A power play with hotel elevators
In hospitality and gaming facilities, elevators are a vital system—a system that relies on power quality. Generally, the height of these facilities makes the use of stairs an alternative for only a few patrons. One of the keys to operation of elevators is the electric motor and the quality of supply that feeds them.
A large player in the elevator business indicated that power quality issues accounts for a high percentage of the “trouble calls” it receives. At the same time, adjustable speed drives (ASD) usage as the means for providing smoother and more efficient operation of the motors is a factor in long-term elevator operation and the power behind it.
The continual increase in non-linear, harmonic-generating load types means an increase in current harmonics, resulting in the propagation of harmonic-based voltage distortion. Some of the losses in an electromagnetic device such as an electric motor are frequency dependent. As the voltage frequency and current signal components increase, the inductive impedance increases, so the power losses increase. In fact, some losses increase as the square of the harmonic number, such as eddy current losses. Since these losses become heat and heat factors into the aging of a motor, the motor’s life expectancy can be shortened when significant harmonic currents are present.
The harmonics in the voltage waveforms are not just induced by harmonics from other equipment. An ASD usually has a full wave, three-phase rectifier as first part of the power-conversion stage. Such a converter is often referred to as a 6-pulse or pole converter (though 12 and 24 pulse converters are on the market). It turns AC into DC, which can then be turned back into AC at a different frequency than the fundamental power frequency, which is usually 50 or 60Hz. Since currents are not drawn in a linear manner, harmonic currents result. Also, as the conversion process goes from one phase conductor to another, there can be an overlap time called the commutation period where two phases are conducting at the same time, resulting in a large current increase and a voltage notch in the waveform. This high-frequency notch can damage the outer windings of the motor and other electronic components if not properly designed. Even more extreme in the pulse-world are pulse-width-modulated (PWM) drives. These devices emit a series of pulses that always have high-frequency components.
Some harmonic currents can generate additional heat in the motor by trying to turn it in the opposite direction of the rotation. The voltage and current in three-phase systems can be represented by a set of vectors or phasors referred to as the sequencing components. In many systems, the harmonic components can be associated with the different sequencing vectors. The table below shows which type of sequencing component that each harmonic is. The negative sequence components (2, 5, 8, 11, etc) are rotating opposite to the fundamental.
Recent surveys on the electrical distribution system have found that the most dominant harmonic is the 5th, which is the most significant harmonic of a 6-pulse converter. A handy formula for determining the harmonics from such as system is the harmonic numbers, h, are equal to the number of poles (p) multiplied by integers (n) plus or minus 1. Hence, h = n × p +/– 1, where n=1, 2, 3, 4. For a 6-pulse converter, 5, 7, 11, 13, 17, 19 ... will be the most dominant harmonics, which is shown in the figure below. Harmonic currents are also being attributed to current flowing through the shafts in motors with non-insulated shafts, bearing damage referred to as fluting.
Many elevator motors face a series of nonfavorable factors that may require derating the motor’s performance and more frequent repairs. Elevator dysfunction is badly regarded by the customer, who is most likely oblivious to the real source of the problem. The savvy electrical contractor can come to the rescue in the preplanning stages with power quality solutions.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.