Ask Greg McMillan - What role do you see dynamic simulation playing in using the best variable frequency drive technology?

Greg's Response:

Dynamic simulations can show how investment in the best drive technology, motor, bearings, input cards, and inverter setup is critical to achieve the best variable frequency drive response (VFD). We start here with discussing the best drive technology.

Pulse width modulation (PWM) is predominantly used today to vary the frequency of the voltage or current to the pump motor. PWM introduces lower noise, has a higher input power factor, and has better low speed performance than older drive technologies. PWM also offers better rangeability from less cogging (torque pulsation) at low speeds. A VFD inverter converts an AC line voltage to a variable frequency voltage for an AC induction motor.

Open loop voltage (volts/hertz) control has the simplest algorithm but is susceptible to varying degrees of slip. Most of the drives provided for pump control use this strategy in which the rate of change of flux, and hence speed, is taken as proportional to voltage. At low speeds the motor losses are larger making the difference between the computed and actual speed (slip) much larger. Some drives make a correction to the voltage to account for estimated motor losses. Ultimately these drives depend upon the DCS to correct for dynamic slip through proportional action and to correct for steady state slip through integral action in process controller(s). The rangeability is normally 40:1 for low static head applications with 0.5 percent speed regulation.

Closed loop slip control has a speed loop cascaded to a torque loop. Speed (tachometer) and torque feedback are shown to be from sensors. The torque feedback may be calculated from a current sensor. A DCS process controller output is the speed set point for the speed controller whose output is the set point to a torque controller. The speed control should stay in the field. Speed control in the DCS is too slow and coordination with the torque control compromised.

The VFD supplier normally addresses the dynamics of speed to torque cascade control so that the VFD response is smooth. The rangeability is normally 80:1 for low static head applications with 0.1 percent speed regulation.

For much more knowledge see ISA Book Essentials of Modern Measurements and Final Elements in the Process Industry and ISA Ask the Automation Pros post “What is the Best VFD Design and Installation Plan”