Showing posts with label Electrical Machines. Show all posts
Showing posts with label Electrical Machines. Show all posts

Monday, September 3, 2018

September 03, 2018

Speed Control Method Of DC Motor

In this article we will study about the speed control of dc series motor.
There are some popular methods of speed control of dc series motor are:

  • Variation of armature resistance. This method is called armature resistance control.
  • Variation of field flux. This method is called field flux control.

ARMATURE RESISTANCE CONTROL

In this case, a external resistance Re is connected in the armature circuit of the dc series motor. When the value of the external resistance is increased, it will cause increased voltage drop. Hence the effective voltage across the armature circuit will be reduced.

Speed will also reduced. So the speed of the motor can be controlled by varying the resistance.  As the value of Re is increased then motor will run at lower speed.
This method suffers from some serious drawbacks:

  • A large amount of power is wasted in the external resistance.
  • Speed can only be reduced below the normal speed with this method. It cannot be used for increase of speed.
FIELD FLUX CONTROL
In this method , a variable resistance called diverter, Rd is connected in parallel across the series field winding. This resistance reduces the current flowing through the series field winding and it results in lesser field flux. Since the speed is inversely proportional to the field flux , so the speed of the motor will increase.
If the value of diverter resistance is decreased then more current will flow through it instead of series field winding and lesser flux will produce which will increase the speed of the motor.
This can also be achieved by providing a tapped series field winding, which is most commonly used for traction purposes.
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Sunday, August 12, 2018

August 12, 2018

The Stepper Motor Basics | Types, Working Operation and Applications

In this article we will study about the various aspects of Stepper motor.
Stepper motor is a special purpose machine , feedback motor in which the rotor of the motor rotates in discrete steps. The angular rotation is determined by the number of pulses fed to the control circuit.
Each input pulse produces the one step of the angular movement. The total rotation of the rotor comprises of various such steps, therefore it is called Stepper Motor.
There are three most popular types of rotor arrangements:

  1. Variable Reluctance (VR) type.
  2. Permanent Magnet (PM) type.
  3. Hybrid type , a combination of VR and PM.
Here we will study only about Variable Reluctance type. Before moving into it , we will focus on a new term called STEP ANGLE.

STEP ANGLE - It is the angle by which the rotor rotates when an input pulse is applied to stator.
Resolution can be defined as number of steps it makes in one revolution of the rotor
Smaller the step angle , higher is the resolution of the positioning of the motor.

VARIABLE RELUCTANCE (VR) STEPPER MOTOR

The principle operation of such motor is based on the property that flux lines always travels/moves through the low reluctance path. The stator and rotor are aligned in such a way that magnetic reluctance is minimum. Variable reluctance motor can be single stack or multi-stack type.
SINGLE STACK VARIABLE RELUCTANCE MOTOR
A variable reluctance motor has a salient pole type stator and winding are placed over it. Stator coils are excited  from a dc source with the help of semiconductor switches , a magnetic field is produced. Due to this the rotor occupies the position which represents the path of minimum reluctance. That is, rotor axis aligns itself to the stator field axis.
Stepper motor , types of stepper motor , variable reluctance stepper motor , single stack reluctance motor
STEPPER MOTOR
As shown in figure above, consider a variable reluctance motor with 4 poles on stator and 2 poles on rotor. Four stator poles A,B,C and D are connected to the dc source with the help of semiconductor switches. These switches are switched in sequence A,B,C,D,A.....
When the winding A is excited , the rotor aligns with the axis of phase A. The rotor gets stable there and cannot move until A is de-energized. Next phase B is excited and A is disconnected . The rotor moves through 90 degree to align with the axis of pole B and in such way this sequence goes on.
The rotor completes one revolution in the 4 steps and rotor moves a step of 90 degree after each pole transition.
Direction of the rotor can be reversed by changing the sequence of the switching i.e. A,D,C,B,A... . Direction does not depend upon the magnitude of the current.
The step angle and resolution of the Stepper motor depends upon the number of poles on the stator and rotor.
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Tuesday, August 7, 2018

August 07, 2018

Construction , Working And Features Of Synchronous Motor

In this post we will study and understand about the various aspects of Synchronous Motor.
Synchronous motor is a type of an AC motor in which the speed of the motor is independent of load and it is synchronized with the frequency of the supply current.
If the frequency of the supply current is kept constant, then it will run at constant speed which is also called synchronous speed.
Synchronous speed is given by:
N = (120*f)/P
where, N = Synchronous speed.
              f = frequency of supply current.
              P = Total number of poles.

CONSTRUCTION OF SYNCHRONOUS MOTOR

Like all motors , it has two main parts called Stator and Rotor.
Stator of the motor carries the 3-phase winding while the rotor is supplied DC.
Stator core is made of thin lamination's to avoid Eddy Current losses.
There are two types of rotor construction
1. Cylindrical type rotor.
2. Salient pole rotor.

WORKING OF SYNCHRONOUS MOTOR

Synchronous motor is doubly excited motor. 3-phase supply is provided to stator while DC is provided to rotor winding.
3-phase supply in stator winding produces a rotating magnetic field while the DC supply produces a constant flux.
Since the rotating magnetic field rotates depending on the frequency. If the frequency is 60 Hz , it means that magnetic field is rotating at 3600 revolutions per minute. At any instant, the rotor poles and the stator poles can achieve same polarity like N-N or S-S , which will result in the repulsion between the poles and at the next instant the poles can achieve same polarity which will result in the attraction between poles.
But due to the inertia of the rotor and rapid attraction and repulsion between the poles, the rotor remains standstill and doesn't moves. That's why synchronous motor are not self-starting.
It is started by some external means and the motor starts to run in the same direction. This causes the magnetic locking between the both fields and the motor continues to rotate even after removing the external means. Since then, due to magnetic locking, motor continues to rotate at constant speed called synchronous speed.

MAIN FEATURES OF SYNCHRONOUS MOTOR

Main features of synchronous motor are ;
1. It runs either at synchronous speed or does not rotate at all.
2. It can work on large range of power factor both lagging and leading, therefore finds its application in power factor improvement.
3. It is not inherently self-starting. It is to be started by some external means.
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