MOTOR DRIVES AND SCR STARTERS BASICS AND TUTORIALS

AC and DC motor drives and SCR starters have been touched on in previous chapters. Here, some of the applications will be discussed, and variable-speed pumps are a good place to start.

Prior to inexpensive variable-speed drives, fluid flow in the chemical process industries and municipal water works was controlled by throttling valves on constant-speed pumps. At low flow rates, the pumps were very inefficient, and the lost energy went into heating the fluid.

With a variable speed drive, the pumps can be speed controlled to meet the fluid flow demand with no loss of efficiency. The same is true for pipeline pumping of liquid fuels where the same advantages accrue in drives to 10,000 hp.

Centrifuges are used for separating liquefied material mixtures. They must be accelerated as quickly as possible and then decelerated quickly when the process step is over. Torque controlled, regenerative variable-speed drives and motors make quick work of the job.

Hoists and elevators can be hoisted smoothly and then regeneratively braked with variable-speed drives to save on energy. Dynamometers have the same requirements. Multiple motors in steel and paper lines can be precisely controlled with constant, synchronized speeds, and they can maintain synchronism when accelerating or decelerating.

In the latest diesel electric locomotives, AC inverters power induction motors on the axles. They provide dynamic braking to minimize brake shoe wear and can immediately reduce torque on a slipping wheel to maintain optimum traction.

Induction motors stand up to the rigors of railroad service much better than DC motors. Ventilation systems operating from variable-speed drives do not have the annoying off-on characteristic of constant-speed fans, since they can be continuously speed controlled to maintain the required air flow.

Furthermore, they reduce wear and tear on the ventilation system by eliminating transient starting torques. Coal-fired electric utility generating stations use variable-speed drives on forced and induced draft fans for boilers, thereby saving the cost of using throttling methods to control air flow.

These units, in the 10,000 hp range, offer important economies in power usage. A kilowatt-
hour saved is a kilowatt-hour the utility can sell. Testing large motor drives is often done with motor generator sets that allow the drive power to be regenerated back to the power line to reduce costs and eliminate the need for a mechanical brake.

The arrangement is referred to as adynamometer. When the input and output voltages of a drive are the same, the drive can be tested at full load and line frequency with a “motorless” dynamometer. A three-phase inductor is used to couple the drive output back to the power line, and the drive electronics are phase locked to the line.

By displacing the relative phases of the drive and line through the control electronics, a full range of load currents can be obtained. The drive, in effect, becomes a synchronous generator tied to the power line through its internal reactance. The cost and losses of a reactor are far less than those of a motor generator set, and the only power required is to support the losses of the drive and reactor.

An auxiliary transformer can be used to match voltages when required. This arrangement has enabled testing of 20,000-hp drives at full load with a 2500-kVA supply rating.

SCR motor starters can often replace variable-speed drives as a lower-cost method of controlling starting currents in large AC motors. In fans and pumps, the torque requirements at low speeds are modest, and starting current may be as little as half of the across-the-line starting current. Furthermore, the starting current may be ramped up so as to permit coordination with capacitors switched in and out to control the voltage.

Another interesting use of AC motor starters is an SCR gating technique that allows the starter to operate induction or synchronous motors for a short time at very low speeds with high torque. This has proven useful in rotary kilns and mixers in the cement industry.

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