pee401- solid state drives and control unit-1 review of electric drives 9/14/20151palanisamy...

PEE401- SOLID STATE DRIVES AND CONTROL

UNIT-1

Review of Electric Drives

04/19/23 1PALANISAMY MTech.,(PhD)

REVIEW OF ELECTRIC DRIVES (Syllabus)

Electric Drives-Advantage of Electric Drives

Selection of Motor power rating

Thermal model of motor for heating and cooling

Classes of duty cycle & Determination of motor rating

Control of Electric drives- modes of operation

Speed control and drive classifications

Closed loop control of drives

04/19/23 PALANISAMY MTech.,(PhD) 2

Definition of Electrical Drives


Advantages of Electrical Drives


Block Diagram of Electric Drive System



Why Power Electronic Devices?

Components in electric drives

Motors• DC motors - permanent magnet – wound field• AC motors – induction, synchronous• 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

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



Power Converters

Control unit

• Complexity depends on performance requirement

• analog- noisy, inflexible, ideally has infinite bandwidth.

• 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:

• Malfunction in power circuit may damage control circuit

• Safety for the operator

• Avoid conduction of harmonic to control circuit



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.

• The term ‘sensorless drives’ is normally referred to the drive

system where the speed is estimated rather than measured.



Modern Electrical Drive System


Choice (or) selection of Electrical Drives


AC and DC Drives


Torque equation for Rotating system



ml m m

dd dJTe T J J

dt dt dt

J – Moment of inertia of the motor-load system referred to the motor shaft (kg-m2)

ωm - Instantaneous angular velocity of motor shaft (rad/sec)

T - Instantaneous value of developed motor torque (Nm)Tl - Instantaneous value of load torque referred to motor shaft

(Nm)

Variable Moment of inertia of the motor-load system

Torque equation for Rotating system with Gears


Four quadrant (multiquadrant) operation of motor using hoist load

m

Te

Te

m

Te

m

Te

m

T

• Direction of positive (forward) speed is arbitrary chosen

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

Quadrant 1Forward motoring

Quadrant 2Forward braking

Quadrant 3Reverse motoring

Quadrant 4Reverse braking

Four quadrant (multiquadrant) operation of motor using hoist load


Torque-speed quadrant of operation

T

12

3 4

T +ve +vePm +ve

T -ve +vePm -ve

T -ve -vePm +ve

T +ve -vePm -ve

• Quadrant of operation is defined by the speed and torque of the motor.

• Most rotating electrical machines can operate in 4 quadrants.

• Not all converters can operate in 4 quadrants.


1st Quadrant (Forward Motoring):

The torque and speed of the motor are in the same direction. Of course, the load torque is opposite to the machine torque.

The electrical machine in this case is operating as a motor. The flow of power is from the machine to the load.

2nd Quadrant (Forward Braking):

The speed direction is unchanged while the direction of the torque is reversed.

Since the load torque direction is in the same direction of speed, the mechanical load is delivering power to the machine.

The machine then receives mechanical energy, converting it in to electrical energy and returning it back to the electric source. The electric machine is thus acting as a Generator. 24

3rd Quadrant (Reverse Motoring):

Compared to the first quadrant, the system speed and torque are reversed in the third quadrant.

Since the torque and speed of the machine are in the same direction, the power flow is from the machine to the load. The machine is therefore acting as a motor rotating in the reverse direction to the speed of the first quadrant.

Bidirectional grinding machine is the good example of the 1st and 3rd quadrant operation. The direction of the load torque of the grinding load is reversed when the speed is reversed (3rd quadrant). A horizontal conveyor belt is another example of this type of operation.


4th Quadrant (Reverse Braking):

The torques remains unchanged as compared to the first quadrant. The speed, however, changes the direction.

From the load perspective, the load torque and the speed are in the same direction. Hence the power flow is from the load to the machine.

The machine is in this case acting as generator delivering the electric power to the source.

The first and fourth quadrant of operation can be explained with the elevator. When the elevator is going upward or downward, the direction of the load torque remains unchanged but the direction of the speed only reversed.


Four quadrant operation of a drive

27

Parameters I quadrant II quadrant III quadrant IV quadrant

Operation of the

Hoist

The hoisting up of

the loaded cage

The hoisting up of

the unloaded cage

The downward

motion of the

unloaded cage

The downward

motion of the

loaded cage

Te+VE -VE -VE +VE

TL-VE +VE +VE -VE

WM+VE +VE -VE -VE

Power +VE -VE +VE -VE

Operation of the

Drive

forward motoring Forward Braking Reverse motoring Reverse Braking

04/19/23 PALANISAMY MTech.,(PhD)

Selection of Motor Power Rating

• Selection of power rating is important to achieve economy with reliability.

