DC & AC Motors: Considering Your Options

How do you go about selecting the right electric motor for an industrial application? What would an expert recommend? What are the advantages and disadvantages to DC & AC Motors? This article holds the answers you've been looking for.

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DC Motors were king in industry up until the late 1980s; These motors were popular because they were able to run to a variable speed setpoint, and they could run at full torque from stall to base speed. A DC motor was powered from either a constant potential supply or a Ward-Leonard system via DC generator. The speed of a DC motor is defined by the voltage applied to the armature and other influencing motor design constants. Speed could be easily measured by calculating Counter-EMF using measurements of armature voltage and current. There are two types of DC motors: brushed and brushless. Brushed is the oldest type of motor and includes a field mounted to the stationary portion or frame, an armature rotating portion, and brushes to carry current to the armature. The direction and speed are controlled by the applied voltage. The torque is measured by the armature current.

Previously, AC motors were utilized for fixed speed applications. AC motors could be order at different rpms of 3600, 1800, 1200 for operation on 60Hz. Pumps and fans were run with across-the-line starters at fixed frequency applied to the AC Motor.  AC Motors started to take over the variable speed market in the 90s due to the wide availability of inverter drives and vector control. Now, variable speed inverter duty AC motors are widely distributed in the United States. AC motors get their ability to change speed because of Variable Speed drives using IGBTs (Insulated-Gate Bipolar Transistor) for the power section, where the frequency can be varied. Torque control is now available with AC vector drives through mathematical modeling of an induction machine where computers calculate the magnetizing current vs. the torque producing current.

AC vs. DC Motors Advantages vs. Disadvantages

When choosing which motor would be best for your project, you need to outweigh the advantages and disadvantages of each motor.

AC advantages: 

  • NEMA standards for frame sizes.
  • At least 95% efficient for typical applications - this translates into electricity cost savings for the customer.
  • On an induction machine, the stator induces the current into the rotor. The only physical contact between the rotor and stator are the bearings. The motors can be commonly ordered “Totally Enclosed”, so it doesn’t get dirty on the inside, leading to less chance of an electrical failure.
  • The prevalence of the significant number of manufacturers in the marketplace - easy to find an AC motor manufacturer. 
  • AC induction motors have no brushes reducing maintenance costs.

AC disadvantages:

  • An AC motor may need a digital encoder or tachometer to get torque from zero speed. Application dependent where required.
  • For low-speed operations with a variable speed drive, an inverter duty rated motor is required.
  • When replacing a DC Motor with an AC Motor, new wires and mounting are upfront costs which can be expensive.
  • Limited operation above motor base speed.

DC advantages:

  • Fairly reliable and widely used in industry.
  • Wide speed range available, often up to 5 times rated base speed.
  • Torque control available from Zero Speed without an encoder
  • Easy to repair and recondition
  • Typically outperforms AC motors for stall operations.

DC disadvantages:

  • 88-92% efficiency range for most motors 
  • To find a DC motor to replace an old one is difficult, and reconditioning can be costly.
  • DC motors are expensive to make and build.
  • Generally open to the environment, allowing dirt and dust in
  • High maintenance cost - Inspection and brush change at least every 3 months
  • Motors are getting older and harder to replace. Fewer manufacturers are making new DC motors for sale. 

How do you go about selecting the right electric motor for an industrial application? What would an expert recommend? 

The experts recommend an AC motor if you’re building something new. If the customer can bear the cost, for most applications in heavy industry - AC conversion from DC is the right choice long term. It comes down to the upfront cost of conversion versus the long-term benefit of offsetting your maintenance cost. The upfront cost would include mechanical conversions and new wiring where the long-term savings would be in reduced maintenance and power savings.

E Tech Group can help its customers analyze a DC to AC upgrade and the associated cost/benefit. If we’re proposing a DC to AC upgrade, we evaluate the existing motor mechanical system/load, its power/drive system, and control application requirements. Analysis is performed to find an AC motor and drive controller that matches the existing DC motor application requirements.

Steve Scheffler is a Senior Engineer & Aaron Muir is an Engineering Manager at E Technologies Group.  E Technologies Group is a certified member of the Control System Integrators Association (CSIA). For more information about E Technologies Group, visit its profile on the Industrial Automation Exchange

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