Drive coupling Indicating needle Spring Drag cup Stator coil Permanent magnet rotor Tachometer generator Indicator Permanent magnet rotor Alternator Synchronous motor Permanent magnet Figure 10-55.  An electric tachometer system with synchronous motors and a drag cup indicator.  A. Pad type B. Screw type ENGINE RPM ROTOR RPM 10 15 20 25 30 5 0 25 30 5 10 40 20 15 35 E T Electrical connector Pole piece Gear wheel Tacho probe Axis of polarization Permanent magnet Sensing coils Pole pieces Figure 10-58.  A tacho probe has no moving parts. The rate of  magnetic flux field density change is directly related to engine speed.  Figure 10-57.  A helicopter tachometer with engine rpm, rotor rpm,  and slippage indications.  Figure 10-56.  Different types of tach generators.  The tachometer probe’s output signals need to be processed  in a remotely located module. They must also be amplified to  drive a servo motor type indicator in the cockpit. They may  also be used as input for an automatic power control system  or a flight data acquisition system.  [Figure 10-58] 10-32