HEV Charging Systems

HEVs utilize the automatic stop/start feature to shut off the engine whenever the vehicle is not moving or when power from the engine is not required. Some HEV systems use a starter/generator unit to perform both functions. The difference between a motor and a generator is the motor uses two opposing magnetic fields and the generator uses one magnetic field that has rotating conductors. The use of electronics to control the direction of current flow allows the unit to function as both a motor and a generator.

A BAS used on some HEVs

FIGURE. A BAS used on some HEVs.

There are two basic designs of the starter/generator. Hie first design uses a belt alternator starter (BAS) that is about the same size as a conventional generator and is mounted in the same way. The second design is to mount an integrated starter/generator (ISG) at either end of the crankshaft. Most designs have the ISG mounted at the rear of the crankshaft between the engine and transmission.

The ISG is usually located at the rear of the crankshaft in the bell housing

FIGURE. The ISG is usually located at the rear of the crankshaft in the bell housing.

The ISG is a 3-phase AC motor that can provide power and torque to the vehicle. It also supports the engine, when the driver demands more power. As seen in Figure, the ISG includes a rotor and stator that is located inside the transmission bell housing. The stator is attached to the engine block and is made up of two separate lamination stacks. The rotor is bolted to the engine crankshaft and has both wire wound and permanent magnet sections.

Rectification is accomplished with traditional diodes. The advantage of this rotor and stator design is that the output at engine idle speed is up to 240 amps. Maximum output can exceed 300 amps.

Generation is done anytime the engine is running. Since the rotor is connected to the crankshaft, it turns at the speed of the engine. Also, during vehicle deceleration the ISG regenerates the power that is used to slow the vehicle to recharge the HV and auxiliary batteries. When the vehicle is traveling downhill and there is zero load on the engine, the wheels can transfer energy through the transmission and engine to the ISG. The energy (AC voltage) is converted to DC voltage and sent to the batteries for storage and use by the electrical components of the vehicle.

Remember that the output of conventional AC generators is dependent upon the intensity of the magnetic field, the number of conductors passing through the magnetic field at any given time, the number of magnets, and the speed at which the lines of flux or the conductors are moving when the intercept occurs. Since the speed at which the magnetic poles move influences the amount of current output, then output will be the lowest at idle. Maximum output will not be achieved until higher engine speeds. It is during the times of low engine speeds that current demand is likely to be at its highest. To increase output, the hybrid rotor has permanent magnets located between the pole pieces of the rotor. The magnet flux from these permanent magnets goes into the pole piece, through the rotor shaft, and then back through the pole piece on the opposite side of the magnet. The permanent magnet fills the gap between the pole pieces, forcing more of the flux from the rotor into the stator windings. This results in an increase of the alternator’s output.

Regulation uses a technique that is referred to as “boost-buck.” At low speed and high electrical loads, the wire wound section is fully energized. This extra magnetic flux then boosts the output of the permanent magnet section. When the engine is operated at a medium speed and with a medium electrical load, the field current is off. During this time only the permanent magnet section is producing the output. During high-speed, low-electrical-load conditions, the field current is reversed. This bucks the permanent magnet’s field and maintains a constant output voltage.

Multiple motors are usually used in a full hybrid

FIGURE. Multiple motors are usually used in a full hybrid.

Full hybrid vehicles that are capable of propelling the vehicle in an electric only mode require HV batteries to power the three-phase AC motors. These batteries may have a capacity of over 300 volts. Many full hybrid vehicles have at least two AC motors located in the transmission or transaxle assembly to operate the planetary gear sets that provide constantly variable gear ratios. These motors can also be used as generators. If the HV battery SOC becomes too low, the engine is started and the crankshaft drives motor “A” to generate high voltage AC current. The current is rectified to DC voltage and sent to recharge the HV battery. Voltage generation can also occur whenever one of the motors slips. In most cases one of the motors is slipping at all times. The slipping causes a cutting of the magnetic field and results in AC current. This current is used to supply electrical energy to the other motor.

Current generated in one motor can be used to power the other

FIGURE. Current generated in one motor can be used to power the other.

Regenerative Braking

About 30% of the kinetic energy lost during braking is in heat. When decreasing acceleration, regenerative braking helps to minimize energy loss by recovering the energy used to slow the vehicle. This is done by converting rotational energy into electrical energy through the ISG or AC motors. Regenerative braking assumes some of the stopping duties from the conventional friction brakes and uses the electric motor to help slow the vehicle. To do this, the electric motor operates as a generator when the brakes are applied, recovering some of the kinetic energy and converting it into electrical energy. The motor becomes a generator by using the kinetic energy of the vehicle to store power in the battery for later use.

When regenerative braking operation is taking place, no friction braking is occurring. Regenerative braking is mainly a function of light brake pedal application, using inputs from the various sensors such as the pedal angle sensor, the vacuum sensor, and the accelerator pedal position sensor. As soon as the accelerator pedal is released, the hybrid control module will initiate regenerative braking. At this point, the electric motors are being turned by the wheels, and act as generators to recharge the HV battery since the rotor is turning within the stator windings. The hydraulic brakes are not used during this phase. If additional vehicle deceleration is required, the hybrid controller can increase the force required to turn the electric motors. This increases resistance to wheel rotation that helps further slow the vehicle. In cases where more braking power is needed, hydraulic brakes are used.

DC/DC Converter

The AC voltage from the motors during regenerative or charging modes is rectified by the inverter module. Since this module converts the high AC voltage into high DC voltage to recharge the HV battery, it cannot be used to recharge the auxiliary battery. An additional function of the inverter is that of a DC/DC converter. The converter allows the conversion of electrical power between the HV direct current system and the low voltage (LV) direct current system. The converter is a bi-directional, solid state, DC conversion device that charges the 12-volt system from the 300-volt direct current system. The converter replaces the function of the engine driven generator while maintaining isolation of the HV system.

The conversion of HV to LV voltage is accomplished through magnetic fields instead of physical wired connections. Sets of coils are used to accomplish the voltage conversion. Hie coils operate as step down transformers to reduce the high DC voltage to the low DC voltage. By sequentially inducing and collapsing the magnetic field of the coils, a smooth output on the 12-volt side is maintained.

When the HEV is placed into the jump assist mode, the converter is required to charge the 300-volt system by use of the 12 volt battery. In this mode, the coils operate as step-up transformers to increase the 12 volts to the 300 volts required to charge the HV battery.

Note: When there is a high rate of current conversion taking place in the inverter/converter module, a significant amount of heat is produced. Many manufactures will use a separate cooling system to run coolant through these modules to prevent damage from the heat.

The inverter is PWM controlled by the hybrid control module or by bus communication from the control module. The hybrid control module requests the required amount of voltage to be applied to the LV system. This will normally be a commanded PWM to the converter that is between 33% and 90%. In this range, the converter increases or decreases the charging voltage between 12.5 and 15.5 volts. In the event that the signal is lost from the control module (or the signal is corrupted in any way), the default low voltage setting is at 13.8 volts.

DC/DC converter coils are used as transformers to reduce the 300 DC volts to about 14 DC volts

FIGURE. DC/DC converter coils are used as transformers to reduce the 300 DC volts to about 14 DC volts.