How does DC motor work?working principle | type of dc motor
Dc motor working principle and Construction, types of dc motors series, shunt and compound dc motors appliaction
Dc motor construction,types and working principle
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| fig: dc motor working principle |
If a current carrying conductor is placed into the field of a permanent magnet, as shown in Fig, a force will be exerted on the conductor to push it out of the magnetic field.
PRACTICAL D.C. MOTORS
DC motor have been used in industrial applications for years. Coupled with a DC drive, DC motors provide very precise control. DC motors can be used with conveyors, elevators, extruders, marine applications, material handling, paper, plastics, rubber, steel, and textile applications to name a few.
Construction of DC motors
DC motor are made up of several major parts which include the following:
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Brushes
2) dc shunt motor
3) dc compound motor
Direct current motors are classified by the way in which the field and armature windings are connected, which may be in series or in parallel.
1) dc Series Motor: Series motor The field and armature windings are connected in series share the same current. The series motor has the characteristics of a high starting torque but a speed which varies with load. Theoretically the motor would speed up to self-destruction, limited only by the windage of the rotating armature and friction, if the load were completely removed.
Figure shows series motor connections and characteristics.For this reason the motor is only suitable for direct coupling to a load, except in very small motors,such as vacuum cleaners and hand drills, and is ideally suited for applications where the machine must start on load, such as electric trains, cranes and hoists.
This characteristic means that the machine will run on both a.c. or d.c. and is, therefore, sometimes referred to as a ‘universal’ motor.
- Frame
- Shaft
- Bearings
- Main Field Windings (Stator)
- Armature (Rotor)
- Commutator
- Brush Assembly
dc motor diagram shown in below figure
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| DC motors |
Contact with the external circuit is made through carbon brushes rubbing on the commutator segments.
Basic Construction
The relationship of the electrical parts of a DC motor is shown in the following diagram. Field windings are mounted on pole pieces to form electromagnets. In smaller DC motors the field may be a permanent magnet. However, in larger DC fields the field is typically an electromagnet. Field windings and pole pieces are bolted to the frame. The armature is inserted between the field windings. The armature is supported by bearings and end brackets (not shown). Carbon brushes are held against the commutator.
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| dc motor parts |
Armature
The armature rotates between the poles of the field windings. The armature is made up of a shaft, core, armature windings, and a commutator. The armature windings are usually form wound and then placed in slots in the core.
Brushes
Brushes ride on the side of the commutator to provide supply voltage to the motor. The DC motor is mechanically complex which can cause problems for them in certain adverse environments. Dirt on the commutator, for example, can inhibit supply voltage from reaching the armature. A certain amount of care is required when using DC motors in certain industrial applications. Corrosives can damage the commutator. In addition, the action of the carbon brush against the commutator causes sparks which may be problematic in hazardous environments.
Basic DC Motor working principle and Operation
Magnetic Fields
there are two electrical elements of a DC motor, the field windings and the armature. The armature windings are made up of current carrying conductors that terminate at a commutator. DC voltage is applied to the armature windings through carbon brushes which ride on the commutator.
In small DC motors, permanent magnets can be used for the stator. However, in large motors used in industrial applications the stator is an electromagnet. When voltage is applied to stator windings an electromagnet with north and south poles is established. The resultant magnetic field is static (non-rotational). For simplicity of explanation, the stator will be represented by permanent magnets in the following illustrations.
In small DC motors, permanent magnets can be used for the stator. However, in large motors used in industrial applications the stator is an electromagnet. When voltage is applied to stator windings an electromagnet with north and south poles is established. The resultant magnetic field is static (non-rotational). For simplicity of explanation, the stator will be represented by permanent magnets in the following illustrations.
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| Construction Basic DC Motor Operation of dc motor |
A DC motor rotates as a result of two magnetic fields interacting with each other. The first field is the main field that exists in the stator windings.
The second field exists in the armature. Whenever current flows through a conductor a magnetic field is generated around the conductor.
Right-Hand Rule for Motors
A relationship, known as the right-hand rule for motors, exists between the main field, the field around a conductor, and the direction the conductor tends to move.
If the thumb, index finger, and third finger are held at right angles to each other and placed as shown in the following illustration so that the index finger points in the direction of the main field flux and the third finger points in the direction of electron flow in the conductor, the thumb will indicate direction of conductor motion. As can be seen from the following illustration, conductors on the left side tend to be pushed up. Conductors on the right side tend to be pushed down. This results in a motor that is rotating in a clockwise direction. You will see later that the amount of force acting on the conductor to produce rotation is directly proportional to the field strength and the amount of current flowing in the conductor.
