Flux Vector Variable Frequency Drive

A flux vector VFD uses feedback from what is happening at the motor to make changes in the output of the variable frequency drive.

However, it still relies on the basic volts per hertz core for controlling the motor. These combined techniques control not only the magnitude of motor flux but also its orientation, thus the flux vector name. The flux vector method provides more precise motor speed & torque control.

Flux vector control improves on the basic V/Hz control technique by providing both a magnitude & angle between the voltage & current. Volts per hertz variable frequency drives control only the magnitude. Vector VFDs come in two types, open loop & closed loop , based on the way they get their feedback information. Open loop is actually a misnomer because it’s actually a closed loop sys tem, but the feedback loop comes from within the variable frequency drive itself instead of an external encoder. For this reason there is a trend to refer to open-loop VFDs as sensor less vector VFDs. Sensorless vector control removes a major source of complexity & simplifies the variable frequency drive installation.

The block diagram of a sensorless flux vector control VFD is shown as:



Its operation can be summarized as follows:

• Slip is the difference between the rotor speed & the synchronous speed of the magnetic field & is required to produce motor torque. The slip estimator block keeps actual motor rotor speed close to the desired set speed.
• The torque current estimator block determines the percent of current that is in phase with the voltage, providing an approximate torque current. This is used to estimate the amount of slip, providing better speed control under load.
• V angle controls the amount of total motor current that goes into motor flux enabled by the torque cur rent estimator. By controlling this angle, low-speed operation & torque control are improved over those of the standard V/Hz VFD.
• The flux vector control retains the V/Hz core & adds additional blocks around the core to improve the performance of the variable frequency drive.
• The current resolver attempts to identify the flux & torque producing currents in the motor & makes these values available to other blocks in the VFD.
• The current limit block monitors motor current & alters the frequency command when the motor current exceeds a predetermined value.

A true closed-loop vector VFD uses a motor-mounted encoder or similar sensor to give positive shaft position indication back to the microprocessor. The motor's rotor position & speed are monitored in real time via a digital encoder to determine & control the motor's actual speed, torque, & power produced. It shows a typical flux vector VFD & motor-mounted encoder used in vector VFD applications. The encoder operates by sending digital pulses back to the VFD indicating both speed & direction. The processor counts the pulses and uses this information along with information about the motor itself in order to control the motor torque & associated operating speed. The most common encoders deliver 1,024 pulses per revolution. To guard against electromagnetic interference (EMI), the cable assembly between the encoder & VFD should be one continuous shielded cable (how to select VFD cable?).

A true closed-loop vector VFD can also make an AC motor develop continuous full torque at zero speed, something that previously only DC drives were capable of. That makes them suitable for crane & hoist in mining applications where the motor must produce full torque before the brake is released or else the load begins dropping & can't be stopped.