Induction Motor Emulation for Variable Frequency Drive Testing

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 3kVA 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 motors 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

2:
Motor-emulating
Figure 1: Three-phase induction Figure
motor
per-phase
equivalent transformer circuit diagram
circuit

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.).

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