Understanding Ke Ratings

It is important to check the Ke (backdrive voltages) from PMBL servo motors to fully understand the health of the motor.  A technician also needs a reference to know if the Ke voltages that they are reading are within specification for that motor model. Ke values that are out of range often indicate an issue with the windings or magnets.  Unfortunately, not all motor brands show a Ke value in their technical documents, and when they do, it often is not specified as to how it was recorded. Here are the different ways the Ke value can be listed:

• Volts RMS: Most commonly listed in technical manuals and what we recommend in testing. 
• Volts Peak (0 to peak voltage): Used by some brands like Allen-Bradley. 
• Volts Peak-to-Peak: Uncommon for higher voltages like motor Ke.
• L-L (Line-to-Line): The voltage is measured between two phases. 
• L-N (Line-to-Neutral): The voltage is measured from phase to neutral. 
• The motor speed at which Ke is rated is also important but not always listed. 1,000 RPM is the standard.

In our test documents, we define the Ke as Volts RMS, L-L, at 1,000 RPM. Most basic voltmeters will provide an RMS readout, and may or may not have the ability to switch between other modes like Peak or Peak to Peak. Below are a few handy conversions for working with different Ke ratings. You can also find online calculators that convert for you: https://www.allaboutcircuits.com/tools/rms-voltage-calculator/ 

Volts Peak to RMS: Divide by the square root of two.

Volts RMS to Peak: Multiply by the square root of two. 

L-L to L-N: Divide the L-L voltage by the square root of three.

L-N to L-L: Multiply the L-N voltage by the square root of three.
 

Rating Conversion Example

Below is an Allen-Bradley motor nameplate. Notice how the manufacturer specifies the Ke value in peak (0 to peak voltage), L-L, kRPM (1,000 RPM).  If measuring the peak voltage with a standard voltmeter that only shows RMS, then you need to convert it. 

• 104 V Peak ÷ √2 = 73.5 V RMS 

Examples of the rating details. 

In the examples below we will compare two possible scenarios; 

A) In the first image listed below we are using a Baldor BSM manual. The manufacturer is listing the ratings in two different values Volts Peak (Vpk/krpm) and also Volts RMS (Vrms/krpm). However it is still missing details describing how to measure those values, if using Line-to-Line or Line-to-Neutral. We normally recommend using Line-to-Line at 1000rpm and compare your results with the technical data. 

B) In the second image we are referring to a Control Techniques Unimotor, not much details are present aside from just a voltage value at 1000rpm but we don't know if it is Volts RMS, Volts Peak or Volts Peak-to-Peak. In those cases we recommend using as reference RMS, L-L, kRPM, but it will be required to test the motor and confirm.

Best Practices

• Always assume nothing is specified unless clearly stated. 
• Use RMS values for consistency. 
• Measure Line-to-Line (L-L) unless instructed otherwise. 
• Convert values when needed to match the manufacturer's method.