Electrical Drives
Lecture 1
Dr. Magdi A. Mosa
Electrical Power and Machines Department
Faculty of Engineering – Helwan University
Electric Drive System
It is electromechanical system that converts the electric energy into mechanical energy and
supply this energy to different parts of the system with electric control.
Applications of Electric Drives
Pumping Traction
Air Crafts Elevators
Applications of Electric Drives
Production Lines
Assembly Lines
Difference between machines and Drives
Machines Courses
Electric Drive Courses
Electric Drive Course Contents
1. Fundamentals of Electric Drive Systems
2. Operation and Control of DC separately excited and shunt motors
3. Operation and Control of DC series motors
4. Operation and Control of three-phase induction motor
Electric Drive References
1- Fundamentals of electric drives
by Mohamed A. El-Sharkawy
2- Electrical Drives Systems
by Adel A. Elsamahy – Mohamed I. El koufouly
Electric Drive System
It is electromechanical system that converts the electric energy into mechanical energy and
supply this energy to different parts of the system with electric control.
Load of Electric Drive System
The load is usually a machine designed to accomplish a certain task such as the
train, the robot, the lift,...etc
Load is always known
1. The rated power, Torque, and Speed
2. The Speed –Torque characteristics
Rated Power of the Load
For Rotational Motion
𝑃𝑜𝑤𝑒𝑟 = 𝑇𝑜𝑟𝑞𝑢𝑒 𝑋 𝑆𝑝𝑒𝑒𝑑
𝑃𝐿 = 𝑇𝐿 ω𝐿
For Linear Motion
𝑃𝑜𝑤𝑒𝑟 = 𝐹𝑜𝑟𝑐𝑒 𝑋 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦
𝑃𝐿 = 𝐹𝐿 𝑉𝐿
Mechanical Load Torque speed C/Cs
A- Constant torque Load
𝑇𝐿
The load torque remains constant throughout the speed range
The horsepower changes linearly with speed for these loads
For example
Conveyors T
Elevators
𝑃𝐿 ∝ 𝑛
Screw compressors
Reciprocating compressors
Positive displacement pumps
𝑇𝑒 P
𝑇𝐿
ω
𝑣
Mechanical Load Torque speed C/Cs
B- Constant Power Load
The horsepower remains constant throughout the speed range
The horsepower changes inversely with speed for these loads
1
𝑇𝐿 ∝
𝑛
𝑃𝐿
P
Mechanical Load Torque speed C/Cs
C- Linear torque Load
𝑇𝐿 ∝ 𝑛
The load torque changes linearly with the speed range
The horsepower proportional to the square of the speed for
For example
Motor generator set T
𝑃𝐿 ∝ 𝑛2
P
Mechanical Load Torque speed C/Cs
D- Quadratic torque Load
The load torque proportional with the square of the speed
The horsepower proportional to the cub of the speed 𝑇𝐿 ∝ 𝑛2
For example
Pumps T
blowers
𝑃𝐿 ∝ 𝑛3
P
Electric Motor
Electric Motor is the heart of Electric drive system
Converts the electric energy into rotational mechanical energy
Most common Types of Electric Motor
There are special machines such as reluctance, stepper,…etc.
Electric Motor Speed-Torque C/Cs
Synchronous
Speed
Separate/Shunt
Induction
Series
Torque
Each motor has advantages and disadvantages which should be considered during motor
selection for certain application
Power Moderator
Controls the flow of power from the source to the motor in such a way that the motor
speed-torque and speed-current characteristics become compatible with the load
requirements.
1- matches the supply to the motor such as rectifiers or inverters,…etc
2- Controls in the motor operation
3-may contains protection elements
Modern is based on Power Electronics Traditional power modulator
Sensing unit
Converts the controlled variable (Speed, Torque, position) into electric signal (Voltage,
Current) suitable for the control unit
Shaft encoder Tacho-generator
Control Unit
Performs three main functions
1) measures the feedback signal from the sensing unit
2) Compares the feedback signal with the reference
3) Provides the control, action to power modulator
How to select the electric dives system components
For optimum matching
1) Study the load
2) Study the available power supply
3) Select the motor
4) Select the power modulator
5) Select control unit
6) Select the sensing unit
Motor Selection
1) Rated power
𝑃𝑚𝑜𝑡𝑜𝑟 = 𝑘𝑃𝐿
𝑃𝑚𝑜𝑡𝑜𝑟 Is the required motor power
𝑃𝐿 Is the rated load power on the motor shaft
𝑘 Is the overload safety factor 𝑘 = 1.25
The Difference between safety factor and capacity factor
𝑇𝐿𝑛 𝑇𝑚𝑛 𝑇𝑚_𝑚𝑎𝑥
𝐴 𝐵
𝑇𝐿𝑛 motor
Overload Safety factor=
Speed
𝑇𝑚𝑛
𝐶
𝑇𝑚_𝑚𝑎𝑥
Overload Capacity factor= 𝑇𝑚𝑛
Torque
Motor Selection
2) Operating Point
Is the nominal operating speed and torque of the motor
𝑇𝑒 = 𝑇𝐿
𝑇𝑒 = 𝑇𝐿
Speed
𝑇𝐿
ω𝑚 = ω𝐿
𝑇𝑒
Torque
Motor Selection
2) Check the system stability
𝑇𝐿
𝐵 motor
Speed
𝐴 𝑇𝑒
Method1 check the stability analytically
∆𝑇𝐿 ∆𝑇𝑒
>
∆ω ∆ω
Metthod2 check the stability graphically Torque
Motor Selection
Speed
𝑇𝐿
Metthod2 check the stability graphically load
ω1
1) For 𝝎𝟏 > 𝝎𝑨 ω𝐴
𝑇𝑒
motor
Form C/Cs 𝑇𝑒1 < 𝑇𝐿1
Then the system will decelerate and the speed
returns to the operating point
2) For 𝝎𝟐 > 𝝎𝑨 𝑇𝑒1 𝑇𝐿1 Torque
Form C/Cs 𝑇𝑒2 > 𝑇𝐿2
Speed
𝑇𝐿
Then the system will accelerate and the speed load
returns to the operating point
ω𝐴
ω2 𝑇𝑒
motor
From 1 and 2 the system will returns to the operating points for
different disturbances
𝑇𝐿2 𝑇𝑒2 Torque
Then the operating point is stable
Motor Selection
𝑇𝐿1
Point (A) is unstable with the first load 𝐵 motor
𝑇𝑒
Speed
Point (A) is stable with the second load 𝑇𝐿2
From that for the same motor the operating
point may be stable or unstable. 𝐴
Torque
Therefore, we cant judge on the stability from motor C/Cs only.
The stability depends on the relative C/Cs between the motor and the load not only the motor
Dr. Magdi A. Mosa
magdimosa@yahoo.com
01282969241
