Induction Motor Emulation for Variable Frequency Drive Testing

Induction Motor Emulation for Variable Frequency Drive Testing Geoffrey Roy, Amber Reinwald, Matthew Geary Advanced Power Electronics and Electric Drives Lab, Center for Clean Energy Engineering and Department of Electrical and Computer Engineering University of Connecticut. , Storrs, CT, USA Abstract Prototype Completion Motor to Transformer Relationship The focus of this project was to design and construct an induction motor emulator. The goal was to simulate an induction motor under various loads in order to test variable frequency drives (VFDs) for Lenze Americas. This was accomplished using a 3 k. VA three-phase transformer and three balanced deltaconnected resistor banks to simulate the output power of an induction motor. These resistors were controlled using electromechanical relays activated by a graphical user interface (GUI) designed in Labview. To complete the prototype power resistors and a three-phase transformer provided by Lenze were used. The Lenze VFD drove the transformer while the output of the system was determined by a relay board and GUI. The corresponding input and output powers were measured using the two watt-meter method. From simulated and experimental data a relationship was created relating a motors slip dependent rotor resistance to the emulating transformers load resistance (Rv). Prototype Research and Simulation Before building a prototype, a simulation in MATLAB and Simulink was built to prove the plausibility of the project. The idea behind the prototype and simulation was that the equivalent circuit of an induction motor and transformer are fundamentally equal. The problem that needed to be solved was how to incorporate a slip dependency in the transformers circuit. The slip dependency was incorporated by including a variable load resistance for the transformer. After running both simulations the motor and transformer outputs were the same. All relevant circuits and simulation diagrams are shown below. Where: Rr’ = a motor’s slip dependent rotor resistance a = the turns ratio of the transformer C = a motor constant to tune the exact load resistance (motor dependent) Graphical User Interface (GUI) Lenze VFD drives the system Transformer output goes to the relay board Relay board activates parallel resistor combinations Figure 5: Flowchart of the finished prototype with pictures of the product Output power changes based on varying resistance A GUI was designed to select the relays that would activate or deactivate the balanced delta-connected power resistors in order to change the output power of the emulator. With the GUI a user can emulate the rising torque of a motor torque vs. speed curve by using the transient option however a user can also emulate the linear region of the torque vs. speed curve using the single slip option. Prototype Testing and Results Table 1: Input and output powers for both resistor banks Figure 1: Three-phase induction motor per-phase equivalent circuit Figure 2: Motor-emulating transformer circuit diagram Figure 6: 1. 5 HP Torque vs. speed plot Figure 8: Front panel of our graphical user interface Conclusion This induction motor emulator design eliminated moving parts and is much lighter than actual induction motors. Thus saving space and energy while simultaneously providing the correct input and output powers of an induction motor. Ultimately providing Lenze with a safer, more efficient method of testing variable frequency drives. Figure 3: Induction machine simulation diagram Acknowledgements Figure 4: Emulation transformer simulation block diagram Copyright © 2015 – Advanced Power Electronics & Electric Drives Lab (APEDL) Figure 7: 1. 5 HP Output power plot We are extremely thankful for the help and guidance of our project manager Chris Johnson and all those from Lenze who mentored us; Mark Collins and Neil Pande as well as UConn Professor Ali Bazzi and UConn graduate students Yiqi Liu (Ph. D. ) and Artur Ulatowski (M. S. ).