Power Electronic Solutions to Improve the Performance of Lundell Automotive Alternators

2.3.1. Mechanical losses

Mechanical losses are generated by brush and bearing friction and losses created by the claw pole rotor and fan (windage losses). The mechanical losses increase considerably at higher speeds (Bosch, 2003). In the current air cooled alternators, one or two fans are used for convection cooling. These fans add to the alternator (aerodynamic) losses and are also responsible for an important part of the alternator audible noise at higher speeds.

2.3.2. Copper losses

As shown in Table 1, the main losses in the alternator are the stator copper losses. Increasing the filling factor will reduce copper losses, however, this is limited by the industrial production constraints (Beretta, 2008).

Other sources of ohmic losses include the losses in the rectifier diodes, regulator losses and and losses occurring due to the contact resistance between the slip rings and brushes.

2.3.3. Magnetic losses

At nominal excitation current and at lower speeds, the output current is low and the stator is fully saturated due to the weak magnetic reaction. This will result in important magnetic losses in the stator core. Increasing the speed and output current while maintaining the same excitation reduces the flux density in the stator. With this demagnetisation effect the magnetic losses become proportional to speed itself rather than the square of speed (as expected in no-load conditions). Reducing the lamination thickness can lead to a significant reduction of the magnetic losses (0.5 mm or 0.35 mm instead of 1 mm).

Since the claw pole parts are made of solid forged iron, the eddy currents can easily circulate. This will also add to magnetic losses at very low speeds and low loads where the flux density in the air-gap (generated mainly by excitation field) is modulated by the stator slot openings. At higher speeds, the stator magneto-motive force (MMF) produces space harmonic fields in the air-gap which are augmented by the slot openings and again produce claw-pole eddy currents (Boldea, 2006). To reduce magnetic losses in the rotor, it is possible to use a laminated material but the assembly process is more complex (Bretta, 2008).

2.3.4. Cooling of the alternator

An efficient cooling method is necessary to limit the temperature in the motor beyond the permissible limits (a junction temperature of 200 C for the diodes and the stator winding).

The thermal dissipation in an alternator is mainly by convection through one or two cooling fans. The small amount of heat generated in the rotor can be dissipated by conduction through alternator bearings.

As an alternative to air cooling, it is possible to cool the alternator more efficiently with a circulation of the engine coolant in the alternator housing. It also reduces the alternator’s noise level by omitting the cooling fan(s).

3.1.1. Electrical modeling of the machine

A 3-phase synchronous machine can be modeled by its equivalent circuit as shown in fig. 4. The parameters of the electrical alternator model are derived from the no-load EMF versus rotor field current characteristic E_V(j_rotor) and the short circuit characteristic I_CC(j_rotor). The stator cyclic inductance (L(j_rotor)) is derived from both tests by applying Eq. 1 where ω is the electrical frequency.

A high rotor field current produces magnetic saturation and the stator inductance value decreases. The accuracy of such a model highly depends on the parameters identification or computation method. The armature phase resistance is measured at the rated temperature rise.

3.1.2. Modeling of the alternator- rectifier

It is possible to evaluate analytically the steady state performance of a 3-phase alternator -rectifier system connected to a voltage source (fig. 5) as shown in (Figueroa et al., 2005). This type of modeling uses the electrical equivalent circuit of the machine to directly determine the steady state copper losses, current harmonic contents and DC bus current.

It is faster than a step by step simulation method as employed by simulators for power electronics (PSIM, Saber) and it is well adapted for use within an optimization loop for system evaluation purposes. However, it uses simplifications which compromise the accuracy of the results including rotor saliency, and magnetic losses in the machine.

To evaluate the simulation method, an experimental set-up is used to compare the output performance of the alternator with various rectifier topologies (fig. 6). Torque is measured by a rotating torque sensor and a strain gauge. The alternator is driven by a 5 kW DC motor via a belt and pulley system. A position encoder is also available. The rectifier has been taken out from the machine and the voltage regulator has been removed so that the excitation current could be imposed using an external current source. The same experimental set-up will allow the comparison of various converter topologies and winding configurations for alternators. It has been used for all the experimental work presented in this chapter.

