An Integrated Starter-Alternator System Using Induction Machine Winding Reconfiguration

G. D. Martin, R. D. Moutoux, M. Myat, R. Tan, G. Sanders, F. Barnes University of Colorado at Boulder, Department of Electrical and Computer Engineering

Abstract-A system designed for use as an integrated starter-alternator unit in an automobile is presented in this paper. The new system utilizes a basic squirrel-cage rotor induction machine with the ability to externally switch the winding configuration for optimal performance. A conventional V/f motor controlling inverter drives the machine in starting mode and rectifies the output of the generator in alternating mode. A system controller manages the inverter and user interface to the system. This system is designed to minimize cost and exhibit good reliability.

I. INTRODUCTION

A trend in some automotive sectors toward high efficiency vehicles is driving development of new components for use in both conventional and hybrid automobiles. Additional factors, including changing voltage standards (from 12 to 42 VDC), the presence of high voltage, high power battery sources, and the trend toward more electrically driven automobile components call for new innovations and implementations to replace old technologies. The engine starter motor and the alternator are electrical components that have received considerable attention. The drive is to combine these now separate parts into a single unit that performs the functions of both.

This paper details the design of a motor-generator system intended to serve as an integrated starter-alternator unit in a conventional or hybrid automobile. The concepts presented in this paper have been applied in the 2007 IEEE International Future Energy Challenge by a joint team of University of Colorado at Boulder and Indian Institute of Technology-Delhi students. To meet the challenging design requirements of the competition, the team has applied machine design and control technologies together in an innovative system.

To be feasible in the automotive industry, an integrated starter-alternator part must be inexpensive, small, and light weight. Further, in order to function as both a starter and an alternator in an automobile, the electric machine must work over a wide speed range. It must produce high starting torque from standstill, and operate with high generating efficiency at speeds ranging from about 2000 rpm to 4000 rpm.

Published concepts, innovations, and patents exist having to do with this design problem. Pole-phase modulation in an AC machine is proposed by J. M. Miller et. al. [1]. A permanent magnet machine solution is proposed by V. Ostovic [2], though while versatile and power dense, tends to be more expensive and less reliable than induction machine solutions. A. K. Jain et. al. provide a method of using direct torque control of an induction machine [3].

The design objectives for the concept developed in this paper are taken from the 2007 IEEE International Future

Energy Challenge. These requirements call for the design and fabrication of an integrated starter generator (ISG) system that interfaces to a 200 VDC (battery) power source. The ISG is to develop at least 30 Nm of torque at standstill, accelerate to 2500 rpm at a constant power of 1 kW, and switch into generating mode to generate 1 kW of 200 VDC power at 3000 rpm with an efficiency of at least 75%. The starter-alternator solution proposed in this paper is comprised of four major hardware components; an induction machine, a winding switching board, a motor controlling inverter/rectifier, and a system control board. Software algorithms loaded in the system micro-controller implement PWM V/f control of the motor and rectifier action during generating, and also manage machine winding switching and the user interface to the system.

II. THEORETICAL DESIGN

The solution selected to overcome the wide speed range challenge involves changing the pole number in the induction machine stator from 8 to 4, changing the number of winding turns of the stator, and controlling the motor with V/f control via an inverter.

The machine selected for this system is a conventional lap-wound squirrel-cage rotor induction machine. This machine was chosen to reduce system cost and facilitate modifications. The machine adheres to NEMA 56 frame size standards, and has a stack length of 8.8 cm. This size was chosen so that the machine had sufficient core iron to supply the necessary torque and power. Fig. 1 shows a cross-sectional diagram of the machine stator and rotor. Table 1 lists the main dimensions and specifications of the machine.

To facilitate winding reconfiguration, the machine was customized by re-winding the stator for both 4- and 8-pole connections and bringing out 48 winding leads to interface to a switch network. The rotor was not customized. Winding reconfiguration is achieved externally to the machine via several electrically driven contactors on a switch board. The machine is designed as an 8-pole induction machine during startup to maximize the starting and low-speed torque, and can then be switched to a 4-pole configuration at higher speeds to maximize generation efficiency.

