ch 8 industrial implementation

8/6/2019 Ch 8 Industrial Implementation

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INDUSTRIAL IMPLELEMENTATION

REGENERATIVE BRAKE

A regenerative brake is an energy recovery mechanism that reduces vehicle speed by converting

some of its kinetic energy into a useful form of energy instead of dissipating it as heat as in a

conventional brake. The converted kinetic energy is stored for future use or fed back into a

power system for use by other vehicles.

Electrical regenerative brakes in electric railways feed the generated electricity back into the

supply system. In battery electric and hybrid electric vehicles, the energy is stored in a battery or

bank of capacitors for later use. Energy may also be stored by compressing air or by a rotating

flywheel.

Regenerative braking is not the same as dynamic braking, which dissipates the electrical energy

as heat and does not maintain energy in a usable form.

MOTOR AS A GENERATOR

Vehicles driven by electric motors use the motor as a generator when using regenerative braking:it is operated as a generator during braking and its output is supplied to an electrical load; the

transfer of energy to the load provides the braking effect.

Regenerative braking is used on hybrid gas/electric automobiles to recoup some of the energylost during stopping. This energy is saved in a storage battery and used later to power the motor

whenever the car is in electric model.

An Energy Regeneration Brake was developed in 1967 for the AMC Amitron. This was a

completely battery powered urban concept car whose batteries were recharged by regenerative

braking, thus increasing the range of the automobile.

Many modern hybrid and electric vehicles use this technique to extend the range of the battery

pack. Examples include the hybrids Toyota Prius, Honda Insight, and the Vectrix electric maxi-scooter.

COMPARISION BETWEEN REGENERATIVE AND DYNAMIC BRAKING

Dynamic brakes (" rheostatic brakes"), unlike regenerative brakes, dissipate the electric energy as

heat by passing the current through large banks of variable resistors. Vehicles that use dynamic

brakes include forklifts, Diesel-electric locomotives and streetcars. If designed appropriately, this

heat can be used to warm the vehicle interior. If dissipated externally, large radiator-like cowls

are employed to house the resistor banks.


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The main disadvantage of regenerative brakes when compared with dynamic brakes is the needto closely match the generated current with the supply characteristics and increased maintenance

cost of the lines. With DC supplies, this requires that the voltage be closely controlled. Only withthe development of power electronics has this been possible with AC supplies, where the supply

frequency must also be matched (this mainly applies to locomotives where an AC supply is

rectified for DC motors).

REGENAERATIVE BRAKING IN STEEL ROLLING MILL

in steel works , roller tables are driven so that slabs, bullets and strips, can be conveyed from one

stage of manufacture to the next. In hot strip mill a reversible drive makes the hot strip pass

through the mill back and forth several times until it is shaped into the desired form and length.

In the drive system where the speed reversal is frequent, a regenerative drive is desired to allow

rapid speed revesel. In the regenerative drive the kinetic energy of moving mass is recovered

while slowing to zero speed, and this results in higher efficiency.

REGENERATIVE BRAKE IN TRAMWAYS AND SUBWAYSHave you ever been stranded on a subway car? The lights flickered wildly, then

the odd humming sounds all came to an abrupt stop, and you sat there exchanging

uncomfortable glances with your fellow passengers? Hey, get this thing going!Ive got an appointment to make! someone shouts at the driver, sequestered in his

cab. The trouble is, he, like the rest of the passengers, is equally helpless in thissituation.

You see, streetcars, subway cars, and light rail cars are all forms of electric

railway cars, and when their source of electricity goes out, so do they. Theyre

similar to the diesel-electric locomotive we looked at last week because they use

electric traction motors for propulsion, meaning to move forward. But their

difference lies in the fact that electric rail cars dont carry their own source ofpower and are entirely reliant on an external source, an electrical substation. See

Figure 1 below.

