ABSTRACT: The present automotive electrical
power system has been in use for more than three decades and has so far
satisfactorily met the requirements of the vehicle electrical loads.
The requirements of increased electrical power,
improved fuel economy and reduced emissions have imposed the necessity
of developing more efficient power systems for the future applications.
The actual trend in automotive design is to transform
classic mechanical and hydraulic systems into electric systems. Examples
of such new loads are: electric water pumps, electric air conditioners,
electric and electrohydraulic power steering systems, electric brakes.
These new systems will provide better performance and customer satisfaction
but they come with the penalty of increased demand of electrical power.
Today's typical luxury class vehicle draws 1,200 to 1,500
W of steady state power from the electrical system and has about 2.5 Km
of wire. It is estimated that the power demand in the year 2005 will be
about 3,000 W for this type of car , and about 7,000 W for a hybrid
electric vehicle. At this level of power demand, the actual electric system
based on the 12V bus voltage becomes inefficient. The wiring harnesses
must be thicker therefore conductor size is increased .For this new level
of power, a new voltage standard is needed in order to enable an efficient
operation of the system.
The present automotive electrical power system
has been in use for more than three decades and has so far satisfactorily
met the requirements of the vehicle electrical loads. This is based on
the claw pole synchronous alternator , also known as the Lundell machine.
The requirements of increased electrical power, improved fuel economy and
reduced emissions have imposed the necessity of developing more efficient
power systems for the automotive applications.
The permanent magnet machine is one of the main candidates
to be used as an alternator in the electric system of an automobile , mainly
due its high power density and increased reliability.
This report presents a new method of design for a permanent
magnet three phase synchronous generator. The design method takes into
account the topology of the converter that will be used in the electric
system in the automobile. The converter must be able to deliver 42V at
the DC load end for a speed range at the generator shaft of 1,800 to 18,000
rpm. The power factor seen by the AC line and the active power supplied
by the PM machine are evaluated and will be used as entry data of the design
algorithm.
The phase rms voltage of the machine is established so
that the load voltage (42V) is obtained at any speed within the range.
The phase rms current is computed based on the equation of the active power
supplied by the machine. The base speed is chosen based on the converter
topology adopted.
The input data of the algorithm is: apparent power of
the machine, power factor, rms phase voltage and current, base speed, number
of pole pairs of the machine, magnet characteristics B-H for any temperature,
recoil permeability, magnet’s maximum operating temperature, iron saturation
flux density, maximum temperature allowable for conductor insulation, maximum
current density of the conductors used, slot filling factor, lamination
stacking factor, conductor’s resistively and temperature coefficient, air
gap length, power and current waveform factors, thermal conductances and
conductivities of the materials used, iron and magnet mass densities, skew
angle of the stator, short pitching angle of the coils in the windings,
rotor diameter, number of stator slots per pole per phase.
The design starts by computing the stator slot pitch ,
then the distribution, short pitching, skewing and the winding factors
of the machine.
The algorithm is iterating the dimension of the machine
in order to achieve a specified efficiency. The algorithm accounts
for the shear stress and is looking to maximize this so that the volume
of the machine is minimized.
The main constraints under which this design method has
been developed are imposed by the automotive requirements: minimum volume
of the machine, imposed efficiency, minimum price.
If your company is a member of the Mechatronic Laboratory, please send the request to receive a copy of any technical report. If you are not a member please send a request to Ali Keyhani, Department of Electrical Engineering, Mechatronics Program at the following address: Ali Keyhani, Ohio State University, Electrical Engineering Department, Mechatronics Systems Laboratory, 2015 Neil Ave., 205 Dereese Lab., Columbus, OH 43210.