24 Questions
What determines the direction of rotation in a reluctance-start motor?
The wide air gap
Why does a reluctance-start motor develop starting torque?
Because the pole flux has two out-of-phase components
What is a characteristic of reluctance-start motors?
They are made in small sizes only
How is speed control accomplished in a reluctance-start motor?
By varying the voltage across the motor
What type of machine is a split-phase motor?
An induction machine
What is a characteristic of split-phase motors?
They are made in larger ratings than shaded-pole and reluctance-start motors
What is a characteristic of the flux in a magnetic circuit containing no iron?
It is in phase with the exciting current
What happens to the flux when the mmf of a coil produces flux in a magnetic circuit made entirely of iron?
It lags behind the exciting current
What is the main difference between a capacitor-start motor and a standard split-phase motor?
The capacitor-start motor develops more starting torque with a lower inrush current during the starting period.
What is the primary application of a capacitor-start split-phase motor?
Small portable hoist applications
What is the main advantage of a two-value capacitor motor?
It retains the benefits of a capacitor during both normal running and starting periods.
What is the typical starting torque of a single-value capacitor motor?
Between 50 to 100 percent of rated torque
What is the main characteristic of a single-value capacitor motor that makes it suitable for applications requiring frequent back-and-forth movement?
Ease of reversing by simply swapping the two identical windings
What type of capacitor is used in a two-value capacitor motor?
A single capacitor unit with a transformed value
What is the primary advantage of a capacitor-start motor over a reluctance-start motor?
Higher starting torque
What type of applications are single-value capacitor motors commonly used in?
Air-moving equipment like fans and blowers
What is the primary function of a centrifugal device in a split-phase motor?
To disconnect the auxiliary winding at the proper time
What is the typical current inrush when starting a split-phase motor directly from the line?
4 to 6 times the rated value
What is the purpose of the capacitor in a capacitor-start motor?
To make the auxiliary and main winding starting currents more out of phase
What is the typical application of standard split-phase motors?
Applications requiring low starting torques and brief starting periods
What is the effectiveness of speed control in split-phase motors?
It is difficult and requires special windings to change the speed
What is the primary advantage of using a capacitor in a capacitor-start motor?
It overcomes the lack of sufficient starting torque
What is the speed variation of split-phase motors between no load and full load?
About 2.5 to 5 percent
What is the key factor in determining the direction of rotation in a split-phase motor?
The connection of the main and auxiliary windings to the source and with respect to each other
Study Notes
Capacitor-Start Motor
- Develops more starting torque with a lower inrush current compared to standard split-phase motor
- Suitable for small portable hoist applications, requiring a control circuit for instant reversal of direction
Two-Value Capacitor Motor
- Uses an autotransformer and a single capacitor unit
- Appears to the primary as having a value of (a²)C, where 'a' is the transformation ratio
- Retains benefits of a capacitor during both normal running and starting periods
Single-Value Capacitor Motor
- Has a permanently connected capacitor in the auxiliary winding circuit
- Frequently used in air-moving equipment like fans and blowers, or in applications requiring quiet operation
- Starting torque between 50 to 100 percent of rated torque
- Ease of reversing by simply swapping the two identical windings
Reluctance-Start Motor
- Direction of rotation is always from the wide air gap to the narrow air gap
- Starting torque is developed due to two out-of-phase components of flux
- Not as widely used as shaded-pole motors, made in small sizes only, and applied where starting and overload torque requirements are low
- Cannot be reversed and must be assembled so that the rotor shaft extends from the motor to drive the load in the proper direction
- Speed control is accomplished by varying the voltage across the motor
Split-Phase Motor
- One of the most widely used types of single-phase motor
- Fundamentally an induction machine, as the rotor power is received inductively
- Mechanism actuated by a centrifugal device or an electromagnetic relay is used to cut out the auxiliary winding at the proper time
- Direction of rotation is determined by the connection of the main and auxiliary windings to the source and with respect to each other
- Starting current inrush may be 4 to 6 times the rated value
- Speed control is difficult and requires special windings to change the speed
- Speed regulation is similar to polyphase motors, with a speed variation of about 2.5 to 5 percent between no load and full load
This quiz covers the characteristics and applications of capacitor-start motors, including their starting torque and operating characteristics.
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