2.what is principle of galvanometr?
    Galvanometer works on the principle of conversion of electrical energy into mechanical energy. When a current flows in a magnetic field it experiences a magnetic torque. If it is free to rotate under a controlling torque, it rotates through an angle proportional to the current flowing through it.


3.

Dynamometer wattmeter:

Construction of dynamometer type wattmeter:

Following figure shows the dynamometer wattmeter for measuring the power. If two coils are connected such that, current proportional to the load voltage, flows through one coil and current proportional to the load current, flows through other coil, the meter can be calibrated directly in watts. This is true because the indication depends upon the product of the two magnetic fields. The strength of the magnetic fields depends upon the values of the current flowing through the coils.

dynamometer wattmeter

Working of dynamometer type wattmeter:

Let

v=supply voltage

i=load current and

R=resistance of the moving coil circuit.

Current through fixed coils, i(f)=I,

Current through the moving coil, i(m)=v/R

Deflecting torque,

• For a DC circuit the deflecting torque is thus proportional to the power.
• For any circuit with fluctuating torque, the instantaneous torque is proportional to instantaneous power. In this case due to inertia of moving parts, the deflection will be proportional to the average power. For sinusoidal alternating quantities the average power is VI COSф, where

V= r.m.s. value of voltage,

I=r.m.s. value of current, and

Ф= phase angle between V and I

Hence an electrodynamic instrument, when connected as shown in figure, indicates the power, irrespective of the fact it is connected in an AC or DC circuit.

Ranges:

i)                   Current circuit: 0.25 A to 100 A with employing current transformers (CTs).

ii)                Potential circuit: 5V to 750 V without employing potential transformers (PTs).

Types of Dynamometer wattmeter:

Dynamometer wattmeters may be divided into two classes:

• Suspended-coil torsion instruments.
• Pivoted-coil, direct indicating instruments.

Suspended-coil torsion wattmeters:

These instruments are used largely as standard wattmeters.

• The moving, or voltage, coil is suspended from a torsion head by a metallic suspension which serves as a lead to the coil. This coil is situated entirely inside the current or fixed coils and the winding in such that the system is a static. Errors due to external magnetic fields are thus avoided.
• The torsion heads carries a scale, and when in use, the moving coil is bought back to the zero position by turning this head; the number of divisions turned through when multiplied by a constant for the instrument gives the power.
• Eddy currents are eliminated as far as possible by winding the current coils of standard wire and by using no metal parts within the region of the magnetic field of the instrument.
• The mutual inductance errors are completely eliminated by making zero position of the coil such that the angle between the planes of moving coil and fixed coil is 90 degree. i.e. the mutual inductance between the fixed and moving coil is zero.
• The elimination of pivot friction makes possible the construction of extremely sensitive and accurate electrodynamic instruments of this pattern.

Pivoted-coil direct-indicating wattmeters:

These instruments are commonly used as switchboard or portable instruments.

• In these instruments, the fixed coil is wound in two halves, which are placed in parallel to another at such a distance, that uniform field is obtained. The moving coil is wound of such a size and pivoted centrally so that it does not project outside the field coils at its maximum deflection position.
• The springs are pivoted for controlling the movement of the moving coil, which also serves as currents lead to the moving coil.
• The damping is provided by using the damping vane attached to the moving system and moving in a sector-shaped box.
• The reading is indicated directly by the pointer attached to the moving system and moving over the calibrated scale.
• The eddy current errors, within the region of the magnetic field of the instrument, are minimized by the use of non-metallic parts of high resistivity material.

Advantages and disadvantages of dynamometer type wattmeter:

The advantages and disadvantages of dynamometer type wattmeters are as under:

Advantages:

1)    In dynamometer type wattmeter, the scale of the instrument is uniform (because deflecting torque is proportional to the true power in both DC as well as AC and the instrument is spring controlled.)

2)    High degree of accuracy can be obtained by careful design; hence these are used for calibration purposes.

Disadvantages :

1)    The error due to the inductance of the pressure coil at low power factor is very serious (unless special features are incorporated to reduce its effect)

2)    In dynamometer type wattmeter, stray field may affect the reading of the instrument. To reduce it, magnetic shielding is provided by enclosing the instrument in an iron case.

4.

