AC Vs. DC Motors

Mark Koch | Technical Support Manager, ALL-TEST Pro, LLC

For those who have experience working with motors, you are likely quite familiar with the difference between AC and DC motors. If you are new to electrical motors or would like a refresher, we will explain. AC (alternating current) and DC (direct current) motors are fundamentally different. Each is comprised of different parts and components, and both produce power through directed electron flow.

The Difference Between DC and AC Motors

At the simplest level, the difference between DC and AC motors is they use different flows of electrons for sending power across lines. We will break down some of the primary differences:

  • DC motors: In a DC motor, electrons are pushed forward in a single direction. These motors are capable of producing high output and are an excellent source for conversion into AC power. DC power is more efficiently stored in batteries and is often used for storing energy.
  • AC motors: AC motors produce alternating current, which means electrons can move forward or backward. AC is the safer of the two for transmitting power over longer distances, as it retains more power when converted through transformers and distributed through a network.

Testing AC and DC Motors

Even with the best maintenance practices, the components in electrical motors have lifespans and will eventually fail. Testing AC and DC motors is a crucial step in ongoing maintenance to ensure their continued operation and optimal output. Even if the motor appears to be working well, an undetected fault could lead to component or system failure if left unaddressed. Typical motor tests include measuring:

  • Shaft and housing vibration
  • Temperatures of components
  • Torque and winding conditions
  • Component position and speed
  • Current and voltage generation

AC Versus DC Motor Tests

While tests for these motors are essentially looking for the same readings, the methods for testing will vary.

Using modern equipment, you can test motors while in an energized or deenergized state. These each have their advantages:

  • Energized testing: Energized testing occurs when equipment is under load to simulate normal operating conditions. This method helps uncover undiscovered or intermittent flaws by generating the heat and vibration standard to motor operation. Energized testing monitors all component performance, checking for wear and abnormal conditions that may require attention.
  • Deenergized testing: Deenergized testing runs diagnostics while machines are powered down. You can use deenergized testing equipment to test a new motor or system before powering on, or as an integral part of your preventive maintenance program. Our advanced testing can perform MCA™ (Motor Circuit Analysis), running complete checks on the entire electrical system.

Testing AC and DC Motors

A complete diagnostic check of your AC or DC motor typically involves multiple tests. Regardless of the type of test performed, always be sure to exercise safety precautions whenever working around electrical equipment. In most cases, testing AC and DC motors includes checking:

  • Current: Measure pull-in current by the shape of the arc and your peak amplitude.
  • Vibration: Look for any excessive vibration from your electrical motor components.
  • Temperature: Take readings of component temperature to check for abnormalities.
  • Alignment: If you have a rotating motor, check the shaft to ensure proper alignment.
  • Windings: Check the condition of your windings to locate damage and electrical shorts.
  • CDT: Track your CDT, or Coast Down Time, to monitor motor performance and degradation.

Advanced Diagnostic Equipment for Testing AC and DC Motors

Testing results will only ever be as good as the equipment used to read them. Visit ALL-TEST Pro for an incredible range of testing tools you can fit in the palm of your hand. We offer an extensive range of equipment for performing energized and deenergized testing. Our products deliver fast results you can rely on for testing the complex electrical systems found in the auto, steel, energy and utility sectors.

For information about purchasing ALL-TEST Pro testing equipment, please visit our online store.

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About the Author

Mark Koch Technical Support Manager, ALL-TEST Pro, LLC

Mark Koch is a member of the Technical Support team at ALL-TEST Pro, LLC (ATP), where he has provided professional support worldwide for users of ATP equipment since 2014. He has 17 years of electrical and reliability maintenance experience. In 2001, he started his career as a construction electrician. During this time, he worked his way up from shop floor apprentice to a foreman in charge of starting and completing remodel and new construction projects. In 2010, he began working at the Metropolitan Sewer District (MSD) of Greater Cincinnati, Ohio, USA, where he learned electrical maintenance best practice and helped implement a predictive maintenance program (PdM). During his service at MSD, his team was the recipient of both the Uptime Magazine’s Best Emerging Maintenance Reliability Program Award and the Uptime Magazine’s Best Asset Condition Management Program Award. At MSD, he learned and provided routine PdM services such as thermography, vibration, lubrication, visual inspection, motor testing, and ultra-sonics.


Education and Certifications:

2001-2006- International Brotherhood of Electrical Workers (IBEW) Apprenticeship Program and the National Electrical Contractors Association (NECA) Inside Wireman Certification.

2013- PdM Visual Testing Level 2 Certification & Level 1 Thermographer Certification; 2014- Vibration Analysis Level 1T Certification.