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Long Live Direct Current Motors

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Reports of the death of DC motors and drives “have been greatly exaggerated” (to borrow from a well-known writer). For over 30 years, industry pundits have been predicting the demise of the DC motor/drive market, in favor of AC. Yet DC stubbornly clings to life.

A DC drive is a direct-current motor speed control system. It modifies the armature voltage or the field current of the motor in order to control its speed. The advantages of using DC drives include:

  • Low losses, less cooling
  • Very compact, less space consumption
  • Thyristor based, less weight due to high current density
  • Wide speed range, smaller motors
  • High starting torque and overload capability, smaller motors and drives
  • Re-use of old equipment like DC motors, short shut down time
  • 12-pulse to lower harmonics
  • Serial sequential for low reactive power consumption

All of which can be summed up into: performance and cost.

Some of the more well-known applications for DC motors include the following markets: cement, cranes, elevators, food and beverage, marine transport, mining, oil rigs, plastic and rubber, printing, pulp and paper, ski lifts, steel rolling, and even theater. Because DC motors and their controls have been around such a long time, the installed base of users around the world is massive, meaning this well-established power technology has enormous marketplace mass. Plus, DC motors deliver constant torque over a wide speed range, and some applications absolutely require this.  For variable-speed applications, basic DC drives are much simpler in design, requiring fewer parts at less cost.  And commissioning the drives and motors is as simple as it gets.

Where the market for DC has seen decreases over the last few decades is in higher-end horsepower applications, where the cost/horsepower breakeven purchase price hovers somewhere around 2 HP.  AC technology dominates the market above that point.

So why have so many otherwise intelligent people been writing off DC motor technology? In a word: brushing. All along, DC motors have relied on metal (originally) or carbon (lately) brushes to quickly turn the current off and on to make the motor rotate. These brushes wear down and must be replaced, entailing a higher degree of maintenance. The newer brushless type of DC (BLDC) motor is rapidly gaining in popularity, especially in some (variable-speed) applications, such as solar and medical; but at a cost premium over traditional DC. In our next blog, we’ll delve further into the advantages of brushless DC technology for commercial applications.

In the meantime, we’re glad to note that the DC motor is still alive and well, thank you very much.

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