DC Motor Drives

Key learnings:
  • 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 are crucial for large applications, ensuring smooth operation of DC motors. These drives are primarily used for reliable speed regulation, frequent starting, braking, and reversing. Let’s will discuss about the different operation of electric drives for different purposes:
We know that, normally the rotor of a DC motor is energized by the commutation process through brushes. So the maximum allowable starting current is determined by the current which can be safely carried out by the brushes without sparking. In general, the motors are such designed that they can carry almost twice of the rated current during starting condition.

For specially designed motors, the starting current can reach 3-5 times the rated current. This surge occurs because when stationary, the motor’s armature resistance is low, causing a large current to flow upon starting.
This is because when the motor is at stand still condition, there is only a small armature resistance present in the circuit so no back emf is generated. That is why when the motor is started with full supply voltage across its terminal, there is a huge current flow through the motor, which may damage the motor because of heavy sparking across the commutators and a huge amount of heat is generated. That is the reason why some precautionary measures are taken during the starting of the DC motors. The speed of a DC motor drive is usually controlled by variable resistance method, which can be also used to limit the starting current as shown in the figure below.

Starting of DC Motor

As the motor speeds up and back EMF builds, resistances are gradually removed from the circuit, maintaining the current within safe limits.

Braking of DC Motors

Braking is a very important operation for DC motor drives. The need of decreasing the speed of a motor or stopping it totally may arise at any moment, that’s when braking is applied. braking of DC motors is basically developing a negative torque while the motor works as generator and as a result the motion of the motor is opposed. There are mainly three types of braking of DC motors :

  1. Regenerative braking
  2. Dynamic or rheostat braking
  3. Plugging or reverse voltage braking.
braking characteristics of a separately excited motor
dc motor
dc motor characteristcs
plugging-speed torque curve

Regenerative braking takes place when the generated energy is supplied to the source, or we can show this via this equation :
E > V and negative Ia..

As the field flux cannot be increased beyond a rated value, so regenerative braking is possible only when the speed of motor is higher than the rated value. The speed torque characteristics is shown in the graph above. When regenerative braking occurs, the terminal voltage rises and as a result the source is relieved from supplying this amount of power. This is the reason why loads are connected across the circuit. So, it is clear that regenerative braking should be used only when there are enough loads to absorb the regenerative power.
Dynamic Braking is another type of braking of DC motor drives where the rotation of the armature itself causes the braking. This method is also a widely used DC motor drive system. When braking is desired, then the armature of the motor is disconnected from the source and a series resistance is introduced across the armature. Then the motor acts as a generator and current flows in the opposite direction which indicates that the field connection is reversed. The diagram for separately excited and series DC motor both are shown in figure below.
When braking is required to occur quickly the resistance (RB) is considered to be of some sections. As the braking occurs and the speed of the motor falls, the resistance are cut out one by one section to maintain the light average torque.

Plugging is a type of braking where the supply voltage is reserved when the need of braking arises. A resistance is also introduced in the circuit while braking takes place. When the direction of the supply voltage is reserved, then the armature current also reserves forcing the back enf to a very high value and hence braking the motor. For series motor only armature is reversed for plugging. The diagram of separately excited and series excited motors are shown in the figure.

Speed Control of DC Motor Drives

The main application of electric drives can be said as the need of braking of DC motors . We know the equation to describe the speed of a rotating DC motor drives is as

Now, according to this equation, the speed of a motor can be controlled by the following methods

  1. Armature voltage control
  2. Field flux control
  3. Armature resistance control
voltage control of dc motor
field flux control of dc motor
torque power characteristic

Among all of these, armature voltage control is preferred because of high efficiency and good speed regulation and good transient response. But the only disadvantage of this method is that it can only operate under the rated speed, because the armature voltage cannot be allowed to exceed rated value. The speed torque curve for armature voltage control is shown below.

When speed control is required above the rated speed, field flux control is used. Normally in ordinary machines, the maximum speed can be allowed up to twice of the rated speed and for specially designed machines this can be allowed up to six times of the rated speed. The torque speed characteristics for field flux control are shown in the figure below.

How the armature voltage control and field flux control is made to operate below and above the rated speed is shown in the figure below.

Lastly, the resistance control method adjusts speed by introducing a resistor in series with the armature, which dissipates power. This inefficient method is seldom used, typically only where brief speed control is needed, like in traction systems.
Speed torque curves of dc motors with
So, the application and types of DC motor drives have been discussed very easily.

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