Types of Dynamometer

Dynamometers can be broadly classified into two types. They are :

1. Absorption Dynamometers
2. Transmission Dynamometers

1. Absorption Dynamometers

Absorption dynamometers measures the power output of the engine to which they are coupled by absorption. The power absorbed is usually dissipated as heat by some medium. Example is Eddy current dynamometer.

2. Transmission Dynamometers

In transmission dynamometers the power is measured by transmission principle which is transferred to the load coupled to the engine and then it is indicated on indicator . These are also called torque meters. Example is strain gauge transmission dynamometer.

Eddy current Dynamometer

Fig. 1: Eddy current dynamometer.

Eddy current is the type of absorption type dynamometer. It consists of non-magnetic solid metallic rotor, which moves in the magnetic field of stator. The stator winding is excited by a D.C. supply as shown in Fig. l. When the solid rotor moves in the field produced by stator windings an emf is produced in it resulting in a large loss of power due to eddy current. This power is dissipated as heat in the rotor and therefore water is circulated through air gap between stator and rotor. Torque on the stator casing may be measured in the usual manner.

Eddy current Dynamometer Advantages

1. They are small in size, as compare to any other of the same capacity.
2. The absorption power can be changed by changing d.c. circuit.
3. It can measure high power output at all speeds.
4. Torque developed is smooth and continuous under all operating conditions.
5. The power range of eddy current brake is up to 300 h.p. and maximum speed is 6000 rpm.

Eddy current Dynamometer Limitations

1. It cannot produce any torque at zero speed.
2. It produces only small torques at low speed.

Strain Gauge Transmission Dynamometer

This is also inline rotating torque sensor, which measures torque. Fig. 2 shows the arrangement of strain gauge dynamometer.

Fig. 1: Strain Gauge Transmission dynamometer.

Strain Gauge Transmission Dynamometer Construction and Measurement

It consists of metal shaft fitted with bonded strain gauge. Here strain gauges are fitted at 45º to shaft axis as shown in Fig. 2.6.2(a). In this type of arrangement 2 strain gauges are subjected to tensile stress and while other is subjected to compressive stress. Strain gauge 1 and 3 must be diametrically opposite to strain gauge 2 and 4. Due to torsion, strain gauge senses compressive as well as tensile deformation. Further these strain gauges are connected to Wheatstone circuit. The output of Wheatstone bridge is proportional to torsion and hence to applied torque on shaft. The bridge power and output of bridge is connected to the sensor through slip ring and brushes as shown in Fig. 2.6.2(b).

Strain Gauge Transmission Dynamometer Advantages

1. It is sensitive to torque.
2. It has full temperature compensation.
3. It provides automatic compensation for bending and axial loads.
4. It gives an instantaneous result.

Strain Gauge Transmission Dynamometer Disadvantages

1. The slip rings may wear out and causes maintenance problems.
2. The device is generally expensive.
3. Initial settings need skill and is time consuming.

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Figure 1: Q Meter.

Circuit Diagram & Working Principle of Q Meter

Fig. 2: Q Meter Series Connection Circuit Diagram.

Fig. 2 depicts the basic principle of operation of a Q-meter. The circuit comprises of two main sections (a) signal generator section and (b) electronic voltmeter. In addition to this, it incorporates two tuning capacitors and a thermocouple high frequency current meter.

The RF signal generator section generates a signal of a desired frequency within a frequency range of 50 kHz to 70 MHz. This frequency range is usually covered in five to six ranges. The signal output voltage of the signal generator is adjusted by means of a potentiometer provided for this purpose. The signal passes a current through a standard resistor (r) and this current is measured by means a thermocouple meter. The signal current is adjusted so that an RF signal of 1 volt develops across the standard resistor. This signal is fed as input to the test circuit.

The coil under test along with the two tuning capacitors form a series circuit and it is tuned to the signal frequency with the adjustment of the two tuning capacitors. The voltage across the tuning capacitors is measured by the electronic voltmeter. Circuit tuning is indicated by maximum reading in the electronic voltmeter.

Q of a coil has been defined as the ratio of the voltage across L or C to the input voltage

\[Q=\frac{{{v}_{C}}\text{  or  }{{\text{v}}_{\text{L}}}}{v}\]

Since v_C has been measured by electronic voltmeter and since input signal voltage has been fixed at 1 volt (i.e. v = 1V), Q is given as,

\[Q=\frac{{{v}_{C}}}{v}={{v}_{C}}\]

Thus the reading obtained in the electronic voltmeter gives a direct indication of the quality factor of the coil under test. If the quality factor of the coil under test is high, then input signal voltage is reduced to 0.1 volt so that the Q factor is 10 times the reading obtained at the voltmeter.

