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TitleElectric Motor Fundamental
TagsElectric Motor Electromagnetic Induction Alternating Current Machines Electrical Components
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Total Pages24
Table of Contents
                            Introduction
Basic Construction and Operating Principle
	Stator
		FIGURE 1: A Typical Stator
	Rotor
	Speed of an Induction Motor
		EQUATION 1:
		EQUATION 2:
		FIGURE 2: A Typical Squirrel Cage Rotor
Types of AC Induction Motors
	Single-Phase Induction Motor
		FIGURE 3: Single-Phase AC Induction Motor With and Without a Start Mechanism
	Split-Phase AC Induction Motor
		FIGURE 4: Typical Split-Phase AC Induction Motor
	Capacitor Start AC Induction Motor
		FIGURE 5: Typical Capacitor Start Induction Motor
	Permanent Split Capacitor (Capacitor Run) AC Induction Motor
		FIGURE 6: Typical PSC Motor
	Capacitor Start/Capacitor Run AC Induction Motor
		FIGURE 7: Typical Capacitor Start/Run Induction Motor
	Shaded-Pole AC Induction Motor
		FIGURE 8: Typical Shaded-Pole Induction Motor
		FIGURE 9: Torque-Speed Curves of Different Types of Single-Phase Induction Motors
Three-Phase AC Induction Motor
	Squirrel Cage Motor
	Wound-Rotor Motor
		FIGURE 10: Typical Wound-Rotor Induction Motor
Torque Equation Governing Motor Operation
	EQUATION 3:
	EQUATION 4:
	FIGURE 11: Typical Torque-Speed Curve of 3-Phase AC Induction Motor
Starting Characteristic
Running Characteristic
Load Characteristic
	FIGURE 12: Torque-Speed Curve – Same Motor with Two Different Loads
	Constant Torque, Variable Speed Loads
		FIGURE 13: Constant Torque, Variable Speed Loads
	Variable Torque, Variable Speed Loads
		FIGURE 14: Variable Torque, Variable Speed Loads
	Constant Power Loads
		FIGURE 15: Constant Power Loads
	Constant Power, Constant Torque Loads
		FIGURE 16: Constant Power, Constant Torque Loads
	High Starting/Breakaway Torque Followed by Constant Torque
		FIGURE 17: High Starting/ Breakaway Torque Followed by Constant Torque
Motor Standards
	NEMA
	IEC
		FIGURE 18: Torque-Speed Curves of Different NEMA Standard Motors
		TABLE 1: Motor Duty Cycle Types as per IEC Standards
Typical Name Plate of an AC Induction Motor
	FIGURE 19: A Typical Name Plate
	TABLE 2: Name Plate Terms and Their Meanings
Need for the Electrical Drive
Variable Frequency Drive (VFD)
	FIGURE 20: Typical VFD
	FIGURE 21: V/f Curve
	VFD as Energy Saver
		FIGURE 22: Typical Centrifugal Pump Characteristics
		EQUATION 5:
		FIGURE 23: Characteristic of Centrifugal Pump with Load – With and Without VFD
Control Techniques
	Scalar Control
	Vector Control
	Direct Torque Control (DTC)
		FIGURE 24: DTC Block Diagram
Summary
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Document Text Contents
Page 1

AN887
AC Induction Motor Fundamentals
INTRODUCTION

AC induction motors are the most common motors
used in industrial motion control systems, as well as in
main powered home appliances. Simple and rugged
design, low-cost, low maintenance and direct connec-
tion to an AC power source are the main advantages of
AC induction motors.

Various types of AC induction motors are available in
the market. Different motors are suitable for different
applications. Although AC induction motors are easier
to design than DC motors, the speed and the torque
control in various types of AC induction motors require
a greater understanding of the design and the
characteristics of these motors.

This application note discusses the basics of an AC
induction motor; the different types, their characteris-
tics, the selection criteria for different applications and
basic control techniques.

BASIC CONSTRUCTION AND
OPERATING PRINCIPLE

Like most motors, an AC induction motor has a fixed
outer portion, called the stator and a rotor that spins
inside with a carefully engineered air gap between the
two.

Virtually all electrical motors use magnetic field rotation
to spin their rotors. A three-phase AC induction motor
is the only type where the rotating magnetic field is

created naturally in the stator because of the nature of
the supply. DC motors depend either on mechanical or
electronic commutation to create rotating magnetic
fields. A single-phase AC induction motor depends on
extra electrical components to produce this rotating
magnetic field.