• Improper selection of motor power rating results extra initial cost and extra loss

of energy due to operation below rated power makes the choice uneconomical.

• Furthermore, induction and synchronous motors operate at a low power factor

when operating below the rated power.

• During operation of the machine, heat is produced due to losses and

temperature rises. An amount of developed heat is dissipated into the

atmosphere. When the dissipation of heat is equal to the developed heat, then it

is said to be equilibrium condition. Motor temperature then reaches a steady

state value.

.

28

• Steady state temperature depends on power loss, which in turn

depends on the output power of the machine. Since temperature

rise has a direct relation with the output power, it is termed thermal

loading on the machine.

• Steady state temperature varies in different parts of the machine. It

is usually high is the windings because loss density in conductors

is high and dissipation is slow; the conductors which are wrapped

in insulating material are partly embedded in slots and thus are not

directly exposed to the cooling air.


Thermal model of motor for Heating and Cooling

• when an electrical motor and drive operates, there is a generation of heat inside the motor.

• The amount of heat generated inside the motor should - accurately as possible. That’s

why thermal modeling of motor is necessary.

• The material of the motors and the shapes and size of the motors are not unique but the

generation of heat does not alter very much depending on these characteristics.

• So, a simple thermal model of any motor can be obtained assuming it to be a

homogeneous body.

• Aim of this modeling is to choose the appropriate rating of a motor so that the

electric motor does not exceed its safe limit during operation.


http://www.electrical4u.com/electrical-motor-types-classification-and-history-of-motor/

http://www.electrical4u.com/electrical-motor-types-classification-and-history-of-motor/

Speed control and Drive Classifications

• Drives - driving motor runs at a nearly fixed speed Constant speed or single speed drives.

• Multi speed drives are those which operate at discrete speed settings- Variable speed drives.

• When a no. of motors are fed from a common converter or when a load is driven by more than one motor – multi motor drive.

• A variable speed drive is called constant torque drive if the drive’s maximum torque capability does not change with a change in speed setting – constant torque mode.

• Speed regulation = (no load speed – full load speed)/

(full load speed)*100%


Closed-Loop Control of Drives:

Feedback loops in an electrical drive may be provided to

satisfy one or more of the following requirements:

•Protection

•Immediate Speed response

•Improvement in steady state accuracy

•Efficiency


Current – limit Control

-

Threshold logic circuit

Controller Converter

Current Sensor

Motor Load

Imax0

I

If

If*+


Closed loop torque control

-

Torque Controller Converter

Torque Sensor

Motor Load

T

T*

+


Closed loop speed control

Δm

-

+

-

SpeedController

Current Sensor

Motor Load

m

+

m*

ConverterCurrent

Controller

Speed Sensor

I*

I

Current Limiter

Current Limiter


Closed loop speed control of multi motor Drives


Phase locked loop (PLL) control


Closed loop position control


Current sensing

Speed sensing

Modes of Operation

An electric drive operates in three

modes,

i.Steady state

ii.Acceleration including

starting

iii.Deceleration including

stopping


ml

dTe T J

dt

Steady state

• Load torque is equal to motor torque

• Change in speed is achieved by varying the steady state motor

speed torque curve so that Load torque is equal to motor torque at desired new speed.

• The motor works as a motor operating in quadrant 1 or 3. 1. frequent maintenance

• 2. lower life

• 3. braking power is always wasted as heat


lTe T

Acceleration and deceleration

Drive depends on the values of T and Tl.

• T > Tl – Acceleration

• T < Tl – deceleration

• During acceleration, the motor torque not only overcome the load torque

but it supplies to dynamic torque J(dωm/dt) to overcome drive inertia.

• In drives with larger inertia, like electric trains, motor torque must exceed

the load torque by a large amount in order to get sufficient acceleration.

When fast transient response required, motor torque should be

maintained at larger rate and the system should be designed with lowest

inertia


Acceleration and deceleration


Motor steady state torque-speed characteristic

Synchronous mch

Induction mch

Separately / shunt DC mch

Series DC

SPEED

TORQUE

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


Synchronous or Reluctance motor:

o They exhibit constant speed characteristics as shown in block curve.

o At steady state conditions, these motors operate at constant speed irrespective of the value of load torque.

DC Shunt/Separately excited motor:

o The characteristic green curve shows that the speed is slightly reduced when the load torque increases.

DC Series Motor:

o The speed is high at light load and low at heavy loading condition as shown in the Blue curve .


Induction Motors:

o They exhibit complex characteristics as shown in red curve.

o During steady state, they operate at the linear proportion of speed torque characteristic, which resembles the characteristic of a DC shunt motor.

o The maximum developed torque of induction motors is limited to Tmax.

In electric drive applications, the selection of the motor should match with the required performance of the loads. For example, the synchronous motor is probably the best option for the constant speed applications. Other motors, such as induction or DC shunt motors can also be used in constant speed applications, provided that feedback circuits are used to compensate for the change in speed when load torque changes.

