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What are the disadvantage/s of single-phase induction motor with respect to three-phase induction motor?
an ununiform rotating magnetic field
Not-Self Started
Poor efficiency & power factor
All of these
All of these
Single-phase induction motors lack a rotating magnetic field in their primary state, producing a pulsating field instead ┬╖ Consequently, they suffer from no starting torque, lower efficiency, power factor, and reduced torque density compared to three-phase induction motors of the same rating.
Single-phase induction motors lack a rotating magnetic field in their primary state, producing a pulsating field instead ┬╖ Consequently, they suffer from no starting torque, lower efficiency, power factor, and reduced torque density compared to three-phase induction motors of the same rating.
TnetтАЛ=TfтАЛтИТTbтАЛ=0┬а(at┬аstandstill) тАФ Net starting torque of a single-phase induction motor
PoutтАЛ=VIcos╧ХтЛЕ╬╖ тАФ Output power relationship considering efficiency and power factor
According to Double Revolving Field Theory, a pulsating single-phase magnetic field can be resolved into two equal and oppositely rotating fields ┬╖ Since the forward and backward torques are equal and opposite at standstill (TfтАЛ=TbтАЛ), the net starting torque is zero, making the motor inherently non-self-starting ┬╖ Additionally, the backward field generates losses, which degrades efficiency and power factor.
Single-phase induction motors produce a pulsating magnetic field rather than a constant-amplitude rotating magnetic field.
They require auxiliary starting mechanisms (e.g., split-phase winding, capacitor-start, shaded-pole) to create a starting torque.
Dipping efficiency and lower power factor arise due to the presence of the backward rotating magnetic field component.
Cheaper to manufacture for low-power applications
Widely suitable for domestic and residential power supplies where 3-phase power is unavailable
Zero inherent starting torque (Not self-starting)
Lower efficiency and power factor compared to 3-phase motors
Non-uniform, pulsating magnetic field resulting in higher vibration and noise
Domestic ceiling fans and blowers
Refrigerators and air conditioners
Small power tools and washing machines
| Feature | Single-Phase Motor | Three-Phase Motor |
|---|---|---|
Rotating Magnetic Field | Non-uniform, Pulsating | Uniform, Constant Magnitude ($1.5 \Phi_m$) |
Self-Starting Capability | No (Requires auxiliary winding) | Yes (Inherently self-starting) |
Efficiency & Power Factor | Poor / Lower | High / Superior |
Cost & Size (Same Rating) | Larger frame, relatively lower rating | Compact, higher torque density |
Option A is correct because the stator produces a pulsating field which decomposes into non-uniform opposing fields.
Option B is correct because the net torque at standstill (s=1) is zero.
Option C is correct due to backward field losses reducing overall efficiency and power factor.
Hence, Option D is the comprehensive correct choice.
D is correct тАФ Single-phase induction motors suffer from non-self-starting capability, pulsating field, and inferior efficiency/power factor relative to 3-phase motors.
Remember that Cross-Field Theory and Double Revolving Field Theory are the two main methods used to analyze single-phase induction motor behavior for competitive exam questions.