Presented by :- 1.Mahima Mittal ( ) 2.Monil Ghadiya ( ) Speed Control of DC Motors DC Machine & Transformer

Contents  What is DC Motor  Types  Definition  Method for Speed Control  Merits/Demerits  Applications

What is motor  The motor or an electrical motor is a device that has brought about one of the biggest advancements in the fields of engineering and technology ever since the invention of electricity. A motor is nothing but an electro- mechanical device that converts electrical energy to mechanical energy.

 The very basic construction of a dc motor contains a current carrying armature which is connected to the supply end through commutator segments and brushes and placed within the north south poles of a permanent or an electro-magnet as shown in the diagram below.construction of a dc motorcurrent

Motor principle  Now to go into the details of the operating principle of DC motor its important that we have a clear understanding of Fleming’s left hand rule to determine the direction of force acting on the armature conductors of dc motor. Fleming’s left hand rule says that if we extend the index finger, middle finger and thumb of our left hand in such a way that the current carrying conductor is placed in a magnetic field (represented by the index finger) is perpendicular to the direction of current (represented by the middle finger), then the conductor experiences a force in the direction (represented by the thumb) mutually perpendicular to both the direction of field and the current in the conductor.Fleming’s left hand rule currentmagnetic fieldcurrent

Types of dc motor

DC Shunt Motor Characteristics

Factors Controlling Motor Speed 

Speed control of DC Shunt Motor  There are three types for controlling speed 1. Variation of Flux 2. Armature or Rheostatic Control Method 3. Voltage Control Method i. Multiple Voltage Control ii. Ward Leonard System

Flux control method

 It is seen that speed of the motor is inversely proportional to flux. Thus by decreasing flux speed can be increased and vice versa.  To control the flux, a rheostat is added in series with the field winding, as shown in the circuit diagram. Adding more resistance in series with field winding will increase the speed, as it will decrease the flux. Field current is relatively small and hence I 2 R loss is small, hence this method is quiet efficient. Though speed can be increased by reducing flux with this method, it puts a limit to maximum speed as weakening of flux beyond the limit will adversely affect the commutation.

Armature control method

 Speed of the motor is directly proportional to the back emf E b and E b = V- I a R a. That is when supply voltage V and armature resistance R a are kept constant, speed is directly proportional to armature current I a. Thus if we add resistance in series with armature, I a decreases and hence speed decreases.  Greater the resistance in series with armature, greater the decrease in speed.

Merits 1.Speed changes with every change in load, because speed variations depend not only on controlling resistance but on load current also. This double dependence makes it impossible to keep the speed sensibly constant on rapidly changing loads. 2.A large amount of power is wasted in the controller resistance. Loss of power is directly proportional to the reduction in speed. Hence, efficiency is decreased. 3.Maximum power developed is diminished in the same ratio as speed. 4.It needs expensive arrangement for dissipation of heat produced in the controller resistance. 5.It gives speeds below the normal, not above it because armature voltage can be decreased (not increased) by the controller resistance.

Voltage Control Method 1. Multiple voltage control:  In this method the, shunt filed is connected to a fixed exciting voltage, and armature is supplied with different voltages. Voltage across armature is changed with the help of a suitable switchgear. The speed is approximately proportional to the voltage across the armature.

1. Ward-Leonard System:

 This system is used where very sensitive speed control of motor is required (e.g electric excavators, elevators etc.) The arrangement of this system is as required in the figure beside.  M 2 is the motor whose speed control is required. M 1 may be any AC motor or DC motor with constant speed. G is the generator directly coupled to M 1. In this method the output from the generator G is fed to the armature of the motor M 2 whose speed is to be controlled. The output voltage of the generator G can be varied from zero to its maximum value, and hence the armature voltage of the motor M 2 is varied very smoothly. Hence very smooth speed control of motor can be obtained by this method.AC motorDC motorgenerator

Uses  Electric excavators  Elevators  Main drives in steel, blooming and paper mills  Colliery winders

Dc series motor  A series wound dc motor like in the case of shunt wound dc motor or compound wound dc motor falls under the category of self-excited dc motors, and it gets its name from the fact that the field winding in this case is connected internally in series to the armature winding. Thus the field winding are exposed to the entire armature current unlike in the case of a shunt motor.shunt wound dc motor compound wound dc motorcurrent

construction  Construction wise a this motor is similar to any other types of dc motors in almost all aspects. It consists of all the fundamental components like the stator housing the field winding or the rotor carrying the armature conductors, and the other vital parts like the commutator or the brush segments all attached in the proper sequence as in the case of a generic DC motor.types of dc motorsthe commutator or the brush segmentsDC motor  Yet if we are to take a close look into the wiring of the field and armature coils of this dc motor, its clearly distinguishable from the other members of this type. To understand that let us revert back into the above mentioned basic fact, that the this motor has field coil connected in series to the armature winding. For this reason relatively higher current flows through the field coils, and its designed accordingly as mentioned below.dc motorcurrent  i) The field coils of dc series motor are wound with relatively fewer turns as the current through the field is its armature current and hence for required mmf less numbers of turns are required.current  ii) The wire is heavier, as the diameter is considerable increased to provide minimum electrical resistance to the flow of full armature current.electrical resistance  In spite of the above mentioned differences, about having fewer coil turns the running of this dc motor remains unaffected, as the current through the field is reasonably high to produce a field strong enough for generating the required amount of torque. To understand that better lets look into the voltage and current equation of dc series motordc motorcurrentvoltage current

Types of dc series motor 1. Flux control method Variations in the flux of a series motor can be brought about in any one of the following ways.  Field divertor  Armature divertor  Trapped field control field  Paralleling field coils 2 Variable resistance in series with motor By increasing the resistance in series with the armature the voltage applied across the armature terminals can be decreased. Series parallel control

 Speed control means intentional change of the drive speed to a value required for performing the specific work process.

Field divertor  A veritable resistance is connected parallel to the series field as shown in fig (a). This variable resistor is called as divertor, as desired amount of current can be diverted through this resistor and hence current through field coil can be decreased. Hence flux can be decreased to desired amount and speed can be increased

Armature divertor  Divertor is connected across the armature as in fig (b).  For a given constant load torque, if armature current is reduced then flux must increase. As, Ta α ØIa  This will result in increase in current taken from the supply and hence flux Ø will increase and subsequently speed of the motor will decrease.

Trapped field control field  As shown in fig (c) field coil is tapped dividing number of turns. Thus we can select different value of Ø by selecting different number of turns.

Paralleling field coils  In this method, several speeds can be obtained by regrouping coils as shown in fig

Variable resistance in series with motor  By introducing resistance in series with armature, voltage across the armature can be reduced. And hence, speed reduces in proportion with it.