# The Physics of Hybrid Vehicles. J. Russell Lemon

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The Physics of Hybrid Vehicles

J. Russell Lemon Russ@FarTooMuch.Info Russ@FarTooMuch.Info

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The Physics of Hybrid Vehicles By J. Russell Lemon Russ@FarTooMuch.Info

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 mass of about 39 kg. 12 batteries in series will supply 144 volts. 12 battery mass will be about 468 kg. 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 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 23.1 cm [9.1 ] Mass is 65 kg [143 lbm] 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 batterys 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 batterys 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 Russ@FarTooMuch.Info

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