# How efficient is a phase converter?

A commonly asked question is what is the efficiency of a phase converter. Efficiency is a function of the useful power output so when expressed as a % it can be highly misleading if the useful load is very small. Instead it is more straightforward to discuss the parasitic power consumption of a phase converter. A typical 4-kW rotary has a fairly constant 500-W of parasitic losses (windage, bearing losses, iron losses, and copper losses). So at full power - supplying a 4-kW load, it is 89% efficient. Clearly as load falls off this efficiency figure worsens, e.g. to 80% for a 2-kW load, and 67% for a 1-kW load. This is one reason why it is better not to buy too large a phase converter as ideally one would like to keep parasitic losses to a minimum.

In the UK the electricity companies charge for Watts consumed rather than current. This is important because the same 4-kW rotary phase converter that consumes 500-W when idling will actually draw about 8-Amps of current rather than the 2-Amps that the parasitic losses would lead one to expect (500W/240V=2A). The other 6-Amps is simply current flowing in and then out of the phase converter as the capacitors charge and discharge, and you do not have to pay the electricity company for it. This is something that causes many clients confusion as they try to understand why their electricity meter is not spinning wildly even though their ammeter is reading a large current.

Outside the UK things are often different and electricity companies may base a proportion of their bill on current. In this case the power factor of the phase converter is important and in these instances our Boosters become even more attractive as we can improve the situation for clients.