Fig. 7 shows the characteristics of the output maximum current and efficiency versus speed provided by the manufacturer along with those obtained by simulation. The simulation slightly overestimates the output current particularly in the central part of the curve. This could be explained by the absence of magnetic losses in the simulation, assumption of a sinusoidal EMF and inaccuracies in parameter identification. Despite these drawbacks, it can be seen that the proposed model is able to provide a current versus speed curve of satisfactory accuracy. In the same figure, it can be seen that the absence of the magnetic and mechanical losses leads to a significant overestimation of the efficiency.

4.2.1. Delta-wye and the series-parallel reconfiguration

One solution is to divide each phase winding in several coil groups and to modify the winding configuration by using several switches. It is then possible to obtain different connections of the coil groups (parallel, series, delta, wye) as a function of the speed. Fig. 13 shows the delta-wye and the series-parallel reconfiguration (Liang et al., 1999).

4.2.2. Multi-winding stator with series/parallel diode rectifier

One can also use a multi-winding stator with different number of turns to adapt the output characteristics (output current and efficiency) with a single voltage output. In this case, the different windings are connected to different diode rectifiers and must be magnetically decoupled (Cros et al., 2003). In order to compare the relative performance, we consider a given stator with the electrical parameters of a single-winding configuration (L₀, R₀ and Φ₀). Equation (3) shows the new electrical parameters of a m-winding configuration in the same stator.

If we consider m=2, it is possible to optimize the first winding system for the lower speeds and the second one for the higher speeds by choosing the right number of turns in each winding while maintaining the same copper volume per slot. For example, one may use N conductors for the first winding and N/2 conductors for the second one, as shown in fig. 14.

Fig. 15. shows a comparison of output characteristics between a single-winding configuration with N conductors per slot and the double-winding configuration of fig. 14. Note that the double-winding configuration provides higher output current when the speed is greater than 2700 rpm but the current is reduced at low-speed.

To improve the performance over the whole speed range, it is more interesting to use the same number of turns in the two winding systems and make parallel/series reconfigurations at the rectifier outputs as shown in fig. 16. This method is easier to implement than the AC phase winding reconfiguration. It uses only one unidirectional switch and two additional diodes. Fig. 17 shows the performances of a double-winding configuration using the same number of conductors per slot as the original single-winding. A series connection provides the same output current as the original alternator, during low-speed operation when diode voltage drop is neglected. Once the speed exceeds 2700 rpm, a parallel connection is used to obtain higher output current and lower copper losses.

4.2.3. Determination of parameters for rewound and multi-winding alternators

The winding scheme of a reconfigured alternator compared to the original one (Delcotron 22SI Type 12V-100A) is depicted in Fig. 18. In order to minimize the magnetic coupling, the first 3-phase winding is wound using half stator (18 slots) and the second 3-phase winding using the other half. The new lap windings have the same coil pitch as the original alternator and 12 conductors per slot. The alternator parameters are given in Table 2. The magnetic coupling between the two 3-phase winding systems has been measured for different rotor positions and the maximal measured mutual inductance between them reaches 4% of the measured self-inductance.

Assuming that magnetic coupling is negligible, the electrical parameters of a multiple three phase winding system can be computed from the original alternator parameters (L₀, R₀, Φ_NL0 with N₀ conductors per slot). When the same stator is rewound with m three-phase windings and N conductors per slot, the new electrical parameters can be expressed as:

The analysis of these equations and the experimental results (Table 2) confirms that both winding systems are magnetically decoupled.

4.3.1. Synchronous rectifier

A synchronous rectifier is an interesting alternative to conventional diode rectifiers. The main advantage of such converters is the reduced rectifier losses particularly at higher speeds if MOSFETs with low on-resistance (e.g. 4 mΩ (Beretta, 2008)) are used. Besides, it may be also employed in applications where bidirectional power transfer is required such as stop-start system (Beretta, 2008). In a start-stop system developed by Citroën, the claw-pole machine performs the functions of both starter and alternator. In the starter mode, the phase current and EMF in each phase are synchronised for maximum torque using a position sensor. In the alternator mode, the converter operates as a synchronous rectifier. The voltage drop may be as low as 0.2 V for an output current of 120 A in contrast to 0.8 V to 1.1 V for a diode. The losses can be 60 % lower compared to a diode rectifier (Beretta, 2008). In both modes of operation the switching losses will be much lower than PWM converters due to lower switching frequency. In the alternator mode the switching frequency may be as high as 2800 Hz (at 4000 rpm), however the drain currents are kept close to zero during MOSFET switching (Beretta, 2008).

