WHICH BRAKING METHOD IS THE BEST
Which braking method is the best among 3 discussed? As in every engineering problem, there is no definite answer to this question since each has requirements, advantages & disadvantages.
In the previous parts, we already discussed the requirements of each method and application details. Therefore, selection of a method can be made by requirement analysis.
The plugging is the hardest and most stressful method for braking. It creates the highest electrical stress over the controller and the motor and the highest mechanical stress over the system. To decide on whether or not to use plugging, following evaluations can be done;
- What is the required maximum braking torque? Can this torque be created by any other braking method?
- Is the deceleration rate critical?
- Is regenerative braking applicable system-wise? Can it generate required braking torque?
- What is the level of electrical & mechanical stress generated by plugging?
The plugging enables full motor stopping in a period of 2 x system’s mechanical time constant. The deceleration rate may be useful in some cases. However, it is a highly energy consuming method and all braking stress is over the motor.
If plugging is not applicable or not required, the second solution may well be regenerative braking. The regenerative braking is capable of generating high braking torque over a wide speed range. Regenerative braking also differs from other methods in the sense that it does not dissipate but accumulates braking energy and yields to significant energy conservation for frequently accelerating & decelerating systems. The main requirement of the regenerative braking is an electrical storage connected the power source bus. If energy storage is not possible, this energy should be dissipated through a load.
Dynamic braking is the safest form of motor braking where braking force is dependent on motor speed, the braking energy is dissipated and minimal stress is created over the system. However, as mentioned in part 1, the braking power is limited and decreases with decreasing speed. Typical dynamic braking stops the motor in a period over 5 times the mechanical time constant of the system. Therefore, it is not suited for systems requiring fast braking.
Of course, it is possible to combine the methods to meet all requirements of the system. For instance, a system may require the benefits of regenerative braking but the power source is 1 quadrant only. Therefore, the user may add a dynamic braking circuit to the system and dissipate the energy generated by the braking. Furthermore, dynamic braking circuit can be programmed to trip when the bus voltage is higher than the maximum possible voltage of the running system. Then the regenerative braking increases the bus voltage and at a point where the bus voltage is higher than maximum source voltage but not critical enough to damage system, the dynamic braking circuit dissipates stored energy and the system works in unison. If such a scheme is applied, it is also possible to remove disconnect circuitry of the dynamic braking since the bus voltage will not fall under the voltage of the power source and dynamic brake circuit will only dissipate excess charge. V-Direct Motion Controllers offer all control and power system to setup a combined regenerative/dynamic braking system. The controllers use regenerative braking whenever available and a programmable trip point dynamic braking controller frequently monitors bus voltage, enabling externally connected braking resistor whenever needed. This scheme provides very flexible braking capabilities to the system while minimizing system level requirements.