Låghastighetsvakt typ TVRD
TVRD är en låghastighetsvakt där varvtalet är inställbart på dels, medurs variabelt stigande respektive fallande varvtal och dels moturs variabelt stigande respektive fallande varvtal.
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- TVRD indikerar på om maskinen i fråga fungerar.
- TVRD indikerar på om maskinen håller den angivna hastigheten.
- Om motorns hastighet stiger eller faller utanför det satta varvtalsområdet kan signal fås för indikering av detta.
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Vakten har skyddsform S43 enligt SEN 21 21. Dammsäkert och striltättutförande.
TVRD har ersatt den tidigare låghastighetsvakten TVRB 12, 14 och 18.
Design
The monitor is comprised of a rotor, a stator and two contact units integrated in a pressure die-cast light-alloy enclosure. The enclosure is provided with two conduit entries thread size Pr 18.6 to allow installation of cables. The cable entries are sealed by two plugs on delivery. The rotor itself is manufactured from a permanently mag-netised aluminium cobalt alloy and is supported by two ball bearings.
The stator is also supported in the same manner as the rotor and can be rotated approximately 15° in both directions from the neutral position. A microswitch is provided at each end position, these microswitches being actuated by the stator. The neutral position of the stator is maintained by two helical springs, the tension on the springs being adjusted by means of two setting screws. Microswitches and adjustment screws are positioned behind a cover in the housing.
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Operation
When rotating, the rotor exerts a torsional moment on the stator of such a magnitude that the contact unit is actuated, the contacts changing position. If the speed falls below a certain limit, the contact unit will reset.
The operating limits are set by means of two setting screws. One contact unit is provided for each direction of rotation.
Applications
• Disconnecting current when counter-current braking.
When counter-current braking, the motor is switched to reversing thus producing a large braking torque. This method of braking is suitable for machines which must be stopped often in order to change work pieces, to carry out measurements and the like, and enables a considerable saving of time. However, the current must be broken at a precise instant. If it is broken too quickly, the desired saving of time is not obtained, and if the current is broken too slowly, the motor may change direction of rotation which is followed by the hazards that such malfunction signifies.
If the speed monitor is set so that contacts execute a changeover at approximately 40-50 r/m, when the speed is decreasing. the machine will come to a standstill virtually at the same time as the current is broken.
• Indication of whether machines are running or not.
• Indication of whether machines are running at the desired speed.
If motors exceed or fall below the speed set, signals can be arranged to indicate these conditions.
