What is Dynamometer ?A dynamometer is an apparatus that measures mechanical force i.e. frictional resistance.

Knowing the frictional resistance, we may obtainThe torque transmitted The power of engine

Types of Dynamometer

Basic types:Absorption DynamometerTransmission DynamometerDriving DynamometerSome Other Types are:Water Brake DynamometerEddy Current DynamometerD.C. DynamometerA.C. Dynamometer

Absorption Dynamometer In absorption dynamometer, the entire

power produced by the engine is absorbed by the friction resistances & is transformed into heat during the process of measurement.

Good for power systems as they measure power of generators, electric motor, turbines & engines.

Types of Absorption Dynamometer

Prony Brake DynamometerRope Brake DynamometerInductor Dynamometer

Rope Brake DynamometerIt is simplest form of

absorption dynamometer as shown in figure;

A rope is wound round a

large wheel called brake drum pulley coupled to the shaft whose power is to be measured.

Explanation

The rope is attached to a spring balance at one end to a deadweight W at the other end.

The torque is T = (W-S) r S = reading of spring balance r = the effective radius r = (D + d) / 2The power is P = 2πNT/60

Horse power H.P. = 2πN (W – S) r / 735 ҳ 60

Advantages & Disadvantages

AdvantagesSimple in operation & easy to makeSuitable for measurement of wide range of

powerSteadier in operation & requires no lubricationDisadvantagesOutput of driving machine is dissipated as heatDo not stabilize the speed of machine under

test as there is no increase in torque with increase of speed

Prony Dynamometer

A rather old device like the rope brake for the measurement of torque and power from machines; as shown in figure

Transmission Dynamometer

In transmission dynamometer, principle involved is that the power measured being transmitted either to or from the dynamometer is not absorbed or dissipated.

The power after measurement is available in useful mechanical or electrical form

Types of Transmission Dynamometer

Epicyclic – train Dynamometer Belt transmission DynamometerTorsion Dynamometer

Torque measurements on (Transmission Dynamometer)

Water Brake DynamometerThe schematic shows the most

common type of water brake, the variable level type.

Water is added until the engine is held at a steady rpm against the load. Water is then kept at that level and replaced by constant draining and refilling, which is needed to carry away the heat created by absorbing the horsepower (which in itself is a measure of power output of the engine). The housing attempts to rotate in response to the torque produced but is restrained by the scale or torque metering cell which measures the torque.

Water Brake Dyno Setup

Advantages & Disadvantages

Advantages: Low Cost Low InertiaDisadvantages:Difficult to Control High Maintenance

Eddy Current Dynamometer

• The eddy current dynamometer shown generates electrical currents by rotating a disk in a magnetic field.

• The resultant currents are dissipated inside the machine.

• An electrical input allows the user to dictate the strength of the field and thus amount of dissipation for a given speed.

Eddy Current Dyno Setup

Advantages & Disadvantages

AdvantagesThe speed-torque characteristics make the eddy

current dynamometer ideal for engine testing, and its versatility also allows effective use in testing transmissions, turbines, electric motors, gears, pumps and many other machines

Precise Control & Low Maintenance Widely accepted in the automotive industry

DisadvantagesHigh Inertia Moderate Cost

D.C. Dynamometer

D.C. Electric type dynamometers so called Cradled dynamometer are widely used for power & torque measurement of IC engines & other mechanical equipments

The basic arrangement is shown in figure:

Some facts:The D.C. machine can act as both generator

and motor. Thus , the D.C. machine can act both as absorption dynamometer and a transmission dynamometer

When D.C. machine is made to work as a dc motor, it acts as driving dynamometer

D.C. Absorption Dynamometer

Electrical Dynamometer Setup

(showing engine, torque measurement arrangement and tachometer)

ExplanationFig shows d.c. generator which act as absorption

dynamometer. The generator is mounted on low friction trunnion bearings & is driven by test machine (IC engine). The rotation is opposed when load is connected to armature circuit of the D.C. machine.

The power absorbed by the resistive load is P = V² / R V = the output voltage of

generator R = the resistance of the load The resisting torque can be controlled by varying either

V or R.The range of d.c. dynamometer when used as an

absorption dynamometer is 5000 H.P.

Advantages & DisadvantagesAdvantages:Fast Response Air Cooled (No water required) Line RegenerativeDisadvantages:High Cost (Dynamometer and Power Amplifier) High Inertia Requires high amperage electrical service

A.C. DynamometerAn AC dynamometer is

similar to a DC dynamometer except it uses an AC motor and is powered by alternating current.

The important features of A.C. Dynamometers include fast response, flexible control & energy conservation since 85 % of the energy absorbed is returned back to the power lines.

Advantages & DisadvantagesAdvantages:Very Low Inertia Very Fast Response Line RegenerativeDisadvantages:High Cost Requires high amperage electrical service

Servo Controlled DynamometerIn this arrangement an engine of an automobile

is automatically controlled using D.C. dynamometer. Tape recordings of engine torque and speed under actual driving conditions are utilized to reproduce these conditions in Lab.

The dynamometer has a load cell which has strain gauges. The output of the strain gauges bridge is proportional to the actual torque.