Siemens Milltronics MFA 4p Operating Instructions Manual page 44

Technical Reference
A.1 Principles of Operation
MFA 4p Operation
The MFA 4p provides a short circuit protected, +24 V DC unregulated supply to the preamp.
In the event that the interconnecting wiring is shorted, output current from the MFA 4p is
automatically limited and the on-board alarm relays are de-energized to indicate failure.
The output current pulses from the pre-amp are super-imposed onto the DC current supply.
These are monitored by Probe LED 1, which is illuminated at the rate of the incoming pulses
and is useful for positioning the probe.
The rate at which the pulses are received by the MFA 4p is compared to a setpoint reference
signal from the time base generator.
Although two pulses within range are required to energize the relays, as long as the
frequency of the incoming pulses exceeds the setpoint frequency (or is less than that of the
setpoint in the case of overspeed detection), the MFA 4p keeps the alarm relays energized.
The reference generator is frequency adjustable by the pulses per minute (ppm) switch and
potentiometer.
The alarm relays will de-energize after two time constants of the setpoint when the frequency
of the incoming pulses falls below that of the setpoint (or exceeds that of the setpoint in the
case of overspeed detection). The relay status is indicated by Relay LED 2, which is
illuminated when the relays are energized (normal).
The MFA 4p has a 0 to 60 second time delay feature, allowing the monitored device to
accelerate to normal running speed before monitoring begins.
This feature is activated when power is applied to the MFA 4p in parallel with the motor
starter contact coil. The time delay circuit simulates normal operating conditions for the
amount of time as set by the Start Delay potentiometer, keeping the alarm relays energized.
If the monitored device does not reach normal speed before the set time period, the relays
will de-energize giving an alarm condition. This feature is not applicable in the overspeed
detection mode.
42
Operating Instructions, 08/2017, A5E33988839_AB
Milltronics MFA 4p