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Questions and Answers
What is the purpose of making the stator flux rotating type in a single-phase induction motor?
Why does a single-phase induction motor require some starting means?
What is the main problem with the pulsating flux produced by the main winding?
What happens if the rotor of a single-phase I.M is rotated in one direction by some mechanical means?
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What is the direction of rotation of the motor due to the forward rotating field?
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What is the percentage of positive slip generated by the forward rotating field?
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What is the purpose of the aluminum bars in the squirrel cage rotor?
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What is the reason for a single-phase induction motor not being self-starting?
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What is the formula to calculate the slip (Sp) in a single-phase induction motor?
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What is the percentage of negative slip generated by the reverse rotating field?
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What is the basis of the behavior of a single-phase induction motor?
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What is the characteristic of a single-phase induction motor when connected to a single-phase a.c. supply?
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What is the formula to calculate the negative slip (Sneg) in a single-phase induction motor?
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What is the theory used to describe the operation of a single-phase induction motor?
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What is the consequence of rotating the rotor of a single-phase I.M by some mechanical means?
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Why is this method of starting not convenient for large motors?
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What is the range of small positive slip that generates larger torque than a large negative slip?
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What is the purpose of the auxiliary winding in a single-phase IM?
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What happens to the auxiliary winding when the rotor reaches synchronous speed?
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According to the Double Revolving Field Theory, what is the magnitude of each rotating component?
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What is the direction of rotation of the forward component of the stator flux?
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What is the relationship between the synchronous speed and the supply frequency?
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What is the resultant of the two components Φf and Φb at start?
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Why are single-phase induction motors not self-starting according to double revolving field theory?
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What is the effect of the rotating field on the rotor, according to the Double Revolving Field Theory?
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What is the resultant of the two rotating components of the stator flux at any instant?
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What happens to the two components Φf and Φb after 90°?
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What is the direction of the torque produced by the interaction of the rotor flux with the forward component Φf?
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What is the instantaneous value of the stator flux at θ = 90°?
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What is the effect of the rotor flux interacting with the backward component Φb?
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What is the net torque experienced by the rotor at starting?
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What can be used to show the two oppositely directed torques and the resultant torque?
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According to the double-field revolving theory, how many constant-amplitude fluxes are produced in a single-phase induction motor?
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What is the relationship between the equivalent circuits of the two motors in a single-phase induction motor?
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What is the input power equation for a single-phase induction motor?
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What is the significance of the double-field revolving theory in understanding single-phase induction motors?
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What is the relationship between the rotor values in the equivalent circuit of a single-phase induction motor?
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What is the condition for a single-phase induction motor to operate in the running condition?
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What is the purpose of the capacitor in a single-phase induction motor?
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What is an advantage of single-phase induction motors?
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Study Notes
Single-Phase Induction Motor
- A single-phase induction motor is not self-starting, unlike a three-phase induction motor, and requires some starting means such as electric or electronic means.
- The single-phase stator windings produce a magnetic field that pulsates in strength in a sinusoidal manner, which does not rotate and therefore cannot produce rotation in a stationary squirrel-cage rotor.
Squirrel Cage Rotor
- The rotor has a laminated iron core with slots, and aluminum bars are molded on the slots and short-circuited at both ends with a ring.
- The rotor quickly accelerates until it reaches a speed slightly below the synchronous speed once it is rotated in one direction by some mechanical means.
Basics of Single-Phase Induction Motor
- A single-phase induction motor inherently does not develop any starting torque and therefore will not start to rotate if the stator winding is connected to a single-phase ac supply.
- However, if the rotor is started by auxiliary means, the motor will quickly attain the rated speed.
Double Revolving Field Theory
- The pulsating flux produced by the main winding can be resolved into two counter-rotating components (forward and backward components).
- The motor can be started in the direction of the forward rotating field, which generates a small (≈1-3%) positive slip.
- The reverse rotating field generates a larger (≈1.95%) negative slip.
- The interaction between the fields and the current induced in the rotor bars generates opposing torque.
Solution to Starting Torque Problem
- The single-phase induction motor can be made self-starting by adding an auxiliary winding in the slots of the stator to shift the EMF.
- The auxiliary winding and main winding are connected in parallel together and with the supply.
- When the rotor moves with speed until the synchronous speed, the auxiliary winding will open from the connection after a few seconds.
Torque/Speed Characteristics
- The two oppositely directed torques and the resultant torque can be shown effectively with the help of torque-speed characteristics.
Equivalent Circuit of Single-Phase Induction Motor
- A single-phase induction motor can be imagined to be consisting of two motors, having a common stator winding, but with their respective rotors revolving in opposite directions.
- Each rotor has resistance and reactance half the actual rotor values.
- The two equivalent circuits are connected in series.
- The current, power, and torque can be calculated from the combined equivalent circuit using network basics.
Review Questions
- What is a single-phase induction motor?
- How does a single-phase induction motor work?
- How do you test a single-phase motor?
- Which capacitor is used in a single-phase motor?
- What are the advantages of a single-phase induction motor?
- What are the applications of a single-phase induction motor?
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Description
Learn about the working of single-phase induction motors, their differences from three-phase motors, and their starting mechanisms.
