An electric motor is probably the best example to explain the majority of issues that power quality experts, facility managers and electricians most frequently encounter. The laws of electricity haven’t changed, nor has the source of the electricity changed much during this time.
Motors still consume more than half the electricity in the United States. Ohm’s and Kirchoff’s Laws ruled the circuits back then and still do. And, while motors still start up the same way, the results can be significantly different.
As a quick review, a large increase in current will result in a significant voltage drop across the source impedance (all that wiring from source of the electricity to the load), leaving less voltage for the load. This is the rule on the load-generated sags.
Electric motors, especially as the horsepower increases, have large inrush currents, typically six to 10 times the running or steady state current levels. This large increase in current, lasting several cycles to several seconds, depending on the size of the motor and the load, is usually enough to result in a voltage sag to other equipment on the same circuit and even sometimes on parallel circuits.
Rotating electromagnetic loads, such as motors, have mechanical inertia that can allow them to ride through such short disturbances without any significant decrease in mechanical output. Resistive loads, such as incandescent light bulbs, may show a brief decrease in illumination, but unless it was a flicker, it had little effect on the light bulbs or those benefitting from its illumination. Hence, loads such as refrigerators, fans, lighting, pumps and conveyor belts kept performing satisfactorily when voltage sags occurred.
Disturbances can fool controls
Today, many electric motors aren’t powered directly from the AC power source. Instead, the AC voltage is rectified and turned into DC, then turned back into AC voltage but at a different frequency, depending on the load requirements. Whereas the adjustable speed drives are an efficient use of electricity, the control circuitry that monitors the drive can be fooled by different types of power quality disturbances.
This can cause the drive to trip keeping the motor from doing its job. The rectifying action is the source of harmonic currents, which can cause problems for electric motors and other equipment due to the heating effects of the harmonics. Some of the resulting voltage harmonics can try to turn the motor in the opposite direction (negative sequence voltages), further increasing the heat and lower efficiencies.
Most of the so-called sensitive electronic loads also have rectifier circuits in their power supplies, to turn the AC voltage into one or more lower DC voltages for the microprocessors and other digital components. Many of these power supplies don’t have the ability to ride through the reduction in voltages that the rotating electromechanical loads can.
Large DC storage capacitors can provide some equivalent of the mechanical inertia, but larger capacitors mean more cost and larger size equipment, which is contrary to the trends in these globally competitive industries. The same sag that blinked the light bulb may have caused the personal computer to crash; the latter having a significant impact on the productivity of today’s workers.
Voltage sag is No. 1 event
The voltage sag (or dip) still remains the number one power quality event in nearly every benchmark study in addition to the overwhelming majority of the data that customers send to me to review. Today’s power quality instruments can record these sags in greater detail, with more characteristics and information about them than the Model 606 could back in the 1970s.
Knowing the magnitude of the remaining voltage, time duration, missing energy, phase angle of initiating and recovery, and what happened on the other conductors in a three-phase wye or delta circuit (including the neutral and ground) can all help determine the cause of the sag and where it originated from the monitoring point.
But, unless the reliability of the source or the susceptibility of the loads makes some significant progress in the near future, sags will still be No. 1 in the power quality arena 30 years from now. EC
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.