When the efficiency for rated load and 3/4 load is given, the following formula may be used to compute a good approximation of the efficiency at any other partial load:
⎜ ⎛1 ⎜η 00
⎜ ⎜ ⎛1 ⎝η 00
⎜ ⎜ ⎝ ⎛ 0.4375 ⋅ − ⎟ ⎟ ⎠ ⎞ η 7 5 1 7 5 . 0 1
L − ⎟ ⎠ ⎞ − ν 1
⎟ ⎟ − ⎠ ⎞ 1
1 +νL ⋅ p
100 η 7 5 with: η ν L , ν 0 p ηp
= Efficiency at rated load (from 0...1 with 1 equals 100%) = Efficiency at 3/4 load (from 0...1 with 1 equals 100%) = Intermediate results = Desired power (relative to rated load, i.e. from 0...1...overload) = Resulting efficiency (from 0...1 with 1 equals 100%)
Efficiency testing methods
There are a number of test methods for determining motor efficiency. Standard methods for testing induction machines are internationally defined in IEC 60034-2-1, which recognizes several methods for determining motor efficiency, each of which has certain advantages as to accuracy, cost, and ease of testing, depending primarily on motor rating. Some of the methods in IEC 60034-2-1 are harmonized with national standards such as CSA C390 and IEEE 112.
The residual-loss method in IEC 60034-2-1 is a defined calculation procedure for segregating the various types of losses from the raw data and smoothing the additional (stray-) load loss by linear regression analysis. This can reduce the effect of errors introduced from making measurements over the range of loads from 25 percent to approximately 150 percent of rated load. It also adjusts the tested ambient temperature to 25°C to reduce variation due to different testing environments.
The common practice for obtaining the raw data for 0,75 to 370 kW is to test the motor with a load machine and a torque meter and to carefully measure the power input and output to determine loss components and thus efficiency.
Adherence to laboratory quality control standards through a national laboratory accreditation program can further assist in minimizing variations in results which occur when a motor is tested at different facilities.
Even with the use of a consistent and accurate efficiency test method, variations in results for the same motor do occur, primarily due to test equipment and instrument characteristics, and in the case of non-automated testing, personnel factors.