Applications of Power Electronic to Automotive Power Generation

The advancement of automotive electrical systems is making an interest in generators giving uncommon levels of the exhibition. Critical qualities of future alternators incorporate higher power and control thickness, higher temperature operation, and better transient response. The application of power electronics to automotive power generation is a new load-matching technique that presents a simple switched-mode rectifier to achieve dramatic increases in peak and average power output from a conventional Lundell alternator, in addition to considerable upgrades inefficiency. Vehicle’s power electronic components, coupled with the overall power management and control system, introduces a new set of challenges for electrical system design. These power electronic components include energy storage devices, DC/DC converters, inverters, and drives. Automotive Power Electronics have found in many applications some of those are mentioned below.

• Fuel injector solenoid driver circuits
• IGBT ignition coil driver circuits
• Electric power steering systems
• 42V Power Net
• Electric/Hybrid drive trains

The Lundell Alternator:

The Lundell is also called Cla-Pole; alternator is a wound-field synchronous machine in which the rotor comprises a pair of stamped pole pieces secured around a cylindrical field winding. Lundell alternator is the most common power generation device used in cars. It is the most used commercial automotive alternators. In addition, the control capability of the built-in bridge rectifier and voltage regulator included with this alternator. It is a wound-field three-phase synchronous generator containing an internal three-phase diode rectifier and voltage regulator. The rotor consists of a pair of stamped pole pieces, secured around a cylindrical field winding. However, the efficiency and output power of the Lundell alternators are limited. This is a major drawback for its use in modern vehicles requiring an increase in electrical power. The field winding is driven by the voltage regulator via slip rings and carbon brushes. The field current is much smaller than the output current of the alternator. The low current and relatively smooth slip rings ensure greater reliability and longer life than that obtained by a DC generator with its commutator and higher current being passed through its brushes. A stator is a 3-phase configuration and a full bridge diode rectifier is traditionally used at the machine output to rectify the 3-phase voltage generator from the alternator machine.

The above-shown figure is a simple Lundell alternator (switched-mode rectifier) model. The field current of the machine is determined by the field current of the regulator which applies a pulse-width modulated voltage across the field winding. The average field current is determined by the field winding resistance and the average voltage applied by the regulator. Changes in field current occur with an L/R field winding time constant that is typically on the order. This long time constant dominates the transient performance of the alternator. The armature is designed with a set of sinusoidal 3 phase back-emf voltages such as Vsa, Vsb, Vsc, and leakage inductance Ls. The electrical frequency ω is proportional to the mechanical speed ωm and the number of machine poles. The magnitude of the back emf voltages is proportional to both frequency and field current.

V = kωi

The Lundell alternator has large stator leakage reactance. To overcome the reactive drops at high alternator current, relatively large machine back emf magnitudes are necessary. A sudden reduction of load on the alternator reduces the reactive drops and results in a large fraction of the back voltage appearing at the output of the alternator before the field current can be reduced. The resulting transient will takes place. This transient suppression can be easily obtained with the new alternator system through proper control of the switched-mode rectifier.

A diode bridge rectifies the ac machine output into a constant voltage source Vo representing the battery and associated loads. This simple model captures many of the vital aspects of the Lundell alternator while remaining systematically tractable. The application of switched-mode power electronics with a redesigned armature can provide a range of improvements to power and efficiency. We can replace these diodes by MOSFETs for better performance. Additionally, MOSFETs require gate drivers, and gate drivers require power supplies, including level-shifted power supplies. So the cost of substituting a full active bridge for a diode bridge is substantial.

In this system, we can also add a boost switch which may be MOSFET followed by Diode Bridge as a controlled switch. This switch is turned on and off at high frequency in pulse width Modulation. In an averaged sense, the boost switch set acts as a dc transformer with a turns ratio controlled by the PWM duty ratio. That assuming current through rectifier relatively constant over a PWM cycle, by controlling the duty ratio d, one can vary the average voltage at the output of the bridge, to any value below the output voltage of the alternator system.

The use of a PWM controlled rectifier instead of a diode rectifier allows for the following main benefits like boosting operation for increasing the output power at low speed and power factor correction in the machine for maximization of output power.

When the electrical load increases due to more current being drawn from the alternator, the output voltage falls, which is in turn detected by the regulator which increases the duty-cycle to increase the field current, and hence the output voltage increases. Likewise, if there is a decrease in electrical load, the duty cycle decreases so that the output voltage decreases. The PWM full-bridge rectifier (PFBR) can be used to maximize the output power with sinusoidal PWM control. A PFBR is a quite expensive and complex solution. It counts for several active switches and requires rotor position sensing or complex senseless algorithms.

However, like a synchronous rectifier, it offers bidirectional power flow control. If bidirectional power flow is not required, we can use Other PWM rectifiers like the three single-phase BSBR structures. It has twice less active switches and all of them are referenced to the ground. Active switches can be reduced to only one using a Boost Switched-Mode Rectifier (BSMR), With this topology, it is not necessary to use a rotor position sensor but the power angle can’t be controlled.