ac motor basic theory rev 1

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AC MotorsBasic Theory

2 www.saftronics.com

Three Phase Motor Construction

Stator

Rotor

Windings – ElectromagnetsRotor bars



Side View

Rotor

Stator

Enclosure

Air Gap

Stator Windings


Just remember “FBI”

F- Force

B- Flux

I- Current


Just remember “FBI”

Force

Flux

F- Force

B- Flux

I- Current

Current



T1

Side View

T2

T2’

T3

T1’

T3’

+

+

+

+ denotes current is moving away from you

denotes current is moving towards you


Three Phase Motor ConstructionThree Phase Motor Construction

Force VectorForce Vector

T1T1

Side ViewSide View

T2T2

T2’T2’

T3T3

T1’T1’

T3’T3’

++++

++



T1

Side View

T2

T2’

T3

T1’

T3’

+

+

+N N

SS

S

N


Rotation of the Motor

T1

T2

T3

One Cycle*

* – current waveform

+

+

+N N

SS

S

N

++

+N N

S S

S

N

+

+

+N N

SS

S

N


Calculating the Speed of the Motor

An induction motor will complete one full rotation at a rate of .....

..... for every one electrical cycle

2

# of poles

Example: 2

2 poles= 1 revolution per cycle



Formula to find number of electrical cycles per minute

60

1/ fWhere:

60 – Represents 60 seconds in one minute

f – Represents the line frequencyExample:

60

1/ 60Hz= 3600 Cycles per minute



Formula to find motor synchronous RPM

N =O

2

# of poles

60

1 / f

Where:

N - Synchronous Speed

- Revolutions per electrical cycle

- Cycles per minute

2

# of poles

60

1 / f

O



N =O

2

# of poles

60

1 / f

In a simplified form

N =O

120 f

P

Formula to find motor synchronous RPM


“What is Slip?”

• To produce torque in an induction motor, current must flow in the rotor.

• To produce current flow in the rotor, the rotor speed must be slightly slower than the synchronous speed.

• The difference between the synchronous speed and the rotor speed (rated speed) is called the slip.

Stator

Flux

RotorSlip


“What is Slip?”

Rotor RPM = Flux RPM

Lines of flux are not cut and neither

current or voltage flow through the rotor

++

+N N

SS

S

N


“What is Slip?”

Rotor RPM < Flux RPM

Lines of flux are cut and current and

voltage flow through the rotor

N

S++

+N N

SS


Calculating Actual Motor Speed

Formula to find motor RPM

N =120 f

P( 1 – s )

Where:

N – RPM of the motor

f – Frequency in Hz

P – Number of poles of the motor

s – (No – N) / No


Speed – Torque Curve

(600%)

Locked Rotor Torque (150%)

Full Load Torque (100%)

Pull Up Torque (125%)

Breakdown Torque (200-250%)

Rated Speed Synch SpeedSPEED

TORQUE CURRENT

No Load Current (30%)

Slip

(300%)

(300%)

(200%)


Typical Nema Design Characteristics

Nema Design A

• High breakdown torque

• Normal starting torque

• High starting current

• Low full load slip

• Used in applications that require:

– Occasional overloads

– Better efficiency

0

100%

SPEED

TO

RQ

UE 200%

300%

100%



Nema Design B• Normal breakdown

torque• Normal starting torque• Low starting current• Normal full load slip

– less than 5%• General Purpose Motor

0

100%

SPEED

TO

RQ

UE 200%

300%

100%

Design B



Nema Design C

• Low breakdown torque

• High starting torque

• Low starting current

• Normal full load slip

– less than 5%


– high breakaway torque

0

100%

SPEED

TO

RQ

UE 200%

300%

100%

Design C



Nema Design D

• High breakdown torque

• High starting torque

• Normal starting current

• High full load slip

– 5 – 13%


– high breakaway torque

0

100%

SPEED

TO

RQ

UE 200%

300%

100%

Design D


Torque Production when Connected to an Inverter



TORQUE

Fout (Hz) 60


Torque Production when Connected to an Inverter



TORQUE

Fout (Hz) 603.0


Design B vs. Design D



Fout (Hz)1.5 3 60 Fout (Hz) 6031.5


Nema Design B

• 1.5Hz – 100%

• 3.0Hz – 150%

Nema Design D

• 1.5Hz – 25%

• 3.0Hz – 50%


Motor Recommendations

Design Key Points Recommendations

A

C

D

• Starting current may be slightly higher than normal

• Starting torque is less than across the line

• Starting current may be slightly higher

• Large amount of slip results in poor speed regulation

• Starting torque is less than across the line

Recommendations when applying an inverter to design A, C, or D motors

• Check inverter rating to handle the high current condition

• Verify that motor torque is large enough to perform the application

• Check inverter rating to handle the high current condition

• Verify that poor speed regulation will not adversely effect the application

• Verify that motor torque is large enough to perform the application


Motor Nameplate Data

XYZ MOTOR COMPANYXYZ MOTOR COMPANY

HP 10

RPM 1750

Hz 60

VOLTS 460

AMPS 15

NEMA DESIGN B

INS. CLASS BPHASE 3

FRAME 215T

S.F. 1.15


Understanding the Nameplate

HP – Horsepower• The horsepower figure

stamped on the nameplate is the horsepower the motor is rated to develop when connected to a circuit of the voltage, frequency and number of phases specified on the motor nameplate.

