DC tachogenerator voltage drops to zero in CCW rotation

[dimitrive]

Hi
I need a help. I do have a DC TachoGenerator whereby i can measure the DC voltage if the shaft rotates.
The problem is:
When I rotate the shaft at CW, I got a fixed plus voltage.But when i rotate to CCW, the voltage is minus but slowly later it decrease to zero. (even the shaft still rotating)
Why?

 

[itsjustme]

well its either the one or the other, the generator or the voltage indicator, thry the indicator on a electric motor.

 

[berkeman]

Do you know how the "tachogenerator" is constructed? Have you looked at its output with an oscilloscope?