MCQs on Electrical Machines

01․ The essential condition for parallel operation of two D.C generators is that they have

The following conditions must be satisfied for the parallel operation of DC generator. They are, 1. The terminal voltage of all generators must be same. 2. The polarities of all generators must be same. 3. The prime movers must have the same and stable characteristics from the rotation point of view. 4. The percent voltage drop with respect to the load variation must be same for all generators at all time.

02․ Which of the following load normally needs starting torque more than rated torque?

The conveyor belts are used to transfer a set of things from one place another place. During running condition, we can’t place any of the materials. So, before starting the conveyor belt, we need to place everything on the conveyor belt. Hence, we need more starting torque to run that load. Basically, in the conveyor belts, three phase slip ring induction motors (or) synchronous motors are used to achieve the high starting torque.

03․ The working principle of transformer depends on

The transformer is working under the mutual inductance principle. This principle is derived from the Faradays law of electromagnetic induction. Hence, it depends on that principle.

04․ The power developed by salient pole synchronous machine is/are

The power developed P = EV/X_d * sinδ + V²/2 * (X_d-X_q)/(X_d*X_q) *sin2δ Where E = Excitation voltage V = Terminal voltage X_d = Direct axis reactance, X_q = Quadrature axis reactance δ = load angle Power due to excitation = EV/X_d * sinδ Power due to saliency or reluctance = V²/2 * (X_d-X_q)/(X_d*X_q) *sin2δ Therefore, the power developed by salient pole synchronous machine contains two components.

05․ In salient pole synchronous machine, if direct axis reactance and quadrature axis reactance is equal, then power developed due to reluctance is

The power developed in salient pole synchronous machine P = EV/X_d * sinδ + V²/2 * (X_d - X_q)/(X_d*X_q) *sin2δ Where E = Excitation voltage V = Terminal voltage X_d = Direct axis reactance, X_q = Quadrature axis reactance δ = load angle Power developed due to reluctance = V²/2 * (X_d - X_q)/(X_d*X_q) *sin2δ Therefore, when direct axis reactance X_d is equal to quadrature axis reactance X_q, the power developed due to reluctance is zero.

06․ In salient pole synchronous machine, the reluctance power varies as

The power developed in salient pole synchronous machine P = EV/X_d * sinδ + V²/2 * (X_d - X_q)/(X_d*X_q) *sin2δ Where E = Excitation value V = Terminal voltage X_d = Direct axis reactance, X_q = Quadrature axis reactance δ = load angle Power due to reluctance = V²/2 * (X_d - X_q)/(X_d*X_q) *sin2δ Therefore, in salient pole synchronous machine, the reluctance power varies as sin2δ.

07․ In salient pole synchronous machine, maximum power occurs at a load angle of

The power developed in salient pole synchronous machine P = EV/Xd * sinδ + V²/2 * (Xd-Xq)/(Xd*Xq) *sin2δ Excitation power varies as sinδ and power due to reluctance varies as sin2δ. Maximum excitation power occurs at 90° and maximum power due to reluctance occurs at 45°. Therefore, the maximum power developed in salient pole synchronous machine occurs at load angle less than 90°.

08․ The maximum power developed in cylindrical synchronous machine is occurs at a load angle of

The power developed in cylindrical synchronous machine is Maximum power developed P_max = EV/X_s Where E = Excitation voltage V = Terminal voltage X_s = Synchronous reactance δ = load angle Therefore, maximum power developed in cylindrical synchronous machine is at a load angle of 90°.

09․ Maximum power developed in a cylindrical synchronous machine depends on

The power developed in cylindrical synchronous machine is P = EV/X_s * sin δ Where E = Excitation voltage V = Terminal voltage X_s = Synchronous reactance δ = load angle Maximum power developed P_max = EV/X_s Maximum power developed when sinδ=1, i.e. when δ = 90°, maximum power developed in cylindrical synchronous machine. Therefore, maximum power developed only depends on excitation and synchronous reactance.

10․ Two generators operating in parallel with rating 50MW and 100MW respectively. Both their respective governor settings are identical as 4%, consider same no load frequency. How will the machine share a common load of 100 MW

System frequency f = f₀ - (f₀ - f₁)/ P_rated *P₁ Where, f₀ is rated power frequency. System frequency f = 50 - 0.04*50/50* P₁ -------------- equation number 1 and also System frequency f = 50 - 0.04*50/100* P₂ -------------- equation number 2 Given that P₁ + P₂ = 100 ----------------equation number 3 By solving the above equations P₁ = 33.33 MW P₂ = 66.67 MW