Chuck Yung
EASA Senior Technical Support Specialist

How much no-load current should I expect when testing a motor? We would like to have a ratio of no-load amps / full-load amps, for quality control purposes. Many of us expect a motor to draw approximately one-third of rated current, when operating from rated voltage on our test panel. That is a good rule of thumb - most of the time. While there are lots of exceptions, most of them are predictable.

The intent of this article is to explain why those statements are valid and, in the process, to offer practical guidelines for assessing no-load current. Many of us apply these principles daily. 
Knowledge is power. We should, whenever possible, improve our knowledge by gathering facts: 

• Use the AC Motor Verification & Redesign Program to check densities before rewinding the motor. 
• Keep records of tests for comparison of identical machines. 
• Get information from the manufacturer to supplement your records.

Topics covered in this article include:

• Practical guidelines
• Effects of applied voltage
• Different designs affect rule
• Flux and air gap
• Number of poles
• Considering scale, manufacturing tolerances
• Exceptions to every rule