induction motor seminar - University of Illinois at Urbana ...energy.ece.illinois.edu/...16-16-induction-motor-seminar-handout.pdf · motor Gramme ring wound stator ... connection

U of I Seminar Page 1 TAK 9 / 16 / 2016

Inductions Motors

Overview of history, physical design, basic theory, and

performance with emphasis on aircraft applications

Typical large induction motor circa 1894

100 HP – 8 pole ‐ 600 RPM – must be 40 hertz

Drip‐proof, self‐vent

Formed coil / chain wound / single coil per slot

(consequent pole)

Wound rotor with spider construction

Slip rings outboard of bearing

Sleeve bearing


1. Description, configuration, features ‐ Other names are asynchronous or squirrel cage motor

‐ Only one of the windings is excited with electrical

input as a requirement

‐ Does it even have a winding on the rotor?

‐ It’s like a transformer with an air gap and the

secondary is shorted out and spins

‐ It’s like a clutch being ridden all the time

‐ It’s simple and robust

‐ It’s synonymous with 3 phase, polyphase, and rotary

magnetic field

‐ One of the 10 best discoveries in the last 130 years.

‐ It’s not just a motor ‐ it’s a brake or a generator

Tesla’s first motor

2 phase (4 wire)

2 pole

Concentrated or

concentric pole wound

Wound rotor ‐ shorted


Induction motors comprise of the following

electrical / magnetic elements:

Stator – the only part normally connected to external electrical

power. Most commonly on the exterior and almost always

stationary.

Stator winding are three phase or single phase today and carry

full power of the motor.

Stators have laminated cores and most have slots for the coils ‐

either formed coils or random wound coils. It’s hard to tell

induction motor and traditional synchronous stators apart.

Early machines quickly moved to the modern stator

configuration.

The second Tesla

motor

Gramme ring

wound stator

Slotted

6 pole?

Solid rotor

2 phase (4 wire)

U of I Seminar Page 4 TAK 9 / 16 / 2016 Rotor – the rotor need not receive external power or excitation

other than from the stator.

Most rotors are “squirrel cage”‐ consisting of bars that are

shorted together on each end to “end rings”.

Cage rotors are cast or fabricated of aluminum, copper, or

other metals. Most rotors have no insulation.

Wound rotors have winding schemes similar to stators –

typically wound for 3 phases and with a pole count matching

the stator.

Slip rings are used to connect a wound rotor to external

resistance to change the motor speed / torque characteristic.

Early Tesla / Westinghouse 2 phase wound rotor motor

8 concentric coils on

pole pieces

2 phase

4 poles (2 coils per

pole)

2 slip rings


3 phase formed coil stator and matching rotor

Transition to 3 phase and many

slotted distributed winding

This machine is formed coil,

concentric, consequent, and

chain wound

15 coil groups, 5 per phase

note 1 group has a special cross

over

10 pole, 120 slot,

4 slots per pole per phase

Small slot opening to reduce

slot leakage but coils inserted

from ends

Circa 1900

Matching squirrel cage

rotor

2 radial vents rotor and

stator)

Fabricated construction

Note open slot tops

90 slots (originally wound?)

How many phases or

poles??


Stator Winding Basics

Core configuration

Barrel slotted, Gramme ring,

or concentrated pole wound

Slot type

Parallel side ‐ formed coil

Semi‐closed ( parallel tooth) – random winding

Coil shape

Loop wound or wave wound


Layers

2 layer lap ‐ 3 groups per pole if 60 degree belts or 3 groups per pole pair if 120 degree belts (low speed of 2 speed motors)

1 layer ‐ 3 groups per pole pair since half as many groups can be

called consequent pole

Loop shape

Knuckled lap coils Concentric / chain coils

Winding repetition

Integral winding – slot per pole per phase ( s/p/ph) is an integer ‐ all groups have same # of coils in series no restrictions in parallels

Fractional winding – s/p/ph is a fraction – parallels limited


Winding factors

Pitch factor – Kp: Coil embrace, pitch, or throw – what part of

pole pitch is covered. Sine of 90 degrees x embrace.

