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Topics: Variable speed drive resistor load on Variable frequency drives
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10-07-2012 10:09 AM

Variable speed drive resistor load

Please comments on the impact on the variable speed drive, using a resistor load bank for testing instead of a motor.
10-07-2012 11:25 AM
Top #2
10-07-2012 11:25 AM
I have taught variable speed drive for many years at a university and one of my favorite demos was to take a 3 phase 230vac drive and wire the motor output leads to three 120vac light bulbs connected in a wye. If you set the VSD to a slow acc rate, you can visibly watch the 3 light bulbs light up in a rotating pattern that gets brighter and faster as the Volts/Hertz increase. It is a poor way to test a drive but it is a good way to visibly demonstrate rotating 3 phase motor speed and direction control.
10-07-2012 02:12 PM
Top #3
10-07-2012 02:12 PM
For a quick and dirty test, I have used a NEMA motor stator--just un-bolt the end bell and pull out the rotor. Once, when this was not practical, I went to an electric supply warehouse and brought back three spools of single-0 welding cable. We hooked them up in a wye configuration as three big air-core inductors and tested our inverter into that load.
10-07-2012 08:33 PM
Top #4
10-07-2012 08:33 PM
A motor that has the current in phase with its back EMF is actually a pure resistor with an series inductive drop. A good starting point is to choose a resistor value corresponding to the rated power output of the motor. A 110 volt, 1/2 hp motor motor would be emulated with 32.4 ohm. (373 watt) The stator resistive loss is negligible compared to this load. However, that stator inductive drop is large at the top running speed. So, for an accurate emulation, you need to add the equivalent stator inductance. A vector drive should increase the voltage and adjust its phase to assure I and V are phased together at the resistor.
10-08-2012 01:14 AM
Top #5
10-08-2012 01:14 AM
motor can be modeled as the series connection of resistor , inductor and back EMF. without inductor, nothing smoothes out the harmonic voltage. that means the variable speed drive will see a lot of harmonic current. without back EMF , nothing counts against the VSD voltage. that means a little voltage will creates a large current, depending on how large the resistor you will use.
10-18-2012 02:19 AM
Top #6
10-18-2012 02:19 AM
this was for accelerated life testing (ALT) where we test the inverter for it's intended life time in a short time period. In our case, we installed 10 or 15 inverters in a temperature/humidity chamber and drove the simulated motor loads at specific intervals 24 hours a day under various temperature/humidity points until we meet the equivalent life of the product. The goal is to identify any hardware components that are prone to failure in the field and improve the robustness of those components and thus improve the reliability of the product.
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