The instrument has an accuracy of about ± 5% for Q measurement upto 15 MHz. In addition of Q measurements, self inductance of a coil may also be determined but no direct scale for this purpose is available. Once the circuit with unknown inductance has been tuned, the signal frequency and tuning capacitance is noted down and value of coil inductance is computed from the relation

\[f=\frac{1}{2\pi \sqrt{LC}}\]

It is also possible to measure unknown capacitance of small capacitors with this instrument. For this purpose a standard inductor is used and the circuit is tuned with tuning capacitor. Now the unknown capacitor is connected in the circuit and tuning capacitor is retuned to obtain maximum reading in the voltmeter. Since the unknown capacitor has been connected in parallel with the tuning capacitor, the decrease in the capacitance of the tuning capacitor has to be made to obtain resonance after unknown capacitance is connected and this equals the value of unknown capacitance.

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Fig. 1: Block Diagram of Standard Signal Generator.

Block Diagram of Standard Signal Generator

It is often used in the measurement of gain, signal to noise ratio, bandwidth, standing wave ratio and other circuit parameters.
The signal generator may be considered as a source of RF energy with a known frequency and amplitude. This energy may be a simple RF carrier or modulated to a required depth of modulation. Amplitude modulation is commonly used in standard signal generators because most of the radio receivers are AM receivers. Sinusoidal modulating signals are commonly used but sometimes the modulating signal may be of square wave or pulse type. The output voltage can be accurately adjusted by means of a calibrated attenuator and read by an output meter. Fig. 1 gives the block diagram of a standard signal generator.

The instrument consists of an RF oscillator that can give sinusoidal signals with a frequency that can be set anywhere in the range of 80 kHz to 30 MHz. This RF signal is given to the wideband amplifier which amplifies this signal. The amplified signal is given to the output terminals through a calibrated attenuator which is set to give RF output of the desired magnitude.

The instrument is provided with a modulation switch. When this switch is put in the OFF position the modulating signal is not connected and the output available is RF carrier. When this switch is put to INT. MOD position, AF signal from the Modulation Oscillator is applied to the wideband amplifier. This amplifier now works as a modulator and gives an Amplitude modulated RF signal to the output. The modulation oscillator usually operates at standard modulating signal frequencies of 400 Hz and 1000 Hz.

In the third position of the modulation switch, the internal modulation is cut-off and we can use external audio signal for modulation. By changing the amplitude of this A.F signal, we can change the depth of modulation. Frequency of modulating signal can also be varied. This is an important requirement when we are measuring the overall fidelity of a radio receiver.
The signal output can usually be adjusted within a range of 1 µV to 1 V in steps.

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Fig. 1: Block Diagram of an Audio Signal Generator.

Block Diagram of Audio Signal Generator

Fig. 1 gives this block diagram of an audio oscillator that is capable of giving sinusoidal as well as square wave output. The Wein bridge oscillator is heart of the instrument and is used to generate sine-wave signal of any desired frequency with in a frequency range of 10 Hz – 500 kHz. The frequency of the oscillator is changed by means of a Range Switch and within a given range, frequency is made continuously variable.

The output of this oscillator is applied to sine wave circuit or a square wave circuit with the help of function switch. In the sine-wave mode, the signal is amplified and given to the output terminal through the attenuator. The amplitude can be set to any desired value by means of attenuator and the magnitude control.

When the function switch is in square-wave position, the oscillator output is given to a wave shaping circuit that converts the sinusoidal signal into square wave signal. The square wave signal in amplified and through an attenuator given to the output terminals. The attenuator is used for varying the amplitude of the square wave output. Fig. 2 shows an audio signal generator used in laboratories.

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The most commonly used output waveforms are the sine wave, square wave, triangular wave and saw-tooth wave with frequencies that can be adjusted from fraction of a hertz to several hundred kilohertz. Different output signals from the function generator may be available simultaneously. Another useful feature of the function generator is to phase lock to an external signal source. A function generator may be used to phase lock another function generator and the two output signals may be displaced in phase according to a requirement. The function generator may be phase locked to a frequency standard. The function generator outputs will then have the same frequency accuracy and stability as the frequency standard.

Block Diagram of Function Generator

Fig. 1: Block Diagram of a Function Generator.

Fig. 1 shows the block diagram of a function generator. The circuit does not employ a conventional oscillator circuit because these oscillators fail to operate at very low frequencies. It uses a different technique for generating signals. Here two constant current sources are used to feed the integrator circuit. When the circuit is switched on, the upper constant current source sends a constant current into the integrator and the output of the integrator starts rising linearly depending upon the magnitude of the constant current supplied by the upper current source. The integrator output is connected to the comparator. When integrator output reaches a pre-determined level, the comparator changes state; the upper constant current source is cut-off while the lower current source is switched on. This current source passes a current in reverse direction and the voltage at the integrator output starts decreasing linearly. When this output decreases to a pre-determined level, the comparator again switches on. The lower current source is switched off and upper current source is switched on and the circuit repeats the same action.

