Tacho of DC-drives

Do tacho's of dc-drives always reverse the output voltage when rotating in the reverse direction? This is because I wanted to reverse the rotation direction of a dc-drive. So I reverse the connection of the armature and the connection of the tacho. But the the drive immediately ran up to maximum speed althoug the setpoint was low.

Yes, a DC tach is simply a DC generator, if you reverse the direction the polarity will change.

More details, type of drive etc.. and maybe we can help you out.

The drive is a Siemens Simoreg 6RA211 two phase power supply, I only see one thyrisor bridge (4 thyristors), so I believe its 2-quadrant (one direction delivering power and braking). Looking at the drawing of the connections and the internal build up of the speed and current controller, it seems that the tacho input always has to be positive and the resulting reference should be negative if the motor has to run. If the resulting reference (main and additional) is positive I suppose the drive is stopped. This because the - of the speed opamp is connectied to the addition of refence and actual value and the + is connected to zero voltage.

some tachos fitted to d.c drives do not reverse the output when direction is reversed and are usually a.c rectified devices whichare a cheaper option than a d.c tacho technodrive motors for example used to have true d.c tachos as an option at greater cost

Thanks Mel, but these are standard Siemens DC-drives, the exact type I don't know. I only know they give 60V at 1000 rpm.

siemens also supply drives with tachos for single quadrant operation manufactured by thalheim
see link: http://www.transdrive.co.uk/dc_motor_selection.htm
siemens motors /options mounted equipement
this explains code no.s etc
hope this will be helpfull

tachs and DC drives

You could try a full wave bridge diode of the proper voltage/current range to keep the polarity the same at the drive input from the tach.
Cheap and dirty may work.

You may well have an AC tach. This would be particularly true if the motor nameplate says STAB SHUNT or COMP SHUNT making the motor basically a unidirectional motor.