Part II_AC Motors

8/10/2019 Part II_AC Motors

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AC Motors

CHUI Chee Kong, PhDControl & Mechatronics Group

Mechanical Engineering, NUS

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What you need to know

Appropriate AC motor type for variousapplications

Variation of torque with speed of motors

Compute electrical and mechanical quantities of

AC motors

Understand the operation and characteristics ofthree-phase induction motors, single-phase

motors, stepper motors and brushless DC

motors.

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Contents

1. Three-Phase Induction Motors2. Single Phase Motors

3. Stepper Motors

4. Brushless DC Motors

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1. Three-Phase Induction Motors

Three-phase induction motors are widelyused in applications require power ratings

over 5 hp.

Examples: pumps, fans, compressors etc.

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1.1 Rotating Stator Field

The field established by the stator windings of a three-

phase induction machine consists of an even number

of magnetic poles. The field rotates at a speed known

as synchronous speed.

The stator of AC motors

contains a set of windings to

which three phase electrical

power is applied.

These windings establish a

rotating magnetic field in the

gap between the stator and

the rotor.

The windings create an

even number of poles P.

In the case of two pole field,the stator contains three

windings (one for each

phase) embedded in slots

cut lengthwise on the inside

of the stator.

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Rotating Stator Field (continue)

Two views of a two-pole stator showing one of the three windings. For simplicity, we

represent the winding with a single turn, but in a real machine, each winding has many

turns distributed around the circumference of the stator such that the air-gap flux varies

sinusoidally with . 6


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Direction of Rotation, Poles and

Synchronous Speed

The direction of

rotation of a three-phase induction motor

can be reversed by

interchanging any two

of the three line

connections to the

three-phase source.

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Direction of Rotation, Poles and

Synchronous Speed (continue)

Synchronous Speed versus Number of Poles for f= 60 Hz.

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1.2 Squirrel-Cage Induction Machines

The rotor conductors of a squirrel-

cage induction machine are

aluminum bars connected to rings

that short the ends together. These

conductors are formed by casting

molten aluminum into slots in the

laminated iron rotor.

The simplest, least expensive, and

most rugged AC machines. The squirrel-cage rotor

Consists of aluminum bars with

shorting rings at the ends

Formed by casting molten

aluminum into slots of laminated

iron core

No external electrical

connections to the rotor

An advantage over DC

Motors: No brushes,

commutators etc. which

means longer service life.

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Supplementary Notes AC Motors

Chui Chee Kong, ME, NUS

AC Motor: Three-Phase Squirel-CageInduction Motor

- Induction motor: magnetic field on the rotor iscreated by an induced current

- Squirel-cage: a ring at either end of the rotor, and

bars connecting the rings over the length of the rotor

Cross sectional view

Rotor

Stator

Stator winding

a

a

bc

b

c

Squirel-cage rotor


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1.3 Slip and Slip Frequency

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The frequency of the rotor current is called t

slip frequency.

In Hz, fslip= sfwhere fis the operating

frequency of the induction motor.


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1.4 Torque-Speed Characteristics

Torque-versus-speed characteristic

for a typical three-phase induction

motor.

First, when s=0, relative velocity

between the conductors and the field is

zero, the induced voltage is zero.Consequently, the rotor currents are

zero and the torque is zero.

For small slips, the inductive reactance

of the conductors sLc are negligible.

Maximum rotor current is aligned with

maximum stator field, which is the

optimum situation for producing torque.Since the induced voltage is

proportional to slip and the

mpedance is independent of slip, the

currents are proportional to

slip. Torque is proportional to current.

Hence, assuming smal l sl ip , torque

s prop or t ional to s l ip11


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Torque-Speed Characteristics

(continue)

Torque-versus-speed characteristic

for a typical three-phase induction

motor.

As the motor slows further, the inductive

eactance eventually dominates. Then,urrent is nearly independent of

lip. Thus, the torque tends to level out

s the motor slows.

ecause the poles on the rotor tend to

ecome aligned with the stator poles,

he torque decreases as the motor

lows to a stop.

tarting torque or stall torque: torque

or zero speed

ull-out torque or the breakover

urque: the maximum

orque

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Example: Slip and Frequency of rotor current in a 3-phase induction motor

A 5-hp four pole 60-Hz 3-phase induction motor runs at 1750 rpm under full-

load conditions. Determine the slip and frequency of the rotor current at full

load.

Estimate the speed if the load torque drops in half.

Slip frequency (in Hz),fslip= s f= 0.02778 x 60 = 1.667 Hz.

In the normal range of operation, slip is approximately proportional to output

power and torque.

At half power, we estimate that s = 0.02778/2 = 1.389%.

This corresponds to a speed of 1775 rpm.


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Example: DC to AC power conversion for 3-phase induction motor

A 4-pole induction motor drives a load at 2500 rpm. This is accomplished by

using an electronic converter to convert a 400-V DC source into a set of 3-phase

AC voltages.

Given that the frequency of the AC voltages is 86.8 Hz assuming that the slip is

4%. The load is 2 hp. If the DC-to-AC converter has a power efficiency of 88%

and the motor has a power efficiency of 80%, estimate the current taken from

the DC source.

