The main difference between transformer and induction motor is

 Q251. The torque of an induction motor is maximum when the slip is equal to?

A) 1
B) R₂/X₂
C) X₂/R₂
D) 0
Answer: B
Explanation: From $T = \frac{sE_2^2R_2}{R_2^2+(sX_2)^2}$, torque is maximum when $R_2 = sX_2 \Rightarrow s = R_2/X_2$.

---

Q252. At starting, the frequency of rotor current in a 50 Hz motor is:
A) 0 Hz
B) 50 Hz
C) 25 Hz
D) 5 Hz
Answer: B
Explanation: At standstill, slip = 1 ⇒ $f₂ = s f = 1×50 = 50 Hz$.

---

Q253. The efficiency of an induction motor increases with:
A) Load
B) Supply voltage
C) Rotor resistance
D) Frequency
Answer: A
Explanation: Efficiency increases as copper and core losses become smaller portion of total power at higher loads.

---

Q254. The maximum torque of an induction motor is:
A) Proportional to rotor resistance
B) Independent of rotor resistance
C) Inversely proportional to rotor resistance
D) Zero
Answer: B
Explanation: $T_{max} \propto V^2 / (2X_2)$, independent of R₂.

---

Q255. The air gap power equals rotor copper loss plus:
A) Rotor core loss
B) Mechanical power
C) Friction loss
D) None
Answer: B
Explanation: $P_{ag} = P_{rotor\_cu} + P_{mech}$.

---

Q256. The torque developed by an induction motor is zero when:
A) Rotor speed = synchronous speed
B) Rotor speed = 0
C) Slip = 1
D) Slip = 0.5
Answer: A
Explanation: No relative speed ⇒ no emf ⇒ no current ⇒ zero torque.

---

Q257. When the supply frequency is doubled, synchronous speed:
A) Halves
B) Doubles
C) Triples
D) Remains same
Answer: B
Explanation: $N_s = 120f/P$, directly proportional to frequency.

---

Q258. The main function of the slip rings in an induction motor is to:
A) Supply power to rotor
B) Insert external resistance
C) Reduce core losses
D) Supply excitation
Answer: B
Explanation: Used to insert external resistance in rotor circuit during starting.

---

Q259. The main advantage of a squirrel-cage induction motor is:
A) High starting torque
B) Low cost and ruggedness
C) Speed control
D) High efficiency only
Answer: B
Explanation: Simple, rugged, low maintenance design.

---

Q260. The torque developed in an induction motor is proportional to:
A) Square of rotor current
B) Supply voltage
C) Rotor copper loss
D) Air gap power
Answer: D
Explanation: Torque ∝ air-gap power / synchronous speed.

---

Q261. The direction of rotation of an induction motor can be reversed by:
A) Changing rotor leads
B) Interchanging any two stator leads
C) Reversing supply voltage polarity
D) Changing number of poles
Answer: B
Explanation: Reversing any two phases changes direction of rotating field.

---

Q262. The induction motor is sometimes called a:
A) Constant speed motor
B) Variable speed motor
C) Synchronous motor
D) DC motor
Answer: A
Explanation: Slip is small, so speed remains nearly constant.

---

Q263. The frequency of the rotor emf depends on:
A) Rotor speed
B) Supply frequency and slip
C) Supply voltage
D) Load
Answer: B
Explanation: $f_2 = s f$.

---

Q264. For maximum starting torque, the value of rotor resistance should be:
A) Equal to standstill reactance
B) Less than reactance
C) Greater than reactance
D) Zero
Answer: A
Explanation: $R_2 = X_2$ for max torque at starting (s=1).

---

Q265. The power factor of induction motor is:
A) Leading
B) Lagging
C) Unity
D) Zero
Answer: B
Explanation: Inductive nature causes lagging PF.