Distribution factor ‐ Kd‐ phase effect of adjacent coils in a group in series ‐ never less than 3/π (.955) for 60 degree belts.

Connection ‐ parallel groups, series groups, wye or delta.

Integral example ‐ 6 pole – 54 slots (60 degree belts, 2 layer)

333333333333333333 = 54 s/p/ph = 3

3 3 3 3 3 3 18 coil groups all the same

3 3 3 3 3 3 6 coil groups per phase

3 3 3 3 3 3 6, 3 , 2, or 1 parallels available

Fractional example ‐ 8 pole – 90 slots (60 degree belts, 2 layer)

433343334333433343334333 = 90 s/p/ph = 3.75

4 3 3 3 4 3 3 3 24 coil groups not all the same

3 4 3 3 3 4 3 3 8 coil groups per phase

3 3 4 3 3 3 4 3 2, or 1 parallels available

Always 4 groups in series

3 + 3 + 3 + 4 = 11 = 3.75 x 4

Example ‐ 36 slot / 4 pole / 2 layer lap winding

3 turn coils / 2 parallel Y / 1 to 7 throw

s/p/ph = 36 / 4 / 3 = 3

pole pitch = 36 / 4 = 9 coil pitch = (7‐1) / 9 = .67 kp= sin ( 90 x .67) = .866

kd =.960 4 groups per phase since 2 parallel there are two groups in series

Series turns = 4 turn x 3 coils per group x 2 groups in series x .866 x .96 = 19.95


Partially wound formed coil stator

Lap wound – 2 coil sides per slot

Bar wound = 1 turn = 1 knuckle

Multi‐turn – 2 knuckle

Multi‐turn formed coils with

corona gradient paint in core area.

Stator coil at loop stage and after

“spreading”

2 – four turn coils in slot, without

and with side‐ways stranding


Random wound stators in process

Aircraft hydraulic

pump motor

Random lap wound

Semi ‐ closed slots

Parallel teeth

Note phase papers


Rotor Construction

Wound rotor

Bar wound

Maybe 4 pole because coil end

turn crosses about 1/8th of

circumference.

Bar to bar connection will be

soldered using clips.

First motor I ever wound was

like this in 1977. Took me 3

weeks.

Traditional fabricated bar rotor are used

for:

1. High efficiency (copper bar)

2. Large size

Copper trade groups tout the

advantages of copper rotors – aluminum

rotors have “bad efficiency”

Fabricated rotor have brazed or soldered

connection and potentially loose bars.

They are most prone to rotor problems

Is efficiency always a good thing?

Rotor slot combo crib sheet‐

black art at its best.

Refer to Gabriel Kron, AIEE, 1931


Comparison of moderate and deep bar designs

Slot opening – carter factor for

effective air gap

Slot leakage (reactance)

Various depth over width ratios

Skin effect

Deep bar effect

Rotor reactance and resistance

varies with slip / speed

24 HP rotor 6 pole machine

Die cast aluminum

½ slot stator skew

Open slot tops

Double tear drop design

65 slot (13 x 5)

56 slot stator

(6 pole x 3 phase x 3 slots/pole/ph = 54)

65 / 54 = 1.20 (greater than 1.2 or less than .8)

R‐S = 9 (not 6, 6+/‐1, or 6+/‐2)


2. Theory and some history

‐ The transformer analogy

‐ Rotary magnetic field

‐ Force mechanism

‐ Logic, slip, and the equivalent circuit


The rotating magnetic field

The first AC machines were alternators (synchronous machines) but

these machines were conceived as an AC replacement for DC dynamos.

They were “2 wire” machines that essentially replaced the commutator

of a DC machine with slip rings. Then the ideal came to place this new

type AC winding on the stator. The alternator was the necessary

companion to the transformer for long distance transmission.

To date no one had tried to operate these machines as motors nor had

they been paralleled. Frequency was not important because nothing

“operated” off frequency. Early machines were single phase ‐ there

was no concept of multiple or polyphases.

