Proceedings of the 2008 International Conference on Electrical Machines
Paper ID 1281
Determination of Three Phase Induction Machine
Efficiency from Start up Data
Johann Peter Bacher, Florian Waldhart
Institute of Drive Technology and Electrical Machines
E.A.M, University of Technology Graz
Kopernikusgasse, 8010 Graz (Austria)l
Tel: (+43)-316-873-8601, fax: (+43)-316-873-7244
Abstract- There are various methods to determine the
efficiency of three phase induction machines. But all of them have
in common that they are to inaccurate or they are impracticable.
In this paper the machine efficiency will be derived from start up
data. For that purpose the variation with time of all stator
currents and all stator voltages have to been measured. Normally
it isn’t possible to mount a speed sensor on the machine. For this
reason only the speed at the end of the start up can be determined.
Practical results with three phase induction machines with
different power ranges are presented.
Nowadays a cost effective operation of three phase induction
machines is absolutely necessary to save energy and
maintenance costs. For this reason a simply method to
determine the machine efficiency is needed. In  fifteen
motor efficiency methods are described. The comparison of
these methods shows that only two of them are useable because
of the error limits.
The air-gap torque method and the shaft torque method have
an anticipated error limit within 0.5%. The first one needs the
no-load point and the torque must be measured by the second
method. Both conditions are very rarely, because the motor
shaft is normally connected and torque measurement shafts are
The described method in this paper bases on the current
circle diagram of an induction machine . The shaft torque
and subsequently the motor efficiency can be derived from
Ossana’s circle. In order to get the diagram two points must be
known at the minimum.