Drive solves overvoltage issue

• control in small package 
• dust-tight IP-66 enclosure

Crippen Manufacturing produces a wide variety of grain processing equipment, including grain shaker tables used to separate freshly harvested grains from husks, leaves, soil and other detritus. An AC motor in the shaker rotates an eccentric cam connected to three large perforated tables. The tables are angled so that grain slides down from table one onto table two, repeats the same motion onto table three, after which it passes to an auger for transport to the next process. Air constantly blows up through the table perforations to help separate lighter from heavier material.

At the machine’s original maximum speed (72 Hz drive output frequency), the oscillation rate was 11 Hz (90 ms period), and directly proportional to the speed of the motor. The eccentric load on the motor produced a sinusoidal load profile, where the motor regenerated for half the cycle and motored for the other half. This load, in turn, produced a DC bus level that oscillated in phase with the load profile. The load magnitude varied exponentially with motor speed — at low speeds, there was relatively little DC bus variation. However, at a running speed of 55 Hz or greater, the DC bus varied enough to cause an "overvoltage" fault in the adjustable frequency drive. A DB resistor to suppress the DC bus voltage was unacceptable to the customer, because the machine is often installed out-of-doors and in environments where it is exposed to explosive grain dust.

The solution was to use a Yaskawa V74X AC drive in a NEMA 4X/12 enclosure. The drive provides V/f or open loop vector control in a small package. Initial testing showed that overvoltage suppression software and other features were successful at eliminating overvoltage trips, but adjustments were still required. By selecting different parameter inputs from the V74X keypad, the drive V/f pattern was modified to reduce the magnetic flux in the motor at speeds above 40 Hz. The voltage applied to the motor at rated speed was lowered, via the keypad, from 460V to 280V and from 230V to 140V. At these voltage levels, the motor provided a softer response to load changes, an effect similar to using a filter. This alteration prevented large swings in DC bus voltage, even at maximum speed. Monitoring of motor current before and after the change in V/f pattern indicated no current increases.

Crippen Manufacturing, St. Louis, MI. 
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Yaskawa, New Berlin, WI. 
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