Phase Converters and Elevators

Most elevators for low rise buildings use a hydraulic system to raise the elevator car, and rely on gravity to lower the car.  The electric motor powering the hydraulic pump is usually a three-phase motor.  Most manufacturers offer single-phase elevators, but the cars tend to be small and they are relatively slow.  For commercial buildings and large homes, a three-phase elevator is a much better solution.

It is common for the elevator to be the only three-phase load in the building, and three-phase service from the utility may be too expensive or not available at any price.  In this situation, a phase converter can be a viable alternative to operate the elevator.

Elevators tend to be demanding applications for phase converters.  The motor operating the hydraulic pump is usually large, 15 Hp and above.  It is also likely to operate near its full load amperage rating, requiring good voltage balance to avoid damage to the motor.  The motor may also require a large inrush current to start the motor under a load.

Rotary Phase Converters and Elevators

Rotary converters are capable of operating elevators, but have

some drawbacks. Most of the power required by an elevator is in the main motor which usually starts under load. A rotary converter will have to be oversized to supply the starting current for this motor. There are usually other small loads in an elevator system such as motors to open and close the doors, lights, ventilation, etc. Even though the main motor runs only when the elevator is ascending, these other loads may require power at all times. Unless the power requirements of the system can be isolated and supplied by the single-phase system, the phase converter must be running at all times. A large rotary converter in this situation would waste significant energy. If the rotary converter is turned off and runs only when the main motor operates, special controls must be designed to start the converter and after a time delay, supply power to the motor. This leads to a delay in elevator service and extra installation cost.

VFDs and Elevators

A variable frequency drive (VFD) can be a good solution for operating an elevator only if the main motor can be isolated from the other loads in the elevator system. The control signal for the main motor must be adapted to control the VFD, which in turn must control the main motor.  The other loads in the

system will likely be damaged if operated by a VFD, so they must be powered by the single-phase service. This complicates the installation and may not be possible. If it is possible, the soft start of the VFD will eliminate the inrush current of the motor when it starts, easing the burden on the single-phase line.  A large VFD may require input filtering to prevent problems associated with harmonic distortion of the incoming current.

Digital Phase Converters and Elevators

A digital phase converter is probably the best choice among phase converters for operating elevators. The perfect voltage balance protects the main motor from damage and the converter is capable of supplying the large starting current requirements. The output voltage of a digital phase converter is a sine wave and is safe to power all the loads in the elevator system.  The converter consumes very little power when it is energized, so can be left on to provide continuous power to the system.