A tacho generator generates a feedback signal proportional to the speed of rotation, which can be used to provide closed loop control of a motors speed.
If such a ‘tacho’ is used and fed to an appropriate controller, then the controller can maintain motor speed very accurately under varying loads: by responding to the feedback signal, the controller automatically compensates for the varying load and maintains the motor at a constant speed.
Low speed performance
Motors have ‘stiction’ – or static friction. A moving surface, such as a motor bearing, has higher friction when it is stationary that when it is moving. The result if this is that it takes a significant voltage to start the motor, but it will then run at some speed. A tacho will automatically compensate for this effect by giving the motor more voltage to start it turning then automatically reducing the voltage to keep the motor just turning.
Without a tacho generator, a speed range of maybe 10:1 may be accomplished. With careful setting up, maybe even 15:1. But speed range is heavily load dependant.
With a tacho generator this speed range can be extended easily to 30:1 or even 50:1 speed range and this can be maintained under varying load.
Tacho generators and suitable controllers
True tacho generators are, however, expensive – so the facility is normally only found on expensive industrial controllers.
Our Pro-120 can be fitted with our tacho feedback board. In theory the same system could be used with our other controllers, but the controllers do not have the necessary connections brought out suitably.
This tacho feedback is done by means of a small, inexpensive additional board which plugs into the optional expansion connector on the Pro-120 or VTX controller.
It is also quite possible to use a much cheaper small permanent magnet motor as a tacho generator. See Speed stability and Tacho generator feedback in our FAQ sheet.