Electronic Brake

[lapira]

Hi,

I have a 3 phase motor connected to an electronic brake and whenever the brake turns on the motor stops normally. However, when the motor starts again and the brake removed I have an overload.
I would like to know how an electronic brake actually works so that i can understand better the braking action.

thanks in anticipation.

 

[busbar]

One type of brake disconnects 3ø AC from the motor stator and applies DC, terminated by a timer or zero-speed switch.

 

[OperaHouse]

The problem may be with the time delay between turning the brake off and turning the motor on. You made it sound like the brake stayed on till the next time the motor started. Is this overload electronic from a VFD? The magnetic field may not have collapsed or the brake is still electrically connected when the motor starts.

 

[jraef]

Assuming it is a DC injection brake (as most stand-alone brakes are) it is wired in paralell with the motor starter, and interlocked so that the 2 are never enegized at the same time. The DCB injects DC onto 2 windings of the AC motor, setting up a stationary magnetic field in the stator. That in turn creates a counter rotating magnetic field in the rotor and produces torque in the opposite rotation as the rotor, which brings the load to a stop.

The strength of that field is directly proportional to the energy applied to it, so the braking power is no different than running / starting power as far as the thermal effect on the motor. In other words at a minimum, each brake cycle needs to be counted as a start cycle, and the overload relay needs to be in the circuit at all times to protect your motor. The fact that it is tripping would indicate that your duty cycle is stressful on your motor. It is tripping because it is trying to prevent a fire!

As I see it you have 3 choices.
1) Lower the brake energy (torque, voltage, current, whatever your DCB has for a setting). This will of course mean that your stopping time will be longer.

2) If this is unacceptable, you will beed to change to a larger motor. To avoid adding starting torque problems you may also consider using a Soft Starter. Many of them come with DCB options as well.

3) Remove the starter and brake and install a VFD with a Dynamic Brake package. It works differently, by converting the mechanical energy of your spinning load back into electricity, and dumping it off into a resistor bank as heat. They tend to work faster than a DCB, but cost more. If price is no object you can go with a Regenerative Brake system, which puts the energy back into the utility grid.



Quando Omni Flunkus Moritati

 

[tmahan]

Your message indicates that the overload is tripping during the restart after braking.

To me this suggest that the thermal overload is in the circuit (this is good) when the brake is actually firing. The DC current usually 2-3x the motor full load current (Jraef, is that right?)

It further sound to me like you are attempting to restart rather quickly after the stop before the motor has a chance to cool down from the heating caused by the brake. DC Injection Braking is from a thermal point of view quite hard on the motor. Large amounts of current is being provided and the cooling fan on the rotor is slowing down.
For this reason the DC braking counts as two starts on the number of starts per hour limitations on your motor.

The thermal element is doing excatly what it is supposed to do. The solution is to change the operational patter if possible or use different equipment as suggested above.

 

[jraef]

tmahan,
Yes the current through the brake can be 2-4X FLA, in older voltage controlled brake designs especially. They had no current feedback and so had not control of the current flow. In that design, as the motor slowed down, the current would rise, and at the end of the braking cycle could even spike up to LRA. Newer DCBs have current control so that the amount of braking torque can be controlled by a steady current application, which can be set as low as FLA. Even then, the current is still seen by the OL relay and contributes to the tripping curve as it should.

Quando Omni Flunkus Moritati