Vector VFDs are commonly employed in applications requiring strong starting torque because they allow a great level of control over the speed & torque of the electric motor.
Closed Loop Vector Control
A vector is a mathematical construct made up of two values that operate at right angles to one another. In complex mathematics, this is the combined impact of a real & imaginary quantity in 90-degree opposed planes. The combined effect of these 2 quantities results in the current provided to the motor.
A vector controlled Variable Frequency Drive (VFD) still use pulse width modulation (PWM) to send a precisely controlled current to the motor coils, but the vector control governs how that current is computed.
Field-Oriented Control (FOC)
In this computation, the two right angle opposing values are the motor’s magnetic flux current and the torque current, which are used to generate current for the field windings. This control style is commonly referred to as field-oriented control (FOC).
In a conventional V/f drive unit, the computation that generates current assumes that every rise in voltage results in a consistent increase in current - which implies perfect linearity. In actuality, this is not the case, and any parameter change, such as friction in an earlier motor, temperature from ambient (or) load conditions, or even changes in the load itself, causes the motor to respond. As a result, the motor will be unable to respond to changes as quickly.
IEC 61800, IEC 61000, NEMA ICS 7, and UL 508C - These standards serve to ensure that vector VFDs are efficient, safe, and do not cause electrical interference, while also increasing motor control & system performance.
When the magnetic flux current and torque current are measured and provided separately, the controller may track those values, as well as known conditions regarding the motor temperature & load, & recalculate an appropriate output to the motor coils on a more regular basis.
More information on the motor is needed for vector control.
Current Value
Using small Hall Effect sensors, the current flux and torque components of the drive can be monitored concurrently.
Motor Position
The motor position must be precisely known in order to send the appropriate current components to each coil. Previously, this would have required an encoder (or) another form of position feedback. In several cases, this is still employed for enhanced accuracy, however contemporary microprocessors can track motion more accurately based on a steady current value and suitable motor parameter input. This is known as open-loop control.
Motor Temperature
The temperature of the motor has a significant impact on efficiency, as much of the current is wasted as heat rather than magnetic force. These temperature variations can be predicted by properly accounting for the motor and system. Yet again, sensor data may improve accuracy.
VFD Control Outputs
The output can be modeled and changed in real time based on the current values, position, and system characteristics. This necessitates controllers capable of doing complex computations, which frequently include a PI (or) PID controller for more accurate response, & must be repeated hundreds of times per second. This is no easy work for simple microprocessors, & until recently, it was more expensive to accomplish.
As control electronics evolve more powerful, less expensive, & smaller, more drive units are likely to rely on vector control’s more accurate output.
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