Could somebody suggest me a method to measure the back emf of a PMDC motor with the specification of 12V,3.5A?

 

Drive it with a constant current source, and record its terminal voltage vs speed.

 

Could you please explain me in detail please?

My intention is to measure the speed which I did not explain earlier.Ok. To your reply which value constant current should I use ? My motor load may vary from 3.0 A to 3.5A.

Please suggest me.

 

Back EMF is a constant that is dependent upon the windings and the field strength. Typical EMF ratings are X volts/1000RPM. Two ways of testing exist. 1) use external drive to spin motor at 1000 RPM and measure voltage;; 2) apply voltage per specification and measure RPM under no-load conditions.
The applied voltage to maintain a fixed RPM will vary depending upon load (higher voltage to maintain speed under heavier load) due to the IR drop of voltage in the windings. Many velocity loop controllers that use armature voltage as a feedback have a special control called IR Comp to compensate for that drop.