Page 326 - Electronic Mechanic - TP - Volume - 2

ELECTRONICS MECHANIC - CITS

1 Fluxcontrolmethod:This methodis used when speeds above normal speeds are required. In the above equation
(1) by decreasing the flux, the speed can be increased and vice versa. The flux of a dc motor can be changed by
changing Ish with the help of a shunt field rheostat. The current in shunt field rheostat is less. I2R loss is small, so
that rheostat is small in size. Armature or rheostat control method:- This method is used when speeds below theno-
loadspeedsarerequired.Inthismethodthevoltageacrossthearmature is varied by increasing a variable rheostat
in series with the armature circuit,asresistanceisincreased,potentialdifferenceacrossthearmatureisdecreased,
there by decreasing the armature speed. This method is expensive (Cu loss more) and unsuitable for rapidly
changing loads because for a given value of Rt, the speed will change with load.
2 Voltagecontrolmethod:-Thismethodisalsousedtocontrolthespeedsbelow theno-load speeds. In this method the
shunt field of the motor is connected across the fixed supply and then by supplying different voltage to the
armature different speeds are obtained. The control voltage is obtained from potential dividers, solid state
rectifier and Ward-Leonard system.
3 For Armature voltage control method:-
• The connections are made as shown in the circuit diagram.
• With the armature rheostat is CUTIN position, the field rheostat in CUTOUT position and the 3-point starter
handle at its initial position, the D.C. supply switch is closed.
• The 3-point starter handle is moved clock wise gradually to cut out the resistance in armature circuit so that
the motor starts and run sat some speed.
• The armature rheostat is brought to cut outposition so that rated voltage is applied across the armature.
• Thefieldcurrentisadjustedtoacertainvaluebyvaryingthefieldrheostat such that the motor runs at nearly rated
speed.
• The armature rheostat is CUTIN gradually so that the armature voltage is varied in steps and the
corresponding speeds are noted in the tabular column.
• Stepno.5isrepeateduntilthearmaturerheostatiscompletelyCUTIN.

• The field rheostat is brought back to CUTOUT position, the armature rheostat to CUTIN position and the
D.C. supply switch is opened.
Armature voltage control

Table
Field current =

Sl.No Armature voltage In Volts Speed In rpm

309
CITS : E & H - Electronics Mechanic - Exercise 218&219