HomeFAQDoes VFD efficiency can be measured as inverter?

Does VFD efficiency can be measured as inverter?

One confusing factor is that inverters are classed by motor power. That is, a 7.5kW inverter is designed to work with a 7.5kW motor (10hp for you guys across the pond). Now this is the mechanical power output from the motor, which has electrical and mechanical losses, so there is maybe 8.0kW or more of electrical power going in at a power factor of 0.8 say. So the inverter is feeding maybe 10kVA. If is 95% efficient, does this mean its losses are 500W, or 375W?

Another problem is that measuring efficiency can be difficult, as it is the difference of two large numbers, one of which (the input) is a non-sinusoidal current, and the output is worse, a PWM voltage waveform and current with ripple on it. You need expensive 3ph kit to make this measurement properly.

All that said, if the technology is much the same (IGBTs, PWM) efficiencies don't vary too much as manufacturers all want the highest efficiencies in their drives if only to keep the heatsinks small and allow high ambient operation.

PV inverters works usually in conditions more stable than variable frequency drive, and therefore it is more easy to fix some rules. In a VFD you can have frequency continuously variable, and in these conditions is very difficult to state how to measure efficiency. Consider that the efficiency alone of a VF inverter should be high, but motor/generator and main interface should be lower, in a certain configuration of parameters. In another setting, inverter efficiency should decrease but other related efficiency can increase.

VFD efficiency is measured by using meeting such as a Multilin PQM on the input and output of the VFD. You use the KW hours record so the longer you let the meters record the more accurate your measurements. then simply divide power out by power in.

Calculating VFD losses you need to consider the motor efficiency since the VFD rating is actually the power it can produce on the motor shaft.

To determine the actual VFD efficiency you need to know the heat generated by the switching and conducting losses. This is a complex thermal model. You should be able to get the efficiency curves from the VFD manufacturer.

In PV inverters there is a good reason to have the maximum inverter efficiency. Instead, in the combination VFD (inverter + motor) it is not interesting to know exactly the inverter efficiency. In certain conditions it is possible that inverter efficiency is lower but motor efficiency is better. The same VFD in another working point could have a better inverter efficiency but a bad motor efficiency. Therefore, selecting a VFD, I will look to total efficiency and never to the inverter efficiency alone.

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