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## ELECTRIC DRIVES

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### ELECTRIC DRIVES

INTRODUCTION TO ELECTRIC DRIVES

MODULE 1

Dr. Nik Rumzi Nik Idris

Dept. of Energy Conversion, UTM

2013

Drives are systems employed for motion control

Require prime movers

Drives that employ electric motors as

prime movers are known as Electrical Drives

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

- About 50% of electrical energy used for drives

- Can be either used for fixed speed or variable speed

- 75% - constant speed, 25% variable speed (expanding)

- MEP 1523 will be covering variable speed drives

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

In

Power out

Power loss

Mainly in valve

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Example on VSD application

Variable Speed Drives

Constant speed

valve

Supply

motor

pump

In

Power

In

Power out

Power loss

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Example on VSD application

Variable Speed Drives

Constant speed

valve

Supply

Supply

motor

pump

motor

PEC

pump

Power out

Power loss

Mainly in valve

In

Power out

Power loss

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Example on VSD application

Variable Speed Drives

Constant speed

valve

Supply

Supply

motor

pump

motor

PEC

pump

Power

In

Power out

Power loss

Mainly in valve

Conventional electric drives (variable speed)

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

- Bulky
- Inefficient
- inflexible

Modern electric drives (With power electronic converters)

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

- Small
- Efficient
- Flexible

Speed sensorless

Machine Theory

Utility interface

Renewable energy

Non-linear control

Real-time control

DSP application

PFC

Speed sensorless

Power electronic converters

Modern electric drives

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

- Inter-disciplinary
- Several research area
- Expanding

- Motors
- DC motors - permanent magnet – wound field
- AC motors – induction, synchronous (IPMSM, SMPSM), brushless DC
- Applications, cost, environment
- Natural speed-torque characteristic is not compatible with load requirements

- Power sources
- DC – batteries, fuel cell, photovoltaic - unregulated
- AC – Single- three- phase utility, wind generator - unregulated

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

- Power processor
- To provide a regulated power supply
- Combination of power electronic converters

- More efficient
- Flexible
- Compact
- AC-DC DC-DC DC-AC AC-AC

- Control unit
- Complexity depends on performance requirement
- analog- noisy, inflexible, ideally has infinite bandwidth.
- digital – immune to noise, configurable, bandwidth is smaller than the analog controller’s
- DSP/microprocessor – flexible, lower bandwidth - DSPs perform faster operation than microprocessors (multiplication in single cycle), can perform complex estimations
- Electrical isolation between control circuit and power circuit is needed:
- Malfuction in power circuit may damage control circuit
- Safety for the operator
- Avoid conduction of harmonic to control circuit

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

- Sensors
- Sensors (voltage, current, speed or torque) is normally required for closed-loop operation or protection
- Electrical isolation between sensors and control circuit is needed for the reasons previously explained
- The term ‘sensorless drives’ is normally referred to the drive system where the speed is estimated rather than measured.

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

DC motors: Regular maintenance, heavy, expensive, speed limit

Easy control, decouple control of torque and flux

AC motors: Less maintenance, light, less expensive, high speed

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Coupling between torque and flux – variable spatial angle between rotor and stator flux

Before semiconductor devices were introduced (<1950)

- AC motors for fixed speed applications
- DC motors for variable speed applications

After semiconductor devices were introduced (1950s)

- Variable frequency sources available – AC motors in variable speed applications

- Coupling between flux and torque control
- Application limited to medium performance applications – fans, blowers, compressors – scalar control

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

- High performance applications dominated by DC motors – tractions, elevators, servos, etc

After semiconductor devices were introduced (1950s)

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

After vector control drives were introduced (1980s)

- AC motors used in high performance applications – elevators, tractions, servos
- AC motors favorable than DC motors – however control is complex hence expensive

- Cost of microprocessor/semiconductors decreasing –predicted 30 years ago AC motors would take over DC motors

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Classification of IM drives (Buja, Kamierkowski, “Direct torque control of PWM inverter-fed AC motors - a survey”, IEEE Transactions on Industrial Electronics, 2004.

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Linear motion, constant M

- First order differential equation for speed
- Second order differential equation for displacement

Elementary principles of mechanics

v

x

Fm

M

Ff

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Tl

Te , m

With constant J,

- First order differential equation for angular frequency (or velocity)
- Second order differential equation for angle (or position)

Elementary principles of mechanics

Rotational motion

- Normally is the case for electrical drives

J

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics

For constant J,

Torque dynamic – present during speed transient

Angular acceleration

Larger net torque and smaller J gives faster acceleration

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics

A drive system that require fast acceleration must have

- large motor torque capability

- small overall moment of inertia

As the motor speed increases, the kinetic energy also increases. During deceleration, the dynamic torque changes its sign and thus helps motor to maintain the speed. This energy is extracted from the stored kinetic energy:

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

J is purposely increased to do this job !

