The process transmitter sending its output signal to the proportional-integral-derivative (PID) controller that acts as the outer loop senses the process dynamics; its output is cascaded to the VFD.
Within the drive electronics are algorithms that control the electrical motor power, frequency, voltage and current. The current and speed set the motor torque. So the drive doesn’t control just the speed, it also regulates the torque delivered to the rotor shaft. This torque produces the rotational force applied to the load (pump, etc.) that powers the process.
Figure 2 -- Class A motor: The curve with load is
Properly understanding the dynamics of a VFD control loop requires considering all elements of the system and how they interact.
Drive Control Strategies
A computer program connected to the drive or a human machine interface (HMI) front panel enables inputting data about the load and the motor as well as setting the drive control strategy, which usually takes advantage of proprietary functionality. This strategy together with the load dynamic behavior determines performance. Loops within the drive electronics can be configured to control speed (through an external encoder), voltage, current and, in some cases, motor flux. These are the inner loops of the process control cascade. When tuning, remember that the inner loops must be at least five times more responsive than the outer loop. Another term to describe performance is bandwidth; it’s inversely proportional to the time constant of the controller/motor with no load.
VFD manufacturers offer models with varying performance and cost. So, assess which is the best drive for the particular application.
Most drives have a defined, configured startup sequence that is to be run with the load disconnected. During this sequence the drive powers the stator and makes measurements that determine the characteristics of the motor. These motor constants then are used to tune the internal electronic program.
Drive control strategies generally fall into three categories :
Volts per Hertz control. This is the simplest method. As seen in the torque speed curve (Figure 2), the region to the right of the peak can be considered linear — therefore controlling the frequency will regulate the shaft speed. The supplied voltage is v = ir + dλ/dt, where λ is the flux linkage. The derivative term is directly related to the rotation; so the rotational speed is proportional to voltage, thereby volts per hertz. At high speeds the ir term is negligible. The speed error is large at less than 10% of the rated rpm. Some configurations can compensate for this. This strategy is recommended for fan and pump applications. The resolution is about 0.5% base speed over a 40:1 range.
Another problem with this strategy is the reduced motor torque at low speeds. This can be a serious problem conveying sticky solids or slurries. The torque decreases because the motor and wire ir drop is a larger percentage of the supplied power. Using a larger wire size can help solve this problem.
Constant-slip current control. This strategy regulates the slip or difference between the electrical speed and the actual speed. This is configured two ways, either optimum torque (maximum torque per amp) or maximum efficiency. Based on the motor constants found during configuration or testing, the drive electronics will calculate the flux linkages to avoid saturation. Two inner loops are used for this strategy. The speed command, usually the output from the outer process controller, is cascaded to a proportional-integral (PI) controller in the drive electronics. This PI controller compares the shaft speed to the set point and provides an output signal to a torque controller. This controller converts the torque set point to a current required to achieve that torque. A current sensor then corrects this current.