Sags from Laser Printers, Copiers and the Like

Please see Electrical Contractor magazine for referenced Figures.


A periodic voltage sag on a single-phase line occurring approximately every 30 to 60 seconds, accompanied by a sharp increase in resistive current and/or a sharp increase in the neutral-to-ground voltage, is probably due to a laser printer, copy machine or similar type load.

One of the more typical power quality disturbances that originate within a facility is from loads that periodically turn on and off. These include such equipment as the heating, ventilation and air-conditioning (HVAC) system, cold beverage vending machines, and even coffeepots. As reviewed in last month’s rule, motors typically have a large inrush current when first energized, which results in a decrease in voltage or a sag at the load. The compressors used in the cooling portion of HVAC systems and vending machines have a similar effect.

The large increase in information technology equipment in offices and factories has introduced another type of load that can have a similar effect, though individually not as dramatic as one caused by a large horsepower motor. Some types of printing equipment, such as laser printers and copy machines, have heating elements within them that turn on periodically to keep the machine’s printing components “warm.”

These types of loads usually have two different power signatures. The first occurs when the heating element turns on, releasing a large increase in current into the resistive heating element, which has a lower impedance than the steady-state condition and is almost purely resistive. Therefore, the current waveform will be nearly sinusoidal, low distortion, and in phase with the voltage waveform (power factor close to 1). The period of the heating element turning on varies from product to product and is based on performance, but typically is once every 30 to 60 seconds for a second or so. This can be seen in the figures (below).

These devices also have electronics in them, so you may also see the classic single-phase, rectified-input, switching-power-supply signature. Since the bridge rectifier only conducts current when the input is larger than the output, it only conducts current in the middle portion of the cycle, when the input voltage sine wave is at a higher voltage than the voltage on the storage capacitor, which is fed by the bridge. This can be seen in the waveforms before the resistive load turns on and after it turns off.

Another lesson from these data is learned by comparing the neutral-to-ground voltage to the line current on a single-phase load. Most of the current in the line conductor flows back through the neutral conductor. (Some current may be diverted to ground, where there are filters and suppression devices on the equipment.) This current multiplied by the wiring impedance (mostly resistive) creates a voltage between neutral and ground. Hence, this voltage waveform and the current waveform have similar shapes. This can be clearly seen in Figures 1 and 2. In addition, since half of the voltage drop in the source impedance is in the line conductor and half is in the neutral conductor, the swell in the neutral-to-ground voltage is equal to half of the line-to-neutral sag, but much like the mirror image, as shown in Figure 1. 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 732.248.4393.

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