Electrical Machines Practice Test 1

1. The effect of poly phase currents in poly phase winding can be compared to _______________

Mechanical rotation of permanent magnets at synchronous speed

Mechanical rotation of either permanent magnets or DC excited field poles, at synchronous speed

Mechanical rotation of DC excited field poles at synchronous speed

None of the mentioned

2. If B_p= peak value of sinusoidal flux density wave, L = armature core length, D = armature diameter, P = number of poles, then which of the following statements are correct?

(i) flux per pole, Φ=2/PB_pLD
(ii) Average flux density, B_av=2/πB_p
(iii) Φ=4/PB_pLD
(iv) B_av=3/πB_p
(v) B_av=PΦ/πDL
(vi) B_av=PΦ/πrL

(ii), (iii), (vi)

(iii), (iv), (v)

(ii), (iii), (v)

(i), (ii), (v)

3. A parallel plate capacitor has a capacitance of 10μF. If the linear dimensions of the plates are doubled and distance between them is also doubled, the new value of capacitance would be __________

10μF

20μF

40μF

5μF

4. Which component of torque in the following equation is called the electromagnetic torque of electromagnetic energy conversion device?

T_e=1/2i_s²dL_s/dθ_r+1/2i_r²dL_r/dθ_r+i_si_rdM_s_r/dθ_r

i_si_rdM_s_r/dθ_r

1/2i_s²dL_s/dθ_r

all of the mentioned

1/2i_r²dL_r/dθ_r

5. The variation of reluctance Rl with space angle θ_r depends on the shape of __________

stator or rotor poles

rotor poles

stator poles

both stator and rotor poles

6. In a doubly excited magnetic system with salient pole type stator and rotor, if the rotor is not allowed to move, then the equation for magnetic field stored energy in establishing the currents from zero to i_s and i_r is __________

W_fld=1/2 i_s²L_s+M_rs i_s i_r

W_fld=1/2 i_r²L_r+M_rs i_s i_r

W_fld=1/2 i_s²L_s+1/2 i_r²L_r

W_fld=1/2 i_s²L_s+1/2 i_r²L_r+M_rs i_s i_r

7. The self and mutual inductances of a doubly excited magnetic system are L_s=0.6+0.20cos2θ_r H and L_r=0.75+0.30cos2θ_r H and M_sr=0.8cosθ_r H. For a stationary rotor at an angular position of θ_r=60°,what is the magnitude of torque when the currents i_s=20A DC and i_r=10A DC?

-233.88N-m

+467.76N-m

+233.88N-m

-467.76N-m

8. In a doubly excited magnetic system of salient pole type stator and rotor, the reluctance torque is present only when _____________

stator current is acting alone

rotor current is acting alone

both stator and rotor currents are acting

any of the stator or rotor currents acting alone

9. For a p-pole machine, which of the following statements are correct regarding the rotating field speed?

f/(p/2) revolutions in one second

The rotating field speed is 1/(p/2) revolutions in one cycle

f/(p/2) revolutions in f cycles

Any of the mentioned

10. For a linear electromagnetic circuit, which of the following statement is true?

Co-energy is zero

Field energy is equal to the Co-energy

Field energy is greater than the Co-energy

Field energy is less than the Co-energy