Electric Railway Car Propulsion System

This substation performs the task of taking the power provided by an electric


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utility power plant and converting it into a form of power that the electric rail car

can use to operate its traction motors. The two are connected via a trolley wire and

the two rails that the car runs on. The railway car has a spring-loaded arm called apantograph on its roof that touches the trolley wire, allowing electrical current to

flow into a speed control system housed under the car. This speed control system

performs the task of varying the flow of electrical current to the traction motors,enabling the car to move, before it eventually exits the motor through its wheels,then back to the substation where it originated, thus completing an electrical

circuit.

Many newer electric railway cars couple a regenerative braking system with a

mechanical one. Their operation is similar in nature to a dynamic braking system

where the traction motors are turned into generators. The difference is that with

regenerative braking systems the current from the traction motors is sent to thetrolley wire through the pantograph as shown in Figure 2 below.

An Electric Railway Car Using Regenerative Brakes

This diagram shows the railcar generating electricity, but it may not be so

obvious how its motion is made to slow down, after all, we see no resistor grids likewe did in last weeks illustration of a dynamic braking system. So how does it

stop? The trick here is that there are other cars running on the rail line at the sametime which are using electrical current to move forward. So what does this have to

do with stoppingit you ask? Lets take a look at Figure 3 for clarification.


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How Regenerative Brakes Help Save Power

In this illustration we see that as Car A goes downhill and the operator applies thebrakes, the regenerative brake will be caused to start pumping electrical current into

the trolley wire. Now, if Car B is on the same rail line going up the other side of

the hill, it will need power to climb that hill, and it will need to draw that powerfrom the trolley wire by way of its pantograph. But instead of drawing allitselectrical current from the substation, Car B will first draw off the current produced

by nearby Car A, and only then will it draw the remainder of its powerrequirements from the substation.

During this braking process the kinetic energy in Car A is converted into electrical

energy by its traction motors. Then Car B uses its own traction motors to convert

the electrical energy drawn from Car A into mechanical energy, enabling it to climb

the hill. Car B has effectively robbed Car A of its energy, so Car A slows down.

As we discovered last week during our discussion of dynamic brakes, regenerative

brakes become ineffective below a certain minimum speed. This is the reason that

electric rail cars need mechanical brakes to complete the job of stopping.

We see that the regenerative braking process is actually quite green. It allows forelectrical energy that would normally be wasted as heat energy escaping into the

atmosphere to be converted into useful energy, taking a significant chunk out of the

demand for new energy off of substations. It also helps the electric railway to save

money when it comes time to paying the electric bills.


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HOIST AND CRANES

Hoists and elevators can injure or kill. Accidents can occur on counterweighted hoisting systemsif the mechanical brake fails while the cage is empty. The counterweight falls; the cage over

speed and crashes. Direct-current hoist motors prevent this type of accident if equipped with

passive electrical braking systems known as dynamic braking. Installing a dynamic brakerequires minimal modifications to the control system and modest expense.

Hoists and elevators have safety features to prevent the cage from falling. Safety catches activate

if the brakes or wire ropes fail. Safety catches, however, are not normally installed on the

counterweight.

Many hoisting systems rely solely on the mechanical brakes to stop the cage in an emergency.

Under normal operation, the electrical drive equipment controls the speed of the hoisting system

while the mechanical brakes only hold the cage at a stopped position. The frequency with which

the mechanical brakes are exercised is minimal when compared to the constant use of the drive

equipment.

History proves that this is not a good assumption. Accidents occurred when the emergency stop

button was pushed--an action that defeated the retarding effort of the hoist motor--when themechanical brakes were inoperative. This allowed the overhauling load to free-fall, with the final

speed limited only by inertia and frictional forces. The high-speed crashes at the travel limitcause extensive mechanical damage and fatal injuries.

The direct-current motors on elevators and hoists can prevent the failure because the electrical

drive and control system can limit the speed of the falling overhauling load. This electrical

source of braking retards the free-fall speed when the mechanical brakes fail.

