Page 436 - The Mechatronics Handbook

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Uneven retarding forces associated with fluctuating coefficients of friction generally make rubbing
friction a poor way to generate drag torque. Nevertheless, because they can be easily constructed, ad hoc
variations of prony brakes, often using only bare ropes or wooden cleats connected by ropes or straps,
find use in the laboratory or wherever undemanding or infrequent power measurements are to be made.
More sophisticated prony brake constructions are used in standalone dynamometers with self-contained
cooling water tanks in sizes up to 746 kW (1000 Hp) for operation up to 3600 rpm with torques to
5400 Nm [23]. Available in stationary and mobile models, they find use in testing large electric motors
as well as engines and transmissions on agricultural vehicles. Prony brakes allow full drag torque to be
imposed down to zero speed.
William Froude (1810–1879) [24] invented a water brake (1877) that does not depend on rubbing
friction. Drag torque within a Froude brake is developed between the rotor and the stator by the momen-
tum imparted by the rotor to water contained within the brake casing. Rotor rotation forces the water
to circulate between cup-like pockets cast into facing surfaces of both rotor and stator. The rotor is
supported in the stator by bearings that also fix its axial position. Labyrinth-type seals prevent water
leakage while minimizing frictional drag and wear. The stator casing is supported in the dynamometer
frame in cradle fashion by trunnion bearings. The torque that prevents rotation of the stator is measured
by reaction forces in much the same manner as with the prony brake. Drag torque is adjusted by a valve,
controlling either the back pressure in the water outlet piping [25] or the inlet flow rate [26] or sometimes
(to allow very rapid torque changes) with two valves controlling both [27]. In any case, the absorbed
energy is carried away by the continuous water flow. Other types of cradle-mounted water brakes,
while externally similar, have substantially different internal constructions and depend on other prin-
ciples for developing the drag torque (e.g., smooth rotors develop viscous drag by shearing and turbu-
lence). Nevertheless, all hydraulic dynamometers purposefully function as inefficient centrifugal pumps.
Regardless of internal design and valve settings, maximum drag torque is low at low speeds (zero at
standstill) but can rise rapidly, typically varying with the square of rotational speed. The irreducible
presence of some water, as well as windage, places a speed-dependent lower limit on the controllable drag
torque. In any one design, wear and vibration caused by cavitation place upper limits on the speed and
power level. Hydraulic dynamometers are available in a wide range of capacities between 300 and 25,000
kW, with some portable units having capacities as low as 75 kW [26]. The largest ever built [27], absorbing
up to about 75,000 kW (100,000 Hp), has been used to test propulsion systems for nuclear submarines.
Maximum speeds match the operating speeds of the prime movers that they are built to test and therefore
generally decrease with increasing capacity. High-speed gas turbine and aerospace engine test equipment
can operate as high as 30,000 rpm [25].
In 1855, Jean B. L. Foucault (1819–1868) [22] demonstrated the conversion of mechanical work into
heat by rotating a copper disk between the poles of an electromagnet. This simple means of developing
drag torque, based on eddy currents, has, since circa 1935, been widely exploited in dynamometers.
Figure 19.51 shows the essential features of this type of brake. Rotation of a toothed or spoked steel rotor
through a spatially uniform magnetic field, created by direct current through coils in the stator, induces
locally circulating (eddy) currents in electrically conductive (copper) portions of the stator. Electromag-
netic forces between the rotor, which is magnetized by the uniform field, and the field arising from the
eddy currents, create the drag torque. This torque, and hence the mechanical input power, are controlled
by adjusting the excitation current in the stator coils. Electrical input power is less than 1% of the rated
capacity. The dynamometer is effectively an internally short-circuited generator because the power
associated with the resistive losses from the generated eddy currents is dissipated within the machine.
Being heated by the flow of these currents, the stator must be cooled, sometimes (in smaller capacity
machines) by air supplied by blowers [23], but more often by the continuous flow of water [25,27,28].
In dry gap eddy current brakes (the type shown in Figure 19.51), water flow is limited to passages within
the stator. Larger machines are often of the water in gap type, wherein water also circulates around the
rotor [28]. Water in contact with the moving rotor effectively acts as in a water brake, adding a nonelec-
tromagnetic component to the total drag torque, thereby placing a lower limit to the controllable torque.
Windage limits the minimum value of controllable torque in dry gap types. Since drag torque is developed

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