AC 3 Phase Induction Motor Brake

Yep, it's just like brakes on a car, they have pads that wear and need periodoc adjusting or replacing. If it was working to stop in mid-travel and no longer is, this is the likely reason.

It also might be that the brake coil is separately energized (uncommon) and the circuit in that brake control is malfunctioning or is not intended to be used in mid-travel, so it has a timer in it. Sometimes on doors, the brake is only intended to be used as a security measure, not to stop the moving door, so the brake coil is only de-energized a few seconds AFTER the motor is turned off. Your first step should be to understand the roll of the brake in your door system.

Gravity and inertia are working against your braking action in the down direction. In the up direction gravity cancels the inertia very quickly and static friction kicks in on the brake pads. When the motion ceases, the static friction provided by the break pads will keep it from moving. In the down direction your breaks are only able to provide dynamic friction which is always lower than static friction.

A good alternate motor would be a servo motor. It does not need a brake but it does need power to hold the door in any position you want. And, it will stop it more precisely than an induction brake-motor.

Very good point No60! I wonder if a worm gear drive with a servo motor would qualify in the event of a power failure?

Generally no. Even those of higher ratios cannot be perfectly depended on to be self-locking.

It can be due to wearing of brake liner or brake needs adjustment. If the brake is D.C then it can be due to residual magnetism in the brake coil which does not release brake instantaneously. This problem can be solved by providing additional contractor in brake coil circuit to stop the residual current.

There may be two brakes, one dynamic and one mechanical.

The dynamic one is there to stop the door, the other to hold it there.

If a contact is burnt or wiring damaged, the dynamic brake will/may not work properly and not slow the door down abruptly as it should, which may explain the symptoms you are seeing......maybe just the mechanical brake is working.

Just a possibility!!

Note.

A dynamic brake is just removing 3 phase power to the motor and shorting the three motor connections together......nothing more, nothing less.....

Many (here!) do not understand it fully, but it is actually extremely simple.....works on both AC & DC motors. It is built into quality electric drills, bot Battery and mains.....if the drill does not immediately stop when required, its cheap crap!!

To overcome the effect of the gravity and inertia when going in downward direction you may consider providing electrical braking through DC injection in any two phases of the motor for a short duration and then applying mechanical brake for holding the door. However, electrical braking will not be needed in the upward direction.

This scheme will ensure fast stoppage of motor in downward direction and less wear and tear of the brake lining.