Overhead cranes braking systems.

Question

An overhead crane&rsquo;s 3-phase slip ring motor can single phase while operator is hoisting/lifting a load. This has a potential of the load suddenly dropping to the floor crashing the operator or damaging equipment. From the principle of operation of a 3-phase induction motor, if it single phases whilst running, motor keeps running (but won&rsquo;t restart if it trips until single phase problem is solved) until it trips on protection, eg, single phasing, fuses blowing, phase imbalance, winding temperature high etc. This therefore means that the motor keeps running but doesn&rsquo;t have enough torque to hold the load. 
 (Obviously during normal operation, the brakes are released but fail safe when operation is cancelled or stopped). 
 
 How best can such a scenario be avoided or how can it be stopped from happening since overhead cranes are used everyday in our industries.

mapj1 · Accepted Answer

It will be an odd design that is neither self breaking (so lowly geared that the maximum design load does not spin the motor) - smaller winch designs are like this, nor does any kind of brake come on under spring pressure if there is a power loss. Unless the loads are so small that no injury could ensue, the brake release circuit needs to be interlocked to the motor power - so if there is no power- the brakes go and stay on, I'm surprised it is not like that. 
 Where in the world are you ? I ask because in the EU that would be a failure to meet machinery directive. 
 
 Mike 
 PS 
 consider this extract from 
 EN 15011:2011 (E) 5.4.2 Braking systems 5.4.2.1 General All power driven motions shall be under the control of a braking system at all times. The braking systems shall be such that movements can be decelerated, the motions can be held and unintentional movements avoided. The systems shall be capable of bringing a fully loaded crane to rest from the highest speed it can attain. 5.4.2.2 Mechanical service brakes in power driven motions Only power released brakes shall be used and they shall maintain their ability to stop the motion, at all times. Brakes shall be protected from the ingress of substances within the environment, which are likely to have a detrimental effect on the performance of the brake. NOTE Where electrical braking systems are used, the associated mechanical brake is only subjected to limited use. Special attention therefore may be neede d to maintain the required mechanical braking torque, see 7.3.3. Mechanical service brakes shall engage automatically in the following cases: the control device returns to its neutral position; the power supply to the brake is interrupted; the emergency stop device is activated 
 
 etc.

AJJewsbury · Answer

Not really my areas, but to get the discussion going... 
 Sounds to me like the motor will have stalled - so will have stopped (at least momentarily as the load transitioned from going up to going down) in any event wouldn't have kept running in any normal sense. 
 The safety systems sound very dubious to me - phase monitoring relays are reasonably cheap and readily available - so I would have thought the machine should have gone into shutdown as soon as one line dropped - probably cutting off all supply to the motor and allowing the brakes to be applied. 
 - Andy.

Overhead cranes braking systems.

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