Early alternator with dynamo exciter

disc wound design

Field stationary

Armature rotating with slip rings – single ckt

First Commercial 3 phase alternator, claw tooth rotor

Lauffen to Frankfort system 110 miles

32 pole, 40 hertz, 2000 volt, 50 volt , 1400 amp

93 hertz at 400 RPM , weighs 9 tons

U of I Seminar Page 15 TAK 9 / 16 / 2016 Galileo Ferraris and Tesla theorized that AC current with a fixed phase

relationship (polyphase) could be placed about a machine geometrically

such that these pulsating MMF’s would result in a wave that rotates

about the machine and allow inducted excitation of a rotor across the

air gap.

Phase A MMF in a machine

Phase B MMF in a machine

120 degrees away from A

Phase C MMF in a machine

120 degrees away from B

Resultant MMF wave

moving or travelling

U of I Seminar Page 16 TAK 9 / 16 / 2016 The result of this idea lead to 3 advancements:

1. The idea that polyphases would allow for a traveling MMF that could be put to use . Mikhail Dolivo‐Dobrovolsky showed three phases (Drehstrom) was more practical than two phase. “AC” was already identified as having a separate advantage because it allowed high voltage distribution via transformers.

2. The invention of the induction motor nearly simultaneously by Galileo Ferraris and Tesla. (Later Mikhail Dolivo‐Dobrovolsky simplified the motor with the squirrel cage rotor.)

3. The understanding that the alternator could also operate as a synchronous motor.

Tesla or Ferraris and who else?

Walter Bailey – 1879 – demonstrated a 2 phase device powered by dry cells and a hand crank commutator.

Ferraris demonstrated polyphase and rotating magnetic fields in 1885. He built induction type motor prior to Tesla.

Tesla received a US patent on May 1st 1888. He made great prototypes and presented the motor like a showman at technical gatherings.

Dolivo‐Dobrovolsky added a lot to practicality – 3 phase theory and rotor construction.

Behrend and De La Tour published practical theory and the circle diagram in 1901.

Steinmetz provided the equivalent circuit we use today ‐ replacing the more complete Heyland circle diagram.

By 1911, Bailey’s book – The induction Motor summarizes modern theory.


Ferrasis’s first 4

motors

1885 and 1886

All two phase

The last one has a

magnetic core

Bailey’s Motor ‐1879

Commutating sequence (partial):


Force Mechanism

Refer to Direct – Current Machines by Michael Liwschitz (Second

Edition 1956) Chapter 14 “Tangential Forces in Electric Machines” for

a detailed presentation of force theory.

Flux created by MMF


Logic, Slip and the Equivalent Circuit

The MMF and flux wave of an induction motor travel the same way that

they do in a synchronous machine thus:

RPM (synchronous) = 120 x Frequency / poles

If the rotor of an induction motor travel exactly in step with the stator‐

created field, then the flux looks to be at stand still when viewed from

the rotor.

If this is the case then no voltage / current can be induced in the rotor

and it can produce no torque to keep itself spinning in lock step with

the field wave.

Induction motors never operate at synchronous speed under their own

power.

When motoring, induction motor run below synchronous speed or are

slipping relative to synchronous speed.

Slip ( S ) = (synchronous speed – actual speed)

synchronous speed


At synchronous speed slip = 0 or 0%

At stall / locked rotor slip = 1.0 or 100%

Example: a 4 pole 60 hertz motor runs at 1740 RPM

RPM synchronous = 120 x 60 / 4 = 1800 RPM

Slip = ( 1800 ‐1740) / 1800 = 60 / 1800 = .033 = 3.3%

Rotor electrical frequency = slip x stator frequency

At synchronous speed Slip = 0, rotor frequency = 0, (No frequency, no induction)

At stall slip =1, rotor frequency = line frequency For our example above rotor frequency = .033 x 60 hz = 2 hertz

Rotors can be made of lower grade steel since operating

frequency is low (and losses are low).

It easy to observe that as load is increased in an induction motor that it

slows down a bit just like DC motors.

The induction motor equivalent circuit is a combination of the

transformer equivalent circuit and the idea of slip. It was developed by

Steinmetz. Prior to the equivalent circuit, motor performance was

calculated using the circle diagram.


Remember that the induction motor was described as being

like a clutch?