---

Q266. The slip of an induction motor at full-load is about:
A) 1–2%
B) 3–5%
C) 10–15%
D) 20–30%
Answer: B
Explanation: For standard 3-phase induction motors, s ≈ 0.03–0.05.

---

Q267. If load is increased, slip will:
A) Increase
B) Decrease
C) Remain constant
D) Become zero
Answer: A
Explanation: To develop more torque, rotor speed decreases → slip increases.

---

Q268. The stator current at no-load is about:
A) 20–40% of full-load current
B) 50–60%
C) 80–90%
D) 100%
Answer: A
Explanation: Magnetizing current and small losses cause 20–40% current at no-load.

---

Q269. When slip = 1, the motor:
A) Runs normally
B) Is at standstill
C) Runs above synchronous speed
D) None
Answer: B
Explanation: s = 1 ⇒ standstill condition.

---

Q270. When slip = 0, the motor:
A) Runs at synchronous speed
B) Stands still
C) Delivers maximum torque
D) Acts as generator
Answer: A
Explanation: s = 0 ⇒ N = Ns.

---

Q271. The torque under running condition is maximum when:
A) R₂/s = X₂
B) R₂ = sX₂
C) R₂ = 0
D) X₂ = 0
Answer: A
Explanation: Condition for maximum torque derived from torque-slip equation.

---

Q272. The speed regulation of induction motor is:
A) Very poor
B) Very good
C) Infinite
D) None
Answer: B
Explanation: Small speed variation with load → good regulation.

---

Q273. If supply voltage is reduced by 10%, torque will reduce by:
A) 10%
B) 19%
C) 21%
D) 25%
Answer: B
Explanation: $T ∝ V^2$ → (0.9)² = 0.81 ⇒ 19% reduction.

---

Q274. A 6-pole motor running on 50 Hz supply has synchronous speed:
A) 1500 rpm
B) 1000 rpm
C) 1200 rpm
D) 3000 rpm
Answer: A
Explanation: $N_s = 120f/P = 120×50/6 = 1000 rpm$ → correction: 1000 rpm ✅
(Corrected Answer: B)

---

Q275. The slip speed of a 4-pole, 50 Hz induction motor running at 1440 rpm is:
A) 40 rpm
B) 60 rpm
C) 80 rpm
D) 100 rpm
Answer: B
Explanation: $N_s = 1500$, so slip speed = 1500 – 1440 = 60 rpm.

---

Q276. When running light, the current drawn by an induction motor is mainly:
A) Reactive
B) Active
C) Resistive
D) Capacitive
Answer: A
Explanation: Mainly magnetizing current, lagging in nature.

---

Q277. The main disadvantage of a slip-ring motor is:
A) Costly and maintenance
B) Low starting torque
C) Poor efficiency
D) High speed
Answer: A
Explanation: Slip rings and brushes require maintenance.

---

Q278. The speed of an induction motor can be controlled by:
A) V/f control
B) Changing supply voltage
C) Rotor resistance control
D) All of these
Answer: D
Explanation: All methods used in speed control.

---

Q279. The slip frequency of a motor running at 960 rpm on 50 Hz, 6-pole supply is:
A) 2 Hz
B) 3 Hz
C) 4 Hz
D) 5 Hz
Answer: B
Explanation: $N_s=1000$, slip=40/1000=0.04 → $f₂=0.04×50=2 Hz$.

---

Q280. The supply current of an induction motor is minimum at:
A) No-load
B) Half-load
C) Full-load
D) Overload
Answer: A
Explanation: Only magnetizing current flows at no-load.

---

Q281. Crawling in induction motors occurs due to:
A) Harmonics
B) Saturation
C) Voltage unbalance
D) Improper supply frequency
Answer: A
Explanation: 7th harmonic field causes crawling at 1/7th of Ns.

---

Q282. Cogging occurs in induction motors due to:
A) Improper slot combination
B) Overloading
C) Low voltage
D) High frequency
Answer: A
Explanation: Magnetic locking between stator and rotor teeth.

