Two Minute Tips  

MCA™ Improves DC Motor Testing Capabilities

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

The ALL-TEST PRO 7™ brings predictive maintenance to DC motor testing. Preventive Maintenance Tasks for DC motors such as commuta- tor & brush inspections, lubrication, are very important for their long-term successful operation. However, these inspections fail to determine the condition of the electrical winding or insulation. Adding periodic electrical tests, such as measurements of the winding resistance and Insulation Resistance to Ground (IRG) provide some insight to possible connection issues & weaknesses in ground wall insulation, but still fail to determine the overall condition of the equipments insulation condition.

By adding MCA™ readings to DC motor testing provides early indication of developing problems within the motors electrical system beyond those detected using a megohm and ohmmeter. MCA tests can be performed quickly from the drive and can confirm or eliminate faults in DC machines.

MCA™ | DC machines | ALL-TEST PRO 7™


Several key points quickly determine the condition of DC Machines



  1. Take Series winding and armature windings readings together
  2. Test motors and generators the same
  3. I/F reading outside of the range of -15 to -50 indicates a winding fault
  4. An increase in temperature corrected winding resistance, accompanied by changes in impedance indicates loose connec- tions
  5. A decrease in temperature corrected resistance accompanied by changes in impedance, inductance, phase angle & current frequency response (I/F) indicates developing winding shorts
  6. Deviations of phase angle or I/F of more than 2 points between like motors indicates the need for a MCA complete analysis
  7. Changes in MCA reading in the armature circuit between test intervals prompts a bar to bar armature test
  8. Changes in MCA readings in the armature circuit taken back to back indicates carbon build up in the armature

By following these simple guidelines using the AT7P™ provides early fault detection before the DC machine fails during operation. Recommended testing intervals should be at least those shown in Table 1.


Table 1: DC Motor Test Frequency

DC Motor Test Frequency

Once a developing fault is detected, it is recommended to reduce the time intervals between tests until the machine can be removed for repair. A complete armature test is recommended in conjunction with preventive maintenance tasks.


Preventive electrical testing of direct current machines is much easier using the DC mode function of the AT7P™. Step by step detailed easy to follow procedures are provided on the large backlit LCD

display to make the testing quick and easy to perform from the motor drive in less than 5 minutes. Addition- al tests and features are available for troubleshooting at the motor to quickly pinpoint the source of the problem. MCA™ testing dramatically improves DC machine testing by saving time and providing more details as compared to traditional techniques and methods.


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Fanklan Steven
3 years ago

How is resistance calculated

Last edited 3 years ago by Fanklan Steven
3 years ago
Reply to  Fanklan Steven

Hello Fanklan,
Resistance is calculated using Ohm’s Law. In order for you to calculate resistance you would need to know the voltage applied to the circuit and the current the circuit is drawing. You will then divide the voltage by the current and that will give you the resistance of the circuit.
Please feel free to contact me at if you have any other questions.

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.