explain in detail the working& construction of motor and dc generator?

Asked by pankajkumarsingh25 | 8th Feb, 2010, 02:28: PM

Expert Answer:

Dear Student

A universal motor can be manufactured in two different ways.

· Non-compensated type with concentrated poles

· Compensated type with distributed field.

The compensated type is preferred for high power rating appliances and the Non-compensated for low power rated appliances. Both the compensated and Non-compensated have construction similar to that of a DC series motor.

 

 

 
 

Non-Compensated motor

The Non-compensated motor has 2 salient poles and it is laminated. The armature is of wound type and the laminated core is either straight or skewed slots. The leads of the armature winding are connected to the commutator. High resistance brushes are used along with this type of motor to help better commutation.

The skew in the armature slots serves for two purposes:

· It reduces the magnetic hum.

· It aids in reducing the locking tendency of rotor, which is called magnetic locking. Magnetic locking is a condition during which the rotor teeth remains locked under the stator teeth due to magnetic attraction between the stator and rotor. This will be considerably reduced by using the Skew

 

 

 
 

Compensated type motor

The compensated type motor consists of distributed field winding and the stator core is similar to that of split-phase motor. We already know that split phase motors consist of an auxiliary winding in addition to main winding. Similar to the split phase motors, the compensated type also consists of an additional winding. The compensating winding helps in reducing the reactance voltage which is caused due to alternating flux, when the motor runs with the aid of an AC supply.

Both the types of motors develop unidirectional torque regardless of the supply with which they run. The supply may be AC or DC but the direction of torque is same.

 

 

 

 
 

Can the direction of rotation be reversed for these types of motors?

 

Yes. The direction of rotation can be changed. For the non-compensated motor having salient pole, the direction of rotation can be reversed by changing the direction of flow of current through the armature or field winding. This can also be done by interchanging the leads on the brush holders.

In case of compensated type motor, either the armature leads or the field leads and shifting the brushes against the direction of rotation of motor. This helps in reversal of rotation.

 

 

 
 

Speed/Load characteristics and Speed Control

Speed/Load Characteristics:

Very similar to that of DC series motor, the universal motor also has varying speed characteristics. The speed is low at full loads. The speed is Very high and dangerous at no-loads. During no-loads the speed is limited only by its own frictional and windage load.

 

 

 

 

Speed control:

Speed control of universal motor is very important and the following methods are employed for the speed control of universal motors.

Resistance method:

In this method of speed control a variable resistance is connected in series with the motor. The amount of resistance in the circuit can be changed. A foot pedal is used for this purpose. Usually this method is employed for motors used in sewing machines.

 

 

Centrifugal Mechanism:

This method is involved whenever the application involves a number of speeds. Best example is home food and fruit mixers. Here a centrifugal device is attached to the motor. If the motor rises above the specified speed set by the lever, the centrifugal device opens the contact and R comes in contact with the circuit. This causes the motor speed to decrease below the set speed. When the motor runs slower than the speed set by the lever, the contact is established and resistance is short circuited. This causes the speed to increase. The variations in speed are noticeable as the process is repeated in a rapid manner.

 

 

Tapping-Field Method:

As the name suggests the field is tapped at various points in this method. This is done by wounding the field pole.

Now how this tapping arrangement is established?

The tapping arrangement is established by two ways

· The various sections in the field pole are wounded and different sizes of wire and taps are brought out from those sections.

· In the second method, resistance made of Nichrome is wounded over the field pole. Various tappings are brought out from this wire.

 

 

 
 

Applications

 

Universal motors are mostly employed in

· Vacuum cleaners

· Portable drills

· Drink mixers

· Sewing machine

Generator

There are no DC generators, One can use rectifiers/filters to get DC from AC.

Generators

If you've ever moved paper clips around with a magnet or killed time arranging metal shavings into a beard on a "Wooly Willy" toy, then you've dabbled in the basic principles behind even the most complicated electric generators. The magnetic field responsible for lining up all those little bits of metal into a proper Mohawk haircut is due to the movement of electrons. Move a magnet toward a paper clip and you'll force the electrons in the clip to move. Similarly, if you allow electrons to move through a metal wire, a magnetic field will form around the wire.



There is a definite link between the phenomena of electricity and magnetism. A generator is simply a device that moves a magnet near a wire to create a steady flow of electrons. The action that forces this movement varies greatly, ranging from hand cranks and steam engines to nuclear fission, but the principle remains the same.

­One simple way to think about a generator is to imagine it acting like a pump pushing water through a pipe. Only instead of pushing water, a generator uses Magnet to push electrons along. This is a slight oversimplification, but it paints a helpful picture of the properties at work in a generator. A water pump moves a certain number of water molecules and applies a certain amount of pressure to them. In the same way, the magnet in a generator pushes a certain number of electrons along and applies a certain amount of "pressure" to the electrons.

In an electrical circuit, the number of electrons in motion is called the amperage or current, and it's measured in amps. The "pressure" pushing the electrons along is called the voltage and is measured in volts. For instance, a generator spinning at 1,000 rotations per minute might produce 1 amp at 6 volts. The 1 amp is the number of electrons moving (1 amp physically means that 6.24 x 1018­ ­electrons move through a wire every second), and the voltage is the amount of pressure behind those electrons.

Regards

Team

Topperlearning

 



Answered by  | 8th Feb, 2010, 05:38: PM

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