Essay on AC and DC Motors

For many years, DC motor technology was the choice for electric lift trucks. Today, the advantages of AC motor technology have made it a cost-effective, highly efficient solution for making electric lift trucks more energy efficient, more productive, more reliable, and less costly to operate than their DC-powered counterparts. To understand why, it helps to know the history of AC in industrial applications, particularly with respect to lift truck control systems.

Historically, applications that required precise motor control over a wide range of speeds have used DC variable speed drives. DC drives offer full torque at zero speed -- this is due to independent controls over speed and torque. However, DC motors are more expensive and require a much higher degree of maintenance than AC motors. The newest technology is an AC drive that offers torque control, an important feature when energy efficiency and motor reliability is demanded. An AC drive initially may be more expensive than an equivalent DC unit, however, an AC motor will provide a faster return on investment (Manias, Ziogas, Olivier, 1985).
AC Drive Advantages

Uses conventional, low cost, 3-phase AC induction motors for most applications
Preferred for tight motor mount applications
Smaller, lighter and more commonly available than DC motors
Suited for speeds over 2,500 r.p.m. since there are no brushes
Used when multiple motors in a system must operate simultaneously at a common frequency and/or speed

DC Drive Advantages

Less complex with single power conversion from AC to DC
Normally less expensive for most horsepower ratings
Long tradition of use as adjustable speed machines
Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque
Capable of providing starting and accelerating torques in excess of 400% of rated.........................

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