# Final Exam Machine 2010

Topics: Electric motor, Armature, Electric motors Pages: 6 (1060 words) Published: December 8, 2011
SULIT

UNIVERSITI TEKNOLOGI MALAYSIA PEPERIKSAAN AKHIR SEMESTER 1 SESI 2010/2011 KOD MATAPELAJARAN MATA PELAJARAN PENSYARAH : : : SEE 3433 MESIN ELEKTRIK DR. AWANG JUSOH EN. NIK DIN MUHAMAD DR. NIK RUMZI NIK IDRIS

KURSUS SEKSYEN MASA TARIKH

: : : :

SEE KL, P. PINANG, JB 2 JAM 30 MINIT

:

JAWAB EMPAT (4) SOALAN SAHAJA. SEMUA PENGIRAAN HENDAKLAH DITUNJUKKAN DENGAN JELAS.

KERTAS SOALAN INI TERDIRI DARIPADA 6 (ENAM) MUKA SURAT SAHAJA

SEE 3433 -2Question 1

(a)

With the help of a simple electromechanical system, explain briefly how energy conversion process occurs in the system. Assume the system is lossless. [6 marks]

(b)

In the electromagnetic relay system, the flux linkage (λ) and current (i) relationship is given by: i = λ3 + 3λ(1- x)2 ; valid for 0 < x < 20cm Find the expression of the force acting on the moving part as a function of λ when x = 3cm. [6 marks]

(c)

An elementary two-pole cylindrical rotating machine with a uniform air gap is shown in Figure Q1(c). The mutual inductance between the rotor and the stator is given by Lsr = 0.5 cos θ Henries

The windings are supplied with either of two sources: Source A: Source B: 3 A DC 6 A r.m.s f = 50 Hz

The rotor is locked at θ = 50°. What is the torque developed for the following source connections: (i) (ii) Source A to both windings Source A to the rotor and source B to the stator. Rotor axis

[6 marks] [7 marks]

θ = 50° (locked)

Stator axis

Figure Q1(c)

SEE 3433 -3Question 2 (a) Briefly explain what do you understand by an armature reaction in a DC machine. What effect does the armature reaction has on: (i) (ii) the flux per pole? generated back emf (Ea)?

(iii) commutation action?

Give two methods to reduce the effects of armature reaction [6 marks]

(b) A DC machine is to have the following rating: 240V, 1500 rpm, 4 poles. Coils to be used to construct the winding of the machine are rated at 5 V and 6 A. If the lap wind is to be used, calculate: (i) (ii) the number of coils needed The kW rating of the machine [6 marks]

(c) A 12kW, 120V, 1000 rpm DC machine operated as shunt self-excited generator has a magnetization characteristic as shown in Figure 2(c) in the Attachment Q2(c). The shunt field winding resistance Rfw and the adjustable field control resistance Rfc are 80Ω and 100Ω respectively. The rated field current is 1 A. Using Figure 2(c) in Attachment Q2(c) :

(i) (ii)

Determine the maximum possible value of the generated voltage Determine the value of Rfc to produce rated terminal voltage (no load connected)

(iii) Determine the Rfc that results in a critical field curcuit resistance (iv) If a rated load is connected to the terminal, what would be the terminal voltage if the no-load terminal voltage is adjusted to 90V ?

SEE 3433 -4Question 3 (a) (b) Explain 3 methods that can be used to control the speed of a DC motor A starting current in DC motor is normally very large and it is necessary to limit this starting current. Explain why the starting current is normally large and briefly describe a method used to limit this current.

(c)

A 10kW, 100V, 1000 rpm DC motor with an armature resistance of 0.1 ohm is connected to a 100V DC supply. (i) (ii) Determine the starting current if no starter is used in the armature circuit If the starting current of 250 A is desired, determine the value of the starting resistance. (iii) If the starting current is to be limited between 100% and 250% of the rated current, determine the value of the resistances required to be used for the starter box.

Question 4 (a) Draw the per-phase equivalent circuit of an induction motor recommended by the IEEE. Briefly explain the meaning of all the parameters in the circuit. [4 marks]

Show how an increase in rotor circuit resistance will increase the starting torque of an induction motor.

(b)

The parameters for a...