The equivalent circuit as a tool

No Load S ≈0 (near synchronous speed)

Locked rotor (stall / standstill) S= 1

R2”/S ≈ ∞

Rotor (secondary) nearly

open circuited

Almost no induced voltage

R2” / S = R2”

Xm and Rc >> other parameters

so they can be ignored

Starting torque ~ I^2 x R2”

Is

Im

Ir’ or I2’


Plot of Equivalent circuit performance


The derivation of the equivalent circuit is quite complex

but the result is quite simple. All inductive parameters

are treated with stator / line frequency though X2

physically experiences variable frequency.

R2 is subject to “skin effect” and “deep bar” effect as slip

increases. As a result R2 is greater at stall than running

thereby increasing motor torque. (R2start > 1.5 x R2run).

X2 is subject to saturation of the slot leakage due to

frequency and high current at stall. As a result X2 is

lower at stall than running. This increased locked current

and thereby increases torque. (X2start < 1.5x X2run).

With these effects varying with slip, the calculated motor

performance looks more like actual performance of the

motor as follows:


With this enhance modeling:

Locked current ( LRA) increases from 69 amps to 92 amps

( X1 + X2 reduced to 75% )

Locked torque (LRT) increase from 21 to 79 in‐#

( R2 increased to 210% of running value)

Locked power factor increase from 19% to 38%

Breakdown torque ( BDT) increases from 97 to 111 in‐#

Some motors exhibit a “belly” in the

torque curve called pull up torque (PUT).

It’s hard to predict or model.

BDT

LRALRTLocked PF

FLT (full load torque)

FLA (full load amps)


Running performance / Load saturation


3. Motor Design and Application:

The equivalent circuit, magnetics, and D^2xL

The previous discussion of the equivalent circuit suggest that motor

performance boils down to designing something that has the

equivalent circuit parameters that will provide performance desired.

How is this tied to magnetics and size or volume of the motor? What

about thermal capability?

The key parameter in motor performance / capability are X1, X2

and R2. All three are controlled by slot dimension, the number of

turns (squared), and the size of the coil. (Slot permeance is a

detailed subject of its own.)

The parameter Xm, R1 are secondary parameters whose effects

we wish to minimize. Ideally we want Xm to be infinite (to

improve power factor) and R1 to be zero. Both of these

parameters are strongly influenced by the size of the machine and

the number of turns in the coils / winding.

R1 and Rc in large part determine the machine losses and have no

positive effect. R2 controls rotor loss but is special in that it

controls locked torque and the slope of the torque curve in the

running section.

When flux loading of a machine is higher it can have less turns in

the primary, having the big effect of reducing the equivalent

circuit parameters and increasing machine capability. On the


down side increasing flux loading decreases Xm and Rc –

increasing magnetic losses.

The idea of D^2xL relates torque capability of a machine to

volume measured at the air gap. Sizing constants (torque per air

gap volume) consider magnetics, losses, heating and experience.

(N= turns per phase)

Φ= total flux per pole in lines


No load saturation Xm, and Rc

If the rotor or secondary current is 38 amps

at unity power factor then:

I = √ (21^2+38^2) = 43.4 amps

PF = 38 / 43.4 = .876

U of I Seminar Page 30 TAK 9 / 16 / 2016 Is low rotor resistance a good thing? Consider if the rotor of the

RJ motor was copper instead of aluminum.

Customer says – “I want a motor with high efficiency – a copper rotor

but it needs to have good starting performance and a decent power

factor when starting.” ??#??+


Shaping the speed / torque curve


How does voltage affect the motor?

Torque profile proportional to V^2 ( 85 / 111.5)^2 = .58

Current proportional to V (85 / 111.5) = .76

Power factor unchanged

Motor slows down – torque margin eroded.

How about at load conditions?

Speed will reduce with lower voltage

Depends on load type (constant HP, constant torque, cube law)

Current and PF depend on load change and dominance of magnetizing versus “working” current.

Simple load model


How does frequency affect the motor?

Torque inversely proportional to (frequency)^2

T@ 650 hz = (400/650)^2 of 400 hz = 38%

Current inversely proportional to frequency

I @ 650 hz = (400/650) of 400 hz = 62%

For both V and F

T ~ ( V / F)^2 I (starting) ~ (V / F)

V / F = flux, magnetic loading / strength

At low frequency R1 and R2 become more dominant and erode

torque and current.


