Matching individual components to a particular drive system has several advantages over pre-configured systems because this allows the engineer to optimise the system to his requirements. Pre-configured systems are always optimised for general applications and can never fit all. If available, an indicator for the efficiency of components is efficiency classes.
The standard EN 50598-2 defines efficiency classes for FC’s. As power electronics can have several configurations the classes IE0-IE2 are defined for Complete Drive Modules (CDM) consisting of rectifier, intermediate circuit and inverter. CDM with ability to feed back e.g. braking energy to the mains are addressed but not covered because they have typically twice the losses.
The IE classes are defined in relation to a reference CDM (RCDM). By having the same scale for all power sizes the classes are defined by relative losses. CDM with relative losses in the range of ±25% of the RCDM is classified as IE1. CDM with higher losses are grouped in IE0 while CDM with lower losses are in class IE2.
The rating does not reflect the CDM efficiency at lower speed / torque as it’s determined at 100% relative speed and 90% relative torque-producing current. For verification the CDM is tested with all included components at a defined test load. Fine tuning or a special test mode is not allowed.
For the power range 0,12-1000 kW, efficiency classes IE1-IE4 for electric motors are defined in the standard IEC/EN 60034-30-1. Although the standard is valid for all motor types some motor constructions (e.g. brake motors) are excluded from the standard. Several countries and regions use the IE class limits to define Minimum Efficiency Performance Standards (MEPS) to restrict the use of low efficiency motors. The efficiency class is related to the nominal operating point of the motor. Efficiencies at full speed but reduced torque must be stated on the nameplate or in the documentation. Limits are different for supply frequencies (50/60Hz) and the number of motor poles (2, 4 or 6 poles).
Classes for motors operated with FC’s are under discussion and will be defined in IEC/EN 60034-30-2.
Frequency Converter + Motor Combination
Efficiency classes for Frequency Converter and Motor combination are defined in the standard EN 50598-2 via an IES rating. Similar to the CDM, the classes of the so called Power Drive System (PDS), which is the motor + FC combination, are related to a reference system. PDS with 20% higher losses than the reference are in class IE0 while systems with 20% lower losses are in class IES2.
The classification is made for 100% relative speed and 100% relative torque. If the FC is designed for a shorter cable or it’s directly mounted on the motor where shorter cable can be used this must be stated in the documentation. In general all kind of optimisations are possible as long they are noted in the documentation. Consequently comparing two PDS ratings is difficult because they will most likely have different bases.
The IES class for FC and drive combinations illustrates the difficulty in optimising a system and that all components must be carefully selected, in order to optimise the application. The difference between pre-configured and non-optimised free combined systems will most often be minor, but matching different components generally allows finer adjustment to the machine, giving the machine builder a competitive advantage.