Fl

Te,

r

r

Tl

v

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Elementary principles of mechanics

Combination of rotational and translational motions

M

Te = r(Fe), Tl = r(Fl), v =r

r2M - Equivalent moment inertia of the

linearly moving mass

m

Motor

Te

n1

Load 1, Tl1

J2

m2

Load 2, Tl2

n2

J1

Elementary principles of mechanics – effect of gearing

Motors designed for high speed are smaller in size and volume

Low speed applications use gear to utilize high speed motors

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

m

Motor

Te

Load 1, Tl1

n1

J2

m2

Load 2, Tl2

n2

J1

Elementary principles of mechanics – effect of gearing

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

m

Motor

Te

Equivalent Load , Tlequ

Tlequ = Tl1 + a2Tl2

Jequ

a2 = n1/n2=2/1

Synchronous mch

Induction mch

Separately / shunt DC mch

Series DC

TORQUE

Motor steady state torque-speed characteristic (natural characteristic)

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

By using power electronic converters, the motor characteristic can be change at will

T~ 2

T~

Coulomb friction

Viscous friction

Friction due to turbulent flow

Load steady state torque-speed characteristic

Frictional torque (passive load)

- Exist in all motor-load drive system simultaneously
- In most cases, only one or two are dominating
- Exists when there is motion

SPEED

TORQUE

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Gravitational torque

Vehicle drive

Te

TORQUE

TL

gM

FL

Load steady state torque-speed characteristic

Constant torque, e.g. gravitational torque (active load)

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

TL = rFL = r g M sin

Torque

Gravitational torque

Load steady state torque-speed characteristic

Hoist drive

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Tl

Steady state

speed

r

r2

r1

r3

Load and motor steady state torque

At constant speed, Te= Tl

Steady state speed is at point of intersection between Te and Tl of the steady state torque characteristics

Torque

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Speed

(rad/s)

100

25

60

10

45

t (ms)

Torque and speed profile

Speed profile

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

The system is described by: Te – Tload = J(d/dt) + B

J = 0.01 kg-m2, B = 0.01 Nm/rads-1 and Tload = 5 Nm.

What is the torque profile (torque needed to be produced) ?

(rad/s)

100

25

60

10

45

t (ms)

Torque and speed profile

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

0 < t <10 ms Te = 0.01(0) + 0.01(0) + 5 Nm = 5 Nm

10ms < t <25 ms Te = 0.01(100/0.015) +0.01(-66.67 + 6666.67t) + 5

= (71 + 66.67t) Nm

25ms < t< 45ms Te = 0.01(0) + 0.01(100) + 5 = 6 Nm

45ms < t < 60ms Te = 0.01(-100/0.015) + 0.01(400 -6666.67t) + 5

= -57.67 – 66.67t

speed

(rad/s)

100

Speed profile

25

60

t (ms)

10

45

Torque

(Nm)

72.67

torque profile

71.67

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

6

5

45

25

10

60

t (ms)

-60.67

-61.67

Torque

(Nm)

70

J = 0.001 kg-m2, B = 0.1 Nm/rads-1 and Tload = 5 Nm.

6

45

25

10

60

t (ms)

-65

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

For the same system and with the motor torque profile given above, what would be the speed profile?

Unavoidable power losses causes temperature increase

Insulation used in the windings are classified based on the temperature it can withstand.

Motors must be operated within the allowable maximum temperature

Sources of power losses (hence temperature increase):

- Conductor heat losses (i2R)

- Core losses – hysteresis and eddy current

- Friction losses – bearings, brush windage

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Electrical machines can be overloaded as long their temperature does not exceed the temperature limit

Accurate prediction of temperature distribution in machines is complex – hetrogeneous materials, complex geometrical shapes

Simplified assuming machine as homogeneous body

Ambient temperature, To

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

p1

Thermal capacity, C (Ws/oC)

Surface A, (m2)

Surface temperature, T (oC)

p2

Emitted heat power

(convection)

Input heat power

(losses)

Power balance:

Heat transfer by convection:

, where is the coefficient of heat transfer

Which gives:

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

With T(0) = 0 and p1 = ph = constant ,

, where

Thermal considerations

t

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Cooling transient

t

Load torque is constant over extended period multiple

Steady state temperature reached

Nominal output power chosen equals or exceeds continuous load

Losses due to continuous load

p1n

t

Thermal considerations

The duration of overloading depends on the modes of operation:

Continuous duty

Short time intermittent duty

Periodic intermittent duty

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Short time intermittent duty

Operation considerably less than time constant,

Motor allowed to cool before next cycle

Motor can be overloaded until maximum temperature reached

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

p1n

t1

Thermal considerations

Short time intermittent duty

p1

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

t

Periodic intermittent duty

Load cycles are repeated periodically

Motors are not allowed to completely cooled

Fluctuations in temperature until steady state temperature is reached

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

heating

coolling

heating

coolling

heating

coolling

Thermal considerations

Periodic intermittent duty

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

t

Periodic intermittent duty

Example of a simple case – p1 rectangular periodic pattern

- pn = 100kW, nominal power
- M = 800kg
- = 0.92, nominal efficiency

T= 50oC, steady state temperature rise due to pn

Also,

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

If we assume motor is solid iron of specific heat cFE=0.48 kWs/kgoC, thermal capacity C is given by

C = cFE M = 0.48 (800) = 384 kWs/oC

Finally , thermal time constant = 384000/180 = 35 minutes

Periodic intermittent duty

Example of a simple case – p1 rectangular periodic pattern

For a duty cycle of 30% (period of 20 mins), heat losses of twice the nominal,

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

T

Torque-speed quadrant of operation

1

2

- T -ve
- +ve

Pm -ve

- T +ve
- +ve

Pm +ve

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

3

4

- T -ve
- -ve

Pm +ve

- T +ve
- -ve

Pm -ve

m

m

Te

Te

T

Te

m

m

4-quadrant operation

- Direction of positive (forward) speed is arbitrary chosen

- Direction of positive torque will produce positive (forward) speed

Quadrant 1

Forward motoring

Quadrant 2

Forward braking

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Quadrant 3

Reverse motoring

Quadrant 4

Reverse braking

Power limit for transient torque

Ratings of converters and motors

Torque

Continuous torque limit

Power limit for continuous torque

Maximum

speed limit

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

Speed

INTRODUCTION TO ELECTRIC DRIVES - MODULE 1

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