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of the mechanical input power is accounted for by the product of the reaction torque and the rotational
speed. In the motoring mode, torque and speed are controlled by adjustment of both field and armature
currents. Modern AC machines utilize regenerative input power converters to allow braking power to
be returned to the utility power line. In the motoring mode, speed is controlled by high-power, solid-
state, adjustable frequency inverters. Internal construction is that of a simple three-phase induction
motor, having neither brushes, slip rings, nor commutators. The absence of rotor windings allows for
higher speed operation than DC machines. Universal dynamometers are “four-quadrant” machines, a
term denoting their ability to produce torque in the same or opposite direction as their rotational velocity.
This unique ability allows the effective drag torque to be reduced to zero at any speed. Universal
dynamometers [25,28] are available in a relatively limited range of capacities (56–450 kW), with com-
mensurate torque (110–1900 Nm) and speed (4500–13,500 rpm) ranges, reflecting their principal appli-
cation in automotive engine development. Special dynamometers for testing transmissions and other
vehicular drive train components insert the DUT between a diesel engine or electric motor prime mover
and a hydraulic or eddy current brake [30].

Measurement Accuracy
Accuracy of power measurement (see discussion in [4]) is generally limited by the torque measurement
(±0.25% to ±1%) since rotational speed can be measured with almost any desired accuracy. Torque errors
can arise from the application of extraneous (i.e., not indicated) torques from hose and cable connections,
from windage of external parts, and from miscalibration of the load cell. Undetected friction in the
trunnion bearings of cradled dynamometers can compromise the torque measurement accuracy. Ideally,
well-lubricated antifriction bearings make no significant contribution to the restraining torque. In prac-
tice, however, the unchanging contact region of the balls or other rolling elements on the bearing races
makes them prone to brinelling (a form of denting) from forces arising from vibration, unsupported
weight of attached devices, or even inadvertently during the alignment of connected machinery. The
problem can be alleviated by periodic rotation of the (primarily outer) bearing races. In some bearing-
in-bearing constructions, the central races are continuously rotated at low speeds by an electric motor
while still others avoid the problem by supporting the stator on hydrostatic oil lift bearings [28].
Costs
The wide range of torque, speed, and power levels, together with the variation in sophistication of asso-
ciated instrumentation, is reflected in the very wide range of dynamometer prices. Suspension systems of
the type illustrated in Fig. 19.49 (for which the user must supply the rotating machine) cost $4000–6000,
increasing with capacity [12]. A 100-Hp (74.6 kW) portable water brake equipped with a strain gage load
cell and a digital readout instrument for torque, speed, and power costs $4500, or $8950 with more
sophisticated data acquisition equipment [26]. Stationary (and some transportable [23]) hydraulic dyna-
mometers cost from $113/kW in the smaller sizes [25] down to $35/kW for the very largest [27].
Transportation, installation, and instrumentation can add significantly to these costs. Eddy current
dynamometers cost from as little as $57/kW to nearly $700/kW, depending on the rated capacity, type
of control system, and instrumentation [24,25,28]. Hysteresis brakes with integral speed sensors cost
from $3300 to $14,000 according to capacity [29]. Compatible controllers, from manual to fully pro-
grammable for PC test control and data acquisition via an IEEE-488 interface, vary in price from $500 to
$4200. The flexibility and high performance of AC universal dynamometers is reflected in their compar-
atively high prices of $670–2200/kW [25,28].

References
1. Pinney, C. P. and Baker, W. E., Velocity Measurement, The Measurement, Instrumentation and Sensors
Handbook, Webster, J. G., Ed., Boca Raton, FL: CRC Press, 1999.
2. Timoshenko, S. Strength of Materials, 3rd ed., New York: Robert E. Kreiger, Part I, 281–290; Part II,
235–250, 1956.

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