D.C Tachogenerators

The D.C Tachogenerators is a type of electrical type’s tachogenerators which can also be used for speed measurement.

The D.C tachogenerator is shown in below figure.

The armature of the D.C Tachogenerator is kept in the permanent magnetic field. The armature of the tachogenerator is coupled to the machine whose speed is to be measured. When the shaft of the machine revolves, the armature of the tachogenerator revolves in the magnetic field producing e.m.f. which is proportional to the product of the flux and speed to be measured. Now as the field of the permanent field is fixed, the e.m.f generated is proportional to the speed directly. The e.m.f induced is measured using moving coil voltmeter with uniform scale calibrated in speed directly. The series resistance is used to limit the current under output short circuit condition. The polarity of output voltage indicates the direction of rotation. The commutator collects current from armature conductors and converts internally induced a.c e.m.f into d.c (unidirectional) e.m.f. while the brushes are used to collect current from commutator and make it available to external circuitry of the d.c tachogenerator.



The advantages of d.c tachogenerator are as follows:

1.The output voltage is small enough to measure it with conventional d.c voltmeters.
2.The polarity of output voltage directly indicates the direction of rotation.


The Disadvantages of d.c tachogenerator are as follows:

1.Because of variations in contact resistance, considerable error is introduced in the output voltage. Hence periodic maintenance of the commutator and brushes is required.

2.Non-linearity in the output of the d.c tachogenerator occurs because of distortions in the permanent magnetic field due to large armature currents. Hence input resistance of meter should be very high as compared to the output resistance of the generator.


Temperature compensation and Error in PMMC:

The basic pmmc instrument is sensitive to the temperature. The magnetic field strength and spring tension decrease with increase in temperature. The coil resistance increases with increase in the temperature. Thus pointer reads low for a given current. The meter tends to read low by approximately 0.2% per Celsius rise in the temperature. Hence the temperature compensation is provided by appropriate use of series and shunt resistance of copper and manganin.

Temperature compensation

The simple temperature compensation circuit for PMMC uses a resistance in series with a movable coil, as shown in the figure. The resistor is called swamping resistor. It is made up of manganin having practically zero temperature coefficients, combined with copper in the ratio of 20/1 or 30/1.
temperature compensation for pmmc

The resultant resistance of the coil and the swamping resistor increases slightly as temperature increases, just enough to compensate the change in springs and magnet due to temperature. Thus the effect of temperature is compensated.

More complicated but complete cancellation of temperature effects can be obtained by using the swamping resistors in series and parallel combination as shown in figure.

advanced temperature compensation for pmmc

In this circuit, by correct proportioning of copper and manganin parts, complete cancellation of the temperature effects can be achieved.

Errors in PMMC Instrument

The basic sources of error in PMMC instruments are friction, temperature and aging of various parts. To reduce the frictional errors ratio of torque to weight is made very high.

The most serious errors are produced by the heat generated or by changes in the temperature. This changes the resistance of the working coil, causing large errors. In case of voltmeters, a large series resistance of very low temperature coefficient is used. This reduces the temperature errors.

The aging of permanent magnet and control springs also cause errors. Opposite errors in PMMC is caused by weakening of magnet and spring cause. The weakening of magnet causes less deflection while weakening of control springs cause large deflection, for a particular value of current. The proper use of material and pre-ageing during manufacturing can reduce the errors due to weakening of control springs.


Permanent magnet moving coil instruments (PMMC)

The permanent magnet moving coil instruments are most accurate type for direct current measurements. The action of these instruments is based on the motoring principle. When a current carrying coil is placed in the magnetic field produced by permanent magnet, the coil experiences a force and moves. As the coil is moving and the magnet is permanent, the instrument is called permanent magnet moving coil instrument. This basic principle is called D’Arsonval principle. The amount of force experienced by the coil is proportional to the current passing through the coil.

The PMMC instrument is shown in the below images.

permanent magnet moving coil

The moving coil is either rectangular or circular in shape. It has number of turns of fine wire. The coil is suspended so that it is free to turn about its vertical axis. The coil is placed in uniform, horizontal and radial magnetic field of a permanent magnet in the shape of a horse-shoe. The iron core is spherical if coil is circular and is cylindrical if the coil is rectangular. Due to iron core, the deflecting torque increase, increasing the sensitivity of the instrument.

The controlling torque is provided by two phosphor bronze hair springs.

The damping torque is provided by eddy current damping. It is obtained by movement of aluminum former, moving in the magnetic field of the permanent magnet.

The pointer is carried by the spindle and it moves over a graduated scale. The pointer has light weight so that it deflects rapidly. The mirror is placed below the pointer to get the accurate reading by removing the parallax. The weight of the instrument is normally counter balanced by the weights situated diametrically opposite and rapidly connected to it. The scale markings of the basic d.c PMMC instruments are usually linearly spaced as the deflecting torque and hence the pointer deflections are directly proportional to the current passing through the coil.

The top view of PMMC instrument is shown in the below image.
pmmc top view

Advantages of PMMC

The various advantages of PMMC instruments are,

It has uniform scale.
With a powerful magnet, its torque to weight ratio is very high. So operating current of PMMC is small.
The sensitivity is high.
The eddy currents induced in the metallic former over which coil is wound, provide effective damping.
It consumes low power, of the order of 25 W to 200 mW.
It has high accuracy.
Instrument is free from hysteresis error.
Extension of instrument range is possible.
Not affected by external magnetic fields called stray magnetic fields.

Disadvantages of PMMC

The various disadvantages of PMMC instruments are ,

PMMC is Suitable for direct current measurement only.
Ageing of permanent magnet and the control springs introduces the errors.
The cost is high due to delicate construction and accurate machining.
The friction is due to jewel-pivot suspension.


Classification of Bridges

The two Types of bridges are,

  1. D.C Bridges
  2. A.C Bridges

The D.C bridges are used to measure the resistance while the A.C bridges are used to measure the impedances consisting capacitance and inductances. The D.C bridges use the D.C voltages as the excitation voltage while the A.C bridges use the alternating voltage as the excitation voltage.

The two types of D.C bridges

  1. Wheatstone Bridge
  2. Kelvin Bridge

The various types of A.C Bridges are,

  1. Capacitance Comparison Bridge
  2. Inductance Comparison Bridge
  3. Maxwell’s Bridge
  4. Hay’s Bridge
  5. Anderson Bridge
  6. Schering Bridge
  7. Wien Bridge
Please comment me, if any further details are required


Project on Governing of Speed

Governing of motor speed is taken as an example for the purpose of study. Details have been shown in the figure.

With the help of a keypad, information regarding the speed to be maintained is fed to the controller.

Now the controller starts the motor. Any digital signal from the controller is converted to an analog signal (firing pulse) by a D/A converter.

This firing pulse is given to the inverter, which controls the frequency of the input supply.

Now the motor speed is changed according to the output frequency of the inverter.

A tachometer is used to provide feedback control. The tachometer converts the motor speed into an electrical analog signal.

The analog signal is amplified and converted to a digital signal using a A/D converter.

Now the controller compares the feedback signal and the required speed and generates an error signal.

This error signal is fed to the D/A converter again.

The process briefed till now is continued until the required motor speed is achieved. Thus the variation of speed is governed.


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