Dynamic braking exploits the ability of the direct-current drive motor to act as a generator. The

motor requires torque and kinetic energy of the falling load to generates electricity that isdissipated as heat in a resistor. The retarding torque limits the speed of the falling overhauling

load. The amount of retardation and the final speed of the cage depend on the motor terminal

characteristics and the resistance value of the dynamic braking resistor.

Hoist motor performance

The direct current motor circuit has the motor armature in series with the power supply. This


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power supply is either a generator or SCR bridge that converts line voltage to a variable directcurrent voltage that controls the speed of the motor. The field of the motor is normally supplied

from a separate source--either fixed (constant potential) or variable (field weakening)--thatcontrols the speed of the hoist motor.

A shunt-wound direct current motor can operate as either a motor or generator. It operates as a

motor when it produces torque in the same direction as shaft rotation. The motor operates as agenerator when the direction of motor torque opposes shaft rotation, as when a load overhaulsmotor torque, thus reversing shaft rotation. Then, the direct current generator--a.k.a. hoisting

motor--converts the energy of the overhauling load into electrical power to be returned to the

grid. This regenerative brakingactively pushes power into the ac power system instead of

dissipating it as heat.

The motor is said to be in the powering mode when motor action is taking place. In the inverting

mode, generator action is taking place. If raising the load produces positive shaft rotation, then

the four quadrants of operation are defined. The motor torque and direction are directly

proportional to the armature current and voltage, respectively.

The constant-speed unbalanced hoisting system operates in quadrants 1 and 4. Quadrant 1represents motor speed and torque acting in the same direction. Thus, the motor supplies a

positive motive force to the load. Quadrant 4 represents the negative direction and positive

torque of a motor that is developing a braking force. During deceleration and acceleration, thehoisting system operates in quadrants 2 and 3, respectively.

Constant-speed counterweighted (balanced) hoisting systems operate in four quadrants. When

the counterweight is heavier than the load, the hoisting system operates primarily in quadrants 2and 3. The motor acts as a generator for part of the hoist cycle and as a motor for another other

part, depending on the load.

The ability of the motor to return power to the grid is what allows the motor to provide a braking

force. Under normal operation, the motor control circuit provides both motive power and braking

power. The mechanical brakes are called upon only to provide a very low-speed stop at the top or

bottom or, in case of an emergency, to provide complete stopping. The mechanical brakes are

called upon very infrequently to completely stop the hoist. However, when they are called upon,

they must provide 100 percent of the stopping force. This places a severe burden on the

mechanical brakes at a critical time.

HYBRID VEHICLES

Hybrid cars are often known as cars of the era. The main feature of the hybrid car is that when

we start the car engine, electrical energy is used. This way it helps in keeping a tab on the tail

pipe emissions. The use of automobiles is increasing in every part of the globe and so is thethreat of toxic pollutants and global warming, thanks to their exhaust ingredients. But if we are

using a hybrid car the decrease in the tail pipe emission will do a great service to the

environment and society. After starting the engine of the hybrid car gasoline engine will take up

the charge. If we want to increase the speed, gasoline is essential to attribute the pace for the

drive. While waiting at the traffic signals, maneuvering your car in a heavy traffic and climbing

on steep slopes, the electrical energy will be again activated. This way hybrid vehicle minimizes

the use of gasoline. We should not forget breaking the notorious fuel consumer. In hybrid cars

while we apply breaks it is re-channeled for the electrical battery charging, known as


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regenerative braking, and a separate energy for battery charging is not required.But presently wecan spot more and more such vehicles on the road. But still they are not produced on commercial

scale. Therefore they are costly. Not all that long ago, hybrid vehicles were still really exotic.Now, you see them more and more frequently on our roads. However, hybrid cars are not mass-

produced as their production costs are still relatively high.

POWER FACTOR IMPROVEMENT

Phase controlled converter are widely used because these converter are simple, less expensive,

reliable and do not require and communication circuit. However, the supply power factor in

phase controlled converter is when the firing angle is large. As the firing angle increases, the

displacement angle between the supply voltage and current increases and the converter draws

more lagging reactive power, thereby decreasing the power factor.

