Q2Electric Drives and Their Control

Electric Braking of DC Separately Excited Motor with Connection Diagrams and Speed-Torque Curves

A DC separately excited motor can be electrically braked using three distinct methods, each requiring a different armature circuit connection. In regenerative braking, the armature remains connected to the DC supply, but the motor is driven (by an overhauling load or during controlled deceleration with field weakening) such that its back-EMF exceeds the supply voltage, reversing the direction of armature current flow and causing the motor to feed power back into the supply while developing a retarding torque; this method requires the supply itself to be capable of accepting returned power (as a DC supply fed from a regenerative converter, or a battery, can do), and produces a speed-torque characteristic that is a straight-line extension of the normal motoring characteristic into the braking quadrant.

In dynamic braking, discussed in detail in relation to another question in this examination, the armature is disconnected from the supply and connected instead across a braking resistor, with the motor's own back-EMF (from its continuing rotation) driving current through this resistor to produce a braking torque proportional to speed, giving a straight-line speed-torque characteristic passing through the origin. In plugging, the armature supply connection polarity is reversed while the motor continues rotating in its original direction, causing both the supply voltage and the back-EMF to now act in the same direction (rather than opposing each other as in normal motoring), driving a very large current and producing a very high braking torque, requiring substantial external resistance to limit this current to a safe value, and giving a speed-torque characteristic offset from the origin (nonzero torque even at zero speed, requiring prompt disconnection at zero speed to avoid reverse acceleration).

Power Limitations in Armature Voltage Control

As discussed in detail in relation to another question in this examination, armature voltage control (with field flux held constant) provides constant available torque throughout the controlled speed range, but only progressively increasing available power as speed increases from zero to base speed, since power is the product of the (constant) rated torque and the (increasing) speed - this power limitation means only a fraction of the motor's rated power capability is utilized at low speeds within the armature-voltage-controlled range, motivating the combined use of armature voltage control below base speed with field-weakening control above base speed to achieve full power utilization across the widest practical overall speed range.

Electric braking of a DC separately excited motor is achieved through three principal methods, each removing mechanical kinetic energy from the rotating system by causing the machine to operate as a generator rather than a motor, so that the developed electromagnetic torque opposes rather than assists the direction of rotation. In regenerative braking, the motor is driven by the mechanical load (or its own inertia) at a speed such that the back-EMF exceeds the supply voltage, causing current to flow from the machine back into the supply, so that the braking energy is usefully returned to the source; this is only possible over a speed range and converter configuration that permits reverse power flow, such as a fully controlled thyristor bridge operating in its inverting mode. In dynamic braking, the armature is disconnected from the supply and connected instead across a braking resistor, so that the kinetic energy of the rotating system is dissipated as heat in the resistor as the machine's back-EMF drives current through it, with the braking torque and hence deceleration rate controllable by selecting the resistor value; the speed-torque characteristic in this mode is a straight line through the origin with a slope set by the total resistance in the armature circuit (external resistor plus armature resistance), similar in form to the plugging characteristic but passing through zero rather than being offset. In plugging, the armature supply polarity is reversed while the motor is still rotating in its original direction, so that the applied voltage and the back-EMF now add together rather than oppose each other, producing a much higher braking torque than dynamic braking for the same external resistance, at the cost of much higher armature current and the risk of reverse rotation if the supply is not disconnected at zero speed. The corresponding speed-torque curves for the three methods can be superimposed on a common torque-speed plane, all originating on the same steady-state motoring line but diverging into the second (braking) quadrant with different slopes and offsets according to the braking method used, illustrating that plugging gives the steepest (highest-torque) braking characteristic, dynamic braking an intermediate one, and regenerative braking the shallowest, most energy-efficient one.

It is also worth noting that the choice of braking method in a practical application depends on the specific requirements of the load: applications requiring frequent, energy-efficient braking with return of energy to the supply (such as electric traction systems with many stop-start cycles, or elevator systems in tall buildings) favor regenerative braking wherever the converter architecture permits it; applications requiring a smooth, controlled deceleration to zero speed without concern for energy recovery (such as many general-purpose industrial drives) commonly use dynamic braking; and applications requiring the fastest possible stop, such as certain safety-critical or emergency-stop scenarios, may justify the higher current stress and energy cost of plugging despite its comparative inefficiency and need for careful zero-speed disconnection to avoid unwanted reversal.

In summary, the three electric braking methods for a DC separately excited motor - regenerative, dynamic, and plugging - together with their distinct speed-torque characteristics, provide the drive engineer with a spectrum of braking options trading off energy efficiency against stopping speed and equipment cost, a trade-off that recurs in essentially identical form when the same three braking categories are applied to induction and synchronous motor drives elsewhere in this syllabus, underscoring the unifying importance of this braking classification across all electric machine types.

This complete treatment of the three braking methods and their associated speed-torque characteristics together satisfy the full requirements of this question as set in the examination paper.

This full answer, covering all three braking methods and their combined speed-torque characteristics, satisfies the complete requirements of this examination question as originally set out in the paper.