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AC and DC Motor Drives: Is One the Clear Choice?

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When engineers are looking to buy a motor, the question most often asked:  Is AC or DC technology better? The answer isn’t that one is better than the other. Rather it is application and cost dependent.  Here we will discuss the options and specifically the advantages and disadvantages of each. Furthermore, it is often a matter of personal preference, and as such we encourage you to let us know your thoughts on the subject.

What They Are

Both AC and DC drives, simply put, control the speed and direction an electric motor. Both technologies have been around for over 100 years. AC drives, also known as adjustable speed drives or variable frequency drives (VFD’s) control motor speed by changing the frequency of the electrical supply to the motor.  With DC drives, the motor turns at a speed directly proportional to the voltage applied to the armature.

Advantages and Disadvantages of AC Drives

Advantages provided by AC drives include: Reduced power line disturbances, lower power demand on start, controlled acceleration, controlled starting current, adjustable operation speed, and adjustable torque. While the installed base and market for AC motors and drives is larger than DC, it is difficult to find an AC solution with a horsepower rating under 1/2.  The current trend for VFD development is to add more (programmable) features and even programmable logic controller (PLC) functionality, making them complicated to commission and operate. These features, while attractive to the experienced user, may place additional pressure on the technical ability of maintenance and operations personnel.

 Advantages and Disadvantages of DC Drives

Advantages of DC drives include: High starting torque, easy installation, speed control over a wide range (both above and below the rated speed), quick starting, stopping, reversing, and acceleration, linear speed-torque curve and accurate step-less speed with constant torque.  DC motors are readily available and in fact dominate the fractional and sub-fractional horsepower installed base and new purchase market.  There is growing demand for 12 and 24V motors to support applications in solar and portable (truck mounted) equipment which DC technology readily supports.  However, DC technology does not lend itself to hazardous or explosive applications.  With DC technology the initial investment (motor and drive cost) is less than AC in general for applications under 1 horsepower; above one horsepower the higher maintenance cost of DC technology (motor brush wear) must be considered but may be offset by the additional personnel skill requirements previously mentioned.

In the end, there is a market for both AC and DC, and there are clearly advantages and disadvantages to both. What do you think? What is your motor drive preference?

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