Semi converter provides the power factor better than the full converter system, although the

improvement is not remarkable. This poor factor operation is a major concern in variable-speed

drives and in high- power applications.

To facilitate analysis, it is assumed that motor current is constant (ripple free) and the ac supply

is ideal (has no supply impedance)

PHASE ANGLE CONTROL

For full converter

The average output voltage is

Ea= Emax Cos

The r.m.s value of supply current I is

I=Ia

The nth harmonic current is

In=[2^(3/2)*Ia]/n

The displacement angle of nth harmonic

n=arctan(an/bn)=-n


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The displacement angle of fundamental harmonic is same as the firing angle i.e. 1=.

The negative sign shows that fundamental current lage the supply voltage.

The supply power factor is

Pf= [2^(3/2)*cos]/

The displacement factor is

DF=cos1=cos

Therefore both power factor and displacement factor decreases with increase in firing angle.


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SEMI CONVERTER


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The average output voltage is

Ea=2^(1/2)v(1+cos)/

The nth harmonic current is

In=[2^(3/2)Ia*cos(n/2)]/n

n=-n/2

PF=[2^(1/2)(1+cos)]/(*(1-/)^(1/2))

DF= cos(/2)

SEQUENCE CONTROL OF FULL CONVERTER

Some application require motoring as well as regenerative braking operation of DC motor. In

this case full converters are required. Because of the absence of freewheeling diodes the

converters cannot be bypassed. Therefore both converters must stay in operation. Sequence

control can be implemented by turning on one converter fully advanced (I.e. =0) or fully

retarded (i.e. =180) and controlling the firing angle of the other converter.

In the rectifying (or motoring) mode of operation, the firing angle of converter 1 is kept fully

advanced (i.e. 1=0) and the firing angle of average output voltage. At 2=0 , both converters are

fully turned on and the output voltage is maximum. At 2=180 , the output voltages of the two

converters cancel each other, and thus the output voltage is zero. Voltage and current waveform

for 1=0 and 2=60 as shown in figure.

In the inverting (or regenerative) mode of operation, the firing angle of converter 2 may be kept

fully retarded (2=180) while that of converter 1 is controlled in the range of

0


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PULSE WIDTH MODULATION

In pulse modulation scheme the thyristor turns on and off several times during each half cycle.

The widths of the pulses are varied to change the output voltage. Lower order harmonics can be

eliminated or reduced by selecting the type of modulation of pulse widths ant the number of

pulses per half cycle. Higher order harmonics may increase, but these are of no concern because

they can be eliminated easily by filters.

A sinusoidal pulse-width control technique is illustrated in fig. in this method firing signals for

the thyristors are obtained by comparing a triangular voltage e1 with a rectified sinusoidal

voltagee3, which is in phase with the supply voltage (v) . the output voltage ea is varied by

changing the amplitude of e3 or the modulation index m, where m is defined by

M=


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To obtain expressions for various performance parameters, the instants of turn-on (i.e., s) and

turn-off (i.e., s) are obtained by using Newtons method11 to solve for intersecting points b/w

the signals e3 and e1.

Once these values (i.e., the s and s) are obtained, expressions for the performance parameterscan be obtained as follows:

Where p=numbers of half pulses per half-cycle.

Because of symmetry in the current waveform , even harmonics are absent and also:


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in sinusoidal modulation of the PWM control scheme, the displacement angle 1 is zero, the

displacement factor is unity, and the power factor tends to remain high. The lowest order

harmonic is the fifth for four pulses per half- cycle, and the seventh for six pulses per half-cycle.

Therefore, lower-order harmonics that are difficult to filter out are eliminated or reduced by

selecting the numbers of pulses of half-cycle. Note that sinusoidal modulation is maintained as

long as the modulation index m is limited to less than unity.

The performance characteristics of phase angle control (PAC), extinction angle control (EAC),

symmetrical angle control (SAC), and pulse-width modulation control (PWM), are compared in

fig. converters using force commutation show definite improvement in performance.

ch 8 industrial implementation

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