DC Braking 1

Can some one explain what is DC braking in 3-phase induction motor and how it works?

 

[jbartos]

Suggestion: Visit

http://www.moderndrives.co.uk/braking-systems.html

for more info

 

[Marke]

Hello Ravi
Firstly, look at how an induction motor works. When an induction motor is connected to the supply, there is a rotating magnetic field generated by the stator. This rotating magnetic field induces a current in the short circuited rotor winding. The frequency of the current flowing in the rotor is equal to the difference between the rotational speed of the stator field and the rotor speed. The rotor current creats another field which is equal in frequency to the stator field (relative to the stator) and ther is an interaction between these two field which causes the torque to be produced. The action of these two field is to generate and acceleration torque to cause the rotor to try to operate at the same speed as the stator field, however there must be some slip for the rotor field to be generated, so synchronous speed is not reached.
Changing the speed of the stator field will change the speed of the rotor. If we now replace the AC voltage applied to the stator with a DC voltage, we have a stationary stator field, or to look at it another way, we have a stator field operating from a frequency of zero Hz. If the rotor is spinning, the stator flux will induce a rotor current and the rotor will try to spin at the same speed as the stator field. - in this case zero. A stationary torque is generated and acts as a break. As with the spinning stator field, the rotor needs to have some slip, so there will be no torque at exactly zero speed. The DC break may not bring the motor to a complete standstill!
Power is still dissipated in the rotor during breaking, and in the stator due to the ixixR losses. Breaking voltages are much lower than the design AC voltage due to the impedance being resistive only.

Regards, Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: The motor application may have to be looked into also. E.g.
Time optimal control for induction motor servo system
- Min-Ho Park Chung-Yeun Won
Dept. of Electr. Eng., Seoul Nat. Univ., South Korea
This paper appears in: Power Electronics, IEEE Transactions on
On page(s): 514 - 524
July 1991
Volume: 6 Issue: 3
ISSN: 0885-8993
References Cited: 7
INSPEC Accession Number: 4006126 Abstract:
A time-optimal position control scheme based on Pontryagin's minimum principle is proposed for a current-source-inverter (CSI) fed induction motor system. The induction motor with field-oriented control is modeled with a fast time-optimal position control system of second order which has two modes of operation: the first bang-bang mode operates until the position is close to target, followed by second DC braking mode which gives asymptotic stability. In order to realize this aim, the time-optimal controller is added instead of the speed controller to the conventional field oriented control loop of the CSI-fed induction motor. The validity of the time-optimal control solution has been verified by an experimental test. Experimental results are in close agreement with a digital simulation.