Transcript Electric Vehicles - Far Too Much . Info

The Physics of
Hybrid Vehicles
J. Russell Lemon
[email protected]
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The Physics of
Hybrid Vehicles
By
J. Russell Lemon
[email protected]
How they work …
And why
Hybrid Vehicles
• What follows is a discussion of the
interaction of the motor & controller with
the battery and an auxiliary source of
electrical power [alternator or generator].
• It is assumed that you have read:
”Physics of Electric Vehicles”
• To go fast and far, minimize your losses.
Battery Pack
• The sources of energy for an hybrid
vehicles are its battery and its alternator.
• They must supply enough current to the
electric motor in order for it to supply
needed torque.
• They must provide enough voltage to force
the needed current through the electric
motor at the desired speed.
• They must have enough energy to supply
the needed power for the needed amount
of time.
12 V Battery
• U. S. Battery makes a 12 volt battery with
a 75 amp discharge time of 72 minutes
called the EV 145 XC. Its 20 hr amp-hr
rating is 145 amp-hrs.
• It has a weight of about 86 lb.
• 12 batteries in series will supply 144 volts.
• 12 batteries will weight about 1031 lb.
• Energy of 12 batteries at 75 amps is
almost 13 kW-hr
• 144 V x 75 A is 10.8 kW
1 lead-acid Cell
2.585 volts maximum charging voltage
2.45 volts soft charge – less stress
2.1 volts fully charged – no current - rest
2.01 volts 75+% DoD – no current - rest
2
volts rated voltage
1.75 volts – 20 hr 0.05 C discharging
1.55 volts – 1 hr 0.6 C discharging
1.40 volts – 30 min 1.0 C discharging
[C is amp-hr rating for 20 hours.]
One Battery
15.5 volts maximum charging voltage
14.7 volts soft full charge – less stress
12.6 volts fully charged – no current - rest
12.1 volts 75+% DoD – no current - rest
12 volts rated voltage
10.5 volts – 20 hr 0.05 C discharging
9.5 volts – 1 hr 0.6 C discharging
8.4 volts – 30 min 1.0 C discharging
[C is amp-hr rating for 20 hours.]
144 volt Battery Pack
186 volts maximum charging voltage
176 volts soft charge – less stress
151 volts fully charged – no current - rest
145 volts 75+% DoD – no current - rest
144 volts rated voltage
126 volts – 20 hr 0.05 C discharging
112 volts – 1 hr 0.6 C discharging
101 volts – 30 min 1.0 C discharging
[C is amp-hr rating for 20 hours.]
Electric Motor
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Series wound direct current motor
In any gear, speed is proportional to RPM
Constant torque for even acceleration
Torque roughly proportional to current
Increasing voltage is necessary to
maintain current to maintain torque as
vehicle speed and motor RPM increase
• Batteries must have enough voltage and
current to maintain desired speed
Electric Motor 2
• The selected electric motor is the Advanced DC
FB1-4001
• Diameter is 9.1”
• Weight is 143 lb
• Max continuous rated current is 180 A
• Max 1 hour rated current is 200 A
• Max 5 minute rated current is 340 A
• Current is limited by motor temperature
• Motor speed should be kept under 6000 rpm
[High rpm causes rapid brush and bearing wear.]
Motor Characteristics
• Torque increases with current.
• Back voltage increases with current
and motor speed [rpm].
• [Motors are also a generator].
Vehicle Characteristics
You select with your foot the current sent to
the electric motor. With a constant current
you have a constant torque. As the vehicle
accelerates from a stop, the controller
increases the voltage on the motor to maintain
that current until there is no more voltage.
[battery voltage reached] As the vehicle
continues to accelerate, current and therefore
torque decrease, causing acceleration to also
decrease until torque is just enough to match
losses and you maintain a constant speed.
Vehicle Characteristics 2
In the following graph, for a given foot
setting, you follow a constant torque line
up to the battery voltage and then follow a
horizontal line to the right as rpm and
vehicle speed increase. Note the
corresponding decrease in torque.
You must have enough battery voltage to
push the current you need to get the
torque you need to go the speed you need.
Hybrid Vehicle (1)
The motor-generator is added to extend
range. Motor generators are made to run
on a variety of different fuels. Commercial
motor-generators include gasoline, diesel,
propane & CNG. Be sure the motor
controller can take the maximum charging
voltage. Voltage should not exceed the
soft battery charging voltage. For long
trips, the motor generator must supply
enough power to maintain freeway speed.
Hybrid Vehicle (2)
The main purpose of the auxiliary
generator is to provide power to the motor.
Therefore the generator must be current
limited as it provides its safe maximum
current to the motor as the motor causes
the system voltage to drop below the
battery’s nominal voltage. When the motor
needs little power, the excess generated
power is used to charge the battery. The
generated voltage must not exceed the
battery’s soft maximum voltage.
Hybrid Vehicle (3)
In other words, the generator first
provides power to the motor and then the
remainder to charge the battery if an
excess is being generated. Direct power to
the motor avoids the loss of converting
electrical power into chemical power and
then back to electrical power.
Note that a three-phase alternator is most
often used to provide electrical power.
Hybrid Vehicle (4)
High-end generators have fold-back.
This means that as system voltage
drops to zero, current output from
the generator is limited to a small,
safe value. [When the short is
removed, the generator returns to
normal operation.]
Hybrid Vehicle (5)
The output of current and voltage from a
three-phase alternator has little ripple.
[Short term peak to minimum ratio]
A full wave rectified single-phase AC
generator has a very high peak to minimum
ratio since the minimum is zero. A large
inductor can smooth out the current and
thus reduce the peak to minimum ratio.
Circuit shown for a ‘240 VAC generator’.
Test Circuits
These circuit diagrams show meters for
measuring average and peak voltage. Simple
RC [resistor-capacitor] filters are used to
decrease the AC component and allow
steady DC meter readings. [especially with
a digital meter] A diode is used to charge a
capacitor for the peak reading. Peak
voltage should not exceed maximum soft
charge voltage. For both configurations,
the generator must limit maximum output
current to a safe level.
Comment
Note that for the same power, 3-phase
alternators are much smaller and lighter
then 60 Hz generators. Since alternators
do not have the 60 Hz limitation, they can
run faster, trading speed for torque that
requires larger unit size. The three 120 V
120 A windings of an alternator provide
more power then 2 120 V 120 A windings of
a single phase generator. The inherit low
ripple of an alternator is a definite
advantage when smooth direct current is
needed.
The End
Physics of
Hybrid Vehicles
by
J. Russell Lemon
[email protected]
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