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Variable Frequency Drives

Get Your Motor Running White Paper Series

Download Get Your Motor Running Part 5: Variable Frequency Drives

Part 5 of five. How a VFD controls motor speed and torque across the operating range, and what changes once the drive stays in control during normal running.

A VFD changes more than the way a motor starts. Because the drive remains in control after starting, the motor no longer operates at one principal speed. Torque capability, cooling, magnetic flux and mechanical limits all vary across the range, and the load may demand more or less torque as they do. A constant-torque load at low speed can draw close to rated current while the motor's shaft-mounted fan turns too slowly to cool it. Inverter-duty construction does not by itself resolve that.

Inside Get Your Motor Running Part 5:

  • Drive architecture, PWM, and V/F, sensorless vector and closed-loop vector control
  • Constant-torque and constant-power regions, and what field weakening costs
  • Selecting on current and duty rather than power rating
  • Harmonics, braking methods, motor cables, bearing currents and synchronous transfer

 

 

The Get Your Motor Running Series

Five technical papers take you from the fundamentals of motor starting through to the selection and application of soft starters and variable frequency drives.

aucom_gymr_handbook The Complete Series: A Practical Guide to Motor Starting and Control The complete guide to motor starting and control. All five Get Your Motor Running papers in one reference, from what determines starting performance through to selecting and applying soft starters and variable frequency drives. Worked examples and ratings guidance throughout. Read the Handbook aucom_gymr_p1 Reduced-voltage starting of three-phase induction motors What happens when starting current is restricted, and why motor design decides the outcome. Nine four-pole 110 kW motors with similar rated power, speed and efficiency are compared at the same current limit: at 300% FLC the available initial torque ranges from 24% to 65% of full-load torque. Covers locked-rotor current, locked-rotor torque and the speed/torque curve. Read Part 1 aucom_gymr_p2 Starting torque requirements of machines and motor loads What the driven machine requires from the motor during starting. Breakaway torque, work torque, acceleration torque and inertia, with worked examples for a punch press, a centrifugal fan, a compressor and a flywheel, plus the equations for full-load torque, acceleration torque and reflected load inertia. Read Part 2 aucom_gymr_p3 Motor starting solutions The principal methods compared: direct-on-line, primary resistance, auto-transformer, star-delta, soft starter and variable frequency drive. Covers open-transition and closed-transition switching, why the transition point matters more than the initial current reduction, and a side-by-side comparison of all six methods. Read Part 3 aucom_gymr_p4 Solid-state soft starters Controlled starting for fixed-speed applications. Open-loop and closed-loop control methods, from timed voltage ramp through to acceleration control. Also covers SCR thermal duty, AC-53 utilisation codes and ratings tables, motor protection, semiconductor fuse coordination, and power factor correction capacitor switching. Read Part 4 aucom_gymr_p5 Variable frequency drives How a VFD controls speed and torque across the operating range. V/F, sensorless vector and closed-loop vector control, the constant-torque and constant-power regions, load characteristics and energy use, drive and motor selection, harmonics and mitigation, braking methods, and synchronous transfer. Read Part 5