MEASUREMENT
OF POWER
Torque
is exerted along a
rotating shaft. By measuring this torque
which is exerted along
a rotating shaft,
the shaft power
can be determined.
For torque measurement
dynamometers are used.
T = F.r
P = 2πNT
Where, T –Torque, F –Force at a known radius r, P
–Power
Types
of dynamometers
·
Absorption dynamometers
·
Driving dynamometers
·
Transmission dynamometers
Absorption dynamometers
The dynamometer absorbs the mechanical
energy when torque is measured. It dissipates mechanical energy (heat due to
friction) when torque is measured. Therefore, dynamometers are used to measure
torque/power of power sources like engine and motors.
Mechanical Dynamometers
In prony brake, mechanical energy is converted into
heat through dry friction between the wooden brake blocks and the flywheel
(pulley) of the machine. One block carries a lever arm. An arrangement is
provided to tighten the rope which is connected to the arm. Rope is tightened
so as to increase the frictional resistance between the blocks and the pulley.
Power dissipated, P
= 2πNT/60
The capacity of proney brake is limited due to wear
of wooden blocks, friction coefficient varies. So, it is unsuitable for large
powers when it is used for long periods.
Eddy Current
Dynamometer
Basically an electrical dynamometer of absorption
type, used to measure power from a source such as engine or a motor. When a
conducting material moves through a magnetic flux field, voltage is generated,
which causes current to flow. If the conductor is a wire forming, a part of a
complete circuit current will be caused to flow through that circuit and with
some form of commutating device a form of A.C or D.C generator may result.
An eddy current dynamometer is shown
above. It consists of a metal disc or wheel which is rotated in the flux of a
magnetic field. The field if produced by field elements or coils is excited by
an external source and attached to the dynamometer housing which is mounted in
trunnion bearings. As the disc turns, eddy currents are generated. Its reaction
with the magnetic field tends to rotate the complete housing in the trunnion
bearings. Water cooling is employed.
Hydraulic or Fluid
Friction Dynamometer
A rotating disk that is fixed to the driving shaft,
Semi-elliptical grooves are provided on the disc through which a stream of
water flows. There is a casting which is stationary and the disc rotates in
this casing. When the driving shaft rotates, water flow is in a helical path in
the chamber. Due to vortices and eddy-currents setup in the water, the casting
tends to rotate in the same direction as that of the driving shafts. By varying
the amount of water, the braking action is provided. Braking can also be
provided by varying the distance between the rotating disk and the casting.The
absorbing element is constrained by a force-measuring device placed at the end
of the arm of radius r.
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