- DC Motor Drives Definition: DC motor drives are systems used to control the performance of DC motors, enhancing operations such as speed, starting, braking, and reversing.
- Starting Mechanisms: Starting DC motor drives involves managing high initial currents to prevent motor damage, typically by varying resistance.
- Braking Systems: Effective braking in DC motor drives is achieved using techniques like regenerative, dynamic, and plugging, depending on the motor’s operation and needs.
- Speed Control: Speed control of DC motor drives can be done through armature voltage, field flux, and resistance methods, each offering different benefits for motor efficiency and performance.
- Regenerative Braking Explained: In regenerative braking, the motor generates power back to the source, used effectively when there are sufficient loads to absorb this power.
DC motors drives control torque, speed, starting, braking and reversal. Modern electric drives use a power converter, current limits and control loops to regulate the motor.
In a brushed motor, the commutator and brushes carry armature current, but the permitted starting current comes from the motor and drive ratings. No fixed current multiple applies to every machine. At start, the drive must limit armature current until the motor develops enough back electromotive force (EMF). This protects the windings, commutator, brushes, converter and mechanical load.
At standstill, back EMF is zero. The armature current is therefore limited mainly by armature resistance, added circuit impedance and any active current control.
Applying full terminal voltage directly can produce damaging current, heat and commutator sparking. A traditional starting of the DC motors scheme inserts series resistance. A converter-fed drive instead ramps the applied voltage and enforces an armature-current limit.
In a resistance starter, sections of resistances are removed as speed and back EMF rise. An electronic drive meets the same current-limiting objective through closed-loop converter control.
Braking of DC Motors
Electrical braking develops torque opposite to motion and slows or stops DC motor drives. The motor operates as a generator during braking, so mechanical energy becomes electrical energy. Depending on the circuit, that energy returns to the supply or is dissipated as heat. The methods used for braking of DC motors differ in how the circuit is reconfigured and where the energy goes. The three common types of braking of DC motors are:
- Regenerative braking
- Dynamic or rheostat braking
- Plugging or reverse voltage braking.
Regenerative braking makes the motor return electrical energy through a receptive converter and supply or DC bus. For a fixed armature supply and field, the condition can be written as:
E > V and negative Ia..
Regeneration can also occur below rated speed when a controlled converter reduces armature voltage or commands negative torque. The source or DC bus must be able to accept the returned energy. If it cannot, the drive needs a braking resistor or another energy-management path.
Dynamic Braking disconnects the armature from the source and connects it to a braking resistor while the field remains excited. The rotating motor then acts as a generator, and the resistor dissipates the generated energy as heat. For a series DC motor, the windings must be reconnected so the generated current produces torque that opposes rotation.
The braking resistance (RB) limits current. Staged resistance can maintain braking current as the generated voltage falls with speed, although braking torque still approaches zero at standstill.
Plugging reverses the armature or field polarity relative to the other. The supply voltage and back EMF then add, so a resistor or active current control must limit armature current. The drive must disconnect near zero speed unless reverse rotation is intended.
Speed Control of DC Motor Drives
Speed control is a main function of a DC drive, while braking of DC motors is one operating mode. For a separately excited motor, the approximate steady-state speed relationship is
This relationship shows three practical speed-control methods:
- Armature voltage control
- Field flux control
- Armature resistance control
Armature-voltage control is normally used from zero to base speed with rated field flux. This region provides approximately constant torque, and the converter limits armature current. The applied armature voltage must stay within the motor nameplate rating.
Above base speed, field weakening reduces flux to increase speed. This region provides approximately constant power, so available torque falls as speed rises. The permitted field-weakening range and maximum speed depend on the motor’s mechanical, commutation, field and drive limits; no universal speed multiple applies.
Combining armature-voltage control below base speed with field weakening above base speed gives a wide controlled speed range.
Armature resistance control reduces speed by placing a resistor in series with the armature. It wastes power as heat and gives load-dependent speed with poor regulation, so it is mainly a legacy or short-duration method.
The appropriate DC motor drives method depends on the required speed range, torque, braking duty, supply and motor ratings.
