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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.