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Motor Generator Sets

For the Paper Car Project, specifications for a 20 hp shunt wound motor were recomputed for a 144 volt battery. (i.e. current was increased in inverse proportion to voltage decrease. Frame size stayed the same.) There are four modes of operation:

           A.   Constant Torque acceleration
           B.   Constant Power acceleration
           C.   Regenerative deceleration
           D.   Resistive deceleration

Modes B & C are above regen speed, the speed at which the motor generates full battery voltage with maximum field current. Modes A & D are below regen speed. In mode A, armature voltage is controlled to maintain the armature current needed for the desired acceleration. In mode B & C, field current is controlled for desired acceleration or deceleration. In mode D, a resistive motor load can assist mechanical brakes to slow the vehicle to a full stop.

Motor/generators can be used to extend range. These are called hybrid vehicles. Unfortunately, the motor/generators are inefficient (about 17%) and do not meet CARB specifications. (Except for the diesel, they are ordinary, carbureted gas motors.)

For the calculation of efficiency, the following conversions were used:
           1 Kilowatt-hour                = 3413 BTU
           1 gallon CA gasoline    about 107,000 BTU
           1 gallon AZ gasoline    about 125,000 BTU
           1 gallon fuel oil       about 139,000 BTU
           1 therm               exactly 100,000 BTU = 29.3 kW-hr

The Honda EB 6500SX (gas) uses a 13hp 389cc engine to generate 5.5 kva, (120V@45.8A or 240V@22.9A) and uses 4.5 gallons in 5.2 hours. Therefore: 5.5*3413/((4.5/5.2)*12500) = 17.4% efficiency, or about 0.16 gal/kw-hr. If gasoline costs $2.00 per gallon, the fuel cost would be about $0.32/kw-hr.

Similarly, the Honda ES 6500 uses a 12hp 359cc to generate 6 kva, using 4.2 gallons in 4.7 hours for 18.3% efficiency, or about 0.15 gal/kw-hr. At $2.00 per gallon that would be $0.30/kw-hr.

The Honda EB12D diesel uses a 20hp 1061cc to generate 10 kva, using 10 gallons in 9 hours for 22.1% efficiency, or .11 gal/kw-hr. But this engine- generator is very heavy. At $1.67 per gallon, the fuel cost would be about $0.18/kw-hr.

These engine-generators consume as much gas as a geo-metro at a steady 55 mph which consumes about 1 gal/hr. (55 mph / 50 mpg) Unless an engine-generator is specifically designed for efficiency and low pollution, a hybrid car would have no advantage over a gas powered car while the engine-generator is running.

If an unturbo-charged Natural Gas engine is used to produce electric power, the conversion efficiency should be about 25%. If the cost of a therm is $1.00, then the electric power produced would cost about $1/(0.25 * 29.3) or about $0.14/kW-hr for the fuel cost.

1. Maximum output torque is determined by motor frame size, and the quality of the magnetic circuit. For a given frame size, the maximum torque is similar for series, shunt and polyphase motors. Permanent magnet motors have much less torque if ordinary permanent magnets are used, or are very expensive if exotic magnets are used. Another problem with permanent magnet motors is if they are overloaded, their maximum torque is permanently lost. (Or at least until the magnet is remagnetized.

2. Maximum output speed is limited by the strength of the armature (or rotor), or by mechanical commutation for DC motors. Mechanical commutation above 5000 rpm in 20 hp motors requires special commutating coils in shunt motors to prevent arcing. In both series and shunt motors, high speed and torque (current) greatly shortens brush life. AC motors with ratings over 400 hp have been designed for speeds in excess of 20,000 rpm. For these motors, power is limited by rotor strength and bearing load.

Year 2000 update

Here in San Diego, SDG&E charges almost $0.10/kw-hr to deliver electric power. For the last couple of years, the cost of energy has been between $0.03 and $0.04 per kw-hr. ($0.13 to $0.14 to user.) Now that the industry has been deregulated, and with a shortage of power, the spot price of power has gone as high as the legal limit (in California) of $0.50/kw-hr. ($0.60 to user) During hot summer days, approximately one third of San Diego's power is imported from out of state. If additional power is not available to import (i.e. demand without supply) then the price of power becomes the legal maximum. (as in 'very expensive') SDG&E offers special meters (for a price) to those with electric vehicles. With these meters inexpensive power is available from midnight to 6 a.m., and then power becomes very expensive during the day.

What this means, that if any new day-time charging units are installed, they will probably be metered. And the cost for a 240 volt - 30 amp connection, could be as much as $3.60 an hour. This definitely tilts the balance in favor of hybrids for extended day-time travel. There has even been talk of using hybrid vehicles to generate power to power a home during the day! Deregulation certainly makes things more interesting.

Note that the current non-baseline cost (SDG&E July 2000) of residential natural gas is slightly more then $1 per therm.