If the thumb, index finger, and third finger are held at right angles to each other and placed as shown in the following illustration so that the index finger points in the direction of the main field flux and the third finger points in the direction of electron flow in the conductor, the thumb will indicate direction of conductor motion. As can be seen from the following illustration, conductors on the left side tend to be pushed up. Conductors on the right side tend to be pushed down. This results in a motor that is rotating in a clockwise direction. You will see later that the amount of force acting on the conductor to produce rotation is directly proportional to the field strength and the amount of current flowing in the conductor.
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Right-Hand Rule for Motors |
What is CEMF
Whenever a conductor cuts through lines of flux a voltage is induced in the conductor. In a DC motor the armature conductors cut through the lines of flux of the main field. The voltage induced into the armature conductors is always in opposition to the applied DC voltage. Since the voltage induced into the conductor is in opposition to the applied voltage it is known as CEMF (counter electromotive force). CEMF reduces the applied armature voltage
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| CEMF (counter electromotive force) |
The amount of induced CEMF depends on many factors such as the number of turns in the coils, flux density, and the speed which the flux lines are cut.
Armature Field
The amount of induced CEMF depends on many factors such as the number of turns in the coils, flux density, and the speed which the flux lines are cut.
Armature Field An armature, as we have learned, is made up of many coils and conductors. The magnetic fields of these conductors combine to form a resultant armature field with a north and south pole. The north pole of the armature is attracted to the south pole of the main field. The south pole of the armature is attracted to the north pole of the main field. This attraction exerts a continuous torque on the armature. Even though the armature is continuously moving, the resultant field appears to be fixed. This is due to commutation, which will be discussed next.
Armature Field An armature, as we have learned, is made up of many coils and conductors. The magnetic fields of these conductors combine to form a resultant armature field with a north and south pole. The north pole of the armature is attracted to the south pole of the main field. The south pole of the armature is attracted to the north pole of the main field. This attraction exerts a continuous torque on the armature. Even though the armature is continuously moving, the resultant field appears to be fixed. This is due to commutation, which will be discussed next.
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| Armature Field |
What is Commutation in dc motor
In the following illustration of a DC motor only one armature conductor is shown. Half of the conductor has been shaded black, the other half white. The conductor is connected to two segments of the commutator.
In position 1 the black half of the conductor is in contact with the negative side of the DC applied voltage. Current flows away from the commutator on the black half of the conductor and returns to the positive side, flowing towards the commutator on the white half.
In position 2 the conductor has rotated 90°. At this position the conductor is lined up with the main field. This conductor is no longer cutting main field magnetic lines of flux; therefore, no voltage is being induced into the conductor. Only applied voltage is present. The conductor coil is short-circuited by the brush spanning the two adjacent commutator segments. This allows current to reverse as the black commutator segment makes contact with the positive side of the applied DC voltage and the white commutator segment makes contact with the negative side of the applied DC voltage.
In position 1 the black half of the conductor is in contact with the negative side of the DC applied voltage. Current flows away from the commutator on the black half of the conductor and returns to the positive side, flowing towards the commutator on the white half.
In position 2 the conductor has rotated 90°. At this position the conductor is lined up with the main field. This conductor is no longer cutting main field magnetic lines of flux; therefore, no voltage is being induced into the conductor. Only applied voltage is present. The conductor coil is short-circuited by the brush spanning the two adjacent commutator segments. This allows current to reverse as the black commutator segment makes contact with the positive side of the applied DC voltage and the white commutator segment makes contact with the negative side of the applied DC voltage.
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| Commutation |
As the conductor continues to rotate from position 2 to position 3 current flows away from the commutator in the white half and toward the commutator in the black half. Current has reversed direction in the conductor. This is known as commutation.
Types of DC Motors
Permanent Magnet Motors
The field of DC motors can be a permanent magnet, or electromagnets connected in series, shunt, or compound.
Permanent Magnet Motors The permanent magnet motor uses a magnet to supply field flux. Permanent magnet DC motors have excellent starting torque capability with good speed regulation. A disadvantage of permanent magnet DC motors is they are limited to the amount of load they can drive. These motors can be found on low horsepower applications. Another disadvantage is that torque is usually limited to 150% of rated torque to prevent demagnetization of the permanent magnets.
Permanent Magnet Motors The permanent magnet motor uses a magnet to supply field flux. Permanent magnet DC motors have excellent starting torque capability with good speed regulation. A disadvantage of permanent magnet DC motors is they are limited to the amount of load they can drive. These motors can be found on low horsepower applications. Another disadvantage is that torque is usually limited to 150% of rated torque to prevent demagnetization of the permanent magnets.
