Electric Drive ( Electrical Engineering ) part-1

Topic includes 

- Electric Drive

- Parts of Electric Drive

- Advantage and Dis advantage of Electric Drive

- Braking

- Regenerative, Plugging and Dynamic braking.

rahulsinghmar.blogspot.com

Electrical Drive

Definition: The system which is used for controlling the motion of an electrical machine, such type of system is called an electrical drive. In other words, the drive which uses the electric motor is called electrical drive. The electrical drive uses any of the prime movers like diesel or a petrol engine, gas or steam turbines, steam engines, hydraulic motors and electrical motors as a primary source of energy. This prime mover supplies the mechanical energy to the drive for motion control.

Parts of Electrical Drive

The main parts of the electrical drives are power modulator, motor, controlling unit and sensing units.Their parts are explained below in details.

Power Modulator – The power modulator regulates the output power of the source. It controls the power from the source to the motor in such a manner that motor transmits the speed-torque characteristic required by the load. During the transient operations like starting, braking and speed reversing the excessive current drawn from the source. This excessive current drawn from the source may overload it or may cause a voltage drop. Hence the power modulator restricts the source and motor current.

The power modulator converts the energy according to the requirement of the motor e.g. if the source is DC and an induction motor is used then power modulator convert DC into AC. It also selects the mode of operation of the motor, i.e., motoring or braking.

Control Unit – The control unit controls the power modulator which operates at small voltage and power levels. The control unit also operates the power modulator as desired. It also generates the commands for the protection of power modulator and motor. An input command signal which adjusts the operating point of the drive, from an input to the control unit.

Sensing Unit – It senses the certain drive parameter like motor current and speed. It mainly required either for protection or for closed loop operation

rahulsinghmar.blogspot.com

Advantages of Electrical Drive

The following are the advantages of electrical drive.

The electric drive has very large range of torque, speed and power.
Their working is independent of the environmental condition.
The electric drives are free from pollution.
The electric drives operate on all the quadrants of speed torque plane.
The drive can easily be started and it does not require any refuelling.
The efficiency of the drives is high because fewer losses occur on it.

Disadvantages of Electrical Drive

The power failure completely disabled the whole of the system.

The application of the drive is limited because it cannot use in a place where the power supply is not available.
It can cause noise pollution.
The initial cost of the system is high.
It has a poor dynamic response.
The output power obtained from the drive is low.
During the breakdown of conductors or short circuit, the system may get damaged due to which several problems occur.

Braking

The term braking comes from the term brake. We know that brake is an equipment to reduce the speed of any moving or rotating equipment, like vehicles, locomotives. The process of applying brakes can be termed as braking. Now coming to the term or question what is braking. First of all we can classify the term braking in two parts

Mechanical Braking
Electrical Braking

rahulsinghmar.blogspot.com

Mechanical braking is left out here because as it is an electrical engineering site, we should only focus on electrical braking here. In mechanical braking the speed of the machine is reduced solely by mechanical process but electrical braking is far more interesting than that because the whole process is depended on the flux and torque directions. We will further see through the various types of braking but the main idea behind each type of barking is the reversal of the direction of the flux.

Brakes are used to reduce or cease the speed of motors. We know that there are various types of motors available (DC motors, induction motors, synchronous motors, single phase motors etc.) and the specialty and properties of these motors are different from each other, hence this braking methods also differs from each other. But we can divide braking in to three parts mainly, which are applicable for almost every type of motors.

Regenerative Braking.
Plugging type braking.
Dynamic braking.

Regenerative Braking

Regenerative braking takes place whenever the speed of the motor exceeds the synchronous speed. This baking method is called regenerative baking because here the motor works as generator and supply itself is given power from the load, i.e. motors. The main criteria for regenerative braking is that the rotor has to rotate at a speed higher than synchronous speed, only then the motor will act as a generator and the direction of current flow through the circuit and direction of the torque reverses and braking takes place. The only disadvantage of this type of braking is that the motor has to run at super synchronous speed which may damage the motor mechanically and electrically, but regenerative braking can be done at sub synchronous speed if the variable frequency source is available.

Plugging Type Braking

Another type of braking is Plugging type braking. In this method the terminals of supply are reversed, as a result the generator torque also reverses which resists the normal rotation of the motor and as a result the speed decreases. During plugging external resistance is also introduced into the circuit to limit the flowing current. The main disadvantage of this method is that here power is wasted.

Dynamic Braking

Another method of reversing the direction of torque and braking the motor is dynamic braking. In this method of braking the motor which is at a running condition is disconnected from the source and connected across a resistance. When the motor is disconnected from the source, the rotor keeps rotating due to inertia and it works as a self-excited generator. When the motor works as a generator the flow of the current and torque reverses. During braking to maintain the steady torque sectional resistances are cut out one by one.