Losses in Induction Motor

A. Definition of energy efficiency

Efficiency is the ratio of mechanical energy output divided by
the electrical energy input. There are different efficiency definitions
that describe the relationship between a motor’s rating and
efficiency test results:

- Tested. This refers to the efficiency measured by testing that
specific motor.

- Nominal or Average Expected. Nominal values are the average
values obtained after testing a sample population of the motor model.

- Nameplate. This refers to the efficiency measured by a specific
standard.

- Minimum. These values are intended to represent the lowest point in
the bell curve of motor efficiency distribution.

- Apparent Efficiency. This is the product of a motor’s efficiency and
power factor.

Figure 2.1 – Typical energy flow of standard motors

B. Motor Losses
Energy losses are the determining factor in motor
efficiency. These losses can be divided in five classes:

Classes of Motor Energy Losses

The main difference between the standards emerges
from the way in which the additional load losses, is
treated. The IEC 34.2 standard assumes a standard value
for the additional load losses at rated load of 0.5% of the
input power. The new proposed IEC 61972 standard
gives two possibilities for the assessment of the
additional losses. The first one is a determination by
means of the measured output power, as in the IEEE 112-
B; the second one gives a fixed amount to every machine
of the same rated power. The Japanese JEC standard 37
completely neglects the additional load losses.