WHY WE USE AC MOTOR INSTEAD OF DC MOTOR - APPLICATIONS OF AC MOTORS AND APPLICATIONS OF DC MOTORS

For electrical technology, Motor is known as the main unit of electrical technology. because of motor use in all industries with all machines and usually all the places normally. the Electrical motor is the main sub-sub-branch of electrical engineering because it is not easy to learn all concepts about electrical motor technology. There are many types and kinds of electrical motors like,

1. AC motors
2. DC motors

and there are also differents kinds in AC motors and DC motors as synchronous motors, induction motors, explosion proof, low voltage and motor with permanent magnet etc.

Where We Use AC Motors:

The alternating current electric motor (AC motors) are ideal for most applications linear, for fans, pumps, compressors, mills, machine tools, boilers, robots, generators and in many other products categories.
As regarding the choice of the type of electric motor for a given application, this is influenced by different factors, starting from the costs of purchase and operation, the yield, the efficiency, the periodic maintenance.

Where We Use DC Motors:

A DC motor is an electric motor that runs on direct current power. In any electric motor, the operation is dependent upon simple electromagnetism. A current carrying conductor generates a magnetic field, when this is then placed in an external magnetic field, it will encounter a force proportional to the current in the conductor and to the strength of the external magnetic field Resources and Information. is a device which converts electrical energy to mechanical energy. It works on the fact that a current carrying conductor placed in a magnetic field experiences a force which causes it to rotate with respect to its original position. Practical DC Motor consists of field windings to provide the magnetic flux and armature which acts as the conductor.

Applications of DC Motors

Series Motors

The series DC motors are used where high starting torque is required, and variations in speed are possible. For example – the series motors are used in Traction system, Cranes, air compressors.

Shunt Motors

The shunt motors are used where constant speed is required and starting conditions are not severe. The various applications of DC shunt motor are in Lathe Machines, Centrifugal Pumps, Fans, Blowers, Conveyors, Lifts, Weaving Machine, Spinning machines, etc.

Compound Motors

The compound motors are used where higher starting torque and fairly constant speed is required. The examples of usage of compound motors are in Presses, Shears, Conveyors, Elevators, Rolling Mills, Heavy Planners, etc.

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INDUCTION MOTOR QUESTIONS FOR PRACTICES

1.      Drive the output equation of a 3-phase induction motor and also explain how the magnetic and electric loading is selected.

2.      Write down the steps to find out the main dimensions of a 3-phase IM. Also draw a flow chart for it.

3.      Explain and find out flux density in stator teeth.

4.      Explain how the rotor is designed of a 3-phase ship ring induction motor. Also draw a flow chart for it.

5.      Estimate the stator slot leakage reactance for 1-Layer winding.

6.      Write down step by step how the circle diagram is drawn for an induction motor?

7.      Calculate the main dimension, turns per phase, Number of conductors, slots, cross-section of the  conductor of a 50 KW, 415 volts, 3 phase,, 50 Hz, 1000 RPM, slip ring  induction motor. Given B_av = 0.52T, η= 0.9, pf =0.89 lagging, =32000 ac /m, δ= 5A /mm².

8.      A 20-kw, 3-phase, 6-pole, 50-Hz, 400 V delta connected cage rotor induction motor has 54 stator slots, each containing 10 conductors. Design suitable number of rotor slots and determine the value of bar and end ring currents. The machine has efficiency of 85% and power factor 0.85 lagging. Also find the bar and end ring sections, if current density is 6.0 A/mm². Assume rotor mmf as 85% of the stator mmf.

9.      Describe the significance of B₃₀ in a  3-phase induction motors. A 75 kw, 3300V, 50 Hz, 8-pole, 3-ph, star connected induction motor has a magnetizing current which is 35% of full load current. Calculate the number of stator winding turns per phase if the mmf required for flux density at 60⁰ from the inter polar axis is 500 A. Assume winding factor = 0.95, full load efficiency = 0.94 and full load power factor = 0.86.

10.  Estimate the main dimensions & air gap length for a 3-phase, 20 HP, 400 V, 6-pole, 50 Hz, 970 RPM induction motor suitable for a star-delta starting. Assume magnetic and electric specific loadings as 0.45 wb/m² and 23000 ac/m respectively, ratio of core length to pole pitch 0.85, full load efficiency 0.88 and power factor 0.89.

11.  Estimate the main dimensions, number of radial ducts, number of stator slots, number of turns per phase, conductors per slot and slot dimensions for 7.5 kW, 415 V, 3-phase, delta-connected, 4-pole, 50 Hz squirrel-cage induction motor. The flux per pole is 0.015 Wb. Assume efficiency 85% and power factor as 0.85 lagging.

