What are the limitations of A.C.Tachometer Generators?

In order to overcome some of the diffculties,A.C.Tachometer Generators are used.The Tachometer has Rotating Magnet which may be either Permanent Magnet or an Electromagnet.

The Coil is wound on the Stator and therefore the problems associated with Commutator (as in D.C.Tachometer) are absent.

The rotation of the Magnet causes an E.M.F to be induced in the Stator Coil.The Amplitude and the Frequency of this E.M.F. are both proportional to the speed of Rotation.Thus either Amplitude or Frequency of induced voltage may be used as a measured of Rotational Speed.

The Output Voltage of A.C.Tachometer Generators is rectified and is measured with a Permanent Magnet Moving Coil Instrument.(PMMCI)

LIMITATIONS:-

01)The difficulty with this system is that at low speed the Frequency of output voltage is low and hence it is very difficult to smooth out the ripples in the output voltage wave-shape and hence A.C.Tachometer Generators are designed to have a large number of Poles,so that the Frequency of output voltage is high,even at low speed.

02)High Speed is also present a problem.At high speeds,the Frequency increases and therefore,the Impedance of the Coil of TachoGeneretors increases. If good linearity is to be maintained in the input Impedance of the display device must be considerably larger than the Impedance of the Coil.

Drag Cup Rotor A.C.Tachogenerators:-

The Tachogenerators consists of a Stator and a rotor.The Stator has two windings mounted at 90 degree to each other.The two Stator windings are,
01)Exitation Winding
02)Sensing Winding

Advantages of Drag Cup Rotor A.C.Tachogenerators:-

01)A Linear Relationship between the output voltage and speed is obtained if the carrier frequency is 5 to 10 times larger than the frequency of the speed signal.For this purpose of excitation winding is supplied with a frequency of 400 hertz.

02)The Drag Cup type tachogenerators are rugged and in-expensive.Also,they requires little maintenance.

03)In some situations these Tachogenerators are very useful since they give a ripple free output.

Disadvantages:-

01)These Tachogenerators are hard to calibrate.

02)The output voltage is proportional to the product of speed and input voltage.