With the refinement and greater control offered by modern AC motor drives and high-performance 3-phase induction motors, mid-range brushed DC motors have pretty much gone the way of the dinosaur. Learn what issues should be considered when choosing between DC and AC motors for your specific application.
Direct current (DC) motors have long been known for their sheer brute strength, wide torque range, and responsive speed control. Although DC motors are often thought of as “simple” because DC electricity is less complex than alternating current (AC), it couldn’t be farther from the truth. DC motors often require more hardware than their AC motor counterparts, including power converters, semiconductor fuses, line/load contactors, additional branch circuit protection, specialized overload and field protection, and interlocking. As the industry is trending away from DC motors, it has become difficult to find and replace these components. These extra devices also increase the electrical complexity of the system.
DC motors may be found in steel industries, hoists, traction applications, propulsion, elevators, printing, and mining.
Over the years, AC induction motors have become the industry standard due to their lower cost and the vector control gained by using modern variable frequency drives (VFDs). Advancements with VFDs mean that AC motors can achieve full torque control throughout the speed range, quick and dynamic speed changes, reduced motor size, and reduced power consumption. Today AC motors are ubiquitous in industry and are found running conveyors, machinery and equipment, fans and pumps, HVAC, utility, and processes.
Cornerstone Automation has performed installations for a web-based process by using AC vector duty motors for the unwind rolls with encoders to maintain product tension by utilizing adjustable torque and speed. In other words, we are able to maintain tension to near zero speed in slow applications.
There are many things to consider if you have DC motors and are contemplating conversion to AC motors. DC motors are known for their constant torque capability throughout their entire RPM range, a feature that AC motors couldn’t match until the advent of modern VFDs. In particularly large applications, the AC motor may need nearly twice the horsepower to generate the torque required. Improvements to AC motor controls mean that high-performance AC motors and VFDs are capable of full torque throughout their entire range and offer fine speed regulation when supplied with speed feedback – even on par with DC motor torque. Despite this, the torque requirements of the application may mean the physical size of a large AC motor will still be like that of the DC motor.
DC motors are more expensive to buy and maintain than AC motors and AC motors with quality VFDs consume less power for the same amount of torque. Even so, DC motors naturally have better constant torque characteristics whereas AC motors require high-performance design and speed feedback to get similar torque results. Though in most situations, AC motors are better at variable torque applications given modern VFD technology like Sensorless Vector Control (SVC).
For AC motors, although pulse width modulation (PWM) and powerful microprocessors have led to a reduction in VFD size and efficiency, the quality of circuits and semiconductors in these smaller packages can lead to significant harmonic noise and the need for additional accessories for VFDs such as inductive reactors, DV/DT, and sine wave output filters, stringent cabling requirements, and harmonic line side filters.
Four-quadrant DC drives may also cause excessive harmonics on the line side when regenerating and filters may be required to protect other nearby drives. Harmonic mitigation may need to be considered or added to existing equipment or during conversion.
To mitigate this, line-side filters may need to be used depending on the power conversion technology used in the VFD, especially where there are other nearby drives and power factor correction equipment to reduce the harmful effects of harmonic distortion. Perhaps the most well-known problem of modern PWM VFDs is the reflected wave phenomenon, sometimes referred to as “ringing.” This vibration can cause premature motor failure, or nuisance VFD trips and is most often aggravated by the length of the wire to the motor.
In summary, the decision to choose a DC or AC motor is not a simple question. The specifics, including harmonics, the available power, and voltage as well as the torque and speed requirements, need to be examined to find the best-fit motor for the application.