12.  Determine the main dimensions, turns per phase, number of slots, conductor section and slot area of 200 H.P, 3 phase, 50Hz, 400 Volts, 1480 r.p.m. slip ring induction motor. Assume Bav = 0.5 wb/m², ac = 30,000 amp conductor / meter, efficiency = 0.9 and power factor = 0.9, current density = 3.5 amp per mm².

13.  Design a 10 HP, 415 V, 3-phase, 50 Hz, 1440 rpm, squirrel cage induction motor. The machine is to be started by a star delta starter. Assume all suitable data yourself.

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SYNCHRONOUS MOTOR - CONSTRUCTION OF SYNCHRONOUS MOTOR

Synchronous Machine

Construction of Synchronous Machines:

Stator:

The stator is similar in construction that of a induction motor.

It is made up of laminated sheet steel having slot on its inner periphery.

Three phase winding is placed in the slot on stator and it serves as armature winding.

Rotor two types Salient:

• for low speed Non-Salient (cylindrical rotor)
• for high speed  Slip rings:

Two slip ring at rotor shaft to provide supply to field winding

Brushed: Two brushes Shaft:

Bearings:

Principle of operation:

Generator: Field produced on rotor by dc current through slip rings

Rotor field is turned at synchronous speed (Ns) by a prime mover.

EMFs induced in stator coils with frequency given by Motor:

The stator is wound for the similar number of poles as that of rotor, and fed with three phase AC supply. 

The 3 phase AC supply produces rotating magnetic field (RMF) in stator.

The rotor winding is fed with DC supply which magnetizes the rotor.

The rotor gets locked to the RMF and rotates unlike induction motor at synchronous speed under all load condition

Why Synchronous motor is not self starting?

How to make it self starting?

Consider a two pole synchronous machine as shown in figure below

Why Synchronous motor is not self starting?

Now, the stator poles are revolving with synchronous speed (lets say clockwise). If the rotor position is such that, N pole of the rotor is near the N pole of the stator (as shown in first schematic of above figure), then the poles of the stator and rotor will repel each other, and the torque produced will be anticlockwise.

The stator poles are rotating with synchronous speed, and they rotate around very fast and interchange their position. But at this very soon, rotor can not rotate with the same angle (due to inertia), and the next position will be likely the second schematic in above figure. In this case, poles of the stator will attract the poles of rotor, and the torque produced will be clockwise.

Hence, the rotor will undergo to a rapidly reversing torque, and the motor will not start.

How to make it self starting?

But, if the rotor is rotated upto the synchronous speed of the stator by means of an external force (in the direction of revolving field of the stator), and the rotor field is excited near the synchronous speed, the poles of stator will keep attracting the opposite poles of the rotor (as the rotor is also, now, rotating with it and the position of the poles will be similar throughout the cycle). Now, the rotor will undergo unidirectional torque. The opposite poles of the stator and rotor will get locked with each other, and the rotor will rotate at the synchronous speed.

Below is comment portion, not just for see, it is for your feedback and questions or suggestions about this topic. So kindly use this comment sections.

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WHAT IS TWO PHASE SYSTEM

The Two phase Motor is the motor which has two winding, The Starting winding and the running winding. and it need 90 degree difference between phases. Often as like servo motor.

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WHY POLE SHOE HAS BEEN GIVEN A SPECIFIC SHAPE

So as you know, pole shoe has been given a specific shape, So why it is need for a specific shapoe? I will share two main problems of this scenario.

1. Pole shoe enlarges the area of armature core to come across the flux and which is necessory to produce larger induced EMF. So to acheive this, pole shoe has been given a particular shape.
   
2. And (for given a shape) It is necessory that  maximum area of the armature comes across the magnetic field produced by the field windings.

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WHAT ARE THE FUNCTION OF YOKE? DEFINE THE FUNCTION AND HOW TO CHOOSE THE MATERIAL FOR YOKE?

So first of all I would like to share some thing (basic) about Yoke, What is Yoke? How to choose material? OKay!!
Yoke is the body part of any motor whether it is dc motor or AC motor. Both have yoke, Yoke is the structure and body of motor.

The Functions of Yoke are:

1. It provide mechanically support to the poles.
2. It serves the purpose of outer most cover of the motor.
3. It provide a path of low reluctance for flux.
4. It forms a part of the magnetic circuits.

And in the last, It is prepared by Cost Iron, For large machines, we used rolled steel, silicon steel which provides high permeability.

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HOW THE REVERSE DIRECTION OF ROTATION OF DC MOTOR.

To reverse the direction of rotation of DC motor, either it is main field's produced by the field winding reverse or the current passing through the armature is reversed, By changing the direction of current passing through the field winding, the direction of the main field can be reversed easily just by changing the direction of armature's current.

#dcmotor
#motor
#electricalmotor

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