The machine will start at standstill as an 8-pole machine operating at a calculated frequency of 27 Hz and a calculated line-to-line voltage of 121 VRMS provided by the power inverter. Calculations show that this operating point should provide the required 30 Nm of starting torque. The drive frequency is then increased from the low frequency value to

Figure 1. Cross-sectional diagram of induction machine.

TABLE 1

PARAMETER AND SPECIFICATIONS LIST FOR INDUCTION MACHINE Stator: One sided air gap (g) = 0.34375 mm Number of stator slots (N1) = 24 slots Outer diameter of stator core (Dos) = 153.2 mm Number of poles (p) = 2 poles Inner diameter of stator core (Dis) = 79.375 mm Axial length of stator core (l) = 88mm Cross-section of one stator slot (AN1) = 78 mm2 Height of stator back iron (hj1) = 21mm Height of one stator slot (hN1 = hz1) = 14.9 mm Opening of stator slot (SN1) = 1.85 mm bz1 = 1.85mm

Rotor: Number of rotor slots (N2) = 34 slots Outer diameter of rotor core (DoR) = 78.5 mm Cross section of one rotor slot (AN2) = N/A Height of rotor back iron (hj2) = 14.5 mm Height of rotor slot (hN2 = hz2) = 14.5 mm Opening of rotor slot (SN2) = N/A Inner diameter of rotor core (di) = 22 mm Total winding turns = 400 turns per phase with AWG #23

50 Hz to accelerate the machine and its load. The inverter is designed to provide up to 1 kW of constant power to accelerate the system. Once the shaft speed reaches approximately 750 rpm, the machine is switched from 8-pole operation to 4-pole operation. The change from 8-pole to 4-pole operation causes the machine to speed up to approximately 1500 rpm, and a speed transient is seen.

In order to increase the speed beyond 1500 rpm, the inverter drive frequency is once again increased, this time from calculated 50 Hz to 75 Hz, causing the machine speed to increase to around 2250 rpm. Because of the back EMF developed above 1500 rpm, the V/f ratio is adjusted to accomplish field weakening. To overcome the back EMF and

Figure 2. Drive control frequency during acceleration.

Figure 3. Shaft speed during acceleration with constant power load.

TABLE 2 SUMMARY OF CONTROLLED MACHINE ACCELERATION

Speed(rpm) Frequency(Hz) Time(sec) Winding Configuration

0-750 27-50 0-1.5 8-pole, N turns 750-1500 50 1.5-2 4-pole, N turns 1500-2250 50-75 2-3.5 4-pole, N turns 2250-3000 75-100 3.5-5 4-pole, N/2 turns

further increase the torque output at high speed, the number of effective stator turns per phase (N) in the machine is reduced by half [4][5][6]. This increases the flux generated by the stator windings, and gives a “torque boost” to the shaft output. Once this winding reconfiguration has occurred, the drive frequency is increased to near 100 Hz, taking the shaft speed past 2500 rpm up to 3000 rpm. Figs. 2 and 3 show the timing diagrams for drive frequency changes and the resultant change in shaft speed, respectively. Table 2 gives a summary of the machine acceleration drive frequencies, winding configurations, and timing.

III. MODELING

A majority of the design development was accomplished using mathematical modeling of the machine torque and inverter response in Matlab. The winding resistances and inductances were calculated and equivalent circuit models were developed based on the machine parameters for all three winding configurations (8-pole, 4-pole, and 4-pole with a reduced number of winding turns). Based on these three equivalent circuit models and the speeds and frequencies in Table 2, speed-torque curves were developed for varying frequencies and winding configurations. This section details the speed-torque modeling for this project.