HP 10RPM

Hz

VOLTS

AMPS

NEMA DESIGNINS. CLASSPHASE

FRAMES.F.



Rotational Horsepower Formula

The formula to calculate horsepower

HP = 6.28 x Torque x RPM

33,000

Where:

6.28 – Number of radians in one revolution (2 )

Torque – Amount of torque in lb.ft.

RPM – RPM of the motor

33,000 – lb.ft. equal to 1 HP per minute


Rotational Horsepower Formula

The horsepower formula in simplified form

HP = Torque x RPM

5250

Where:

Torque – Amount of torque in lb.ft.

RPM – RPM of the motor

5250 – constant obtained by dividing 33,000 by 6.28



RPM – Revolutions per Minute• The RPM value

represents the rated speed at which the motor will run when properly connected and delivering its rated output.

HP 10RPM 1750

Hz

VOLTS

AMPS


FRAMES.F.




Voltage• The rated voltage

figure on the motor nameplate refers to the voltage of the supply circuit to which the motor should be connected, to produce rated horsepower and RPM.

HP 10RPM 1750

Hz

VOLTS 460

AMPS


FRAMES.F.



The effects of Voltage Variation on the Motor at 60Hz

Low Voltage

• Higher than normal current

• Higher than normal motor temperature

High Voltage

• Higher than normal current

• Lower than normal power factor



Phase• The phase figure on

the motor nameplate describes the alternating current system that the motor has been designed for.

HP 10RPM 1750

Hz

VOLTS 460

AMPS

NEMA DESIGNINS. CLASSPHASE 3

FRAMES.F.




Hz-Frequency• The frequency figure on

the motor nameplate describes the alternating current system frequency that must be applied to the motor to achieve rated speed and horsepower.

HP 10RPM 1750

Hz 60

VOLTS 460

AMPS


FRAMES.F.




Amps• The amp figure on the

motor nameplate represents the approximate current draw by the motor when developing rated horsepower on a circuit of the voltage and frequency specified on the nameplate.

HP 10RPM 1750

Hz 60

VOLTS 460

AMPS 15


FRAMES.F.




NEMA Design• The NEMA design

rating specifies the speed torque curve that will be produced by the motor.HP 10

RPM 1750

Hz 60

VOLTS 460

AMPS 15

NEMA DESIGN BINS. CLASSPHASE 3

FRAMES.F.




Insulation Class• The insulation class

letter designates the amount of allowable temperature rise based on the insulation system and the motor service factor.

HP 10RPM 1750

Hz 60

VOLTS 460

AMPS 15

NEMA DESIGN BINS. CLASS BPHASE 3

FRAMES.F.



Insulation Class Information

Most common insulation classes are class B and F

INS. Class Service Factor Ambient Temp Temp Rise Total Temp

B

F

1.0

1.0

1.15

1.15

40 C

40 C

80 C

90 C

105 C

115 C

120 C

130 C

145 C

155 C



S.F. – Service Factor• The service factor

represents the motor’s capacity to deliver continuously, horsepower beyond its nameplate ratings under rated conditions.Example – a 10HP motor with a service factor of 1.15 will deliver 11.5 horsepower continuously without exceeding the allowable temperature rise of its insulation class

HP 10RPM 1750

Hz 60

VOLTS 460

AMPS 15


FRAMES.F. 1.15




Frame• The frame designation

refers to the physical size of the motor as well as certain construction features such as the shaft and mounting dimensions.

HP 10RPM 1750

Hz 60

VOLTS 460

AMPS 15


FRAME 215TS.F. 1.15



Types of Motor Enclosures

• Open Drip-proof (ODP)

• Totally enclosed non-ventilated (TENV)

• Totally enclosed fan cooled (TEFC)

• Totally enclosed blower cooled (TEBC)



ODP• Open drip-proof• Ventilating openings permit

passage of external cooling air over and around the windings of the motor. Small degree of protection against liquid or solid particles entering the enclosure.



TENV• Totally enclosed non-

ventilated• Totally enclosed enclosure

with no means of external cooling.



TEFC• Totally enclosed fan-

cooled• Totally enclosed enclosure

with external cooling means, such as a shaft connected fan.



TEBC• Totally enclosed blower-

cooled• Totally enclosed enclosure

with external cooling means such as a separately controlled motor/blower


Motor Basics Exercise

Answer the following questions using your knowledge and/or the documents from this class.

1) Write down the formula to calculate synchronous motor speed ( please use the simplified form ).

2) To calculate the actual motor speed what has to be included in the above formula?

3) The majority of electric motors used in general purpose applications in industry today are what NEMA design?

4) A NEMA design D motor is characterized by very high breakdown torque, and a large percentage of slip? ( True or False )

5) To produce torque in the induction motor, _____________ must flow in the rotor. Without ___________ , current would not flow in the rotor and no torque would be produced.


SAFTRONICS TSG Department, Training Section

Saftronics

ac motor basic theory rev 1

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