4.3.2. PWM controlled full-bridge rectifier (PFBR)

In order to exploit the alternator at its maximum capabilities, it is interesting to set an optimum power angle as proposed in (Liang et al.,1999) and (Liang et al.,1996). This can be achieved with a PWM full-bridge rectifier (PFBR) as already shown in fig. 8. For each point of operation, the modulation index k and the angle θ between phase voltage and back-EMF are adjusted 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 sensorless algorithms (Boldea, 2006). However, like a synchronous rectifier, it offers bidirectional power flow control.

Fig. 19 shows the maximum output current and efficiency obtained by using a sinusoidal PWM and a control technique maximizing output power. It also shows the output current curves obtained with original and rewound alternators connected to a conventional diode rectifier. If compared to the rewound alternator with CDR (RACDR), the PFBR connected to the same alternator increases the output power for speeds below 4000 rpm. (For winding parameters see Table 1). During high speed operation, it is preferable to operate in synchronous rectification mode.

The comparison of the output current characteristics with the original alternator shows that the current generated with the PFBR is lower in the range of 1000 to 2000 rpm (Fig. 19). Note that idle power requirement is not satisfied with the PFBR. This is partly due to the magnetic saturation and significant voltage drops across active switches.

4.3.3. Other PWM rectifiers

If bidirectional power flow is not required, the three single-phase BSBR structure shown in fig. 20 is a simpler solution. 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), shown in fig. 21. With this topology, it is not necessary to use a rotor position sensor but the power angle can’t be controlled.

4.3.4. High frequency ripple reduction of the output current using interleaving

All presented PWM rectifier topologies deliver an output DC current with abrupt current steps and high di/dt’s. These high-amplitude fast-moving current transitions generate RF noise that flows to the battery and then pollutes other loads connected to the battery. Attenuation of these current variations to an acceptable level requires a very large output capacitor. This can lead to considerable efforts to comply with EMC standards (Maxim, 2001). Battery current ripple results in battery heating and a corresponding rise in temperature.

Benefits of interleaved converters on current ripple, components stress and EMI reduction are well known for several different applications (Consoli et al., 2004), (Crebier et al., 2005). The main constraint of interleaved structures is the magnetic coupling between the different windings. Both converters must be connected to different three-phase windings that are not magnetically coupled to avoid a decrease of performances. Fig. 22 shows an example of two interleaved BSMRs with two identical three-phase winding systems having a same back-EMF. Q1 and Q2 are driven by two signals having the same duty-cycle and the same frequency but the phase is shifted by 180 degrees.

Interleaving decreases the current ripple for any value of duty-cycle and it allows for total ripple cancellation when D = 0.5. Equation (5) gives the optimal duty-cycle for a number m of interleaved rectifiers where k_i is an integer.

4.3.5. PWM BSMR with high frequency ripple cancellation

The structure of fig. 23 could be also implemented using the double-winding stator system presented above and two BSMRs operated with fixed optimal duty-cycle (D = 0.5). The rectifier control is thus extremely simple and no position sensing is required. Experimental current waveforms are shown in fig. 23 (left). When comparing the output current ripple with the one obtained with the non-interleaved converter presented in fig. 23 (right), one can appreciate the considerable output current ripple reduction due to interleaving. (from 66A rms to 6.1A rms).

4.3.6. PWM BSBR with high frequency ripple cancellation

It is also interesting to operate two interleaved BSBRs with a fixed duty-cycle of 0.5 to obtain an ideal ripple cancellation. Hence the simplest control mode consists in driving all the switches of the first converter with the same gate signal and all the switches of the second converter with the same complementary signal. This method doesn’t require additional rotor position sensing. Since the duty cycle is fixed at 0.5, no more control of the power angle is achievable.

The BSBR has a voltage drop per device less than the BSMR. This leads to better performance over the whole speed range