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5.4 Torque Measurement
5.4 Torque Measurement
5.4.1 Swinging Suspended Motor or Brake
With a driven machine the driving motor may be suspended in bearings, allowing it
to swing. The driving motor torque on the machine then generates a reaction torque
equal to the driving torque, apart from the bearing resistance. The reaction torque
is balanced with a lever arm and a force. Force determination may be performed
by a balance or a force transducer. A force transducer functions like a pressure
transducer. With a turbine, a brake is driven and similarly the reaction torque on the
brake is measured. The brake principle may take various forms. Most obvious is an
electrical generator with power supply to the grid or with dissipation by resistors.
Eddy current brakes are frequently used as well. Power is then dissipated by Foucault currents and the dissipation heat is removed by cooling. Older braking systems
applied friction on a drum (Prony brake) or a water ring impacting a blade system
(Froude brake). These older systems are less precise and difficult to adjust.
5.4.2 Calibrated Motor
Swinging motor suspension cannot be applied on a driven turbomachine with integrated driving motor, as often used with smaller fans and pumps. A solution may
consist in first taking off the electric motor and putting it on a test rig with a swinging suspended brake, as described above. The characteristics of the motor, together
with its controller may then be determined. A frequency-controlled asynchronous
motor is often used. For set frequency, torque and rotational speed are determined
as a function of output power. An example is detailed in Sect. 5.7.
5.4.3 The Torque Transducer
This device is mounted between the shafts of the driving and the driven machine.
An internal bar gets twisted by the torque transferred. Angle torsion is read out.
The problem with a torque transducer is the necessity to measure the torsion in a
rotating system. There are systems with strain gauges, so resistive determination,
with capacitive determination and with inductive determination. The latter features
four cored coils. Due to torsion, cores get deeper in two coils and less deep in the
other two. The coils are mounted in a Wheatstone bridge. The set-up is similar for
resistive and capacitive determination. The power supply of the bridge may be by
AC, transferred at high frequency by a rotary transformer. The generated voltage is
sent outside by a second rotary transformer. This type has the advantage of being
contactless. Slip ring systems exist as well. With those, power supply and read-out
may be by DC for resistive determination
5.4 Torque Measurement
5.4 Torque Measurement
5.4.1 Swinging Suspended Motor or Brake
With a driven machine the driving motor may be suspended in bearings, allowing it
to swing. The driving motor torque on the machine then generates a reaction torque
equal to the driving torque, apart from the bearing resistance. The reaction torque
is balanced with a lever arm and a force. Force determination may be performed
by a balance or a force transducer. A force transducer functions like a pressure
transducer. With a turbine, a brake is driven and similarly the reaction torque on the
brake is measured. The brake principle may take various forms. Most obvious is an
electrical generator with power supply to the grid or with dissipation by resistors.
Eddy current brakes are frequently used as well. Power is then dissipated by Foucault currents and the dissipation heat is removed by cooling. Older braking systems
applied friction on a drum (Prony brake) or a water ring impacting a blade system
(Froude brake). These older systems are less precise and difficult to adjust.
5.4.2 Calibrated Motor
Swinging motor suspension cannot be applied on a driven turbomachine with integrated driving motor, as often used with smaller fans and pumps. A solution may
consist in first taking off the electric motor and putting it on a test rig with a swinging suspended brake, as described above. The characteristics of the motor, together
with its controller may then be determined. A frequency-controlled asynchronous
motor is often used. For set frequency, torque and rotational speed are determined
as a function of output power. An example is detailed in Sect. 5.7.
5.4.3 The Torque Transducer
This device is mounted between the shafts of the driving and the driven machine.
An internal bar gets twisted by the torque transferred. Angle torsion is read out.
The problem with a torque transducer is the necessity to measure the torsion in a
rotating system. There are systems with strain gauges, so resistive determination,
with capacitive determination and with inductive determination. The latter features
four cored coils. Due to torsion, cores get deeper in two coils and less deep in the
other two. The coils are mounted in a Wheatstone bridge. The set-up is similar for
resistive and capacitive determination. The power supply of the bridge may be by
AC, transferred at high frequency by a rotary transformer. The generated voltage is
sent outside by a second rotary transformer. This type has the advantage of being
contactless. Slip ring systems exist as well. With those, power supply and read-out
may be by DC for resistive determination