Two sets of electromagnets are formed inside any motor.
In an AC induction motor, one set of electromagnets is
formed in the stator because of the AC supply connected
to the stator windings. The alternating nature of the sup-
ply voltage induces an Electromagnetic Force (EMF) in
the rotor (just like the voltage is induced in the trans-
former secondary) as per Lenz’s law, thus generating
another set of electromagnets; hence the name – induc-
tion motor. Interaction between the magnetic field of
these electromagnets generates twisting force, or
torque. As a result, the motor rotates in the direction of
the resultant torque.

Stator

The stator is made up of several thin laminations of
aluminum or cast iron. They are punched and clamped
together to form a hollow cylinder (stator core) with
slots as shown in Figure 1. Coils of insulated wires are
inserted into these slots. Each grouping of coils,
together with the core it surrounds, forms an electro-
magnet (a pair of poles) on the application of AC
supply. The number of poles of an AC induction motor
depends on the internal connection of the stator wind-
ings. The stator windings are connected directly to the
power source. Internally they are connected in such a
way, that on applying AC supply, a rotating magnetic
field is created.

FIGURE 1: A TYPICAL STATOR

Author: Rakesh Parekh
Microchip Technology Inc.
 2003 Microchip Technology Inc. DS00887A-page 1

Page 2

AN887
Rotor

The rotor is made up of several thin steel laminations
with evenly spaced bars, which are made up of
aluminum or copper, along the periphery. In the most
popular type of rotor (squirrel cage rotor), these bars
are connected at ends mechanically and electrically by
the use of rings. Almost 90% of induction motors have
squirrel cage rotors. This is because the squirrel cage
rotor has a simple and rugged construction. The rotor
consists of a cylindrical laminated core with axially
placed parallel slots for carrying the conductors. Each
slot carries a copper, aluminum, or alloy bar. These
rotor bars are permanently short-circuited at both ends
by means of the end rings, as shown in Figure 2. This
total assembly resembles the look of a squirrel cage,
which gives the rotor its name. The rotor slots are not
exactly parallel to the shaft. Instead, they are given a
skew for two main reasons.

The first reason is to make the motor run quietly by
reducing magnetic hum and to decrease slot
harmonics.

The second reason is to help reduce the locking ten-
dency of the rotor. The rotor teeth tend to remain locked
under the stator teeth due to direct magnetic attraction
between the two. This happens when the number of
stator teeth are equal to the number of rotor teeth.

The rotor is mounted on the shaft using bearings on
each end; one end of the shaft is normally kept longer
than the other for driving the load. Some motors may
have an accessory shaft on the non-driving end for
mounting speed or position sensing devices. Between
the stator and the rotor, there exists an air gap, through
which due to induction, the energy is transferred from
the stator to the rotor. The generated torque forces the
rotor and then the load to rotate. Regardless of the type
of rotor used, the principle employed for rotation
remains the same.

Speed of an Induction Motor

The magnetic field created in the stator rotates at a
synchronous speed (NS).

EQUATION 1:

The magnetic field produced in the rotor because of the
induced voltage is alternating in nature.

To reduce the relative speed, with respect to the stator,
the rotor starts running in the same direction as that of
the stator flux and tries to catch up with the rotating flux.
However, in practice, the rotor never succeeds in
“catching up” to the stator field. The rotor runs slower
than the speed of the stator field. This speed is called
the Base Speed (Nb).

The difference between NS and Nb is called the slip. The
slip varies with the load. An increase in load will cause
the rotor to slow down or increase slip. A decrease in
load will cause the rotor to speed up or decrease slip.
The slip is expressed as a percentage and can be
determined with the following formula:

EQUATION 2:

FIGURE 2: A TYPICAL SQUIRREL CAGE ROTOR

Ns 120
f
P
---×=

where:
NS = the synchronous speed of the stator

magnetic field in RPM
P = the number of poles on the stator
f = the supply frequency in Hertz

slip
Ns Nb–

Ns
--------------------x100=%

where:
NS = the synchronous speed in RPM
Nb = the base speed in RPM

Conductors End Ring

Shaft

Bearing

Skewed Slots

Bearing

End Ring
DS00887A-page 2  2003 Microchip Technology Inc.

Page 12

AN887
Recently, NEMA has added one more design –
Design E – in its standard for the induction motor.
Design E is similar to Design B, but has a higher
efficiency, high starting currents and lower full-load
running currents. The torque characteristics of Design
E are similar to IEC metric motors of similar power
parameters.

