ASD: Dual Meetings

Adjustable speed drives or ASDs, are often categorized as "sensitive" to power quality phenomena, which means ASD could also stand for "another sensitive device." I am still not sure what "sensitive" means relative to hardware. Susceptible seems like a far better choice of words. But IEEE Std 1100, the Emerald Book, probably states the case the best, when it refers to equipment that "can be both a contributor to and a victim of powering and grounding incompatibilities in the power system." That statement would describe most ASDs.

ASDs can be used in conjunction with pumps, compressors, process controls and other applications with DC, synchronous or induction AC motors. ASDs can improve product quality by having more precise speed control. For example, in a textile fiber process, each of the drums used in the production of the fiber will turn at slightly different speeds and tensions to stretch the fiber to the proper thickness.

ASDs also can be beneficial when used for energy conservation, as they only provide as much power as needed at a given time. For example, there is no need to run the fan in an HVAC at full speed all the time, especially when it is only 70 degrees outside on a summer's day.

ASDs are potential contributors to power quality problems. As sources of harmonics, they can create significant voltage notches and associated high frequency negative impulsive transients, and they can be the source of sags during start up.

As potential victims, ASDs can be susceptible to impulsive transients from lightning, oscillatory transients from power factor capacitor-switching events, voltage sags from the start up of other motors and large current loads on the circuit, voltage unbalance, and the effects of harmonics and notches on the motors used with ASDs.

Lets look at the victim side first. By definition, the ASD is often used to control the speed of the process. Anything that alters that speed could result in serious consequences to the process. In the aforementioned textile fiber production, a drum turning at the wrong speed, producing slack that causes the fiber to break when the control system tries to recover, could be a problem. An increase in speed would be problematic too, as it could produce too much torque and, again, break the fiber.

In a fiber optic process, too much tension could cause the fiber diameter to be too small, and the entire production reel would have to be rejected. Tests conducted at Clemson University with Duke Energy, using both a textile mill simulator and actual plant operation, found that it may not only be the power to the motors that is affected by the sag. Instead, the speed, load and torque transducers that are part of the process-control system were generating incorrect outputs, which fed incorrect information back to the process-control system. This resulted in changes to the motor speed that should not have occurred.

In other cases, an interruption or severe sag can cause the drive's output drive to drop its output power, resulting in the loss of motor torque and speed as well. The process may then trip offline, a significant problem in itself, but it can also be a problem for the restart, as ASDs without "flying restart" capabilities cannot do so until the motor has come to a complete stop, increasing the downtime.

Voltage unbalance is a problem for motors, causing excessive current on one or more of the phases, resulting in increased heating that requires the motor to be derated. A similar problem exists when there is an ASD driving the motor. The excessive current into the ASD from the unbalanced input voltage may cause its overload-protection circuits to trip. In addition, a power factor capacitor switching transient can result in a transient overvoltage condition that can cause the protection circuits to trip, especially on smaller ASDs.

As a potential contributor, harmonics are probably the most significant external effect. The front end of an ASD is a power converter, turning the 60 (or 50) Hz power frequency AC into DC and back into AC at a variable or adjustable frequency. The rectifier circuitry in the power converter will usually result in significant harmonic currents, particularly the 5 and 7, 11 and 13, and 17 and 19 harmonics for six-pole converters (some newer ones use 12-pole converters so the harmonics are 11 and 13, 23 and 25, but much smaller in current.) This can affect the harmonic pollution throughout the facility and even back onto the distribution system. In addition, the commutation period of the rectifying devices overlapping between two phases can result in a significant voltage notch.

On the output side-for PWM type drives-the pulses' fast rise time can cause the motor-winding insulation to break down over time. Lastly, harmonic currents can flow through the bearings, resulting in fluting and, eventually, premature bearing failure.

So, whereas ASDs can be just "another sensitive device," they can also be a source of aggravation for other "sensitive" or susceptible devices and equipment operating on the same circuits. Care should be taken when swapping out plain old motors with ASDs, check and see that what worked before still works, and that the ASDs meet your expectations for energy savings and performance. EC

BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.


About the Author

Richard P. Bingham

Power Quality Columnist
Richard P. Bingham, a contributing editor for power quality, can be reached at 732.287.3680.

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