Induction Motors and Synchronous Speed

Induction Machines

• Induction machines are the most rugged and widely used in industry.
• They consist of a stator and a rotor.
• Both stator and rotor windings carry alternating current (AC).
• The machine can operate as both a motor and a generator.
• However, it is not usually used as a generator for supplying electrical power.
• The induction machine's generator characteristic is not satisfactory for most applications.
• Poly-phase induction motors are extensively used in industrial drives.
• They offer several advantages and disadvantages.

Advantages

• Simple and extremely rugged construction (especially squirrel cage type).
• Low cost and highly reliable.
• High efficiency in normal running, reducing frictional losses (no brushes needed).
• Reasonably good power factor.
• Requires minimal maintenance.
• Starts from rest without external starting. Simple starting for squirrel cage type motors.

Disadvantages

• Speed cannot be easily changed without sacrificing efficiency.
• Speed decreases with increased load (similar to DC shunt motor).
• Speed control is not easily achieved.
• High starting current.
• Low power factor during light loads.
• Lower starting torque compared to DC shunt motors.

Classification of Electrical Machines

• Electrical machines are classified as static (e.g. transformers) and rotating (e.g. DC and AC motors).
• Rotating machines are further grouped as DC and AC machines.
• AC machines are categorized as synchronous and asynchronous machines (induction machines).
• Induction machines come in single-phase and three-phase varieties.

Construction

• Induction motors have two main components: stator and rotor.
• The stator is a stationary steel frame supporting a hollow, cylindrical core built from stacked laminations(0.4-0.5mm thick).
• Slots are evenly spaced in the laminated core to house the stator winding.
• The laminations are insulated from each other (e.g varnish, oxide).
• The stator houses three-phase winding fed from a three-phase AC supply.
• The number of poles determines the speed—more poles, slower speed.
• The stator windings generate a magnetic field that rotates at synchronous speed.

Rotor

• The rotor is composed of stacked laminations producing slots to house the rotor winding.
• It can be of two types: wound-rotor or squirrel-cage.
• Squirrel-cage rotor: consists of copper or aluminum bars shorted at both ends by conducting rings. These bars are permanently short-circuited, so no external resistance is required for starting. The rotor slots are skewed to reduce magnetic hum.
• Wound-rotor: similar winding structure as the stator, but the rotor winding terminals are connected to slip rings, allowing for external resistance addition during starting, for enhanced starting torque, lower starting current and speed adjustment.

Principle of Operation

• Three-phase stator windings, when energized by a three-phase supply, create a rotating magnetic field of constant magnitude.
• This rotating field interacts with the rotor, inducing an emf in the rotor windings.
• The relative speed between the rotating field and the rotor windings creates the induced current.
• The direction of this induced current is determined by Fleming's right-hand rule.

Induction Motor Speed and Slip

• The rotor never rotates at the synchronous speed.
• Slip: the difference between synchronous speed and the rotor speed.
• Slip is defined in terms of percentage (the difference between synchronous speed and motor shaft speed, expressed as a percentage of synchronous speed).
• Slip = [(synchronous speed - shaft speed)/synchronous speed] x 100%
• If the rotor is stationary, slip = 1

Starting of Induction Motors

• Most induction motors start directly across the line, but large motors may experience excessive voltage drops and low starting torque.
• Reduced voltage starting methods (e.g. stator resistance, autotransformer, star-delta) are employed for large motors for improved performance during startup.
• Wound-rotor motors allow for external resistance addition during starting by means of slip rings (adjustable starting torque and current). Once running at rated speed the resistance are shorted out.
• Different starting methods are employed depending on the motor size, power line capacity, and load type.

Transformer

• Induction motors and transformers both operate on the principle of induced voltage.
• A transformer's induced voltage in the secondary winding is directly related to that of the primary winding.
• The induction motor's rotor winding voltage varies due to its rotation, unlike the transformer's secondary winding.