EET 323 – Electrical System Design Lecture 13: Motor Circuits Radian Belu, PhD

Lecture Objectives

Alternating Current (AC) Motors Direct Current (DC) Motors Type of Electric Motors Classification of Motors Electric Motors Alternating Current (AC) Motors Direct Current (DC) Motors Synchronous Induction Three-Phase Single-Phase Self Excited Separately Excited Series Shunt Compound Electrical Equipment/ Electric Motors

Induction Motor Operation 3-Phase Induction Motors – The Most Common and Used Types of Electric Motor 3-Phase Induction Motors – Robust, Reliable, Standardized Construction, Economical Motor Speed easily controllable using V/f or Field Oriented Controllers Have replaced DC Motors in areas where traditional DC Motors cannot be used such as mining or explosive environments Of two types depending on motor construction: Squirrel Cage or Slip Ring Only Disadvantage: Most of them run with a lagging power factor

IM Construction Stator Rotor Motor Control System

Construction The three basic parts of an AC motor are the rotor, stator, and enclosure. The stator and the rotor are electrical circuits that perform as electromagnets.

Construction (Stator Construction) The stator is the stationary electrical part of the motor. The stator core of a National Electrical Manufacturers Association (NEMA) motor is made up of several hundred thin laminations. Stator laminations are stacked together forming a hollow cylinder. Coils of insulated wire are inserted into slots of the stator core. Electromagnetism is the principle behind motor operation. Each grouping of coils, together with the steel core it surrounds, form an electromagnet. The stator windings are connected directly to the power source.

Construction (Rotor Construction) The rotor is the rotating part of the electromagnetic circuit. It can be found in two types: Squirrel cage Wound rotor However, the most common type of rotor is the “squirrel cage” rotor.

Construction (Rotor Construction) Induction motor types: Squirrel cage type: Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings Simple, low cost, robust, low maintenance Wound rotor type: Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes. Easy to control speed, more expensive.

Rotating Magnetic Field Balanced three phase windings, i.e. mechanically displaced 120 degrees form each other, fed by balanced three phase source A rotating magnetic field with constant magnitude is produced, rotating with a speed Where fe is the supply frequency and P is the no. of poles and nsync is called the synchronous speed in rpm (revolutions per minute)

Rotating Magnetic Field

Rotating Magnetic Field

Single-phase Induction Motors

Table 6.2 is Table 310.16 of NEC

Motor Protection The overload elements are seized on the full-load nameplate current rating of the motor, not the full-load motor current (Table 13-2) For continuous-duty motors, Section 430.32 of the NEC requires separated overload protection not exceeding the following percentages of the full-motor ratings:

Table 13.2

Thank you for listening THE END of the Lecture Thank you for listening Questions? Comments