Dynamometer: Introduction and Types

 

A dynamometer is a device used for measuring the torque and brake power required to operate a driven machine.
Dynamometers can be broadly classified into two types. They are:

1. Power Absorption Dynamometers: Power Absorption 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.
   Examples of power absorption dynamometers are Prony brake dynamometer, Rope brake dynamometer, Eddy current dynamometer, Hydraulic dynamometer, etc.
2. Power Transmission Dynamometers: In power transmission dynamometers the power is transmitted to the load coupled to the engine after it is indicated on some scale. These are also called torque meters.

Some of the different types of dynamometers are discussed below in brief.

Prony Brake Dynamometer:

Pony Brake is one of the simplest dynamometers for measuring power output (brake power). It is to attempt to stop the engine using 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.

The Prony brake shown in the above consists of a wooden block, frame, rope, brake shoes and flywheel. It works on the principle of converting power into heat by dry friction. Spring-loaded bolts are provided to increase the friction by tightening the wooden block.
The whole of the power absorbed is converted into heat and hence this type of dynamometer must the cooled.
The brake power is given by the formula

Brake Power (bp) = 2π NT

Where T = Weight applied (W) × distance (l)

Rope Brake Dynamometer:

The rope brake as shown in below figure is another device for measuring brake power of an engine. It consists of some 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 side to a loading device. The power is absorbed in friction between the rope and the drum. Therefore drum in rope brake requires cooling.

Rope brake dynamometers are cheap and can be constructed quickly but brake power can’t be measured accurately because of change in the friction coefficient of the rope with a change in temperature.
The brake power is given by the formula

Brake Power (bp) = π DN (W − S)

where D is the brake drum diameter,
W is the weight of the load and
S is the spring balance reading.

Eddy Current Dynamometer:

The working principle of eddy current dynamometer is shown in the figure below. It consists of a stator on which are fitted some 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 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 in internal combustion engine testing is controlled by regulating the current in the electromagnets.

The following are the main advantages of eddy current dynamometers:

1. High brake power per unit weight of dynamometer.
2. They offer the highest ratio of constant power speed range (up to 5 : 1).
3. Level of field excitation is below 1% of total power being handled by the dynamometer. Thus, they are easy to control and operate.
4. Development of eddy current is smooth hence the torque is also smooth and continuous under all conditions.
5. Relatively higher torque under low-speed conditions.
6. It has no intricate rotating parts except shaft bearing.
7. No natural limit to size, either small or large.

Hydraulic Dynamometer:

A hydraulic dynamometer as shown in the figure below works on the principle of dissipating the power in fluid friction rather than in dry friction.

• In principle, hydraulic dynamometer construction is similar to that of a fluid flywheel.
• Hydraulic dynamometer consists of an impeller or inner rotating member coupled to the output shaft of the engine.
• The impeller in this dynamometer rotates in a casing filled with a fluid.
• Due to the centrifugal force developed in the outer casing, tends to revolve with the impeller, but is resisted by a torque arm supporting the balance weight.
• The frictional forces generated between the impeller and the fluid are measured by the spring balance fitted on the casing.
• The heat developed due to the dissipation of power in Hydraulic dynamometer is carried away by a continuous supply of the working fluid.
• The output power 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 the casing and the impeller.

Absorption Dynamometers:

Transmission dynamometers are also called torque meters. They 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 the strain the of the strain gauge. A four arm bridge is used to reduce the effect of temperature, and the gauges are arranged in pairs such that the effect of axial or transverse load on the strain gauges is avoided.

Above figure shows the transmission dynamometer which employs beams and strain gauges for a sensing torque. Transmission dynamometers measure brake power very accurately and are used where continuous transmission of the load is necessary. These are mainly used in automatic units.