Make the Most of Variable Frequency Drives

Optimum performance depends upon proper installation and control.

By Robert Heider, Washington University, and Clay Lynch, French Gerleman Electric Co.

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This strategy relies on three cascaded controllers. To obtain stable behavior each inner control loop must be several times faster than the loop it’s fed from. Anti-reset windup should be used for each loop. To accommodate the sluggish torque response, the drive performs more slowly than with the volts per hertz strategy. The stator current is controlled at or just slightly above its rated peak value during start up. This strategy has 0.1% of base speed regulation across an 80:1 range.

Blending issue
Figure 3 -- Blending issue: Original VFD led
to inaccuracies in component addition.
Click on illustration for larger image.



Field-oriented control. The basis of this strategy is that the maximum torque between the rotor and stator magnetic fields occurs when the rotor current vector is perpendicular to the stator field (per Lorenz’s force equation). The current control is calculated from the torque command and an estimate of flux. The strategy can be implemented two ways: direct and rotor-oriented where Hall effect transducers measure the flux, and indirect where the flux is estimated. Industrial applications don’t employ flux sensors. This method offers improved transient performance. A 50-hp motor with load reaches full speed in 2 sec. and peak current remains at a steady-state value. High performance drives using this strategy have 0.001% base speed regulation across a 120:1 speed range. Because this method uses estimated machine constants, performance will deteriorate if the constants are incorrectly entered. The problem is most severe at low speeds. If the online machine parameter estimates aren’t correct, the drive will experience “hunting.”

Regardless of control strategy, as already noted, the drive changes the voltage or current to the motor via modulation. The IGBT is a switch either on or off. PWM produces a change in value by alternating the switching sequence. In this manner the output stator current will resemble a sine wave with steps inserted. The switching frequency, usually a configured entry, will determine how well the waveform will track. It’s preferable that phases are sequenced, not all switched at once. This reduces the effective switching speed and results in an offset between the actual and desired current, therefore necessitating some closed-loop strategy.

The resulting waveform produces a quantized output. The root-mean-square value actually will vary in discrete intervals. From a closed-loop perspective this injects a non-linear term in the analysis that will create limit cycles, especially at high process gains.

DC Motors
These motors are classified according to how they are wound. Motor performance, usually shown as a torque speed curve, differs with winding design.

In the case of a shunt wound or separately excited motor, a constant DC voltage is applied to the stator winding while a variable voltage is applied to the rotor winding. A permanent magnet DC motor behaves similarly to a separately excited one. A shunt wound DC motor has a single voltage source that powers both the rotor and stator. The shunt wound, permanent magnet and separately excited motors have a linear torque speed curve and, thus, can provide better speed regulation with varying loads. They come in several winding variations, such as compound windings.

In a series motor, the stator is wired in series with the rotor. The torque speed curve for this motor has a parabolic relationship. This motor frequently is used for high mechanical inertia loads, elevators, cranes, etc.

Proceed With Caution

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