About EOT crane hoist motor

Hi,

You are applying the Plugging type of braking. This makes the revesre the polarity of the motor and hence the reversal of rotation.

N.Suresh

<...Why free fall does not happen??...>

Is there a worm-gear drive somewhere in the mechanism (worm gears cannot drive the input shaft from the output shaft)?

Hoisting Drive motor coupled thro a reduction gear box , out put speed is very less , it never allow the load free.Whenever operator lowering the load he will be taking care by inching operation stop & start not pressing the button permanently (unless it is a safe location)some cases thruster mechanism take care of brakes(it will not rlease the brake fully) Mr.Pw slack also indicated the same point.I hope I cover up ,Further doubt pm me

The manufacturer designs the crane to pay out to prevent two-blocking. Generally there is a primary upper limit switch, and a back-up limit switch (refered to as the plugging switch). When hoisting up, and there is a failure of the primary limit switch, initiation of the plugging switch will reverse the hoist motor to prevent two-blocking of the hoist. Now this is a good feature, as long as your energy phases are correct. If someone working on the crane mis-wires it or somehow the the power supply (e.g. power in the building) for the crane is reverse-phased, the exact opposite will occur when the plugging switch is initiated, the hoist will go up resulting in two-blocking.

This all happens because the primary limit switch and plugging switch are wired in Series with the hoist up directional contactor. If you want to provide an adequate degree of safety, it would probably only take a simple alteration to wire the plugging switch to a circuit different from the primary limit switch. Wire it in series with all pendant control functions (i.e. hoist up, hoist down, trolley left, trolley right, bridge forward & bridge aft). In this configuration, actuation of the plugging switch will shutdown the crane operational power & the controls will not function. For this alteration, you will need to incorporate a by-pass switch (key switch) which will allow you to restore power to the crane. Hope this helps.

v/r

Michael "B"

Thanks all but it seem s that I was unable to explain my question.

@ Suresh, raj and guest While lowering the load plugging is not happening. motor continuously runs in reverse direction. when operator keeps master controller in 0th notch, thruster brake is applied.

@ pw slack, i dont know about gears but I have seen many free falls ( due to electrical problem)

If something wrong happens in rotor ckt then free fall happens. My question is whenever lowering the load motor runs in reverse direction ( plus gravity) and rated voltage is given (No drive is there) so what is there which prevents the free fall

there are always gear reducers between motor and hoist drum. The bigger the ratio of the gear reducer the more difficult is to drive the reducer from the output shaft. Since the hoisting drums are connected to the output shaft, the free fall of the load decreased but not 100% stopped. The motor produces a monment during rotating in the lowering direction. This moment is lowering the load. In case of drive of the hoist drum with slip ring electric motors the lowering speed can be controlled by resistors. The motorr moment prevents the speed accelaration due free fall.

As I understand the original question was "why there is no free fall while lowering load".

The strong magnetic field inside stator of motor infect holds the load and prevent free falling. The logic is like that there is a slight time delay before brake opening and application of voltage across terminals. If your motor is running by a VFD then you see that your brake pads have enormous life that is because starting and stopping is controlled by drive itself through magnetic field, first motor rpm become zero then brake applied.

The hoist controller of a typical DC crane is dealing with an excess of counter EMF. It actually uses the resistors in lowering to"brake " the motor, which is acting like a generator at this time. Less resistance = slower lowering