As a sequel to the discussion of adjustable speed drives (ASDs) in last month’s column, it seems appropriate to show how ASDs are a classic example of the often-quoted line from IEEE 1100 (also known as the Emerald Book). According to IEEE Std 1100-1992, Recommended Practice for the Grounding and Powering of Sensitive Electronic Equipment, equipment “can be both a contributor to and a victim of powering and grounding incompatibilities in the power system.” ASDs are both AC and DC drives, since the speed of an AC motor or DC motor can be controlled through each, respectively.
As a disclaimer, this article is not intended to open the debate of which is better to use. There are pros and cons to consider, especially with respect to the application and overall initial purchase as well as operating and maintenance costs, required range of speed, power factor penalty tariffs, distances, horsepower regulation, etc. But one thing AC and DC drives have in common is the harmonic current produced by their rectifying inputs and their vulnerability to voltage transients and rms voltage variations. In addition, their effect on the quality of the supply for other equipment and even the motors they drive also are factors to consider when installing them.
The figure above shows the voltage waveform and rms values recorded near a 500-HP DC drive. Though only one phase is shown for clarity, all phases had similar severe voltage distortion (>8 percent), notching (1.8V/sec) and transients (786V). These disturbances were propagated throughout the plant on the 480V system and continued for hours. The plant had many other motors that were not on drives, so they would be exposed to these transients and harmonics as their direct source of power.
As I have mentioned in previous articles on motors, three things make motors unhappy: distortion, transients and unbalance. Unhappiness in a motor translates into heat and winding damage.
The rectifier’s operation in the drive’s power supply is the primary source of the harmonic currents and resulting waveform distortion, along with the notches and transients. In many drives (especially DC), the rectifiers are gated or controlled semiconductor switches. The control mechanism turns them only for the portion of the cycle relative to the amount of current needed; hence, the user experiences energy savings. But as a result, the rectifiers no longer draw current in a sinusoidal manner; hence, harmonic distortion occurs. In addition, the control mechanism can precisely control when they are turned on, but the turn off occurs at the point of zero current flow.
In a single-phase system, zero current flow occurs when the current waveform passes through the zero crossing. In a multiphase system, it occurs when another phase is turned on, which briefly creates a short circuit between the two phases, called the commutation period. Though it is very short, usually measured in microseconds, this overlap period results in large current transients. This, in turn, causes large voltage transients, mostly in the negative direction (notches). However, there is some inductive kickback resulting in positive transients—evil for motor windings.
The order of the harmonic currents that are caused by such rectification depends on the number of paths of conduction or poles. A three-phase, full-wave rectifier will have six poles calculated by multiplying the number of phases by two. The resulting harmonics use the h = n × p +/–1 formula, so 5, 7, 11, 13, 17, 19, etc., are the primary harmonic currents.
By going to a different front-end scheme with multiple zigzag transformers with different phase shifts, the number of poles can be increased with each pole carrying less current. The result is lower harmonic distortion, though the harmonic order is higher. For example, a 12-pulse converter has 11, 13, 23, 25, 35, 37, etc., as the dominant harmonic currents, but the distortion is far less by an order of magnitude.
These harmonic currents turn into harmonic voltages and become a distorted source, affecting the other equipment. Besides the losses in the incoming transformers and wiring from the harmonics, other motors powered from such have increased eddy current losses and may have increased heating from the negative sequence harmonics, which are those that try to turn the motor backward from the intended rotation.
Heat also makes the HVAC run more, using more electricity and lowering overall efficiency. Then there is the power-factor reduction, since the useful watts can become significantly less than the volt-amperes provided by the source (especially with DC drives), further decreasing efficiency.
As AC and DC drives continue to be used with more electrical motors (which consume approximately 65 percent of the electricity in the United States), the advantages and drawbacks they pose should be considered and addressed prior to any installation. There are a number of mitigation techniques, which the vendors provide, that can help keep them from becoming more of a victim and source of power quality problems.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.