The IEC Torque-Speed Design Ratings practically
mirror those of NEMA. The IEC Design N motors are
similar to NEMA Design B motors, the most common
motors for industrial applications. The IEC Design H
motors are nearly identical to NEMA Design C motors.

There is no specific IEC equivalent to the NEMA
Design D motor. The IEC Duty Cycle Ratings are
different from those of NEMA’s. Where NEMA usually
specifies continuous, intermittent or special duty
(typically expressed in minutes), the IEC uses nine
different duty cycle designations (IEC 34 -1).

The standards, shown in Table 1, apart from specifying
motor operating parameters and duty cycles, also
specify temperature rise (insulation class), frame size
(physical dimension of the motor), enclosure type,
service factor and so on.

TABLE 1: MOTOR DUTY CYCLE TYPES AS PER IEC STANDARDS

No. Ref. Duty Cycle Type Description

1 S1 Continuous running Operation at constant load of sufficient duration to reach the thermal
equilibrium.

2 S2 Short-time duty Operation at constant load during a given time, less than required to reach
the thermal equilibrium, followed by a rest enabling the machine to reach a
temperature similar to that of the coolant (2 Kelvin tolerance).

3 S3 Intermittent periodic duty A sequence of identical duty cycles, each including a period of operation at
constant load and a rest (without connection to the mains). For this type of
duty, the starting current does not significantly affect the temperature rise.

4 S4 Intermittent periodic duty
with starting

A sequence of identical duty cycles, each consisting of a significant period of
starting, a period under constant load and a rest period.

5 S5 Intermittent periodic duty
with electric braking

A sequence of identical cycles, each consisting of a period of starting, a
period of operation at constant load, followed by rapid electric braking and a
rest period.

6 S6 Continuous operation
periodic duty

A sequence of identical duty cycles, each consisting of a period of operation
at constant load and a period of operation at no-load. There is no rest period.

7 S7 Continuous operation
periodic duty with electric
braking

A sequence of identical duty cycles, each consisting of a period of starting, a
period of operation at constant load, followed by an electric braking. There is
no rest period.

8 S8 Continuous operation
periodic duty with related
load and speed changes

A sequence of identical duty cycles, each consisting of a period of operation
at constant load corresponding to a predetermined speed of rotation,
followed by one or more periods of operation at another constant load
corresponding to the different speeds of rotation (e.g., duty ). There is no rest
period. The period of duty is too short to reach the thermal equilibrium.

9 S9 Duty with non-periodic
load and speed variations

Duty in which, generally, the load and the speed vary non-periodically within
the permissible range. This duty includes frequent overloads that may
exceed the full loads.
DS00887A-page 12  2003 Microchip Technology Inc.

Page 13

AN887
TYPICAL NAME PLATE OF AN
AC INDUCTION MOTOR

A typical name plate on an AC induction motor is
shown in Figure 19.

FIGURE 19: A TYPICAL NAME PLATE

TABLE 2: NAME PLATE TERMS AND THEIR MEANINGS

ORD. No.

<Name of Manufacturer>

<Address of Manufacturer>

286T

1.10

415

60

01/15/2003

95B

1N4560981324

HIGH EFFICIENCY

42

42

1790

CONT

F

TYPE

H.P.

AMPS

R.P.M.

DUTY

CLASS
INSUL

FRAME

SERVICE
FACTOR

VOLTS

HERTZ

DATE

NEMA
NOM. EFF.

NEMA
DESIGN

3 PH

Y

4 POLE

Term Description

Volts Rated terminal supply voltage.

Amps Rated full-load supply current.

H.P. Rated motor output.

R.P.M Rated full-load speed of the motor.

Hertz Rated supply frequency.

Frame External physical dimension of the motor based on the NEMA standards.

Duty Motor load condition, whether it is continuos load, short time, periodic, etc.

Date Date of manufacturing.

Class Insulation Insulation class used for the motor construction. This specifies max. limit of the motor winding
temperature.

NEMA Design This specifies to which NEMA design class the motor belongs to.

Service Factor Factor by which the motor can be overloaded beyond the full load.

NEMA Nom.
Efficiency

Motor operating efficiency at full load.

PH Specifies number of stator phases of the motor.

Pole Specifies number of poles of the motor.

Specifies the motor safety standard.

Y Specifies whether the motor windings are start (Y) connected or delta (∆) connected.
 2003 Microchip Technology Inc. DS00887A-page 13

Page 23

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Printed on recycled paper.
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DS00887A-page 24 2003 Microchip Technology Inc.

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