Electrical Drives fundamentals

1. Review of Basic Electrical Drives
Unit-1 Lecture session-1:

2. The need for variable speed drives:
• Match the speed of a drive to the process requirements
• Match the torque of a drive to the process requirements
• Save energy and improve efficiency

3. What is Drives
• Systems employed for motion control are called drives and may employ any of the
prime movers such as, diesel or petrol engines, gas or steam turbines, steam
engines, hydraulic motors and electric motors, for supplying mechanical energy for
motion control.

4. Main types of variable speed drive (VSD) for industrial applications
a) Typical mechanical VSD with an AC motor as the prime mover;
b) Typical hydraulic VSD with an AC motor as the prime mover;
c) Typical electromagnetic coupling or Eddy Current coupling;
d) Typical electrical VSD with a DC motor and DC voltage converter;
e) Typical electrical VSD with an AC motor and AC frequency converter;
f) Typical slip energy recovery system or static Kramer system;

5. (a) Typical mechanical VSD with an AC motor as the prime mover;
(b) Typical hydraulic VSD with an AC motor as the prime mover;
(c) Typical electromagnetic coupling or Eddy Current coupling;

6. (d) Typical electrical VSD with a DC motor and DC voltage converter;
(e) Typical electrical VSD with an AC motor and AC frequency converter;
(f) Typical slip energy recovery system or static Kramer system;

7. • In contrast to the mechanical and hydraulic variable speed control methods,
electrical variable speed drives are those in which the speed of the electric motor
itself, rather than an intermediary device, is controlled.

8. Advantages of Electrical Drives
1. The electric drive has very large range of torque, speed and power.
2. Their working is independent of the environmental condition.
3. The electric drives are free from pollution.
4. The electric drives operate on all the quadrants of speed torque plane.
5. The drive can easily be started and it does not require any refueling.
6. The efficiency of the drives is high because fewer losses occur on it.

9. What is Electric Drives
• Systems employed for motion control are called drives and may employ any of the
prime movers such as, diesel or petrol engines, gas or steam turbines, steam
engines, hydraulic motors and electric motors, for supplying mechanical energy for
motion control.
• Drives employing electric motors are known as electric drives.
Classification of Electric Drives:
DC drives and AC drives

10. What is Electric Drive ?
Electric drive is a form of machine equipment designed to convert electric energy
into mechanical energy and provide electrical control on this process
Sensing
unit
Motor
Power
Modulator
Source Load
Input command
Control
unit
Fig . Block Diagram of Electric Drive

13. Electrical Motors
Electrical Motors
Motors commonly used in electrical drives are
1. Dc motors-shunt, series, compound and permanent magnet.
2. Induction motors- squirrel cage, wound rotor and linear
3. Synchronous motors- wound field and permanent magnet
4. Brushless dc motors- similar to permanent magnet synchronous motor,
lower cost, simpler and cheaper motor.
5. Stepper motors
6. Switched reluctance motors
.

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Types of Electrical Drives

16. The power electronic modules are used to control the electrical power by performing tasks
such as changing phase, duty cycle, voltage, current etc.

18. Motor Selection
1. Cost
2. Thermal capacity
3. Efficiency
4. Torque-speed profile
5. Acceleration
6. Power density, volume of the motor
7. Ripple, cogging torque
8. Peak torque capability
9. Suitability for hazardous environment
10. Availability of spare parts

19. Choice of Electrical Drives
It depends on a number of factors. Some of the important factors are:
1. Steady state operation requirements- Nature of speed torque characteristics,
speed regulation, speed range, efficiency, duty cycle, quadrants of operation,
speed fluctuations and ratings.
2. Transient operation requirements- Values of acceleration and deceleration,
starting, braking and reversing performance.
3. Requirements related to the source- Type of source, and its capacity, magnitude of
voltage, voltage fluctuations, power factor, harmonics and their effect on other
loads, ability to accept regenerated power.
4. Capital and running cost, maintenance needs, life.
5. Space and weight restrictions if any.
6. Environment and location.
7. Reliability.

20. Losses in Electrical Drive System:
1. Electrical transmission losses: These losses depend on the drive power factor and
harmonics in the line current.
2. Conversion losses in the power modulator (or converter): The semi-conductor
converter usually has low conversion losses.
3. Electric motor losses to convert electric power into mechanical power: These are
determined by choice of motor (quality of its design and selection of right rating) and
quality of supply (voltage variations, unbalance, frequency variations and harmonics).
4. Mechanical losses in the parts of the transmission system such as bearings, gears,
clutches and belts.
5. Losses in the load: Load is a machine required to perform a specified task such as fan,
pump and train.

21. 6. Mechanical transmission losses, such as friction losses to move material from one
location to another. Losses in pipe line carrying fluid is one such example.
Note: The efficient operation of an electrical drive, with minimum consumption of
energy, requires that the drive and the driven machinery (or load) operate efficiently.
This calls for careful design of both electrical and mechanical systems.