation of a breaker with a much shorter current-time characteristic than the starter that controlled the lift motor, and thus caused the consequent nuisance trips. At his expense, the contractor installed a 200A breaker and reliable lift operation was restored while still retaining short circuit protection.
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