- Electrical Machines Tutorial
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- Basic Concepts
- Electromechanical Energy Conversion
- Energy Stored in a Magnetic Field
- Singly-Excited and Doubly Excited Systems
- Rotating Electrical Machines
- Faraday’s Laws of Electromagnetic Induction
- Concept of Induced EMF
- Fleming’s Left Hand and Right Hand Rules
- Transformers
- Electrical Transformer
- Construction of Transformer
- EMF Equation of Transformer
- Turns Ratio and Voltage Transformation Ratio
- Ideal and Practical Transformers
- Transformer on DC
- Losses in a Transformer
- Efficiency of Transformer
- Three-Phase Transformer
- Types of Transformers
- DC Machines
- Construction of DC Machines
- Types of DC Machines
- Working Principle of DC Generator
- EMF Equation of DC Generator
- Types of DC Generators
- Working Principle of DC Motor
- Back EMF in DC Motor
- Types of DC Motors
- Losses in DC Machines
- Applications of DC Machines
- Induction Motors
- Introduction to Induction Motor
- Single-Phase Induction Motor
- Three-Phase Induction Motor
- Construction of Three-Phase Induction Motor
- Three-Phase Induction Motor on Load
- Characteristics of 3-Phase Induction Motor
- Speed Regulation and Speed Control
- Methods of Starting 3-Phase Induction Motors
- Synchronous Machines
- Introduction to 3-Phase Synchronous Machines
- Construction of Synchronous Machine
- Working of 3-Phase Alternator
- Armature Reaction in Synchronous Machines
- Output Power of 3-Phase Alternator
- Losses and Efficiency of 3-Phase Alternator
- Working of 3-Phase Synchronous Motor
- Equivalent Circuit and Power Factor of Synchronous Motor
- Power Developed by Synchronous Motor
- Electrical Machines Resources
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- Electrical Machines - Discussion
Speed Regulation and Speed Control
Speed Regulation of Induction Motors
The speed regulation of induction motor is defined as the change in the motor speed with change in load. It is expressed as a fraction or percentage of full-load speed, i.e.,
$$\mathrm{\mathrm{Speed\:regulation}\:=\:\mathit{\frac{N_{nl}-N_{fl}}{N_{fl}}}\times 100\%}$$
Where,$\mathit{N_{nl}}$ is the no-load speed of the motor and $\mathit{N_{fl}}$ is the full-load speed of the motor.
The speed regulation of an induction motor is about 3% to 5%. Due to this small speed regulation, the induction motors are classified as the constant speed motors.
Speed Control of Three-Phase Induction Motors
The speed of a three-phase induction motor is given by,
$$\mathrm{\mathit{N_{r}}\:=\:\left ( 1-\mathit{s} \right )\mathit{N_{s}}\:\cdot \cdot \cdot (1)}$$
Where,s is the slip and $\mathit{N_{s}}$ is the synchronous speed in RPM.
$$\mathrm{\mathit{N_{s}}\:=\:\frac{120\mathit{f}}{\mathit{P}}\:\cdot \cdot \cdot (2)}$$
From equations (1) & (2), it is clear that the speed of a three-phase induction motor can be varied by changing the following −
Frequency (f) of AC supply,
Number of stator poles (P), and
Slip (s).
In practice, the change in supply frequency is generally not possible because the commercial electric supplies have a constant frequency. Hence, the speed of a three-phase induction motor can be changed either by changing the number of stator poles (P) or the slip (s). We shall now discuss the speed control of squirrel-cage and slip-ring induction motors.
Speed Control of Squirrel-Cage Induction Motors
The speed control of squirrel-cage induction motors is changed by changing the number of stator poles. By the pole changing method, there are only two or four speeds are possible.
In a two-speed induction motor, one stator winding is provided, which may be switched through a suitable control equipment to provide the two speeds. Where, one speed is half of the other. For example, the stator winding may be connected for either 4 or 8 stator poles, giving synchronous speeds of 1500 RPM and 750 RPM, when the motor is supplied from a source of 50 Hz AC supply.
In a four speed induction motor, two separate stator windings are provided each of which provides two speeds.
Following are the major disadvantages of pole changing method of speed control −
This method cannot be used to obtain gradual continuous speed control.
It makes the motor design and switching of the interconnection of stator windings more complicated.
This method can provide a maximum of four different speeds for any one motor due to design and interconnection complications.
Speed Control of Slip-Ring Induction Motors
The speed of a slip-ring induction motor can be varied by changing the motor slip. The following methods are employed for changing the slip and hence the speed −
By the changing the stator line voltage.
By changing the resistance of the rotor circuit.
By adding and changing a foreign voltage in the rotor circuit.
Numerical Example
For a three-phase induction motor, the no-load speed of the motor is 900 RPM and its full-load speed is 880 RPM. Find the speed regulation of the motor.
Solution
Given data,
$\mathit{N_{nl}}$ = 900 RPM
$\mathit{N_{fl}}$ = 880 RPM
$$\mathrm{\therefore\mathrm{Speed\:regulation}\:=\:\mathit{\frac{N_{nl}-N_{fl}}{N_{fl}}}\times 100\%}$$
$$\mathrm{\Rightarrow \mathrm{Speed\:regulation}\:=\:\frac{900-880}{880}\times 100\%\:=\:2.273\%}$$
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