Electric Drive Systems Explained

Posted on May 16, 2025 in Electronics

What is an Electric Drive?

An electric drive can be defined as an electromechanical device for converting electrical energy into mechanical energy to impart motion to different machines and mechanisms for various kinds of process control.

An electric drive is an industrial system which performs the conversion of electrical energy into mechanical energy or vice versa for running and controlling various processes.

An electric drive is defined as a form of machine equipment designed to convert electrical energy into mechanical energy and provide electrical control of the processes. The system employed for motion control is called an electric drive.

Applications of Electric Drives

  • Traction systems
  • Lifts, cranes, electric cars, etc.

Electric drives are available in a wide range of torque, speed, and power. They can be started instantly and can immediately be fully loaded. They can operate in all four quadrants of the speed-torque plane.

Advantages of Electric Drives

  • Cost is low compared to other drive systems.
  • The system is simple and clean.
  • Control is very easy and smooth.
  • Flexible in layout.
  • Facility for remote control.
  • Transmission of power from one place to another can be done with the help of cables instead of long shafts, etc.
  • Maintenance cost is quite low.
  • It can be started at any time without delay.

Disadvantages of Electric Drives

  • The application of the drive is limited because it cannot be used 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.

Power Modulators in Electric Drive Systems

In the electric drive system, the power modulators can be any one of the following:

  1. Controlled rectifiers (AC to DC converters)
  2. Inverters (DC to AC converters)
  3. AC voltage controllers (AC to AC converters)
  4. DC choppers (DC to DC converters)
  5. Cycloconverters (frequency conversion)

Factors for Choosing an Electric Drive

  1. Steady State Operating Conditions Requirements: Nature of speed-torque characteristics, speed regulation, speed range, efficiency, duty cycle, quadrants of operation, speed fluctuations if any, ratings, etc.
  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 regenerative power.
  4. Capital and running cost, maintenance needs, life.
  5. Space and weight restrictions if any.
  6. Environment and location.
  7. Reliability.

Classification of Electric Drives

Generally classified into 3 categories:

  1. Group Drive
  2. Individual Drive
  3. Multimotor Drive

Group Drive

If several groups of mechanisms or machines are organized on one shaft and driven or actuated by one motor, the system is called a group drive or shaft drive.

Group Drive Advantages

  • Most Economical

Group Drive Disadvantages

  1. Any fault that occurs in the driving motor renders all the driving equipment idle.
  2. Efficiency is low because of losses occurring in the energy transmitting mechanisms (Power loss).
  3. Not safe to operate.
  4. Noise level at the working spot is high.
  5. Flexibility is limited. Single motor drives a number of machines through belts from a common shaft.

Individual Drive

If a single motor is used to drive or actuate a given mechanism and it does all the jobs connected with this load, the drive is called an individual drive. All the operations connected with operating a lathe may be performed by a single motor. Each machine is driven by its own separated motor with the help of gears, pulleys, etc.

Individual Drive Disadvantages

  • Power loss occurs (in transmission).

Multimotor Drive

Each operation of the mechanism is taken care of by a separate drive motor. The system contains several individual drives, each of which is used to operate its own mechanism. Separate motors are provided for actuating different parts of the driven mechanism.

Multimotor Drive Advantages

  1. Each machine is driven by a separated motor; it can be run and stopped as desired.
  2. Machines not required can be shut down and also replaced with a minimum of disruption.
  3. There is flexibility in the installation of different machines.
  4. In the case of motor fault, only its connected machine will stop, whereas others will continue working undisturbed.
  5. Absence of belts and line shafts greatly reduces the risk of accidents to the operating personnel.

Multimotor Drive Disadvantages

  • Initial high cost.

Types of Loads in Electric Drives

Loads in electric drives are classified into 2 categories:

  1. Active Load Torques
  2. Passive Load Torques

Active Load Torques

Load torques which have the potential to drive the motor under equilibrium conditions are called active load torques. Load torques usually retain their sign when the drive rotation is changed.

Passive Load Torques

Load torques which always oppose the motion and change their sign on the reversal of motion are called passive load torques. Torque due to friction or cutting is a passive torque.

Components of Load Torques

  1. Friction Torque (TF): The friction torque (TF) is the equivalent value of various friction torques referred to the motor shaft.
  2. Windage Torque (TW): When a motor runs, the wind generates a torque opposing the motion. This is known as the windage torque.
  3. Torque Required to Do Useful Mechanical Work (Tm): Nature of the torque depends on the type of load. It may be constant and independent of speed, some function of speed, may be time invariant or time variant. The nature of the torque may change with the change in the load’s mode of operation.

