Sags from Motor Starts

A voltage sag originates at the startup of a motor. if the voltage drops quickly and steadily increases, you have sag.

Some signatures of power quality (PQ) phenomena are difficult to discern from the megabytes of data that PQ monitors can produce in a relatively short time. Motor starts, fortunately, are easy to determine, and quite common. In industrial facilities, especially older ones, motor starts are probably the leading cause of sags that originate on the load side, or from within a facility. Motors driven by electronic controls, such as adjustable speed drives, have changed the picture a bit, but there are still plenty of electric motors connected directly to the distribution circuits within a facility to make lights blink and, sometimes, processes stop.

A large newspaper printing facility in the northeast wanted to add a significant number of computers and other information technology equipment (formerly known as “sensitive electronics”) to the second floor of its facility. The plant manager needed to know if there were any unforeseen issues. Not only was the harmonic impact of such to be considered, but the effect of the motor starts on the computers shouldn’t be overlooked either. A simple PQ survey over a week’s time would have given them the data they needed to check for problems, before they installed all of the computers and had unexplained lock-ups or restarts.

A motor sitting at rest has a different electrical characteristic than one already rotating. To get the motor to overcome standstill requires supplying the current not only for the rotational torque but also for the magnetic fields. The impedance of a motor, as seen by the electrical source, is much lower once the motor is turning. A lower impedance means that more current will flow when connected to the same voltage then during the steady-state condition. In a motor, this “inrush” current is often six to 10 times higher than current present while running.

If we go back to our Ohm’s and Kirchoff’s Laws, a large increase in current will result in a larger voltage drop across the source impedance, leaving less voltage for the load. This is the rule on the load-generated sags. What is unique is what follows the initial few cycles of the sag. The current decreases steadily in an exponential fashion to the steady-state value, rather than changing abruptly or linearly. As a result, the voltage recovers from the sag, although not back to the pre-start level.

Though the energization of other magnetic equipment, such as transformers, also has a similar curve, the current and voltage waveforms during this time remain sinusoidal for the most part. Transformers tend to distort the waveforms during the energization period. Different size motors with different mechanical loading and in different wiring configurations will produce slightly different plots, but the general shape will be similar to those in the accompanying figures. Use of soft-start circuitry to reduce the inrush will also alter the pictures. EC

BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., 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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