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What’s The Difference Between Linear And Non-Linear Electrical Loads?

AC electrical loads are referred to either as linear or non-linear depending on how they draw current from the mains power supply waveform.  

Loads fall into one of the following categories:

  • Inductive: where the current waveform lags the voltage waveform, with a potentially high in-rush current at start-up, for example a transformer, motor, or SMPS (switch mode power supply).
  • Capacitive: where the current waveform leads the voltage waveform, for example a blade server.
  • Resistive: a load without inductance or capacitance, for example a resistive load bank. Typically, the device will have no initial switch-on surge and the current drawn rises immediately to a steady state.


How Do Linear & Non-Linear Loads Differ?

With a linear load, the relationship between the voltage and current waveforms are sinusoidal and the current at any time is proportional to the voltage (Ohm’s law). Examples of linear loads would include transformers, motors and capacitors. 

On the other hand, with a non-linear load the current isn’t proportional to the voltage and it fluctuates based on the alternating load impedance. 

Common examples of non-linear loads include rectifiers, variable-speed drives and electronic devices such as computers, printers, TVs, servers and telecoms systems that use SMPS power conversion technologies. They are also typically found with blade servers.

Non-linear loads draw in currents in abrupt short pulses. These pulses distort the current waveforms, which in turn generates harmonics that can lead to power problems affecting both the distribution system equipment and the loads connected to it.  

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.  

Several Riello UPS uninterruptible power supplies are designed to specifically handle non-linear loads. These include the Sentinel Dual, Sentinel Tower, and Multi Sentry series. 


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