Load Characteristics in Electric Drives

Constant Torque Loads

Most of the working machines that have a mechanical nature of work like shaping, cutting, grinding, or shearing, require constant torque irrespective of speed. Similarly, cranes during hoisting and conveyors handling constant weight of material per unit time also exhibit this type of characteristic.

Torque Proportional to Speed Loads

Separately excited DC generators connected to a constant resistance load, and eddy current brakes have speed-torque characteristics given by T=kω.

Torque Proportional to Speed Squared Loads

Another type of load met in practice is the one in which load torque is proportional to the square of the speed. Examples include fans, rotary pumps, compressors, and ship propellers.

Torque Inversely Proportional to Speed Loads

Certain types of lathes, boring machines, milling machines, steel mill coilers, and electric traction loads exhibit hyperbolic speed-torque characteristics.

Four Quadrant Operation of Electric Drives

In the I (first) quadrant, power developed is positive and the machine is working as a motor supplying mechanical energy. The I quadrant operation is called Forward Motoring.

II (second) quadrant operation is known as Braking. In this quadrant, the direction of rotation is positive, and the torque is negative, and thus, the machine operates as a generator developing a negative torque, which opposes the motion. The kinetic energy of the rotating parts is available as electrical energy which may be supplied back to the mains. In dynamic braking, the energy is dissipated in a resistance.

The III (third) quadrant operation is known as Reverse Motoring. The motor works in the reverse direction. Both the speed and the torque have negative values while the power is positive.

In the IV (fourth) quadrant, the torque is positive, and the speed is negative. This quadrant corresponds to braking in the Reverse Motoring mode.

Applications of Four Quadrant Operation

Compressor, pump, and fan type loads require operation in the I quadrant only. As their operation is unidirectional, they are called one-quadrant drive systems. Transportation drives require operation in both directions. If regeneration is necessary, application in all four quadrants may be required. If not, then the operation is restricted to quadrants I and III, and thus dynamic braking or mechanical braking may be required. In hoist drives, a four-quadrant operation is needed.

Steady State Stability of Drives

Equilibrium speed of a motor-load system is obtained when motor torque equals the load torque. The drive will operate in steady-state at this speed, provided it is the speed of stable equilibrium. The concept of Steady State Stability of Drive has been developed to readily evaluate the stability of an equilibrium point from the steady-state speed-torque curves of the motor and load, thus avoiding solution of differential equations valid for transient operation of the drive. In most drives, the electrical time constant of the motor is negligible compared to its mechanical time constant. Therefore, during transient operation, the motor can be assumed to be in electrical equilibrium, implying that steady-state speed-torque curves are also applicable to the transient operation.

Continue reading about Steady State Stability of Drives.

Criteria for Steady State Stability

Let the equilibrium of the torques and speed be TM, TL, and ω, and the small deviations are ΔTm, ΔTL, and Δω. After the displacement from the equilibrium state, the torque equation becomes…

Closed Loop Control of Electric Drives

In a closed loop system, the output of the system is fed back to the input. The closed loop system controls the electric drive, and the system is self-adjusted. Feedback loops in an electric drive may be provided to satisfy the following requirements:

  1. Enhancement of speed or torque
  2. To improve steady-state accuracy.
  3. Protection.

Current Limit Control

This scheme is used to limit the converter and motor current below a safe limit during transient operation. The system has a current feedback loop with a threshold logic circuit. The logic circuit protects the system from maximum current. If the current is raised above the maximum set value due to transient operation, the feedback circuit becomes active and forces the current to remain below the maximum value. When the current becomes normal, the feedback loop remains inactive.

Closed Loop Torque Control

Such types of loops are used in battery powered vehicles, rails, and electric trains. The reference torque T* is set through the accelerator, and this T* is followed by the loop controller and the motor. The speed of the drive is controlled by putting pressure on the accelerator.

Closed Loop Speed Control

The block diagram of the closed loop speed control system is shown in the figure below (Note: figure is not provided in the text). This system uses an inner control loop within an outer speed loop. The inner control loop controls the motor current and motor torque below a safe limit. When the current limiter saturates, the drive decelerates in a braking mode. When the current limiter becomes desaturated, the drive is transferred from braking to motoring.

Speed Control of Multi Motor Drives

In such types of drives, the load is shared between several motors. In this system, each section has its own motor which carries most of its load. The rating of the motor is different for different types of loads, but all the motors run at the same speed. If the torque requirement of each motor is fulfilled by its own driving motor, then the driving shaft has to carry only small synchronizing torque.