Type of dc motor:
1) dc series motor2) dc shunt motor
3) dc compound motor
Direct current motors are classified by the way in which the field and armature windings are connected, which may be in series or in parallel.
1) dc Series Motor: Series motor The field and armature windings are connected in series share the same current. The series motor has the characteristics of a high starting torque but a speed which varies with load. Theoretically the motor would speed up to self-destruction, limited only by the windage of the rotating armature and friction, if the load were completely removed.
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| speed load characteristics |
Figure shows series motor connections and characteristics.For this reason the motor is only suitable for direct coupling to a load, except in very small motors,such as vacuum cleaners and hand drills, and is ideally suited for applications where the machine must start on load, such as electric trains, cranes and hoists.
Reversal of rotation may be achieved by reversing the connections of either the field or armature windings but not both.
This characteristic means that the machine will run on both a.c. or d.c. and is, therefore, sometimes referred to as a ‘universal’ motor.
2) Dc shunt motor: Shunt motor The field and armature windings are connected in parallel . Since the field winding is across the supply, the flux and motor speed are considered constant under normal conditions.
In practice, however, as the load increases the field flux distorts and there is a small drop in speed of about 5% at full load, as shown in Fig. The machine has a low starting torque and it is advisable to start with the load disconnected. The shunt motor is a very desirable d.c. motor because of its constant speed characteristics.It is used for driving power tools, such as lathes and drills. Reversal of rotation may be achieved by reversing the connections to either the field or armature winding but not both.
3) dc compound motor: Compound motor The compound motor has two field windings – one in series with the armature and the other in parallel.
If the field windings are connected so that the field flux acts in opposition, the machine is known as a short shunt and has the characteristics of a series motor. If the fields are connected so that the field flux is strengthened, the machine is known as a long shunt and has constant speed characteristics similar to a shunt motor. The arrangement of compound motor connections is given in Fig. The compound motor may be designed to possess the best characteristics of both series and shunt motors, that is, good starting torque together with almost constant speed. Typical applications are for electric motors in steel rolling mills, where a constant speed is required under varying load conditions.
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| speed load characteristics |
In practice, however, as the load increases the field flux distorts and there is a small drop in speed of about 5% at full load, as shown in Fig. The machine has a low starting torque and it is advisable to start with the load disconnected. The shunt motor is a very desirable d.c. motor because of its constant speed characteristics.It is used for driving power tools, such as lathes and drills. Reversal of rotation may be achieved by reversing the connections to either the field or armature winding but not both.
3) dc compound motor: Compound motor The compound motor has two field windings – one in series with the armature and the other in parallel.
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Speed/Torque Curves of dc motor
The following chart compares speed/torque characteristics of DC motors. At the point of equilibrium, the torque produced by the motor is equal to the amount of torque required to turn the load at a constant speed. At lower speeds, such as might happen when load is added, motor torque is higher than load torque and the motor will accelerate back to the point of equilibrium. At speeds above the point of equilibrium, such as might happen when load is removed, the motor’s driving torque is less than required load torque and the motor will decelerate back to the point of equilibrium.
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| speed torque curv of series compound shunt dc motor |
Applications of series,shunt,compound Dc motors
When applying a DC drive and motor to an application it is necessary to know the horsepower, torque, and speed characteristics of the load. The following chart shows typical characteristics of various loads
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Loads generally fall into one of three categories
Constant Torque:The load is essentially the same throughout the speed range. Hoisting gear and belt conveyors are examples.
Variable Torque:The load increases as speed increases. Pumps and fans are examples.
Constant Horsepower:The load decreases as speed increases. Winders and rotary cutting machines are examples
Winders/Coilers
DC motors offer superior characteristics at low speed for winder and coiler operation and performance. In winder applications maintaining tension at standstill is a very important operation. DC motors offer a wide speed range at rated torque. On many winder applications that run in an extended speed range a smaller horsepower DC motor could do the same job as a larger horsepower AC motor.
Marine Applications
DC drives offer several advantages in marine applications. Compact sizing is one of the biggest advantages. DC drives also adapt well from generator supplies such as found in the marine industry.
Crane/Hoist
DC offers several advantages in applications that operate at low speed, such as cranes and hoists. Advantages include low speed accuracy, short-time overload capability, size, torque proving control, and load sharing
Mining/Drilling
DC is often preferred in the high horsepower applications required in the mining and drilling industry. DC drives offer advantages in size and cost. They are rugged, dependable, and proven in the industry
Extruding
Extruding is a price competitive industry. DC offers economical solutions in the 60 to 1000 HP range which is commonly used in extruding applications.
References:Siemens DC Drives
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