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Curran919

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I see a lot of specsmanship on tach sensors concerning this feature. I don't understand exactly how its supposed to work. As I assume, if you have a 1 pulse/rev signal, all a ZSS should be able to do is count how long it has been since a pulse. But if its been 10s since a pulse, that doesn't mean there is zero speed. It just means the speed is less than 6rpm.

Is that 'time since pulse' threshold all there is to it?
John from PA

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Reply with quote  #2 
Curran, go to http://www.metrixvibration.com/tenants/metrix/documents/1079330.PDF#page222 and you will see how at least one OEM provides for a zero speed monitor.  The method is typical, in that it requires two speed sensors.  Not only can zero speed be detected, but reverse rotation can then also be detected.
Curran919

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Reply with quote  #3 
So i understand direction from phase quadrature.

But as far as I can tell, this monitor is only using two sensors as a type of redundancy. I don't see anything that fundamentally requires 2 speed sensors in order to determine zero speed.
Danny Harvey

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Reply with quote  #4 
Most zero speed sensors that I have used over the years use a disc with 20 or so magnets and one proximity sensor.

So your idea that they do not detect actual zero speed is true but off by a divisor of about 20.


Curran919

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Reply with quote  #5 

Danny, the assumption from my first post was for a single pulse per revolution application (like a keyphasor). When you have multiple pulses per revolution (like a hall effect sensor targeting a gear wheel), you can get below the same low speed threshold in less time and therefore 'assume' zero speed faster, as you say.

However, the zero speed attribute is being given to the specific sensor (or preprocessing thereof) with no indication of how many pulses/rev there could be for a given application.

John from PA

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Reply with quote  #6 
As far as the GE/BN 3500/50 Tachometer Module the following is from the maintenance manual:

The Zero Speed function requires inputs from two transducers. Voting logic between the two transducers minimizes false Zero Speed indication in the event of a transducer failure. To meet the Zero Speed alarming requirement, both transducers must have a valid input below 100 rpm, both transducers must observe the rotor speed fall below the Zero Speed setpoint for three consecutive readings, and the Zero Speed Enable switch must be enabled. If both transducers read zero rpm input for one minute and Zero Speed alarming is enabled, a Zero Speed alarm will occur.
Curran919

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Reply with quote  #7 
wow, so it really is trivial and has nothing to do with the transducer at all (as opposed to the tacho / frequency convertor).
Shurafa2

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Reply with quote  #8 
I dealt with zero speed pickups as an end user. The application I'm familiar with is on large turbines (power generation). These turbines had also key phasors and other speed control sensors.

The term zero speed is somehow misleading but it is the term that used for this kind of application. A zero speed pickup is used when the specifications of the other speed pickups are not suitable at low speeds, typically below 100 rpm. A zero speed pick does read speeds between 0-200 rpm with high reliability.  So, in other words, it is a proximity probe just like a key phasor but with special low-speed capability. Refer to the manual number below for more details.

If I recall correctly, the purpose of its installation is for low-speed operational logic requirements like slow roll, engaging/disengaging turning gears, starting stopping pumps, etc.

Regards- Ali M. Al-Shurafa
BN 3300 50 Zero Speed Manual.png 

OLi

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Reply with quote  #9 
There are examples of inductive pickup versions used also at least here in hydro power.
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Curran919

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Reply with quote  #10 
I think I figured it out. Zero speed is used to describe both the sensor and the tachometer/monitor.

In the sensor, zero speed sensors are the active sensors like hall effect and active eddy current probes where the amplitude is not related to the speed of the shaft. A non-zero speed sensor is the magnetic pickup (VR sensor) which is passive (unpowered) and gives a signal amplitude that is proportional to the shaft speed. Therefore at low speeds when the teeth of a gearwheel or reluctance disk pass the probe field volume, the rate of change of reluctance is low and therefore the amplitude of the [unamplified] signal is low. At some [low] speed, the TTL signal becomes unreliable.

In the monitor, zero speed sensing is simply a subset of underspeed detection. It is a binary/alarm output when the speed passes below 100rpm or 200rpm (so high!), regardless if the sensor + target pair is capable of reliably measuring lower speeds. Actually measuring 0 rpm is still functionally impossible.
OLi

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Reply with quote  #11 
There are various "creep" monitors for hydropower that may not measure 0 RPM but alarm for nearly anything but 0 RPM, I don't remember what the limit for creep is but it is minimal as it will indicate a leak or so in the water control system in a hydro turbine and that is not acceptable.
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