INTRODUCTION TO ELECTRICAL DRIVES
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.
The block diagram of the electrical drive is shown in the figure below.
The electrical load like fans, pumps, trains, etc., consists the electrical motor. The requirement
of an electrical load is determined regarding speed and torque. The motor which suited the
capabilities of the load is chosen for the load drive.
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.
Advantages of Electrical Drive
The following are the advantages of electrical drive.
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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.
1. The application of the drive is limited because it cannot use in a place where
the power supply is not available.
2. It can cause noise pollution.
3. The initial cost of the system is high.
4. It has a poor dynamic response.
5. The output power obtained from the drive is low.
6. During the breakdown of conductors or short circuit, the system may get
damaged due to which several problems occur.
Application of Electric Drive
It is used in a large number of industrial and domestic applications like
transportation systems, rolling mills, paper machines, textile mills, machine tools,
fans, pumps, robots and washing, etc.
Block diagram explanation of chopper fed DC drives
A chopper is a device that converts fixed DC input to a variable DC output
voltage directly. Essentially, a chopper is an electronic switch that is used to
interrupt one signal under the control of another.
CHOPPER FED DC DRIVES
• A dc chopper is connected between a fixed-voltage dc source and dc motor to
vary the armature voltage. A chopper is a high speed on/off semiconductor switch
which connects source to load and disconnects the load from source at a fast
speed.
Working principle of DC chopper
DC chopper works on DC voltage. They work as a step up and step down
transformers on DC voltage. They can convert the steady constant DC voltage to
a higher value or lower value based on their type. DC choppers are more efficient,
speed and optimized devices.
Principle of Operation
The principle of operation of chopper can be understood from the circuit diagram
below. The circuit consists of a semiconductor diode, resistor, and a load. For all
type of chopper circuit, the output voltage value is controlled by periodic closing
and opening of the switches used in the circuit.
The chopper can be viewed as an ON/OFF switch that can rapidly connect or disconnect the
source to load connection. Continuous DC is given as source to the chopper as Vs and chopped
DC is obtained across the load as V0.
Vs = Source DC voltage applied as input to the chopper
V0 = Chopped DC voltage obtained across the load
Output Voltage and Current Waveforms
Above are the output voltage and current waveforms of a chopper circuit. From the voltage
waveform, it can be seen that during the period of TON the load voltage V0 is equal to the
source voltage Vs. But when the interval TOFF occurs, the DC voltage step downs to zero,
thus making the load short-circuited.
Output Voltage and Current Waveforms
In the current waveform, it can be seen that during the interval TON the load
current rises to the maximum value. During the interval TOFF, the load current
decays. In TOFF condition the chopper is off so, the load voltage becomes zero.
But load current flows through the diode FD, making the load short-circuited.
Thus, the chopped DC voltage is produced at the load. The current waveform is
continuous which rises during TON state and decays during TOFF state.