Solution:

Pout,motor= 2 hp x 746 = 1492 W

The input power to the motor is:

Pin,motor= Pout,motor/motor= 1492/0.80 = 1865 W

Pout,converter= Pin,motor

The input power to the converter is:

Pin,converter= Pout,converter/convertor= 1865/0.88 = 2119.32 W

Finally, the current taken from the 400-V source is:

I = Pin,converter/400 = 2119.32/400 = 5.298 A


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2. Single-Phase Motors

Single phase motors are importantThree phase power is not available for homes

and most offices

The stator of single phase motor has one main

winding

An auxiliary winding is needed for starting

It has a squirrel-cage rotor that is identical to the

rotor of the three phase induction motor

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OperationofaSimpleInductionMotor

NSStator Rotor

IsI

s

Is

Time

Is

B

B

(due

to

field

current

Is)

(Isis an AC current,

and is increasing)

Faradays Law: current is induced on the rotor.

Lenzs Law: direction of induced current is such that Binducedopposes B.

Binduced

+

.

+

.

Apushat

thetop

end

of

rotor:


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Is

Time

Is

B

B

Binduced

..

+

+

..


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2.1 Torque-Speed Characteristics

The main winding produces two

counter-rotating flux components

each of which induces torque in

the rotor. The main winding alone

induces no net starting torque.

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Rotor spins

counterclockwisewith speed m.


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The net starting torque is zero

Hence the main winding will not start a load from a

standing start Once started, however, the motor develops torque and

accelerates loads within its ratings to nearly synchronous

speed

Its running characteristics (in the vicinity of synchronous

speed) are similar to those of the three phase induction

motor Because of the symmetry of its torque-speed

characteristics, the basic single phase induction motor is

capable of running equally well in either direction

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2.2 Auxiliary Windings

Lack of starting torque is a serious flaw Need to incorporate methods of providing starting

torque

Two windings that are 90apart physically and carrying

currents 90apart in phase produce a rotating magnetic

field.

All single phase motors have an auxiliary windingrotated in space by 90ofrom the main winding

Various provisions can be made to achieve the

requisite phase shift between the current

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2.3 Split Phase Motors

Wind the auxiliary winding with smaller wire that has a higher ratio of

resistance to inductive reactance than the main winding.

Then the current in the auxiliary winding has a different phase angle thanthat of the main current

The auxiliary winding is only used at the start and, after speed picks up,

a switch turns off the auxiliary winding

A common failure is for the switch to fail to open and then the auxiliary

winding overheats and burns out.

Due to the "two step" approach, there is a lot more vibration.

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3. Stepper Motors

Stepper motors are used for accurate, repeatable positioning

applications Read/write heads of a computer floppy drive

Moving the head in an ink-jet printer

An electronic controller applies electrical pulses to the stator winding

so the rotor moves at steps

Motor shaft can be rotated in either directions

Steps range from 0.72 (500 steps per revolution) to 15 (24 steps

per revolution).

Rotational accuracies on the order of 3% of a step which is non-

cumulative as the motor is stepped back and forth.

Speed can be controlled by controlling the rate of the pulses

Direction can be reversed by reversing the switching sequence

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StepperMotorOperation

Off

Off

On

On

N

S

On

On

On

OffOff

Off

Off

Off

Off

Off

Off

Off

Off

Off

Off

N

N

N

N

t= t0 = t4 t= t1

t= t2t= t3

Half stepping to double the resolution.

On


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3.1 Variable-reluctance Stepper Motor

Stator has 8 poles that are 45

degrees apart, contains 4

windings.

Rotor has 6 poles 60 degrees

apart.

Controller applies power to one

of the coils at a time

When current is applied to A and held, the rotor is held in position 1

If power is removed from A and applied to B, the rotor moves 15 degrees

clockwise so that 2 is aligned with B.

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3.2 Permanent-magnet Stepper Motor

Has a cylindrical rotor that is permanentlymagnetized with north and south poles

alternating around its circumference

The stator is similar to that of the

reluctance motor

The rotor position is stepped by applying a

sequence of pulses to the stator windings

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4. Brushless DC Motors

Conventional DC motors are useful in applications thatrequire high speeds and when DC power is available

Due to commutators and brushes, conventional DC

motors have several disadvantages

Short service lives due to brush and commutator wear

Arcing as the brushes move between commutator

segments can pose a hazard in explosiveenvironments and can create severe radio

interference

Brushless DC motor, provides an excellent alternative

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Brushless DC Motors (continue)

Brushless DC motors are essentially permanent-magnet stepping

motors equipped with position sensors and enhanced control units.

Power is applied to one stator winding at a time

When the position sensor indicates that the rotor has

approached alignment with the stator field, the controller

switches power to the next stator winding so that smooth motion

continues

Speed can be controlled by varying the amplitude and duration

of the pulses

Characteristics similar to those of a conventional shunt dc motor.

They are used primarily in low-power applications

Advantages: High efficiency, long service life with little

maintenance, freedom from radio interference, ability to operate in

explosive chemical environments, and capability for very high

speeds (50,000 rpm or more).22


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Comparison

of

Electric

Motors

SteppermotorSmallpowerdevice;Accurate,repeatablepositioningapplication,forexample,

movingtheheadinaninkjetprinter.

BrushlessDCmotorpermanentmagnetsteppingmotorwithpositionsensorandenhancedcontrol

unit.Lowpowerapplicationindifficultenvironment.


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T

nm

Holdingtorque Cutoff speedStepper motor

AC3 haseinductionmotor

ACsinglephaseinductionmotor

withauxiliarywindingforself

starting

=Splitphasemotor

Thesynchronousmotorhaszero

startingtorqueandwouldnot

beabletostartahighinertia

load.

Theseriesfielddc

motorshouldnotbe

operatedwithouta

loadbecause

its

spee

becomesexcessive.

ns

Date post:	02-Jun-2018
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Part II_AC Motors

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