30 Questions
What is the main purpose of the split-phase induction motor discussed in the text?
To develop a starting torque
Why does the rotor of the induction motor continue to turn once started?
Because of the magnetic pull between the two fields
What happens when the rotor of the motor is rotated 90° by an outside force?
A rotary force is generated between the fields
Why are split-phase and shaded-pole motors named as such?
Based on their methods of starting
Why is the start winding disconnected in a split-phase induction motor when it reaches 75% of its rated speed?
As part of the starting process
What differentiates a capacitor-start split-phase motor from other types of split-phase motors?
The incorporation of a starting capacitor
What is the primary purpose of the squirrel-cage windings in a synchronous motor?
To provide the initial torque for starting the motor
How is the rotor of a practical synchronous motor typically constructed?
With salient poles
What is the primary disadvantage of a synchronous motor?
It requires a DC exciter voltage for the rotor
How does the rotor of a synchronous motor interact with the stator field?
The rotor's salient poles produce alternate north and south magnetic poles that are attracted to those produced in the stator
What is the primary characteristic of the synchronous motor's operation?
It operates at exactly synchronous speed with no slip
How is the DC exciter voltage for the rotor of a synchronous motor typically obtained?
From an external source or internally, depending on the motor design
What is the primary advantage of a synchronous motor over an induction motor?
Synchronous motors maintain a constant speed regardless of load variations
What is the maximum torque a synchronous motor can develop without losing synchronism called?
Pull-out torque
How does a synchronous motor overcome the problem of slip and speed variation with varying loads?
By using permanent magnets or DC excited currents in the rotor
What is the primary disadvantage of a synchronous motor compared to an induction motor?
Synchronous motors require a starter mechanism to bring them up to speed
In what types of applications are synchronous motors ideally suited?
Applications requiring constant speed or synchronization of multiple motors
What component is often built into the main rotor of a synchronous motor?
A squirrel-cage rotor
What distinguishes the capacitor start motor from the permanent-split capacitor motor?
The presence of a starting winding
In resistance-start motors, how is the electrical phase shift between the two windings obtained?
By making the impedance of the windings unequal
Why do split-phase motors generally come only in small sizes?
As the auxiliary winding is only a light winding
What is the starting torque like for a permanent-split capacitor motor compared to its full-load torque?
40% of full-load torque
How is the permanent-split capacitor motor's running performance and speed regulation tailored?
By selecting an appropriate capacitor value
What distinguishes split-phase induction motors from resistance-start motors?
The position of the starting winding
What is the main reason that single-phase induction motors are used more often than other types of motors?
They are the least expensive and require the least maintenance
What is the main difference between the stator field in a single-phase induction motor and a polyphase induction motor?
The stator field in a single-phase motor alternates polarity between poles, while in a polyphase motor it rotates
What happens to the phase voltages and phase currents when a motor is connected in delta configuration instead of star configuration?
Both the phase voltages and phase currents increase by a ratio of 3
What happens to the line current when a motor is connected in delta configuration instead of star configuration?
The line current increases by a factor of 3
What happens to the rotor's magnetic field in a single-phase induction motor?
The rotor's magnetic field rotates in the opposite direction to the stator field
What must be done to the rotor of a single-phase induction motor in order for it to start rotating?
The rotor must be rotated by an external force, such as a twist of the hand
Study Notes
Single-Phase Induction Motors
- The rotor is attracted to the stator poles, resulting in a rotary force that turns the rotor towards magnetic correspondence with the stator.
- The rotor continues to turn once started due to the continuous alternation of the two fields.
Split-Phase Induction Motors
- A type of induction motor that incorporates a starting device, using inductance, capacitance, or resistance to develop a starting torque.
- The start winding is disconnected when the motor reaches 75% of its rated speed.
- Capacitor-Start Split-Phase Motors: uses a capacitor to develop a starting torque.
- The capacitor is disconnected when the motor reaches 75% of its rated speed.
Synchronous Motor
- Operates at exactly synchronous speed with no slip.
- The rotor is of a constant polarity (either a permanent magnet or an energized electromagnet) and the windings of the stator are wrapped in a way that produces a rotating magnetic field.
- Provides very little torque at zero speed, and thus needs a separate starting apparatus.
- Runs at speed regardless of load variations up to a point called the pull-out torque.
- The maximum value of torque that a motor can develop without losing synchronism is called its pull-out torque.
Applications of Synchronous Motor
- Ideal for applications in which constant speed is necessary or two or more motors need to be in sync.
- Overcomes the problem of bringing the motor up to speed when connected to a 50-Hz commercial line with the extra expense of a starter mechanism.
Split-Phase Motors
- Come only in small sizes due to the auxiliary winding being only a light winding, which does not develop sufficient torque to start heavy loads.
- Used for many applications in which high starting torque may not be required.
Capacitor Start Motor
- Can be recognized by the bulbous protrusion on the frame where the starting capacitor is located.
- The starting torque is quite low, roughly 40% of full-load, making it suitable for low-inertia loads such as fans and blowers.
- Running performance and speed regulation can be tailored by selecting an appropriate capacitor value.
Permanent-Split Capacitor Motors
- The capacitor is left in series with the starting winding during normal operation.
- No centrifugal switch is required.
Resistance-Start Motors
- Has a starting winding in addition to the main winding, which is switched in and out of the circuit.
- The electrical phase shift between the currents in the two windings is obtained by making the impedance of the windings unequal.
- The starting winding is positioned at right angles to the main winding.
- Can be connected in delta mode if the star connection has sufficient torque to run up to 75% or 80% of full load speed.
Test your knowledge on starting synchronous motors and the role of squirrel cage windings. Learn about the process of energizing the stator and rotor fields to achieve full torque and drive the load. Explore the mechanical switching device that operates on centrifugal force.
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