Choosing measurement points for the load and no-load tests when testing induction motors for energy efficiency

This is the third in a series of articles for members of the new International Testing Centres Network and others who are interested in sharing experience of energy efficiency testing of industrial three-phase electric motors.

Effective motor testing depends on both manufacturers and independent test centres undertaking tests as consistently as possible. The aim of this series of articles is to encourage discussion between technical experts from test labs around the world on interpreting the new international test method standard IEC 60034-2-1. Other topics that will be covered include measurement techniques, procedures and equipment. One of the intended outcomes is to encourage improvements in motor compliance and check testing programs around the world.

This article discusses two clauses of the new international test method standard (see footnote).

Choosing measurement points - Clause 6.4.4.2 – Load curve test

Clause 6.4.4.2 – Load curve test describes testing the motor under different loads. It states that measurements should be taken at six different load points.

Four of these points need to be selected between 25% and up to and including 100% load. The standard states that these points should be approximately equally spaced, that is, approximately at 25%, 50%, 75% and 100% load.

The other two test points are chosen to be above 100% load – up to, but not exceeding 150% load. According to the standard, these points should also be equally spaced. Some laboratories choose 125% and 150% load for the two test points.

Choosing measurement points - Clause 6.4.2.3 - Friction and windage losses, iron loss

Clause 6.4.2.3 Friction and windage losses, iron loss describes testing the motor under no-load conditions. The motor first needs to be stabilised when run at rated frequency and voltage, although if this test is carried out immediately after the load-test, then the motor is considered to be stabilised.

According to the standard, the no-load test requires testing at least seven different points, where the voltage is varied. The rated voltage needs to be included as one of these test points.

Four of the test points need to be between 60% and 125% of rated voltage. Ideally, these points should be equally spaced, for example 60%, 80%, 100% and 125%.

The other three test points need to be between 20% and 50% of rated voltage, also equally spaced. An example of this is 20%, 35% and 50% of rated voltage.

When these points are plotted on a curve, clause 8.2.2.3.2 Friction and windage losses states that only the points that show no significant saturation effect should be used.

Questions for discussion

Question 1 For the load test, which load points does your laboratory use, and why? Do you think that the load points specified are the best possible?

Question 2 For the no-load test, which supply voltage values does your laboratory use, and why? Do you think that the voltage values specified in the standard are the best possible?

Question 3 What correlation coefficients do you normally get when fitting a line to your data? In your opinion, which parts of the measurement process most affect the correlation coefficient?

These questions are just to start a discussion about measurement points. Feel free to add any other comments you would like to make – the discussion is not restricted to these questions. Please post any answers or comments below.

We look forward to hearing from you!

* IEC 60034-2-1 Rotating Electrical Machines – Part 2-1: Standard methods for determining losses and efficiency from tests (excluding machines for traction vehicles). IEC standards can be purchased from the IEC website .