#1
İ
*2
# Turkish Aerospace Industries, Inc.
ODTU Teknokent, TAI B-Blok, 06531 Ankara, TURKEY
1 kaydin@tai.com.tr
* Gazi University, Electrical & Electronics Engineering Department
Maltepe-Ankara, TURKEY
2 aydemirmt@gazi.edu.tr
Abstract
Mechanical batteries have been drawing interest due to their longer life time and bigger depth-of-discharge ratings.
Also, they do not have the adverse effects on the environment as the electro-chemical batteries. In mechanical batteries, an electric machinery is run in motor mode at a constant speed, storing kinetic energy in its inertia. When there is a need of energy, the machine is operated in the generator mode and the kinetic energy is returned to the system as electrical power again.
These systems are also being considered for space systems for the future. In these systems, regenerated voltage needs to be regulated at the bus level of the space vehicle. Using the inverter that drives the motor as a controlled rectifier in the generator region is proposed in this paper. The system is explained and experimental results are given for a small system to demonstrate the concept.
speed, the best option to increase the amount of stored energy is to increase the speed [1]. This high speed operation dictates the use of a brushless motor. Due to the difficulties in the generator operation region, induction motors are not typically considered. Permanent magnet synchronous machines (PMSM) and brushless dc machines (BLDC) also have the advantage of high power density and high efficiency. Thus these types of motors are the best candidates for the application [2,3].
During the motor operation mode an inverter is used to drive the motor. During the generator operation mode, a separate diode bridge can be used to rectify the alternative voltage waveform generated by the machine. It is also possible to utilize the anti-parallel diodes of the inverter as an uncontrolled rectifier in this mode. However, in either case the rectified voltage is unregulated. Therefore, it is suggested in this paper to utilize the three-phase inverter as a controlled rectifier. The biggest advantage of this is the elimination of the additional equipment needed for bus regulation. Also, as there are already position sensors inside the motor, the signals coming out of these sensors can be easily used to determine the switching instants, simplifying the control.
I.
INTRODUCTION
Mechanical batteries are electromechanical devices that store electrical energy at a flywheel rotating at high speeds.
Recently, there have been an increasing interest in these batteries due to the disadvantages of chemical batteries. Most important disadvantages of chemical batteries are their short life times, low depth of discharge, and their negative effects on environment. Uninterrupted power supplies are the most common application area of mechanical batteries today.
Sun is the energy source for space vehicles. Photovoltaic panels generate dc voltage that is needed by various loads on board. Typically, energy is stored in electrochemical batteries to be used when the solar power is not available [1].
Mechanical batteries have also been considered for space applications to replace these heavy and bulky electrochemical batteries. During the charging period a satellite receives solar energy a motor can be rotated at high speeds, storing energy in its inertia [1]. When the satellite is in the dark region, this stored energy is supplied back to the electrical bus of the system. However, as the speed of the machine reduces, the generated voltage is also reduced. Therefore, a converter to convert the unregulated output voltage of the mechanical battery to the bus voltage is needed [1]. The application investigated in this paper is related to this concept.
Mechanical batteries do not have a load to drive and therefore torque requirement is low. As the mechanical stored energy is proportional to the inertia and the square of the
II.
MECHANICAL BATTERY FOR SPACE SYSTEMS
Figure 1 shows the system under investigation. Input voltage source represents the battery or the solar panel output terminals. This power source normally supplies various loads of the satellite represented by “Load.”. At the same time, a
BLDC motor is rotated at high speed. A three-phase inverter is used for this purpose.
Fig. 1. The DC bus and BLDC motor-driver assembly
978-1-4244-5794-6/10/$26.00 ©2010 IEEE 1204

Once the BLDC motor reaches the final speed, its energy requirement becomes very low. When the stored energy needs to be returned (in this application, when satellite enters the dark region and solar panels can not generate any power) inverter operation is ended and the BLDC machine starts to slow down. This is the generator mode of operation and during this mode the voltage magnitude continuously decreases as the speed reduces. Therefore, this generated voltage needs to be regulated at the bus voltage (V bus
) level.