The rate of rise/fall of the integrator output depends upon the magnitude of the current supplied by the upper / flower constant current sources. Changing the magnitudes of these currents would, therefore, change the frequency of the integrator output. The frequency control network controls the magnitudes of current sources and hence the frequency of the output. For this reason, this network is termed as the frequency control network.
The integrator output is of triangular waveform. The waveform may be amplified by an output amplifier and obtained at the output.
The comparator output which is used to switch over function of the current sources is of square wave-shape. This square wave output may be amplified and used as a square wave output.
Lastly, the triangular wave produced by the integrator is given to resistance-diode wave-shaping circuit which converts this triangular signal into a sinusoidal signal.
There are two selector switches S₁ and S₂. S₁ decides the input signal given to output amplifier 1 and S₂ gives the input signal to amplifier 2. We may obtain any two waves in the output by putting these switches in the appropriate positions.

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Figure 1: Pressure Thermometer.

Construction & Working of Pressure Thermometer

The operation of pressure gauge thermometer is based upon the expansion of liquid, gas or vapour in the bulb of the thermometer. It consists of bulb containing a liquid, gas or vapour, which is immersed in environment. The bulb is connected by capillary tube to some pressure measuring device, this pressure measured is calibrated to indicate temperature.

Pressure Bulb or Sensor:

The sensing element is pressure bulb, which contains a thermometer fluid and is placed where temperature is measured. The bulb is made of cylindrical piece of metal tubing closed at one end and connected to capillary tubing at the other end. The size of bulb used depends upon the type of fluid used, temperature span of system and the length of capilary tube used with it.

Capillary Tube:

A capillary tube is used to connect the bulb with pressure measuring device. The length of capillary tube is made as short as possible in order to improve dynamic performance. The capillary tube is made of copper or steel for systems using fluids other than mercury. When mercury is used as the transmitting fluid, stainless steel capillary tube is used.

Pressure Measuring Device:

The pressure measuring device may be bourdon tube, diaphragm, or bellow. The displacement of this is amplified to operate a pointer over a stationary scale. The scale may be calibrated to give temperature.

Advantages of Pressure Thermometer:

1. They are most economical and versatile for the measurement of temperature.
2. They are rugged in construction and have little chance of damage.
3. Remote indication upto 60 m is possible.
4. Accuracy is ± 1%.

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Fig. 1: Eddy current dynamometer.

Working Principle of Eddy Current Dynamometer

Eddy current dynamometer is electrical absorption dynamometer working on principle that, when isolated conductor moves through magnetic flux, it induces eddy current, which get dissipated in the form of heat.

Construction & Working of Eddy Current Dynamometer

It consists of toothed non-magnetic solid metallic rotor connected to the shaft whose power is to be measured. The non-magnetic rotor rotates inside smooth cast iron stator.

The stator is provided with exciting coil of D.C. source. The stator is mounted such that it permits free swing about its axis (Cradled) provided with torque arm, which measures torque.

To dissipate the generated heat, water is supplied in stator casing. During operation of dynamometer, rotor turns and causes constant change in flux density at all points of stator, resulting formation of eddy current, which opposes the motion of rotor. This opposing resistance is measured by brake drum in the form of torque, from which shaft power can be calculated.

Advantages of Eddy Current Dynamometer

1. It can measure high power output at all speeds therefore it is used to test automobile and aircraft engines.
2. It is compact as compared to other dynamometer of same capacity.
3. The torque developed is smooth and continuous under all operating conditions.
4. The absorption power can be changed by changing D.C. current.

Disadvantages of Eddy Current Dynamometer

1. It can not produce any torque at zero speed.
2. It produces small torque at low speed.

Applications of Eddy Current Dynamometer

1. It can measure power upto 300 HP with maximum speed 6000 rpm.

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Figure 1: Liquid in Glass Thermometer.

Construction & Working of Liquid in Glass Thermometer

Liquid in Glass Thermometer consists of glass envelope filled in with liquid and calibrated indicating scale marked on it. The envelope comprises a thick walled glass tube with a capillary bore and a spherical or cylindrical bulb filled with liquid. The two parts are fused together and top of capillary tube is sealed. The size of capillary tube depends on:

1. Size of sensing bulb.
2. Liquid filled in it.
3. Temperature range of thermometer.

The change in temperature causes fluid to expand and rise up the stem. The volume enclosed in stem above liquid may contain vacuum or it may be filled with inert gas. The liquid filled in thermometer is called thermometric fluid.

Table 1: The different thermometric fluids used

Generally, liquid used is mercury.

Advantages of Mercury

1. It has linear coefficient of expansion.
2. It does not stick to glass surface.
3. It is clearly visible in transparent glass.