Speed control is an important term for DC motor operation. DC Motors are used in various applications, starting from household appliances to industrial applications. Somewhere constant speed is required, but in most of the cases, the motor speed is required to vary in a wide range of speed. Imagine that, you are turning ON a fan. The speed of the fan must be controllable by adjusting the regulator. In the same way, other industrial motors should be controllable by the operator. Because, most of the industrial applications requires a wide range of motor speed. On extension with the torque and working principle of D.C. motor, we must now deal with the most essential topic regarding the Speed Control of a D.C. Motor. The back e.m.f in a motor can be expressed as Eb= φZNP / 60A = kφN Volt ,
the symbols denoting:
Z= no. of conductors
P=no. of poles
A= no. of parallel paths in the armature
φ = flux per pole
k= constant for particular machine.

This back e.m.f in the motor is always less than the applied e.m.f by IaRa the armature voltage drop, so that Eb= E – IaRa,
or, k φ N= E- IaRa
or, Speed in r.p.m, N = (E – IaRa) / kφ

The above expression reveals that the speed may be controlled by varying the applied voltage E, the armature current Ia, the armature circuit resistance, Ra, the flux per pole φ. So now we can easily say that according to the principle of speed control as we mentioned it above the following are the methods used in practice:
A. Rheostatic control.
B. Field control.
C. Series-Parallel control.
D. Special control method is Ward Leonard system.

Rheostatic control or Armature resistance control:

In DC motor speed N is depended upon the armature voltage and field current. So, it is possible to vary the speed by changing the armature voltage and field current. We know that, the supply voltage is constant for DC Motors and changing the supply voltage of Motor is quite difficult. So, the armature voltage is controlled instaed of directly controlling the supply voltage. A variable resistance is provided with the armature in series. The armature voltage is controlleed by varying the variable resistance or rhrostat. This method of speed control is used for both DC series motor and DC shunt motor.

Let us take a resistance Re be the resistance ( variable resistor or rheostat) placed in series with the armature circuit. Then, speed, N= {E-Ia(Ra+Rse)}/kφ. This shows that the speed of the motor decreases as the series resistance Rse is being increased. More simply, if the resistance of the rheostat is increased, less amount of voltage will applied to the armature. Similarly, if the resistance o the theostat is decreased, more amount of voltage will applied to the armature circuit, and automatically yhe speed will increase. The field curent is kept constant in this technique. The chief disadvantage of this method is that there is considerable power dissipitation in the series resistance. This method is useful where reduction in speed is required for a short period. This difficulty can be avoided to some extent by the use of Armature Diverter resistance. For a given load torque, if Ia is reduced due to the diverter connected in parallel to the armature, φ must increase, which is T=ktφIa. Thus this increase in flux φ is associated with a reduction in speed as N ∝ (1/φ).

Field Current Control

The main field fluxes are produced bt the field current. So, the speed of DC Motor is easily controlled by varying the field current. They are of two types:

• Field Rheostat: This method is mainly used to control the speed of DC Shunt motor. Here an additional resistance is connected in series with the shunt field circuit. The field current is controlled by varying the rheostat. This is the most satisfactory and economical way to control the speed of D.C. motors. Since the field current is usually very small, the losses in the resistance may be considered negligible. By increasing the resistance, the field is weakened and thereby a considerable variation in speed of shunt motor above normal may be obtained.

• Field Diverter: In this case of a series motor, a diverter i.e. a variable resistance is connected in parallel with the series field. This will divert a part of the load current and thus the field is weakened with a result that the speed of the motor is increased. The parallel resistor is termed as diverter.

• Tapped field method: In this method, the ampere turns are varied by changing the field turns The motor runs at its minimum speed when the full winding is effective. Cutting of the field turns in steps increases the speed of the motor. In this method, it is specially employed for the speed control of traction motors. By this method full and half speed may be obtained without any rheostatic loss. Other speeds may be obtained when an auxiliary series rheostatic control is used. The resistance is gradually cut out as the train speeds up. Then the motors are connected in parallel with the resistance in series which is again cut out gradually.

Special control method is Ward Leonard system

A very special and delicate method of speed control is this Ward Leonard System which is used for controlling the speed of a D.C. motor over a wide range from crawling speed to full speed particularly for those drives where rapid reversibility is an additional requirement. A brief discussion on this topic particularly has been done separately, kindly move to it for more information.

These are the conventional methods of speed control of DC Motor.