Series DC Motor

[Paul.Downie]

Hi, Any help would be grateful.

I have a question regarding dynamic braking.

Motor Details. Series DC Motor The supply to the motor when the dynamic brake resistor is connected to the armature in parallel is 216vdc battery fed.

I understand that the Back emf is dissipated into the resistor to protect the VFD and the motor. What I would like to grasp is how the resistor slows the motor down. The supply 216vdc is held onto the motor until a limit switch has been activated and stated the motor has reached home position.

The dc motor resistance is 4.8ohms and the dynamic resistor is 18 ohms 500 watt.

Voltage is proportional to speed so I assume that the volatge drops can anyone explain this in more detail withs some calculations to back up theory.

Thanks Paul.

 

[jcormack]

some basic equ's

some basic equ's

From this site http://zone.ni.com/devzone/cda/ph/p/id/48


Dynamic braking


Consider a shunt motor whose field is directly connected to a source Es, and whose armature is connected to the same source by means of a double-throw switch The switch connects the armature to either the line or to an external resistor R (Fig. 5.17).

When the motor is running normally, the direction of the armature current I1 and the polarity of the cemf Eo are as shown in Fig. 5.17a. Neglecting the armature IR drop, Eo is equal to Es

If we suddenly open the switch (Fig 5.17b), the motor continues to turn, but its speed will gradually drop due to friction and windage losses. On the other hand, because the shunt field is still excited, induced voltage Eo continues to exist, falling at the same rate as the speed In essence, the motor is now a generator whose armature is on open-circuit.

Let us close the switch on the second set of contacts so that the armature is suddenly conneted to the external resistor (Fig. 5.17c). Voltage Eo will immediately produce an armature current I2. However, this current flows in the opposite direction to the original current /1 It follows that a reverse torque is developed whose magnitude depends upon I2. The reverse torque brings the machine to a rapid, but very smooth stop.



Figure 5.17a Armature connected to a dc source Es.


Figure 5.17b Armature on open circuit generating a voltage Eo.


Figure 5.17c Dynamic braking.

In practice, resistor R is chosen so that the initial braking current is about twice the rated motor current. The initial braking torque is then twice the normal torque of the motor.

As the motor slows down, the gradual decrease in Eo produces a corresponding decrease in I2. Consequently, the braking torque becomes smaller and smaller, finally becoming zero when the armature ceases to turn. The speed drops quickly at first and then more slowly, as the armature comes to a halt. The speed decreases exponentially, somewhat like the voltage across a discharging capacitor. Consequently, the speed decreases by half in equal intervals of time To. To illustrate the usefulness of dynamic braking. Fig. 5.18 compares the speed-time curves for a motor equipped with dynamic braking and one that simply coasts to a stop.