BRUEL & KJAER 4915 - Magnetic Transducer MM 0002; Use with Line Trigger Source

For further information on the MM 0012 Photoelectric Tachometer Probe, a separate
Product Data sheet is available.
3.3.3. Magnetic Transducer MM 0002
This is shown in Fig. 3.5 and is a variable reluctance type magnetic transducer which may
be used as a contact-free, velocity-sensitive pickup for triggering the 4913. It is ideal for
use in dirty environments, where triggering by optical means using the Photoelectric
Probe MM 0012 is likely to be restricted by oil and grease etc.
2700 12
Fig.3.5. The Magnetic Transducer MM 0002 plus associated dimensions
For optimal sensitivity the Magnetic Transducer should be mounted at a distance 1 to
2 mm away from the moving object to be investigated. If the object is neither ferromag-
netic nor electrically conductive, then one of the high-µ discs supplied with the Transduc-
er or a thin strip of copper should be cemented to the object or attached by means of ad-
hesive tape. Also supplied is a 1,2 m miniature coaxial cable with screw lock connectors
which may be used to connect the Transducer to the EXT. TRIGGER INPUT of the
Stroboscope when this is fitted with a 10-32 UNF to BNC Input Adaptor JP 0145. The
MM 0002 is a passive device and therefore does not require a power source.
For further information on the MM 0002 Magnetic Transducer, a separate Product Data
sheet is available.
3.4. USE WITH LINE TRIGGER SOURCE
With the "Line" TRIGGER SOURCE mode, the flash rate is automatically synchronized
with the line frequency of the mains supply used to power the 4913. This is of benefit for
investigations on electric motors and other mains powered equipment. Provided that the
motion frequency of the equipment is an exact integer multiple (eg., 1, 2, 3 etc.) or sub-
multiple of. the supply frequency, then it will provide a stationary image of the equipment
without the need for manual adjustment to the 4913 or having to use a separate external
trigger source. For determining the exact motion frequency of the equipment, use can be
made of the "Phase Delay" mode as described in item 7 of section 3.2.1.
Where induction motors or friction coupled machine parts which are subject to slip are
examined, the motion frequency will not be an exact integer multiple or sub-multiple of
the supply frequency. Provided that the slip frequency is less than 5 Hz (ie., 300 r/min),
then a stationary image can be obtained utilizing the "Slow Motion" mode with the "Line"
mode. If the coupling ratio Kr and synchronous frequency f s of the motor are known (see
manufacturer's specifications), then the percentage slip S
1
can be determined using the
relation:
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