MOTOR LECTURE

By: Erman Çağan Özdemir

INTRODUCTION

• Electromechanical device that converts electrical energy to mechanical energy

• Mechanical energy used to e.g. •Rotate pump impeller, fan, blower

•Drive compressors

•Lift materials

What is an Electric Motor?

ELECTRIC ENERGY UTILIZATION

Electric Motors 55%

Lighting 21%

Other 20%

Computers 4%

KAYNAK: University of Wisconsin, Permanent Magnet Machines and Drives: Principles, Design and Application, 2011

SOME FIGURES ABOUT POWER ELECTRONICS

• In developed countries ;

– 3kW of electric motor power per person,

– Only %10 of electrical motors are driven by power

electronics,

– Motor market enlarges by %4, motor driver market

%9

– In 10 years projection, ~%50 of motors will be driven

by power electronics

KAYNAK: Ian Boldea et al., Induction Machine Handbook, CRC Press LLC

HOW MOTORS WORK (1)

F = q(v × B)

Lorentz Force: When a particle with charge q and velocity v, enters a magnetic field which has a strength of B, a force develops. All motors have 2 basic parts, regardless of design; • Rotor (Moving Part) • Stator (Stationary Part)

When doing mechanical design, always remember motor also creates the same torque in opposite direction in the stator.

ROTATING MAGNETIC FIELD

Six Step Commutation

ROTATING MAGNETIC FIELD (2)

Sinusoidal Commutation

TYPE COMPARISON

AC INDUCTION Motor Brushless AC Motor Brushed DC Motor

Low cost Higher cost Higher cost

Simple motor but large Usually smallest size Smaller than IM

Simple wiring Extra electronic equipment costs

Electronic costs are less than BLAC

On / Off control (Low cost coarse speed control

possible)

Most suitable for precision control

applications

Can be directly operated from battery -

Good speed / torque control

Wide product variety Usually low power Usually low power

Robust Robust + More efficient Brushes wear / Arc

production

• Usually cost and/or size factor dominates • If precision requirement dominates, usually BLAC

SERVO MOTORS

• Servomechanism motors, or servomotors for short, are motors that power devices that employ feedback or error-correction signals to control its mechanical speed, position, or other parameter.

• The feedback or error signal is typically supplied by some type of encoder, a device that changes the parameter to be controlled into a measureable signal.

• In a simple servomechanism, the measured signal is compared to a desired value that represents the intended operation of the servomechanism.

• If the measured and desired signals do not match, an error signal is sent to the servomotor to move the mechanism to the desired position.

𝑉 = 𝐴 ∙ 𝑡/3

𝑋 = 2 ∙ 𝑉 ∙𝑡

3

𝑥 = 2 ∙ 𝐴 ∙ 𝑡/3 2

TYPICAL MOTOR PARAMETERS

0

1

2

3

4

5

6

7

8

9

0 500 1000 1500 2000 2500

Speed [rpm]

To

rqu

e [N

m]

0

200

400

600

800

1000

1200

1400

1600

1800

0 500 1000 1500 2000 2500

Speed [rpm]

Po

we

r [W

]

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500

Speed [rpm]

Effic

ien

cy [%

]

0

10

20

30

40

50

60

70

0 500 1000 1500 2000 2500

Speed [rpm]

Cu

rre

nt [A

]

TYPICAL PMSM MOTOR CHARACTERISTICS

Back Emf Constant (Ke) & Torque Constant (Kt)

• As motor turns back-EMF is induced.

• Ke= volts[v]/speed[rad/sec]

• Kt= tork[Nm]/ampers[I]

Motor Parameters • Thermal resistance: motor’s ability to dissipate heat (C°/W) Rth=L/(k*A) or 1/(h*A)

• Friction torque: bearing+cogging+windage

• Continuous Torque: Continuous torque is the torque a motor can produce continuously

without exceeding the thermal limit of the winding insulation

• Peak Torque: The torque at which the Kt rolls-off (reduces) by 10% as a function of saturation

• Stall Current (Is): This is the current at stall (locked rotor) with rated voltage applied. Is = Volts/Rt • Stall Torque (Ts): This is the actual torque at the output shaft under stall (locked rotor)

conditions. Ts = (Kt*Is)-Tf. • Inductance (L): Inductance is a coefficient between magnetic flux and voltage. It is

analogous to spring in mechanical system (Henry)

Motor Parameters

• Time Constant, Electrical (Te) winding current to reach 63.2% of its steady state conditions • Time Constant, Mechanical (Tm) for an unloaded motor to reach 63.2% of its final velocity • Time Constant, Thermal (Th) to reach 63.2% of its final temperature under known input and load

conditions • Motor Constant (Km): torque/input power

MOTOR PROTECTION NOMENCLATURE

MOTOR LIMITATIONS

• Stator insulation life (due to current passing through windings)

• Demagnetization of magnets (as temperature increases, BH curve changes, increasing

risk of demagnetization for rare-earth magnets)

INSULATION CLASSES

OPERATING CONDITIONS

OPERATING CONDITIONS (2)

MOTOR FEEDBACK DEVICES

• Current feedback sensors

• Velocity loop and feedback sensor

• Position loop and feedback sensor

Current feedback sensors

• The inner most servo loop is the torque, or current loop. Current sensors are actually located in the drive.

Position Loop and Feedback Sensor

• Hall effect devices

• Encoders

• Resolvers

Incremental Encoders

• Provide incremental motor position information via two channels

• A and B are 90° apart

• Quadrature Encoders

• Sine Encoders

Quadrature Encoders

• There are actually four states available per electrical cycle of these signals

cycle: 0/4 1/4 2/4 3/4

Channel A OFF1 OFF2 ON1 ON2

Channel B ON2 OFF1 OFF2 ON1

cycle: 0/4 1/4 2/4 3/4

Channel A OFF1 ON2 ON1 OFF2

Channel B ON2 ON1 OFF2 OFF1

Sine Encoders

• interpolating each complete sine wave greatly increases the system's resolution

• A & B channels are interpolated

ABSOLUTE ENCODERS

• Similar to incremental encoders in that a rotating disk interrupts a photodetector to produce an output signal

• Every position of an absolute encoder is unique

• Absolute encoders do not loose position when power is removed

Hall Sensors

• used to sense the presence of magnetic fields

• used as position feedback when

six-step commutation is employed

• imbedded within the motor windings

• one sensor for each motor phase, aligned with the stator winding

Hall Effect Sensors

Resolver

• rotary transformer

• primary is fed with an AC voltage

• dual secondaries designed to couple the input voltage ratiometrically according to its shaft position

• position feedback signal is provided by the two sinusoidal secondary signals, Sine and Cosine

Resolver

• durable and tolerate heat very well

• Converted digital signals by R/D converters

RESOLVER