From another technological era, a typical adjustable-speed drive that uses a motor-generator set. Driven by the ac motor on the right, the dc generator produces an adjustable voltage for a dc motor. The magnitude of the voltage depends on the voltage applied to the generator field. The insert shows a product based on more modern technology — a solid-state dc drive.
Select figure to enlarge.

Maybe you feel, as do a lot of others, that there is no pressing reason to replace your venerable motor-generator (M-G) sets with more up-to-date solid-state controllers. After all, they may have been faithfully performing for years with only minimal maintenance requirements. But, if you check out the advantages of conversion, you may be in for a shock. In typical applications, delaying the move to solidstate d-c drives is not only costing you a lot of money, but it probably is also affecting production quality and efficiency.

Why convert?

One obvious advantage of solid-state, dc controllers is that they save on maintenance costs. They eliminate the need to periodically lubricate and replace bearings and replace brushes. Also, you can forget about periodically rewinding and overhauling the dc generator.

Solid-state controllers are inherently reliable and are unaffected by many ambient conditions that shorten the life of M-G sets. Also, solid-state drives are modular, which means that replacing a unit is as simple as taking out one unit and replacing it with another. Should a drive-related problem occur, their modularity, along with the self-diagnostics available with solidstate controllers, will get you up and running with a minimum of downtime.

Meeting various application needs is only a matter of reprogramming controller software rather than extensive hardware modification. Also, a communication link between controllers is often available to network multiple drives in a coordinated system.

Speed regulation of solid-state drives using digital tachometer feedback is about 100 times better than using an analog tachometer feedback system as typically used with motor-generator sets — 0.01% for a digital system versus 1% for analog. This means that converting to a digital controller system will increase machine precision, possibly increase productivity, and reduce scrap.

Economics of conversion

Perhaps the most compelling reason to convert to solid-state dc drives is the opportunity for dramatic energy savings and reduced operating costs. For example, a motor- generator set’s two rotating elements operate with an efficiency of only 72% to 81%. But, solid-state drives, when used in conjunction with a line transformer, operate with 95% to 97% rectifier-transformer efficiency. By substituting a solid-state, dc drive for a motor-generator set, and using the existing dc motor, drive efficiency can be improved from 18% to 33%.

When an energized solid-state rectifier operates at no load (when the equipment is stopped), the typical loss rate is 0.6% to 0.7% of the power supply’s power rating. But, a spinning motor-generator set experiences a no-load loss of between 10% and 12% of its full-load rating due to friction, windage and excitation losses.

Example: A solid-state drive replaces a motor-generator set, but the dc motor is retained. The dc motor is rated at 100 hp with an efficiency of 88%. The machine operates 10 hr/day, 6 days/week and over 3,000 hr/year. The cost of electricity is $0.06/kWhr. Excluding power factor penalties, operating cost is:

Where:
C_o = Operating cost, dollars
C_e = Electricity cost, $/kW-hr
P = Power rating of motor, hp
T = Time of operation, hr
μm = Motor efficiency, %
μc = Converter efficiency, %

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