AC electrical loads are referred to either as linear or non linear load depending on how they draw current from the mains power supply waveform.
Linear Load
UPS loads where the voltage and current waveforms are sinusoidal. The current at any time is
proportional to voltage.
Linear load is: POWER FACTOR IMPROVEMENT CAPACITORS, INDESCENT LAMPS, HEATERS ETC.
Non Linear Load
Applies to those UPS loads where the current is not propotional to the voltage. Foremost among loads
meeting their definition are gas discharge lighting having saturated ballast coils and thyritor (SCR) controlled loads.
The nature of non linear load is to generate harmonics in the current waveform. This distortion of the current waveform leads to distortion of the voltage waveform.
Under these conditions, the voltage waveform is no longer proportional to the current.
Non Linear Load is: COMPUTER, LASER PRINTERS, SMPS, REACTIFIER, PLC, ELECTRONIC BALLAST, REFRIGERATOR, TV ETC.
Difference Between Linear load and Non Linear Load
| S/N | LINEAR LOADS | S/N | NON LINEAR LOADS |
| 1 | Ohms law is applicable | 1 | Ohms law is not applicable |
| 2 | Crest Factor= 1Peak/1 RMS =√2=1.41 | 2 | Crest Factor could be 3 to 4 |
| 3 | Power factor=Watts/VxI=Cosφ | 3 | Power factor=Watts/VxI≠Cosφ = Displacement factor X Distortion Factor |
| 4 | Load current does not contain harmonics. | 4 | Load current contains all ODD harmonics. |
| 5 | Could be inductive or capacitive. | 5 | Can’t be categorized. As leading or lagging Loads. |
| 6 | Resistive, Inductive or capacitive. | 6 | Usually an equipment with Diode and Capacitor. |
| 7 | Zero neutral current if 1 Ph. loads are equally balanced on 3Ph. Mains (Vector sum of line current) | 7 | Neutral current could be 2.7 times the line current even if 1Ph. loads are equally balanced on 3 Ph. Mains |
| 8 | May not demand high inrush currents while starting. | 8 | Essentially very high inrush current (20 time of I Normal) is drawn while starting for approx. One cycle. |
Harmonics can cause problems such as distortion of the mains supply voltage, equipment overheating, nuisance tripping of circuit breakers, and misfiring of variable speed drives.
Effects of Harmonics on Neutral
In a 4-wire three phase system, the fundamental currents at any instant will always add up to zero in the neutral. However, the third harmonic of each phase is always in phase with those of the other two phases. As a result, rather than canceling each other (as is the case with the fundamental), they are additive and may well lead to serious neutral loading problems. As an example: a three phase system has 100 amperes load and each phase contains 30% third harmonic. The harmonic current flowing through the neutral will be three times. 30% of 100, or 90 amperes at the three times of the fundamental frequency (150 Hz for 50 Hz systems).
Therefore, it is recommended to use neutral of minimum double the capacity since total load on the system in consideration of Linear loads and Non Linear loads. It is also advised to use single phase UPS/Loads wherever possible with individual neutral in place of using three phase UPS/Loads.
