182
5 Performance Measurement
5.5 Rotational Speed Measurement
5.5.1 Pulse Counters
Instruments for measuring rotational speed are called tachometers. Most function
by generating and counting electric pulses. Pulse generating methods are mostly
magnetic or optical. With a magnetic pulse tachometer, there is a toothed wheel in
front of a magnetic transducer, which is a coil around a permanent magnet. Inductance is different depending on whether a tooth or a void is in front of the magnetic
transducer. An optical pulse tachometer is a disc with holes on the rotating shaft.
The light of a lamp reaches a photocell. The resistance of the photocell depends
on the incidence or the absence of light. The counter transfers its result to a digital
display. Handheld devices normally function with a laser beam. A reflective plate is
fixed onto the shaft. The reflected beam reaches a photocell.
5.5.2 The Speed Transducer
The term speed transducer mostly refers to an instrument with a magnetic transducer in front of a toothed wheel. Within the coil of the magnetic transducer an
alternating voltage is generated with a frequency and amplitude proportional to the
rotational speed. With a digital device, pulse counting supplies the read-out to a
display. With an analogous device, the variable voltage drives a needle.
5.5.3 Electric Tachometer
The term electric tachometer mostly refers to a DC generator producing a voltage
proportional to the rotational speed.
5.6 Laboratory Test of a Pelton Turbine
5.6.1 Test Rig
The Pelton turbine has a rotor with 110 mm mean diameter, provided with 16 buckets. The injector has a 20 mm outlet orifice and a manually operated regulating needle. The rotor is made of accurately cast bronze and fixed on a stainless steel shaft,
running in covered ball bearings. The shaft power is about 300 W at 1600 rpm, with
a 21 m head and a 2.15 l/s flow rate. Specific speed Ω s = 0.15 (see Chap. 7 for definition of specific speed). The water is supplied by a single-stage centrifugal pump.
5 Performance Measurement
5.5 Rotational Speed Measurement
5.5.1 Pulse Counters
Instruments for measuring rotational speed are called tachometers. Most function
by generating and counting electric pulses. Pulse generating methods are mostly
magnetic or optical. With a magnetic pulse tachometer, there is a toothed wheel in
front of a magnetic transducer, which is a coil around a permanent magnet. Inductance is different depending on whether a tooth or a void is in front of the magnetic
transducer. An optical pulse tachometer is a disc with holes on the rotating shaft.
The light of a lamp reaches a photocell. The resistance of the photocell depends
on the incidence or the absence of light. The counter transfers its result to a digital
display. Handheld devices normally function with a laser beam. A reflective plate is
fixed onto the shaft. The reflected beam reaches a photocell.
5.5.2 The Speed Transducer
The term speed transducer mostly refers to an instrument with a magnetic transducer in front of a toothed wheel. Within the coil of the magnetic transducer an
alternating voltage is generated with a frequency and amplitude proportional to the
rotational speed. With a digital device, pulse counting supplies the read-out to a
display. With an analogous device, the variable voltage drives a needle.
5.5.3 Electric Tachometer
The term electric tachometer mostly refers to a DC generator producing a voltage
proportional to the rotational speed.
5.6 Laboratory Test of a Pelton Turbine
5.6.1 Test Rig
The Pelton turbine has a rotor with 110 mm mean diameter, provided with 16 buckets. The injector has a 20 mm outlet orifice and a manually operated regulating needle. The rotor is made of accurately cast bronze and fixed on a stainless steel shaft,
running in covered ball bearings. The shaft power is about 300 W at 1600 rpm, with
a 21 m head and a 2.15 l/s flow rate. Specific speed Ω s = 0.15 (see Chap. 7 for definition of specific speed). The water is supplied by a single-stage centrifugal pump.