This can be achieved by using a separate dc-dc controller.
However, there is already a power converter (inverter) in the system, and its use as a controlled rectifier in the reverse direction is proposed in this paper.
Although using the ac/dc converter in the reverse direction is not a new concept [4,5], existing examples are related to low speed applications such as wind turbines, and complex control algorithms are used in these system. In the application presented here, the speed level is much higher, and a simple control algorithm utilizing the Hall effect signals is suggested.
One of the biggest challenges of controlled rectifier applications is phase synchronization. Synchronized gate signals to the phase voltages need to be generated. When the frequency of the voltage varies this problem is more serious.
This is the case in the mechanical battery application during the discharge mode. Speed continuously goes down, and so does the frequency. Fortunately, on the other hand, Hall sensors located in the rotor assembly of the BLDC machine generates signal that yields information about the rotor position, and this information can be used to determine which windings need to be switched.
III.
CONTROL OF BOOST RECTIFIER
Figure 2 shows the general control technique of a boost rectifier. In order to regulate the output, the bus voltage is measured and this information is used as a feedback signal to compare to the reference voltage. Phase reference signals then are generated by using the error between these two signals. It is also desired that the phase current references and phase voltages be in the same phase. This is achieved by measuring the phase voltages, and multiplying them by the error signal to obtain the synchronization. Alternatively, the error signal can be first put through a compensator and then the multiplication is executed on it [6,7]. Phase reference signals are used to generate PWM switching signals [6]. Figure 3 shows the generated reference signals.
Fig. 2. Control blocks
Fig. 3. Reference signal and generated PWM waveform
In the proposed system structure, the converter is operated as a boost rectifier during the discharge mode, stepping up the back emf voltages generated by the BLDC machine.
Therefore, it should operate in voltage source mode. The voltage source in this case is the BLDC machine windings.
The biggest advantage of this structure is that there is no need to measure the phase voltages to achieve synchronization.
BLDC machine has Hall sensors to detect the rotor position.
The signals generated by these sensors are naturally synchronized with rotor position, and thus phase voltages.
Therefore, they are not affected by the variation of frequency
(speed). Figure 4 shows the system configuration during the discharge mode. High speed machines are used in this application, and phase inductance values are typically low for these machines. Therefore, additional series inductors need to be inserted in the lines so that the boost function can be accomplished.
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Fig. 4. System under investigation during discharge mode
The switch T1 is always off in this interval. When T6 is turned on, the current flows through Phase-a, T6, anti-parallel diode of T4 and Phase-b. When T6 is turned off, current diverts from T6 to the anti-parallel diode of T3. So, in the first part of this interval energy is stored in the line inductors, and in the second part it is transferred to the load. In each mode only one switch is turned on. Current path is completed through the anti-parallel diodes.
The converter can be viewed as a separate boost dc-dc converter at any instant. The equivalent circuit is shown in
Figure 7. V
1
represent the instantaneous voltage of the active ac input lines. For example, for the case seen in Figure 6 it is equal to the instantaneous value of v ab
.
Which switches will be turned on is decided depending on
Hall sensor signals. Figure 5 shows the line voltages and corresponding Hall sensor signals. By looking at these waveforms the lines with the most positive and most negative voltages are determined, and then corresponding switches are turned.
Fig. 5. Hall sensor signals and corresponding line voltages
A boost rectifier can be viewed as a system consisting of three boost converters operating successively. Figure 6 shows the equivalent structure when the phase-a (HI) and phase-b
(LO) are active.
Fig. 7. Boost converter
Phase currents are used as the real current values in the current control loop. Current is measured by using the sense resistors placed at the emitter terminals of the bottom side switches as seen in Figure 8. Measurement can not be performed on the current flowing through the lines as seen in
Figure 6 because, for example when Phase-a and Phase-b are active, although Phase-a current and switch current (I ch
) are equal in the charge mode, they are not the same during discharge mode. On the other hand, phase current flows through the sense resistor of switch T6 on both cases.
