Electrical Machines and Controls

Spring 2014

Paul I-Hai Lin, Professor of ECET

 

Three-Phase Induction Motors

 

Reference

[ 1]     Chapter 7 Three-Phase Induction Motors, Electrical Power and Controls, by Timothy L. Skvarenina and William E. DeWitt, 2004, Prentice Hall

 

Introduction

 

 

 

IM Construction Features

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Totally Enclosed, Air Over (TEAO) – Figure 7-7 (ODP Motor, Baldor)

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Nameplate Data

Figure 7-9 Typical induction motor nameplate information

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Figure 7-10 Induction of rotor currents by rotating stator magnetic field

fg07_01000.jpg                                                 001F8BC5Macintosh HD                   BC848919:

 

 

 

 

Example 7-1. What is the apparent three-phase power drawn by the motor operating at 460V.

Answer:

 

|S| = √3 * |VL||IL|

 

Source: www.allaboutcircuits.com

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Example 7-2 How much torque does the motor of Figure 7-9 deliver when operating at rated conditions?

Answer:

The torque is the application of a force at a distance to cause something to turn: T = F* r

From equation 1-1 (Page 14):  P = ωr*T

 

In SI Unit:

 

 

 

 

 

 

 

 

 

In English Unit:

 

 

 

 

 

 

 

 

Example 7-3 What is the expected starting line current for the motor of Figure 7-9 when operating from a line-to-line voltage of 460V?

Answer:

From NEMA Locked-rotor codes for three-phase induction motor:

Code letter G, kVA/HP at locked rotor (5.6-6.29)

Find     |SLR|

 

|SLR| = √3 * |VL||ILR|

 

|ILR| = |SLR| / √3 * |VL|

 

 

 

 

 

 

 

 

 

 

 

Principles of Operation

 

Example 7-4. A six-pole, 60 Hz IM operates at 3% slip, At what rate do the stator field, the rotor, and the rotor field revolve? What is the frequency of the rotor currents?

 

 

 

 

 

 

 

 

 

 

 

 

 

Example 7-5. The 60 Hz motor whose nameplate is shown in Figure 7-9 has a rated speed of 1765 RPM and is a NEMA design B, which means it is a general-purpose motor. How many poles does the motor have? 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Torque-Speed Characteristic

 

NEMA Motor Designs

 

Induction Motor Efficiency

 

Motor Application Information

·         Ambient temperature

·         Altitude

·         Voltage variation

·         Frequency variation

·         Total voltage magnitude and frequency variation

·         Voltage magnitude unbalance

·         Proper mounting and coupling

·         Encloses not seriously interfering with ventilation of the motor 

 

 

Three-Phase Motor Connections

 

Starting Consideration for Induction Motors

 

Ea = Vm sin(ωt + 0º)

Eb = Vm sin(ωt + 120º)

Ec = Vm sin(ωt + 240º)

 

 

Connection of the phase windings in a Y-connected Generator

 

 

 

 

Conventional diagram of a Y-connection

 

 

 

Phasor diagram for showing the relation of phase and line voltage

 

 

 

 

 

Connection of the phase windings in a Delta-connected Induction Motor

 

 

 

Conventional diagram of a Delta connection