Improving Efficiency in AC drives: Comparison of Topologies and Device Technologies

New standards for power losses measurements for variable speed drives are presented. The power losses of typical variable speed drives are analyzed and future alternatives to increase their efficiency investigated. Solutions with Silicon (Si) technology in Three-Level or Silicon/Silicon Carbide (SiC) as well as full SiC systems in Two-Level topology are shown to increase the inverter efficiency. Introduction Gradually standards have been created defining methods to measure the efficiency of variable speed drives VSD and motors at different speed and load torque values. Future requirements of efficiency classes for general purpose drives (GPD) are also foreseen, similarly what has been established for industrial motors in Europe since 2009 with the Ecodesign Directive [1] [2]. Once those techniques and requirements for measuring the efficiency of VSD are established, the drive companies will focus on increasing the efficiency of their products accordingly. This will be achieved through the use of modern and more efficient power switches, more efficient pulse width modulation PWM techniques, more complex power circuit topologies, and the reduction of the energy losses in other components of the drive system. This article analyses the distribution of power losses in three typical VSD applications: general purpose drives (GPD) used in industry, VSD for elevators (lifts), and VSD for highspeed motors. These drives applications show different requirements for the power semiconductors. Possible scenarios are discussed based on current and future power switching devices and topologies, and using typical operating conditions for the different drive types. The impact of new components and technologies is compared for the mentioned application fields. Applicable Standards and Requirements The current requirements from IEC are focused on the influence of the VSD on the motor efficiency at speeds close to motor rated speed. The IEC 60034-25 defines the method of summation of losses in order to evaluate the impact of a VSD control on the efficiency of a motor with sinusoidal supply. In March 2013, the C838-13 standard was issued in Canada [3]. This standard defines a methodology based on the output/input power measurement to evaluate the efficiency of VSD and motors up to 750V AC at different speed and torque values. All the requirements from AC power supply, the instrumentation and the dynamometer used to impose the load torque in the motor shaft are carefully defined in this standard. In this way the differences of the results from test laboratory to test laboratory are minimized and the results comparable. PCIM Europe 2014, 20 – 22 May 2014, Nuremberg, Germany ISBN 978-3-8007-3603-4 509 © VDE VERLAG GMBH · Berlin · Offenbach Typical VSD losses and Efficiency values Three different VSD types driving 22 kW motors and fed from 400V three-phase line are analyzed and their typical loss distribution is shown. The main components of the VSD, the operating conditions and the main motor data used in the subsequent analysis are collected in the Table I. The selected IGBT modules represent the current state-of-the-art solution. TABLE I Current state-of-the-art reference VSD components and conditions for three different applications Parameter GPD Elevator High Speed Line supply 400 V / 50 Hz / Z=1 % 400 V / 50 Hz / Z=1 % 400 V / 50 Hz / Z=1 % Rectifier DDB6U144N16 DDB6U144N16 DDB6U144N16 DC reactor 6 % 6 % 6 % DC capacitor 1410μF/400V 1410μF/400V 1410μF/400V IGBT inverter FS75R12KT4 FS100R12KT4 FS150R12KT4 fSW [kHz] 5 10 16 Udc [V] 621 621 621 Uout [V] 400 400 400 Modulation index (m) 0.91 0.91 0.91 Heatsink temperature [°C] 90 90 90 Motor type AC-Induction AC-Induction PM Fundamental Frequency [Hz] 50 5