Anna University Chennai 2005 B.E Electrical and Electronics Engineering SOLID STATE DRIVES EE-432 - Question Paper

is 0.5V-sec/radians. For a rated load torque of 15 N-m at 100 rpm and for continuous ripple free
current, determine (1) the firing angle delay of the armature converter (2) input power factor of
the armature converter. [MARKS 8]

(OR)

(b) (i) Suggest a SCR based power circuit, from a dc supply to control the speed of a separately
excited dc motor in both directions and discuss its working. [MARKS 8]

(ii) A 230 volts 960 rpm 200 Amps separately excited dc motor has an armature resistance of
0.02 ohms. The motor is fed from a dc source of 230 volts through a chopper. Assuming
continuous conduction (1) compute the duty ratio of chopper for motoring operation at rated
torque and 350 rpm. (2) If maximum duty ratio of chopper is limited to 0.95 and maximum
permissible motor speed obtainable without field weakening. [MARKS 4+4]


13. (a) (i) Using a diagram and torque speed curve, discuss the stator voltage control scheme for the
speed control of a 3 phase induction motor. [MARKS 6]

(ii) Bring out the limitations of the above speed control scheme. [MARKS 3]

(iii) discuss how voltage and frequency are varied in voltage source inverter fed induction motor
drives. [MARKS 7]

(OR)

(b) (i) Induction motor speed control with constant supply voltage and decreased supply frequency is
rarely used in pratice. -Justify [MARKS 5]

(ii) define the stator frequency control scheme for the speed control of 3 phase induction
motors. [MARKS 6]

(iii) Why in this method of speed control an induction motor is stated to be working in the field
weakening mode. [MARKS 5]


14. (a) (i) discuss using a power circuit how static rotor resistance control serves to vary the speed of a
3 phase slip ring induction motor. [MARKS 7]

(ii) Bring out its advantages and limitations. [MARKS 3]

(iii) Derive the expression for torque of the induction motor, when it is working in above scheme.
[MARKS 6]

(OR)

(b) (i) Draw and discuss the slip power recovery scheme applicable for 3 phase slipring induction
motors. [MARKS 8]

(ii) Show how the above shceme can offer both subsynchronous and super synchronous ranges
of speed. [MARKS 4]

(iii) Compare slip power recovery shceme with rotor resistance control. (4)


15. (a) (i) define using a circuit the self contrlled mode of operation of a synchronous motor. [MARKS 10]

(ii) Bring out the need and advantages of closed loop control in a synchronous motor drive. [MARKS 6]

(OR)

(b) (i) Suggest a scheme for speed control of synchronous motors using current source inverters and
discuss its working. [MARKS 10]

(ii) explain how a synchronous motor operating in an overexcited state does not require a
commutation circuit and serves to improve the input power factor. [MARKS 6]

Earning:   Approval pending.