The overall speed-torque diagram is shown in Fig. 4 with four natural speed-torque characteristics, along with the characteristic for starting at 121 VL-Lrms and 27.5 Hz. The

stator current at 0 rpm is Istart = 24Apeak = 17Arms. As the start-up occurs and system gains speed, the required current supplied to the stator windings will decrease due to the frequency increase and the associated speed increase from 0 rpm to 750 rpm. Fig. 5 illustrates the transitional speed-torque characteristics from 27.5 Hz to 50 Hz at 8-pole operation. It is assumed that the internal combustion engine has started – that is, it develops a motoring torque – at about 750 rpm and no torque from the electric starter is required; that is the starter is loaded by inertial, frictional, and windage torques only.

Fig. 6 illustrates the transition from natural characteristic #1 to natural characteristic #2, which occurs via pole switching from p1=8 poles to p2=4 poles. The transitional speed-torque curves from the natural characteristic #2 to the natural characteristic #3 are depicted in Fig. 7. At the natural characteristic #3 the number of turns (Nrat) is halved with the switch network. From natural characteristic #3 to natural characteristic #4 flux weakening is used, as seen in Fig. 8.

Figure 4. Natural speed-torque and starting characteristics curves.

Figure 5. Speed-torque characteristics from 27.5 Hz to 50 Hz, 8-pole.

Figure 6. Transitional speed-torque curves from characteristic #1 (8-pole) to

characteristic #2 (4-pole) via pole-switching.

Figure 7. Transitional speed-torque curves from characteristic #2 to

characteristic #3 based on V/f control, by increasing inverter frequency from 50 Hz to 75 Hz.

Figure 8. Transitional speed-torque curves from characteristic #3 to

characteristic #4 based on stator phase winding turns reduction with V/f control by increasing inverter frequency from 75 Hz to 100 Hz.

IV. HARDWARE BUILD AND INTEGRATION

Four main components were integrated together in the fabrication of this system. They are the induction machine, the motor controlling inverter, winding switching board, and the

system micro-controller. These components were purchased and modified or built up from off-the-shelf components as described here.

A. Induction Machine A Dayton 4B237 compressor-grade motor was acquired. It

was rated for 3 horsepower at 3500 rpm and 60 Hz, 240/480 volts AC, with a maximum current of 8.5 amps. Other specifications are included in Table 1.

No-load and locked-rotor tests were carried out with a 240 volts AC, 60 Hz input. From this test data, the winding specifications for the required customization were calculated. The number of turns was determined to be 400 turns per phase with AWG #23 wire. The machine was re-wound as an 8-pole / 4-pole machine, with a total of 48 leads brought out of the machine to accommodate pole changing and winding switching. Once customized, the machine was subjected to the same locked-rotor and no-load tests as before. The expected machine operation in 8-pole (delta), 4-pole (double wye) and 4-pole (double wye) with half the number of turns was tested and verified. Figs. 9 and 10 illustrate the winding configuration connections for 8-pole delta and 4-pole double wye configurations, respectively.

B. Winding Switching Board The 48 machine leads were connected to a switching

network that implements the winding reconfigurations. This

Figure 9. Eight pole delta winding connection diagram.

Figure 10. Four pole double wye winding connection diagram.

Figure 11. Diagram of winding switch network.

board was built from electrically actuated mechanical contactors. The switches are controlled by signals established by the system microcontroller. Fig. 11 is a diagram of the winding switching network.

C. Motor Controlling Inverter The induction machine is controlled by an off the shelf

inverter board purchased from ST microelectronics. The inverter uses IGBTs to implement the inverter bridge that provides a PWM motor control waveform to the machine. The control method for the inverter PWM is constant V/f control. The system microcontroller interfaces closely with the inverter board to apply the correct waveforms to the machine during all operation modes.

At the pre-specified speed of 2500 rpm, the system switches from motoring to generating mode. This change is coordinated by the system controller. Three 100µF exciting capacitors connected across the machine input phase leads provide quadrature excitation current to the machine, allowing it to act as a generator. The inverter acts in reverse as a controlled rectifier, regulating the generator output to 200 volts DC.

