Suggestion: In very simple terms and hardware, the motor has to be connected to resistors, while the motor power supply is disconnected or its voltage reduced. Some AC motor drives have this feature. Visit
http://www.designrouge.com/demo/qds/regen.htm
http://www.ipcresistors.com/db.htm
http://www.cressall.com/PAGES/DYNBR.HTM
http://www.cressall.com/PAGES/DBRLFLT.HTM
http://www.railpage.org.au/ausrail/98april/0138.html
http://www.filnor.com/resistors.htm
http://www.gridcom-mining.com/
etc. for more info.

Refer to the Standard Handbook for Electrical Engineers, Fink & Beaty, 13th edition - Section 20.  Standard methods of braking AC motors are -
- Dynamic braking by application of DC to one phase of the motor after disconnecting from the AC supply
- Plugging, by reversing the motor connections to reverse the direction of rotation - needs auxiliary equipment such as zero-speed switches

Additional info is available from the manufacturers websites - check Cutler Hammer, ABB, GE etc.

what's the purpose of these brakes ?
is it to investigate the characteristics of that motor, or maybe to decrease it's power e.t.c. ?

ognen, it is to stop the motor.  Think about an elevator.  How do you stop it at the floor you want?

Lewish,

Not only that but braking may be required in an emergency in certain situations!

Regards,

Lyledunn

Lewish, Elevators use an external brake (like a car drum/disc brake) to stop the motor. This is because the motor not only needs to be stopped, but also needs to be "locked" when it is stationary, to prevent it from creeping.
Torque, if you need the motor to be locked solid while stationary, this might be the best way to do it.

ooge, I know they "lock" at the floor with an external brake, but I am also aware of some that brake the motor during slow down.  It extends brake life.  And, heat is easier to change than brake shoes.

Hi Everyone,

These are common on machine tools where a person caught up in the rotating parts contacts a micro switch with a rod attached. My own workshop milling machine is equipped and if a sleeve is caught the vertical rod is touched and the milling head stops in less than 30 degrees, the motor may be doing 5 to 10 revs.

The hardware is a changeover switch in the line from the motor starter to the motor. When this changes over it short circuits two of the motor terminals say U & V, and connects a DC capacitor to the terminals V & W. The charge in the capacitor is adjustable and this alters the stopping time. The manufacturers recommend that a machine is not stopped too fast to avoid damage to the gears if the braking is too fierce. The changeover switch is a magnetic switch or contactor tripped by the microswitch. The circuit is on the inside of the cover, it's not too complex. One manufacturer in the UK I think is called Instastop.
James