VFD-fed AC Induction Machine
Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.
Variable frequency drives (VFDs) are a type of adjustable speed drive, which are becoming standard equipment to power most AC induction motors. VFDs are common in industrial and automation applications and typically provide robust control of the motor in speed, torque, or position modes. The VFDs tested and simulated in this experiment focus on speed and open-loop control with constant voltage to frequency ratio (V/f) control. The induction motor typically operates at a rated stator flux, and this flux is approximately proportional to the V/f ratio. To maintain constant stator flux, the voltage and frequency applied to the stator are maintained at a constant ratio, which is the V/f ratio. The VFD used in this experiment is a 1 hp Yaskawa V1000 drive, but the procedure applies to most commercially available general purpose drives.
1. Make sure the three-phase disconnect switch is off.
2. Check that the VARIAC is at 0%.
3. Perform the following connections at the machine and VARIAC terminals:
- Connect the induction machine stator terminals to the drive output (right-side connectors, when looking at the front of the drive).
- Connect the drive input (left-hand set of connectors, when looking at the front of the drive) to the VARIAC output.
- Connect the VARIAC input to th
VFDs typically provide a constant voltage-to-frequency ratio to maintain stator flux in an induction machine close to a constant. If a machine is rated at 60 Hz and 208 V (line-to-line, RMS), then the V/f ratio is 208/60 = 3.467 V/Hz. Therefore, when the machine is run at a lower frequency to reduce its speed, the voltage is weakened to maintain a V/f ratio at a constant. For example, if the machine is run at 30 Hz, voltage should be reduced to 104 V. Or, if the machine
VFDs have a wide use in commercial, industrial, and automation systems, and they can save significant amounts of energy, as they adjust the operating point of a motor to draw as much energy as needed under variable speed operation. Inverters used in VFDs are also common in many motor control applications including transportation systems with more electric vehicles, in heating, ventilation, and air conditioning applications, and many others.
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