---

Q283. Slip-ring motors are used when:
A) High starting torque needed
B) Low cost preferred
C) Constant speed needed
D) No speed control required
Answer: A
Explanation: External resistance allows high starting torque.

---

Q284. The rotor copper loss at full-load equals:
A) s × Air-gap power
B) (1–s) × Air-gap power
C) Total input
D) Output
Answer: A
Explanation: Rotor Cu loss = s × P₂.

---

Q285. The rotor mechanical power developed is:
A) (1–s) × Air-gap power
B) s × Air-gap power
C) Air-gap power
D) Input power
Answer: A
Explanation: $P_{mech} = (1–s)P_{ag}$.

---

Q286. The core loss in induction motor occurs mainly in:
A) Stator
B) Rotor
C) Both
D) Bearings
Answer: A
Explanation: Alternating flux induces losses mainly in stator core.

---

Q287. Induction motor operates at lagging power factor because:
A) It takes magnetizing current
B) Rotor resistance is high
C) Slip is small
D) Load is light
Answer: A
Explanation: Magnetizing current lags supply voltage.

---

Q288. In deep-bar rotor, starting torque is:
A) Improved
B) Reduced
C) Constant
D) Zero
Answer: A
Explanation: Skin effect increases effective resistance at start → improved torque.

---

Q289. Double-cage rotor motors are preferred for:
A) High starting torque and good efficiency
B) Low cost
C) Low torque
D) Constant torque
Answer: A
Explanation: Outer cage gives high R₂ (start), inner cage gives low R₂ (run).

---

Q290. The slip for maximum torque in double-cage rotor is:
A) High
B) Low
C) Medium
D) Zero
Answer: A
Explanation: High resistance outer cage causes higher slip for max torque.

---

Q291. The equivalent circuit of induction motor is similar to:
A) Transformer
B) Alternator
C) DC generator
D) Synchronous motor
Answer: A
Explanation: Energy transfer by induction → transformer analogy.

---

Q292. The main difference between transformer and induction motor is:
A) One has rotating part
B) Frequency difference in rotor
C) Slip present
D) All of these
Answer: D
Explanation: All differences are true.

---

Q293. The current in rotor bars of squirrel-cage motor is produced by:
A) Induction
B) Direct connection
C) Slip rings
D) Brushes
Answer: A
Explanation: Induced emf causes rotor current.

---

Q294. The type of rotor most commonly used in 3-phase induction motor is:
A) Squirrel cage
B) Wound rotor
C) Salient pole
D) Cylindrical rotor
Answer: A
Explanation: Simple and maintenance-free.

---

Q295. When load increases, the speed of induction motor:
A) Increases
B) Decreases
C) Remains same
D) First increases then decreases
Answer: B
Explanation: More torque required → higher slip → speed drops slightly.

---

Q296. The induction motor equivalent circuit neglects:
A) Core losses
B) Stator resistance
C) Rotor leakage reactance
D) Magnetizing branch
Answer: A
Explanation: Core loss often neglected for simplicity.

---

Q297. The stator copper loss is given by:
A) I₁²R₁
B) I₂²R₂
C) (1–s)P₂
D) sP₂
Answer: A
Explanation: Power loss due to stator current.

---

Q298. The main reason for heating in induction motor is:
A) Copper and iron losses
B) Mechanical friction
C) Windage
D) Overload
Answer: A
Explanation: Both electrical losses cause heat generation.

---

Q299. The slip in a 3-phase induction motor is always:
A) Less than 1
B) Equal to 1
C) Greater than 1
D) Zero
Answer: A
Explanation: s between 0 (sync) and 1 (standstill).

---

Q300. The speed of rotating magnetic field in stator is:
A) Constant
B) Varies with load
C) Proportional to slip
D) Depends on voltage
Answer: A
Explanation: Ns depends only on supply frequency and poles, hence constant.