Advantages of Liquid in Glass Thermometer

1. Simple to use and relatively cheaper.
2. No auxiliary power is required.
3. Wide range of measurement.
4. Good accuracy.

Disadvantages of Liquid in Glass Thermometer

1. They are fragile in nature.
2. Can not be adopted for automatic recording.
3. Time lag in measurement.

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Fig. 1: Bimetallic Thermometer (Bimetallic strip in spiral form)

Working Principle of Bimetallic Thermometer

Two different materials having different coefficient of thermal expansion rigidly joined together, one on other to form a bimetallic strip. 

Construction & Working of Bimetallic Thermometer

When bimetallic strip is fixed at one end, and heated from free end, it bends in the direction of material having low thermal coefficient of expansion.

Fig. 2: Bimetallic Thermometer (Bimetallic strip in spiral form.)

The bending moment of free end is connected to the pointer, which moves over calibrated scale. Usually bimetallic strip is wound in the form of helix form (see Figure 1) or in spiral form (see Figure 2). Its one end is fastened permanently to outer casing to form stopper and other end is connected to pointer.

A pointer moves over a circular dial as helix coils and uncoils with temperature variation. A typical bimetallic thermometer used between – 40°C to 320°C consist of Invar (36% Ni, 64% Fe) has low coefficient of expansion when joined to Ni-Mo alloy. A thermal well is provided at its outer tubing made of brass, steel or stainless steel for protection against corrosion and breakage.

Advantages of Bimetallic Thermometer

1. Rugged construction.
2. Simple and convenient design.
3. No maintenance is required.
4. It gives ± 1% accuracy.
5. Overloading can be tolerated.
6. Low cost.

Disadvantages of Bimetallic Thermometer

1. Material is subjected to creep at high temperature.
2. Speed of response is low.

Applications of Bimetallic Thermometer

1. Frequently used in simple ON-OFF temperature control devices (Thermostats).
2. In clocks to compensate temperature changes in clock mechanism.
3. In circuit breakers to protect circuit from excess current.
4. Time delay relays.
5. Lamp flashers.

Advantages of Bimetal Helix Thermometer

1. Low cost.
2. Tough, not easily broken.
3. Easily installed and maintained.
4. Wide temperature range.
5. Deflection is more, hence sensitivity is more.

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Figure 1: McLeod Gauge.

Working Principle of McLeod Gauge

The operating principle of McLeod gauge is that, a gas of known volume is compressed to a smaller volume. This reduction in volume increases the pressure which is measured using a manometer.

Construction of McLeod Gauge

McLeod gauge is actually a modified form of mercury manometer. It consists of two limbs and a mercury reservoir. One limb consists of a large bulb over which a capillary (C) is connected. This capillary has a uniform bore of cross sectional area (A). The other limb consists of a capillary tube ‘B’ towards its left side. The length and area of this tube are equal to the length and area of the capillary tube ‘C’. The tube C and B are positioned at same height. Thus, the McLeod gauge consists of three tubes A, B and C. The tube B is taken as the reference column, The vacuum pressure to be measured is applied through tube A.

Working of McLeod Gauge

Figure (2): McLeod Gauge.

The pressure to be measured is applied to the reference capillary tube (B). The mercury level is first kept at a position as shown in figure 2(a) by drawing the plunger. Now compress the mercury level by pushing the piston to a position shown in figure 2(b). This compression of piston seals off the gas present in the bulb and capillary tube A. Again the piston is pushed until the mercury level goes tip to the zero mark in capillary tube B. This is shown in figure 2(c). The volume of the gas in the capillary tube is read by the scale. This becomes tite measure of applied vacuum.

According to Boyle’s law,

P_i . v  = P A_t h

Where

P_i = Applied pressure

v = Gas volume (known)

A_t = Area of the capillary tube A

h = Height or difference between the mercury levels present in the capillary tubes A and B.

\[{{\text{P}}_{\text{i}}}=\frac{\text{ }\!\!\gamma\!\!\text{ }{{\text{A}}_{\text{t}}}{{\text{h}}^{2}}}{\text{v}-{{\text{A}}_{\text{t}}}\text{h}}\]

Thus, pressure is measured using the above equation. McLeod gauges are used to measure the pressure of gases about 10^-4 torr.

Advantages of McLeod Gauge

1. This is considered as a standard device for measuring vacuum, since the pressure can be calculated from the dimensions of the gauge.
2. Its operation does not depend on the mixture of gauges.
3. It is used as a reference standard in the calibration of other vacuum gauges.
4. Corrections need not be applied to the McLeod gauge.

Disadvantages of McLeod Gauge

1. It cannot be used to measure pressure of gases which doesn’t obey Boyle’s law.
2. Continuous measurement is not possible with this instrument.
3. The performance gets affected by the moisture

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