Modified since wave inverters

Dr Brown (Robert) is currently investigating the operation of ‘modified sine wave’ (MSW) inverters with some domestic appliances. The investigation corelates with a legal case where the use of a MSW inverter is alleged to have caused the malfunction of a portable ventilator when in use within an ambulance


An ‘inverter’ is a piece of electrical equipment that converts a direct current (DC) source of electrical energy, to an alternating current (AC) source of electrical energy.


Common sources of DC electrical energy are batteries and a common sources of AC electrical energy is a standard domestic electrical supply. So it follows that to use a domestic appliance, say a kettle etc. within a vehicle, a suitable inverter has to be available such that the DC battery energy can be converted to an AC format to supply energy to the appliance.


When converting DC to AC the inverter ‘attempts to emulate’ the ‘format’ of an AC supply seen/supplied in a normal domestic/industrial circumstance. That is the AC supply should be of a sinusoidal format. A sinusoidis a mathematical curve that describes a smooth periodic oscillation.


Attributes of an inverter to note are,

  • the magnitude of the inverter output voltage, i.e. 220 or 230 or 240 volts or whatever,
  • the frequency of the inverter output supply, i.e. 50 Hz or 60 Hz or whatever
  • The ‘power’ rating of the inverter i.e. 500 Watts, 1kWatt (1000 Watts) etc, or whatever


A further attribute of the output of an inverter is the format of the output in respect to time (t) and whether the output resembles the aforementioned sinusoidal format. In some inverter circumstances the output is not ‘pure’ sinusoidal and can resemble a ‘square wave’. The output of such inverters is referred to as modified sinusoidal or modified sine wave (MSW) and hence the inverter a MSW inverter.


Generally MSW inverters are the cheaper option when compared to pure since wave (PSW) inverters, due to less complexed, inherent electronic circuits and hence the number of components used within the inverter. However, when some ‘complexed’, ‘sensitive’, appliances such as a ventilator are connected to source electrical energy from a MSW inverter the appliance/s can and do malfunction.


Robert’s investigation is ongoing yet purpose is to give light to why to some and which appliances, ‘do not like’ MSW inverters.


Eur Ing Dr Robert Brown (Robert) is the Executive Director of Fraser George and Associates Limited and is a Consultant Engineer in the fields Electrical Electronic and Control Engineering. Robert is also an accomplished professional Expert Witness having prepared and presented many court compliant reports and presented oral evidence within the High Court, Crown Courts and County Courts.


For further information please contact Robert via;

Email,…   or

Tel Land: +44 (0)1777 709175

Tel Mobile: +44 (0) 7976250624