Induction Heating Consultant

Induction Heating

When Is a Kilowatt NOT a Kilowatt?

Power ratings for induction heating equipment are expressed in "kilowatts per minute". In other words, the power supply can deliver that amount of power to a load in one (1) minute's time.

However, the operative word is deliver.

Most industrial induction heating power supply ratings are based on the combination of voltage and current available at the power supply terminals.

Kilowatts = (Current x Voltage) / 1000

This is based on the rating system that older induction heating motor-generator systems used for determining power. This figure does not allow for the losses that exist between the generator terminals and the part to be heated. These losses include the tank circuit (capacitors and inductance/transformers), the leads and the work coil.

Testing induction heating power equipment for delivered power (per IEEE Specification #54) calls for ratings based on the power delivered to a calorimeter. A calorimeter is used to measure actual delivered power.

A calorimeter is used to measure actual delivered power.

As illustrated above, a calorimeter is a steel, water-cooled load. The water flow through the calorimeter and the difference between the water temperature entering the calorimeter and the water temperature leaving the calorimeter determine the amount of power in BTU's that have been delivered to the load. There are 57 BTU's per minute per kilowatt. Since the calorimeter is the load in this test, this measurement allows for all the losses between the power supply terminals and the part being heated. Thus, it is a measure of the actual delivered power.

Delivered power, while providing a more accurate idea of the capability of a power supply, is not the only factor that should be considered when purchasing an induction heating system. Circulating current in the coil, load coupling and lead design are all factors in selecting an induction heating power supply. These will be discussed in future articles.

© 2003 Stanley Zinn

Illustration from F.W. Lewis, "High Frequency Induction Heating", McGraw-Hill, New York, 1950

Stanley Zinn

Stanley Zinn • Induction Heating Consultants • Tel: 585-385-0530 • Fax: 585-385-0536
2604 Elmwood Ave. #214, Rochester NY 14618 • email

Stanley Zinn