D. System Micro-controller The overall system is managed by a system micro-controller

purchased from ST microelectronics. The microcontroller has three main functions:

1) Send signals to the switch network to change the winding configuration.

2) Control the gate drives of the IGBTs in the inverter during motoring and generating operation.

3) Provide a simple user interface to turn the system on and off.

The micro-controller and associated circuitry are controlled via a software algorithm loaded onto the micro-controller using a PC interface. The software executes the timing of the winding changes and PWM control of the machine. Speed is determined by sensing of the output voltage and current on the inverter board. From this feedback information, the micro-controller manages the system through an entire start, acceleration, and generate run with no additional input from the user.

V. TEST RESULTS

At the time of this publication, complete operational testing had not been completed. Using a 200 VDC battery source, the inverter board, switch network and micro-controller, initial tests were run. Preliminary results showing current and torque performance for the 8-pole locked rotor (starting torque) and no-load up to 3000 rpm have been accomplished. Fig. 12 shows the current input to the motor for the locked rotor standstill test. Fig. 13 shows the current input to the motor for the no-load acceleration test (top bar) and the start-up current with no-load (bottom).

Figure 12. Locked-rotor test machine input current with 8-pole delta

configuration.

Figure 13. No load test machine input current with 8-pole, 4-pole, and N/2 changes.

The test results indicate that about 21 Nm of standstill torque

is developed in the locked-rotor test. The torque is limited by the battery source voltage drop during high current draw. This issue is being resolved by adding additional batteries to the source to boost the voltage. The no-load test indicates that the machine starts up and smoothly executes switching between the three winding configurations to accelerate the machine up to 3000 rpm.

VI. ACKNOWLEDGMENTS

The authors acknowledge and thank Professor Ewald Fuchs for technical leadership and support of this project.

We also would like to thank the University of Colorado Department of Electrical and Computer Engineering, the University of Colorado Engineering Excellence Fund, and the Bernard Gordon Prize for funding support.

We also thank the Boundless Corporation for the use of the 200 volt battery source, and Boulder Electric Motor Company for electric motor parts and support.

VII. REFERENCES [1] J. M. Miller and V. Stefanovic and V. Ostovic and J. Kelly, “Design

Considerations for an Automotive Integrated Starter-Generator With Pole-Phase Modulation,” Industry Applications Conference, 2001. Thirty-Sixth IAS Annual Meeting, Vol. 4, Pages 2366-2373, Oct 2001.

[2] Ostovic, V. “Memory motors-a new class of controllable flux PM machines for a true wide speed operation,” Industry Applications Conference, 2001. Thirty-Sixth IAS Annual Meeting. Conference Record of the 2001 IEEE, Vol.4, Iss., 30 Sep-4 Oct 2001, Pages:2577-2584.

[3] Jain, A.K.; Mathapati, S.; Ranganathan, V.T.; Narayanan, V.; “Integrated starter generator for 42-V powernet using induction machine and direct torque control technique,” Power Electronics, IEEE Transactions on Volume 21, Issue 3, May 2006 Page(s):701 - 710.

[4] E. F. Fuchs and J. Schraud, "Analysis of Critical-Speed Increase of Induction Machines via Winding Reconfiguration with Solid-State Switches," Submitted to IEEE Transactions on Energy Conversion, Paper No. TEC-00375-2005.

[5] J. Schraud and E. F. Fuchs, "Experimental Verification of Critical-Speed Increase of Induction Machines via Winding Reconfiguration with Solid-State Switches," Submitted to IEEE Transactions on Energy Conversion, Paper No. TEC-00376-2005.

[6] E. F. Fuchs, "Speed Control of Electric Machines Based on the Number of Series Turns of Windings and the (Induced) Voltage/Frequency Ratio," Invention Disclosure, University of Colorado, Boulder, CO, July 1999.

[7] E. F. Fuchs and M. A. S. Masoum, Power Quality of Electric Machines and Power Systems, Elsevier, Inc., 2007.