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A three-contactor bypass allow a motor to be run directly from the incoming line, bypassing the VFD. This is useful during VFD servicing, and can be used to run the motor at constant speed at a higher efficiency than with the VFD in circuit.

35. Three-contactor bypass: A VFD accessory that allows motor operation across the line or through the VFD.

One contactor is installed between the incoming line and VFD input; another (bypass) contactor is installed between the incoming line and motor.

A third is installed between the VFD output and motor.

A three-contactor bypass allows the motor to be run directly from the incoming line, bypassing the VFD. This allows for VFD servicing while the motor is being run from the incoming line, and can also be used to run the motor at constant speed at a higher efficiency than with the VFD in circuit.

36. V/F mode: Also known as volts-per-hertz mode, this is a simple control method for ac induction motors via a VFD.

A ratio is established in accordance with the base voltage and motor base frequency ratings. This ratio yields a linear pattern that the VFD follows to produce rated motor torque. The ratio of voltage to frequency is the flux level in the machine, which in turn dictates the amount of torque that the machine produces at a given operating point.

37. Open-loop vector: A complex but effective motor-control method that allows VFDs to realize the best characteristics of dc drive control (accurate torque control over a wide speed range) without the brush maintenance and high initial cost of dc motors. For optimum performance, the position or deflection of the motors rotor must be known or accurately estimated.

The lack of actual shaft position feedback in open-loop vector control necessitates the calculation of rotor position by other means ¡ª but the cost savings of eliminating the feedback device, VFD input, and associated cabling offsets the slight loss of motor performance, as compared to closed-loop vector operation.

38. Closed-loop vector: A complex but highly effective method for motor control that allows VFDs to realize dc drive control benefits without the physical limitations of dc motors.

Feedback devices such as encoders or resolvers supply necessary motor slip information to close the loop between VFD output frequency and actual motor shaft speed.

39. PID: The proportional, integral, and derivative control algorithm is widely used throughout industrial control.

When a process loop is created by adding feedback (from a variable such as airflow, pressure, or level) and sent to the VFD, regulation of the variable is possible via PID loop control.

The VFD's PID algorithm uses mathematical properties to determine reaction to changes between the system setpoint and its actual state as measured by feedback.

40. Auto tuning: A process in which the VFD tests an attached and unloaded motor to determine the best tuning parameters.

This glossary contains important VFD definitions, but is not exhaustive. For more information about variable frequency drives, or how they're integrated into larger motion designs, contact the authors through yaskawa.com.