Classification of Electrical Drives or Types of Electrical Drives

The classification of electrical drives depends on the feature being compared. By motor allocation, a single-motor drive powers one machine or axis, a group drive uses one motor for several mechanically linked loads, and a multi-motor drive assigns separate motors to different sections or axes of one process. Drives can also be classified by motor type, converter topology, control method, feedback, duty and speed-torque quadrant. One simple direction-based division is:

1. Reversible drives, which can command both shaft-speed directions
2. Non-reversible drives, designed for one normal shaft-speed direction

Reversal is a mechanical speed-direction function, not a definition based on reversing magnetic flux. A fuller quadrant classification also asks whether motor torque can oppose speed for braking and whether energy can return to the source.

Parts of Electrical Drives

An electrical drive system includes the driven load, motor, power converter or starter, control and protection functions, source, sensors and mechanical transmission where used. The load’s torque-speed curve, inertia, duty cycle and braking demand determine the ratings and control functions required from the other parts.

Electric motors are selected for torque, speed range, duty, efficiency, environment and control response. Traditional DC motors include the shunt wound DC motor, series wound DC motor, compound wound DC motor and permanent magnet DC motor. Major AC classes include induction motors and synchronous motors, with several rotor and excitation forms. Induction designs include squirrel cage and wound-rotor motors. A drive can also use stepper motors, switched-reluctance motors and other specialist machines.

The available types of electric motors support different drive requirements. Before modern power electronics, controlled-speed electrical drives often used DC motors, while line-fed induction and synchronous machines commonly ran near fixed speed. Semiconductor converters now give induction motors wide speed control and strong dynamic performance. This has displaced many DC systems, but the correct choice still depends on the full load, control and lifecycle requirements.

Squirrel-cage induction motors are rugged and have no brushes, but efficiency and maintenance comparisons must use matched motors and duty points. Wound-field and permanent-magnet synchronous motors can offer high efficiency and controlled power factor, with trade-offs in converter needs, rotor technology, cost and field weakening.
Brushless DC motors and permanent-magnet synchronous motors share permanent-magnet rotor technology but differ in back-EMF and control conventions. They serve many servo applications without the brush commutation of DC servo motors. stepper motors suit incremental positioning, while switched-reluctance motors can provide robust variable-speed operation with a dedicated converter.
Power Modulators – are power-electronic or electromechanical stages that control the voltage, current or frequency supplied to electrical drives. A broad historical grouping is:

1. Power converters
2. Variable-impedance control circuits
3. Switching, starting and braking circuits

Power converters transform electrical power between AC and DC forms or change controlled voltage, currents and frequency. Common functional classes are:

1. AC-to-DC rectifiers and active front ends
2. AC-to-AC voltage controllers
3. DC-to-DC converters, including a DC Chopper
4. DC-to-AC inverters
5. Cycloconverters and other direct AC-to-AC frequency converters

An AC-to-DC stage can provide an uncontrolled or controlled DC link from an AC source. A complete variable-frequency drive commonly follows the rectifier with an energy-storage link and inverter.

An AC voltage controller changes RMS output voltage without independently setting output frequency. An autotransformers or tap-changing transformer can also change voltage, but it is not the same converter topology.

A chopper controls DC voltage or current by high-frequency switching. Modern implementations use power transistors such as IGBT’s and power MOSFET’s, selected for voltage, current, switching frequency and loss.

An inverter synthesises AC from a DC link. Pulse-width-modulated semiconductors control output voltage and frequency; they do not literally invert current alone.

A cycloconverter directly produces lower-frequency AC from a fixed-frequency AC source. Its output voltage and frequency are controlled by the switching sequence. Line-commutated Thyristors are common in traditional high-power designs.

Variable Impedance circuits change motor voltage or current by inserting controlled resistance or reactance. They can provide simple starting or speed adjustment but often waste energy and offer less control than electronic conversion. Inductors or reactors can limit starting current in suitable AC systems.
Switching circuits start, stop, reverse, brake and protect motors and electrical drives. Contactors, breakers, interlocks and protective relays must coordinate with the converter and motor. Four-quadrant operation needs a suitable converter and a way to dissipate or regenerate braking energy.
Sources may be single-phase, three-phase or DC and can span low to medium voltage. A motor’s synchronous speed is 120f divided by pole count, so a 50 Hz two-pole field is 3000 rpm and more poles give lower speed. A converter can supply other frequencies within motor mechanical, thermal and insulation limits. Source voltage must match the complete drive rating rather than a universal list of values.

Control Unit –
The controller generates switching commands from setpoints, feedback and limits. It can implement current, torque, speed or position loops and must coordinate starting, stopping, braking, protection, monitoring and communications.
The resulting drive is a system. Converter, motor, controls and driven equipment must be selected and tested together for the required duty.

Advantages of Electrical Drives

Electrical drives can control motion and match motor operation to the load. The main advantage of Electrical drives is flexibility when the system is correctly selected:

1. Available motor and converter combinations cover a wide range of torque, speed and power
2. Control can shape starting, steady-state response, speed, torque, position and electric braking for the load
3. Enclosures, cooling and ratings can be selected for many environments, but no drive suits every condition
4. A suitable converter can support all four speed-torque quadrants, including reversal and regenerative or dissipative braking
5. No combustion exhaust occurs at the point of use, although lifecycle environmental effects still depend on electricity and equipment production
6. Many drives start on command without fuel handling or combustion preheating, subject to motor and load limits
7. Electric supply integrates readily with automation; energy cost and emissions depend on tariff, generation mix, efficiency and duty

These advantages of electrical drives explain their broad use, but selection should compare total system efficiency, control performance, reliability, maintenance, safety and lifecycle cost at the actual operating points.