safety relay and zero speed detector

Discussion in 'General Electronics Chat' started by ghanim, Jan 17, 2010.

  1. ghanim

    Thread Starter New Member

    May 21, 2009
    Hey guys I am new here and i had some problems with a safety relay (f121 tesch certainly) and i want a detailed description for its sematic and function, shortly i want to understand how it works and how i can diagonise its problems,
    also i have a zero speed detector and i want to know how it works and what are its applications
    The two devices are installed in a grinding machine and the zero speed detector is connected to the mill motor
  2. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    A quick Google search for the f121 tesch provided a data sheet explaining the functions and various connections.

    Is your zero speed switch an electronic unit supplied with voltage from a tachometer, or is it one of the mechanical units with the rotating magnets inside an aluminum cup like this one ?

    It detects zero speed of rotating equipment. In milling machines, it is frequently used to indicate full or nearly full stop of the rotating spindle. This may, in turn, provide a signal to the control system that would allow other functions such as an automatic tool change. In a manual milling machine, it is typically used to indicate that the motor speed has been reduced to zero or near zero speed in order to drop the plugging function on stop. (A plugging sequence would be 1;drop the forward contactor. 2;Pick up the reverse contactor applying reverse power to the motor {possibly through resistors} 3; monitor the zero speed switch in order to know when to drop the reverse contactor.)

    Most zero speed switches are adjustable and if not adjusted properly may result in the signal being sent either too early or too late. In the plugging sequence, if too late, the motor may actually begin to rotate in the reverse direction.

    As with most devices such as these, a drawing of how they are connected to the rest of the equipment would allow a much better explanation.
  3. Duane P Wetick

    Senior Member

    Apr 23, 2009
    Safety relays generally have a double contact set that cannot fail in a closed state. Zero Speed Switches (ZSS) are generally applied to the driven shaft, rather than the driver shaft (motor). Their application is to ensure the driven shaft actually comes up to the rated speed, before sealing in a motor starter contact, for example. They can also be used to detect overloads in a process. Does this information help you in any way?

    Regards, DPW [ Everyone's knowledge is in-complete...Albert Einstein]
  4. ghanim

    Thread Starter New Member

    May 21, 2009
    thanks guys this is a good start of learning about the two devices, I'm more concerning right now about the safety relay, i searched for f121 tesch and i found a 2 pages data sheet which gives a general idea about it i did attach this document to illustrate what about to say, in the document there are four wiring examples the one that is identical to my case here is the wiring example no. 1 with a little additions, first in the connection between s33 and s34 there is alot of NC points of different contactors which are controlling the process of the different motors in the machine and at the end there is a relay which is connected to the s34 point, also the connection between s21 and s22 is going through the safety bush button which consists of two NC switches, the NO contacts point 23-24 is conected to the DI in the plc then there is DO point which controls the process of relay which is located in the path of the connection between s33 and s34 mentioned earlier, I did attached the circuit in order to be more clear
    Now can any one tell me exactly the sequence of the events which are suppposed to happen if I press the safety push button? And what exatly these points mean (s21, s22, s33, s34, s11, s52) and how the dual channels work? because from the diagram in the front of the product they seem to me as they are the same channel
    please help me out I'm very fresh at these stuff and I really want to learn in details and quickley.
  5. Duane P Wetick

    Senior Member

    Apr 23, 2009
    I think that you should get an explanation of how this device is supposed to work in your system by the manufacturer. It seems there are a number of different control schemes that can be accomodated, and its up to the manufacturer to use the one that best fits the application. Its a good idea not to override any of the safety devices as someone may be hurt and any warranty may be voided. Sorry, I can't be more specific.

    Regards, DPW [ Everything has limitations...and I hate limitations.]
  6. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    Directly from the data sheet explanation:

    The dual channel operation shown in
    wiring example 1 includes crossfault
    monitoring between both E-stop circuits.
    That means in case of shorts between
    the two E-stop channels the F121 will
    de- activate the outputs. This is achieved
    by an electronic protection circuit in the
    safety relay. After elimination of the malfunction,
    the F121 is ready for operation
    The application with monitored start
    checks the start circuit (S33/S34) and will
    only activate the F121 if there is a
    leading edge in this circuit.

    If wired for autostartfunction the F121
    will be activated automatically by the
    supply voltage if the E-stop circuits and
    the feedback loop (X5/S33) are closed.
    In applications, where both E-stop
    circuits are not closed simultaneously,
    (e.g. safety gates) channel 2 has to be
    activated before channel 1.
    If the inputs will be activated with
    external 24VDC , the negative pole has
    to be connected to A2 (Light curtain
    The open collector semiconductor output
    may transfer the status of the F121
    via the terminals Y31-Y32 direct to a
    To control NC contacts from external
    contactors the feedback loop has to be
    wired in series between the start button
    respectively X5 and S33 (Autostart).

    I really couldn't explain it any better. The series connections of Q14.1 through Q24.1 will all need to be closed before K5.1 being closed will allow a start. Other than that, it is all covered on the data sheet.