Load steady state torque-speed characteristic

SPEED

TORQUE

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.

T~ C

Constant torque load

T~

Generator type load

T~ 2

Fan type load


TL

Te

Vehicle drive


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

SPEED

TORQUE

Gravitational torque

gM

FL



Hoist drive

Speed

Torque


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

TlTe

Steady state speed

r

Torque

Speedr2r3r1

Classes of motor duty



Paper mills compressorsConveyersfans

Continuous duty (constant load and variable load)

Load torque is constant over extended period multiple

Steady state temperature reached

Nominal output power chosen equals or exceeds continuous load

T

t

Ap n1

L

Losses due to continuous load


Operation considerably less than time constant,

Motor allowed to cool before next cycle

Motor can be overloaded until maximum

temperature reached


Crane drivesHousehold appliancesMachine tools for position control


Cutting Drilling Machines

Periodic intermittent duty

p1

t

heating coollingcoolling

coolling

heating

heating

Load cycles are repeated periodically

Motors are not allowed to completely cooled

Fluctuations in temperature until steady state

temperature is reached



Metal cuttingDrilling tool drivesMine hoists


Billet Mill drive Manipulator driveMachine tools

Continuous duty with intermittent periodic loading

• Consists of periodic duty cycles, each consisting of a period of

running at a constant load and period of running at a no load.

• Load period and no load period being too short for respective

temperatures to be obtained.

• This duty distinguished from the intermittent periodic duty by period

of running by constant load and period of running by no load instead

of rest condition.

• Pressing, cutting and drilling machine drives


Continuous duty with starting and braking

• Consists of periodic duty cycles, each consisting of a

period of starting, period of running at a constant load and

period of braking, there is no rest condition.

Continuous duty with periodic speed changes

• Consists of periodic duty cycles, each consisting of a

period of running at one load and speed, and another

period of running at different load and speed. There is no

period of rest.


Determination of motor Rating

Broadly classified into,

Continuous Duty

Fluctuating loads

Short time and intermittent duty


Continuous Duty

• A motor with next higher power rating from commercially available

ratings is selected.

• Motor speed should also match load’s speed requirements.

• Necessary to check whether the motor can fulfill starting torque

requirement and can continue to drive load.

04/19/23 PALANISAMY MTech.,(PhD)66

T

t

Ap n1

L

Losses due to continuous load

Fluctuating loads & intermittent loads

load diagram of Fluctuating load

Motor loss P1 consists of two

components- constant loss Pc

which is independent of load and

contains core & friction loss; and

load dependent copper loss.

This fluctuating load consisting of

n values of motor currents I1, I2, I3,

…In for the durations of t1, t2,

t3,...tn respectively. Then the

equivalent current is,

Equivalent current is,

If the current varies smoothly over a period T,

When torque is directly proportional to current and power is

directly proportional to torque,

In DC motor- allowed to carry larger current than the rated

current for short duration- short time overload capacity of load.

Normally DC motor designed to carry up to 3 times of rated

current. Because the higher currents sparking between the

brushes and commutator.

λ >= (Imax / Irated)


Short Time Duty


In short time duty, time of motor

operation considerably less than the heating

time constant & motor is allowed to cool

down to ambient temperature.

If a motor with continuous duty power

rating of Pr is subjected to a short time duty

load of magnitude Pr’ - then motor temp rise

will be far below the maximum permissible

value & motor can be overloaded by a

factor K (K>1).

• When the duration of running period

in a duty cycle with power KPr is tr,• If the motor losses for powers Pr and (KPr) be P1r nd P1s respectively.

• Pc is the load independent loss

and Pcu load dependent loss.

Then,

The overloading factor for short time duty,


Intermittent periodic duty

04/19/23 PALANISAMY MTech.,(PhD)72

During a period of operation, if the speed

changes in wide limits, leading to changes

in heating and cooling conditions, equ.

current, torque or power are not employed.

Temp rise will fluctuate between a

maximum value Tmax and a minimum

value Tmin. Temp at the end of the working

interval will be given by,

fall in Temp rise at the end of standstill interval ts will be

• Time constants of motor for working and standstill conditions

Tmax= Tmin, then P1r and P1s will be,

• The overloading factor for intermittent periodic duty is,


Applications of Electric Drives

Transportation Systems

Rolling Mills

Paper Mills

Textile Mills

Machine Tools

Fans and Pumps

Robots

Washing Machines etc


pee401- solid state drives and control unit-1 review of electric drives 9/14/20151palanisamy...

Documents

phdpalanisamy mtech

dc drives

loop control of drives

selection of electrical

power circuit

phd sensorssensors voltage

term sensorless drives

rotating system