Dynamic breaking with soft starter? 1

[soft22]

We are using soft starters for blower application and
self developed the Reduced voltage soft starter suitable with aplication upto 100Hp ( 3 anti parallel thyristors ). we want to avoide the free wheeling due high inertia of blower . Can any one give me idea_techeque to introduce dynamic breaking / Dc breaking action in it

Niranjan

 

[Marke]

Hello soft22

There are two main ways to apply braking to the motor.
One is DC braking and the other is plug braking.

DC braking is achieved by applying a low voltage DC to two of the motor terminals. The smoother the DC is, the better the result. If there is a high ripple current, there will be a resultant torque at thew speed of the ripple current. If the ripple current is at twice line frequency (Full wave rectification) then there will be a torque field at twice rated speed. If you use half wave rectification, you will have a torque field at rated speed. You can have problems if the ripple current is at line frequency and the rotational speed is close to synchronous speed because the motor can synchronise onto the ripple frequency rather than the DC field.

You can use the SCRs of the soft starter to develop half wave rectified DC by turning on the + SCR of phase A and the - SCR of phase B. These SCRs must be phased back to control the current. - remember you need a reduced voltage for braking. Also note that the ripple will be at line frequency using this type of DC braking and there will be a tendency for the motor to want to continue at rated speed due to the ripple frequency.

Plug braking is achieved by soft starting with two phases reversed. This will create a reverse rotating torque field and the motor will rapidly slow. You must remove the voltage from the motor when it reaches close to zero speed or it will accelerate to full speed reverse. This can be done by the use of a rotation sensor.

Both of these systems of braking dissipate the load energy in the rotor of the motor. You must be careful to ensure the motor can withstand the high dissipation during start and stop.

Best regards

Mark Empson

http://www.lmphotonics.com