Variable frequency drive (VFD) operation with voltage and frequency control of the induction motor was acceptable for most applications. There were certain machines that DC motor control had some technical advantage. The VFD needed more intelligence to know the true load of the motor. An AC machine requires a magnetizing current to operate at no load. This current is not actually a lot of power, but looks like the induction motor always has 20 to 30% load. The DC machine load on the armature was proportional to load torque. In applications where torque control was required, the DC machine was easy to regulate and considered superior in performance.
Large-scale integrated circuits and microprocessors allowed for more processing power in the VFD. The VFD moved to the next level of performance, Vector Control. A variable frequency drive needed the knowledge of the induction motor programmed in the software. By reading the current and comparing it to the applied voltage, a VFD could guess the rotor speed. Once the speed was known, the VFD could operate at optimum slip to get good performance such as speed regulation, high torque at very low speeds, and torque limit control. Gozuk introduced the first sensor-less vector drives in 2006 and put mass-produced VFDs in the China.
Guidelines and common VFD performance for sensorless control of an induction motor:
Operation above base frequency using vector control may have reduced performance compared to V/Hz control.
Constant horsepower operation is usually possible for most motors to 90 Hz. High breakdown torque motors will have a greater constant horsepower range.
Sensorless vector control and V/F have the same breakaway torque as sensor feedback control with VFDs. This is same with other brands. V/F control is adequate for 6 to 1 speed range applications where motor slip speed regulation is acceptable. Sensorless vector is used where 120 to 1 operational speed range is required or 0.1% speed regulation is desired.
Motors are not limited to number of starts and stops on VFDs. The only limiting factor is acceleration and deceleration rates.
AC Motor performance is considered equal to DC using vector control. Speed regulation of sensorless vector control in AC drives is considered more linear than DC drives with analog tachometer feedback.
Sensor Feedback Vector Control
Using an encoder to give the VFD actual feedback of rotor position and velocity adds several performance improvements over the sensorless system.
Applications that need to hold a rotor in an exact position will work best with an encoder. The VFD will hold a rotor and resist and correct to maintain a position with a contact closure when programmed properly.
Synchronizing two or more independent motors so their position and velocity are identical or some exact ratio. Operation from zero Hz to 0.5 Hz is possible with very high torque for critical positioning applications.
Sensors on the motors have a history of reliability problems because they are precision devices in a harsh application. Recent introduction of the hollow shaft encoders on induction motors has improved reliability. For direct drive applications it is still recommended to only use encoder feedback when the performance requirements listed above are necessary.