Dynamometer | Types Of Dynamometer used For Torque measurement.
Measurement of brake power:
The brake power measurement involves the determination of the torque and the angular speed of the engine output shaft. The torque measuring device is called a dynamometer.
Dynamometers can be broadly classified into two main types,
- power absorption dynamometers and
- transmission dynamometer.
These dynamometers measure and absorb the power output of the engine to which they are coupled. The power absorbed is usually dissipated as heat by some means. Example of such dynamometers is prony brake, rope brake, hydraulic dynamometer, etc.
In transmission dynamometers, the power is transmitted to the load coupled to the engine after it is indicated on some type of scale. These are also called torque-meters.
(a) Prony brake dynamometer
One of the simplest methods of measuring brake power (output) is to attempt to stop the engine by means of a brake on the flywheel and measure the weight which an arm attached to the brake will support, as it tries to rotate with the flywheel.
It consists of wooden block mounted on a flexible rope or band the wooden block when pressed into contact with the rotating drum takes the engine torque and the power is dissipated in frictional resistance. Spring-loaded bolts are provided to tighten the wooden block and hence increase the friction.
The whole of the power absorbed is converted into heat and hence this type of dynamometer must the cooled. The brake horsepower is given by
BP = 2p NT
where, T = W × l
W being the weight applied at a radius l.
(b) Rope brake dynamometer
It consists of a number of turns of rope wound around the rotating drum attached to the output shaft. One side of the rope is connected to a spring balance and the other to a loading device. The power is absorbed in friction between the rope and the drum. The drum therefore requires cooling.
Rope brake is cheap and easily constructed but not a very accurate method because of changes in the friction coefficient of the rope with temperature.
The bp is given by
bp = p DN (W − S)
where, D is the brake drum diameter, W is the weight in Newton and S is the spring scale reading.
(c) Hydraulic Dynamometer
Hydraulic dynamometer works on the principle of dissipating the power in fluid friction rather than in dry friction.
-In principle its construction is similar to that of a fluid flywheel.
-It consists of an inner rotating member or impeller coupled to the output shaft of the engine.
-This impeller rotates in a casing filled with fluid.
-This outer casing, due to the centrifugal force developed, tends to revolve with the impeller, but is resisted by a torque arm supporting the balance weight.
-The frictional forces between the impeller and the fluid are measured by the spring-balance fitted on the casing.
-The heat developed due to dissipation of power is carried away by a continuous supply of the working fluid, usually water.
-The output can be controlled by regulating the sluice gates which can be moved in and out to partially or wholly obstruct the flow of water between impeller, and the casing.
Eddy Current Dynamometer
It consists of a stator on which are fitted a number of electromagnets and a rotor disc made of copper or steel and coupled to the output shaft of the engine. When the rotor rotates eddy currents are produced in the stator due to magnetic flux set up by the passage of field current
in the electromagnets. These eddy currents are dissipated in producing heat so that this type of dynamometer also requires some cooling arrangement. The torque is measured exactly as in other types of absorption dynamometers, i.e. with the help of a moment arm. The load is
controlled by regulating the current in the electromagnets.
The following are the main advantages of eddy current dynamometer:
(a) High brake power per unit weight of dynamometer.
(b) They offer the highest ratio of constant power speed range (up to 5 : 1).
(c) Level of field excitation is below 1% of total power being handled by dynamometer, thus, easy to control and programme.
(d) Development of eddy current is smooth hence the torque is also smooth and continuous under all conditions.
(e) Relatively higher torque under low speed conditions.
(f) It has no intricate rotating parts except shaft bearing.
(g) No natural limit to size-either small or large.
(e) Swinging Field d.c. Dynamometer
Basically, a swinging field d.c. dynamometer is a d.c. shunt motor so supported on trunnion bearings to measure their action torque that the outer case and filed coils tend to rotate with the magnetic drag. Hence, the name swinging field. The torque is measured with an arm and weighing equipment in the usual manner.
Many dynamometers are provided with suitable electric connections to run as motor also. Then the dynamometer is reversible, i.e. works as motoring as well as power absorbing device.
-When used as an absorption dynamometer it works as a d.c. generator and converts mechanical energy into electric energy which is dissipated in an external resistor or fed back to the mains.
-When used as a motoring device an external source of d.c. voltage is needed to drive the motor.
The load is controlled by changing the field current.
It is also an absorption type of dynamometer in that when driven by the engine it absorbs the engine power. Such dynamometers are useful mainly for rough testing and running. The accuracy of the fan dynamometer is very poor. The power absorbed is determined by using previous calibration of the fan brake.
Transmission dynamometers, also called torque meters, mostly consist of a set of strain-gauges fixed on the rotating shaft and the torque is measured by the angular deformation of the shaft which is indicated as strain of the strain gauge. Usually, a four arm bridge is used to reduce the effect of temperature to minimum and the gauges are arranged in pairs such that the effect of axial or transverse load on the strain gauges is avoided.
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