Thursday, April 2, 2009

Losses and Efficiency of 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

- 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.



Genetic Algorithms in Induction Motor Efficiency

Many current techniques of calculating induction motor efficiency
are difficult, expensive, or inaccurate in the field. Induction motors
consume a large percentage of the electricity used in the US.
Accurate calculations of the efficiency of these motors would allow
savings in both energy and cost. One major obstacle in the
calculation of efficiency is that it is often difficult to measure the
output power accurately and safely while the motor is running,
say in a factory. It would be of interest to find a way to estimate
the output power using only easily measured quantities, such as
input current and voltage.


Effective Estimation of Induction Motor Field Efficiency
Based on On-site Measurements
This paper proposes the effective technique for
estimating efficiency of existing three-phase induction
motors in the field. This technique focuses on the
operating efficiency of motors without the need for
removing the motors and without the need for measuring
the output power or torque. This paper describes the use
of a few sets of data (voltage, current, power, speed)
measured from the motor (on-site) coupled with the
genetic algorithms for evaluating the motor parameters
instead of using the no-load and blocked rotor test results.
Once these parameters are known it is possible to obtain
the estimated efficiency of the motor. To illustrate how
well the estimated efficiency match that of the calculated
obtained from the standard evaluations, the results of
various induction motors rating 10 up to 100 hp are
presented. Test results indicate that this proposed
technique has a high accuracy, and then it could be
suitable for conducting on-site energy audits of existing
motors in order to support a decision to replace operating
motors with a higher-efficiency model.

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