Constant V over F operation

Torque fade at locked rotor

U of I Seminar Page 35 TAK 9 / 16 / 2016 Wound rotor and high resistance rotors

Typical applications:

Speed Control ( historic, inefficient)

Punch presses, crushers, grinders

Amusement park rides


Brine resistors for rotor circuits can be 100 gallons or more!


4. Motoring, Generating, and Braking / Plugging

So far we have talked about operation between standstill and

synchronous speed (1 to 0 slip).

But what about speeds above synchronous speed or if the

motor is turning backwards (wind milling) when it is started?

The model is equally valid for these situations:

Above synchronous speed the motor is in the generating

regime. Slip is negative and the power of R2 becomes

negative.

At negative speed slip is greater than 1 and the rotor resistance and torque is less than that at stand still.

R2 / S < R2 if S > 1

Induction generators must be connected to voltage for

excitation.

Vehicle induction motor drives braking (AC traction

locomotives and road vehicles).

Windmills with low frequency power fed into the wound

rotor.

Small hillside hydropower units.



5. Specifications – NEMA and Aircraft

NEMA – National Electrical Manufacturers Association

NEMA MG ‐1 specifies many of the details of commercially

available motor (AC / DC, induction, synchronous, etc.).

Induction motors are the biggest impact area.

Key specification areas include:

Lead / terminal marking requirements – dual voltage, starting

modes, direction of rotation, and field winding for DC motor.

Frame dimensions – motor frame sizes are describe by NEMA.

48 frame, 56 frame, 184TD, 324U….. C flange, D flange

Integral frame induction motor – certain horsepower and speed

combinations are assigned to specific frame sizes.

Induction motor “types” – type B torque curve. Type D etc.

Type B is standard BDT = 200%, LRT = 130%, PUT = 100% for a 25

HP motor.

Locked rotor Code ‐ locked KVA per horsepower. Code G = 5.6 to 6.3 locked KVA per HP Code F = 5 to 5.6 locked KVA / HP

Temperature rise requirements – for different classes of

insulation

Enclosure requirements – open drip‐proof, TEFC ( totally enclose

fan cooled), TENV ( totally enclosed non‐ventilated)

Efficiency standards‐ big area today, high efficiency, premium

efficiency….


Aircraft Electrical Specifications:

Standards:

MIL‐STD‐704 ‐ Aircraft Electrical Power Characteristic

MIL STD‐M‐7969 – General Specification for Aircraft Alternating

Current Motors, 400 Cycle 115 / 200‐Volt System

RTCA DO‐160 ‐ Environmental Conditions and Test Procedures

for Airborne Equipment

Systems:

115 / 200 VAC (most commercial aircraft)

230 / 400 VAC ( Airbus A350 and Boeing 787) 28 VDC ( 24 volt battery) 270 VDC ( 1.35 x 200 VLL)

CF ‐ (constant frequency) ‐ 400 hertz ( constant speed drive) NF – (narrow frequency) – 360 to 650 hertz ( tied to engine speed) WF – (wide / wild frequency) – 360 to 800 hertz

Key Terms:

POR – point of regulation UET – utilization equipment terminals

Normal / Abnormal / Emergency ‐ Voltage and Frequency

Essential Bus / Non‐Essential Bus RAT – Ram Air Turbine

PMP – Pump Motor package

ACMP / DCMP – ( AC or DC motor pump)

U of I Seminar Page 41 TAK 9 / 16 / 2016 Key ACMP Attributes and Requirements

Rotary Piston Pumps

Regulated via hanger angle and hydraulic control system

Regulation curve to work in parallel with other pumps

Max electrical load defined in amps

Maximum weight specifications

Temperature ‐65F to 225 F

Altitudes to 51,000 ft. Explosion proofness

Documents

induction motor seminar - University of Illinois at Urbana ...energy.ece.illinois.edu/...16-16-induction-motor-seminar-handout.pdf · motor Gramme ring wound stator ... connection