Electrical Grid interface for an Induction Motor

• Author / Creator

  Haque, A R N M REAZ U

• The 3-phase induction motor is the workhorse of modern industry and is widely regarded as a highly reliable electromechanical device. Two motor operational modes were of interest in this thesis work, motor starting and steady state operation when connected directly to the electrical grid and operating at the same grid frequency. Typical industrial applications include pumps, compressors and fan loads. Motor starting on isolated or weak grid systems is a highly dynamic process that can cause damage to the motor and load as well as grid voltage fluctuations. During steady-state operation, the induction motor draws reactive lagging currents and is exposed to variable grid voltages that reduce the motor operating efficiency and lifetime expectancy. Hence, the prime purpose of the thesis is to present power electronics that connect the induction motor to the grid and that can control the motor voltage above and below the grid voltage. As a result, the power electronics can provide a range of operational features such as: motor soft start, VAR compensation, improved power conversion efficiency, increased operational lifetime expectancy. The power electronics presented consists of a 3-phase floating H-bridge that is connected in series with the utility grid and a cage induction motor to provide series voltage compensation. By injecting a series voltage in each phase, the proposed system can be used to control the motor voltage during starting and hence limit the motor starting current. The voltage injection can provide a voltage sag ride through capability and operate with a leading grid power factor under steady state, hence generates VARs into the grid. The 3-phase H-bridges produce 5-level pwm motor line voltages with a pwm frequency up-to four times the switching. This compares with the 3-level line voltages produce in a standard VFD at twice the switching frequency, using larger voltage step sizes. The 3-phase H-bridge system therefore results in lower high frequency pwm induced iron losses and Cu losses in the motor. Besides soft start and reactive power generation capability, the proposed system has many other desirable operating features, such as; improved motor operating efficiencies, reduced motor losses. Floating capacitor converters can set the motor voltage at a fixed desired value, above or below the grid voltage, under transient or continuous steady-state conditions, and over the entire range of the motor load. This is useful when the motor is connected to a grid whose nominal voltage differs from the machine’s rated value or that may fluctuate over time (sag or swell). A variety of control options exist to lower the losses of an induction motor, the approach presented is based upon measuring the motor electrical input power and using readily available motor nameplate data to control the motor voltage. The conversion efficiencies of both the motor and the power electronics power can be improved over the entire motor load range. This results in lowering the motor’s operating temperature to improve lifetime expectancy, and avoids derating the motor power rating. The cooling requirements of the power electronics can be reduced, lowering their size, cost and weight. Motor voltage control with bridge voltage control is explained. Simulation and theoretical analysis is presented to predict operation of system as well as theoretical performance curves for the 3-phase H-bridge is presented for motor control modes. Relationship between motor winding temperature rise and motor loss is also established. A 5 HP experimental testbed is used to validate the concepts. Soft start, the grid voltage ride through capability feature and the reactive power generation characteristics are verified. Experimental results show that the proposed system can successfully soft start a standard squirrel cage induction machine under different modes and load conditions. Also the experimental performance of the power electronics, motor and total system is presented with respect to power losses, system efficiency and the motor output power. For applications where frequency control is not required, the proposed 3-phase H-bridge system is a viable cost effective solution.

• Subjects / Keywords

  • Soft start
  • Performance improvement
  • Voltage control
  • Induction Motor

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R36688X1Q

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.