What is the Difference between a Pure-Sine wave and a Modified-Sine wave with respect to Inverters?  

In this regard, a sine wave or sinusoidal wave is a voltage supply in the form of AC (Alternating Current). In South Africa, a household plug socket supplies a voltage of ±230 Volts AC. Where ‘230’ is the amplitude of the wave.

Figure 1 below represents what a sine wave looks like measured over a period of time.Figure 1: 230V Sine wave.

As depicted in figure 1, the amplitude alternates between +230V and -230V. The frequency of alternation of the supply in South Africa is 50 Hertz, meaning 50 times per second.

How is the voltage from the battery converted to AC?

The voltage supply from a battery connected to a general household inverter will be in the range of 12-24 Volts. However, this voltage is DC (Direct Current) and must be converted to AC in order to run household appliances, hence and Inverter is required.

Figure 2 (a) depicts a general DC voltage of 24 Volts. (b) Follows a square wave created from the DC voltage that switches from positive to negative at specific time intervals.  (c) The square wave is then transformed into a modified sine wave.

Figure 2: a) 24V DC, b) 24V square wave, c) 24V- 3 step modified sine wave.


The inverter then amplifies the modified sine wave to 230V allowing compatible home appliances to be powered directly from the inverter.

What is the difference between a Modified Sine Wave and a Pure Sine Wave?

As shown in figure 1, a sine wave is a smooth alternating voltage. Figure 1 in essence is a pure sine wave. But when dealing with inverters the sine waves are not generated by a motor/generator/turbine but rather a DC voltage is modified electronically to produce a sine wave as in figure 2. Therefore manufacturers will refer to the inverters output voltage as a “Modified Sine Wave”. This is more of an estimation of what a sine wave should be.

However, some manufacturers distinguish their inverters that produce much less distortion than the wave shown in figure 2 (c) by calling it a ‘Pure sine wave inverter’. I.e. more steps are used between the positive and negative alternation. The more steps used, the smoother the signal and the more ‘pure’ the wave appears to be. However the more steps involved, the more complex the inverter will be and hence more expensive.

Figure 3 shows a comparison of a sine wave , a simple modified sine wave and a pure sine wave that some complex inverters are capable of.

Figure 3: Overlay of a Sine wave, Modified Sine wave and Pure Sine wave.

For general household appliances such as a TV, PC, lights, etc… a modified sine wave is perfectly suitable.

What devices will not operate on a Modified Sine Wave?

Any appliances resistive in nature (e.g light bulb) or use a switch mode power supply (e.g motor) will run on a modified sine wave.

Devices with a built in clock that derive their time from the incoming frequencies may not function correctly. The function that provides the switching can be triggered at the steps instead of at the zero crossover point. This applies to biomedical devices that are used to monitor heart rate and oxygen cylinders.

Some cheaply made power supplies may also not function correctly. This will present itself in the charger heating up beyond its normal operating temperature.

A device called a line conditioner can be used between the output of the inverter and the non-operational device if needed. A line conditioner or power conditioner provides a “clean” AC power to sensitive electrical equipment [2]. This is an additional device that is separate from the inverter. Currently Switchman Products does not supply line conditioners.

For more information on Inverters and change over panels, please contact joshb@switchman.com

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