Consider a separately excited DC motor which is coupled to a shunt dc generator (under no load condition) and acts as a prime mover. If we set the speed of the DC motor to some certain value (for example 1500 rpm) and increase the field current of the generator, there is a slight reduction in the DC motor's speed. What's the reason for this? Is this related to the armature reaction?

I couldn't find the reason in the textbooks or on the Internet.