Therefore, in general, it can be stated that by measuring the current on the sense resistor connected to lower switch of the phase with lower voltage among the active lines would yield the right current value to be used in current control loop.
Fig. 8. Current measurement
Fig. 6. Equivalent system when phase A (HI) and phase B (LO) are active
IV.
EXPERIMENTAL WORK
A simple experimental set-up has been designed to demonstrate the concept. A 50W BLDC motor of Maxon
Company was selected as the high speed machine. Technosoft
MCK2812 has been used as the BLDC driver. As a requirement of the motor, 32 VDC source was used at the
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input. The switching frequency has been selected as 20 kHz.
Considering the low current rating and low line inductance value of the motor, a series inductor of 500 uH has been added to each line to increase the performance. Even in this case the inductor current is discontinuous. A flywheel with a 4.83e-4 kg-m 2 inertia was attached to the shaft of the motor. DC link capacitor value of the system is 100 uF. The block diagram of the system is shown in Figure 9.
By using this inequality, 2.5 A is obtained as the limit.
Maximum value of the input rms current of the rectifier was measured to be approximately 906 mA in the experiments
(Fig. 10). Based on the results it can be said that the rectifier is stable.
Figure 11 shows the bus voltage (upper curve) and line-toline BLDC machine voltages (lower curve) in the generator mode. The regulator starts functioning after the motor mode
(at the generator mode) and keeps the voltage constant at 13 V until the next motor mode is started.
It should be noted that high maximum speed and low losses are the most important requirements for mechanical batteries.
For long discharge times especially friction losses should be minimized. This can be done by using high quality mechanical ball bearings, or even better, magnetic bearings. In the set-up used in the experiments normal bearings were used, and therefore discharge period is short.
Fig. 9. System block diagram
The system was operated in consecutive modes of charge
(motor) and discharge (generator.) In the charging mode the motor is accelerated to a speed of 10 krpm, and the input power source is disconnected. As soon as the disconnection occurs the generator mode starts and the speed of the BLDC machine starts decreasing. A 250 ohm resistor has been used as the experimental load. The control system works to obtain a regulated dc bus voltage from the generated ac voltages. As seen in the figure, current regulation loop works inside the voltage loop. PI coefficients of are K p voltage controller are K p
=2, K i
=50, K i
=200 for the
=500 for the current controller.
In the motor operation region BLDC motor is driven by current control. For 30 s a constant current reference is used followed by a linearly increasing reference until the motor reaches 10 krpm. In the generator mode, the regulation level has been selected as 13 V in this work. At the transition from motor to generator mode the speed is 10 krpm, and the boost operation region is entered. Considering the voltage constant of the BLDC motor (1220 rpm/V), this means that the regulation is performed below 7.8 V peak line-to-line backemf voltage.
Stability limit is important for this operation and it is defined in (1) [6,7].
I
S
<
2 *
K
R p
∗
* V
K p
∗
+ cos ϕ
L s
∗ K i
(1) where:
I s
= Input RMS rectifier current (measured as 906 mA)
V = Input RMS voltage (measured as 5 V)
K p
K i
= Proportional gain of voltage controller
= Integral gain of voltage controller
L
S
= Rectifier input inductance
(phase inductance, ~525 uH)
R = Rectifier input resistance
(phase resistance, ~1ohm)
cos φ = Input power factor (=1)
DC Line
Voltage
Fig. 10. Current waveform
Phase-
Phase
Voltage
Fig. 11. Experimental results. Upper curve: Bus voltage. Lower curve: Lineto-line back-emf voltage.
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V.
CONCLUSION AND FUTURE WORK
A simple mechanical battery has been built to show that the inverter used in driving the machine in motor mode can also be used as a rectifier in the generator mode. In the generator mode, the devices of the bridge can be switched to boost the back-emf voltages generated by the machine to regulate the bus voltage. Control algorithm is described and the results are given for a 50 W BLDC machine. Performance can be greatly improved by using ceramic or magnetic bearings along with a flywheel having a larger inertia.
The experimental set-up will be improved for future work.
A matching pair of driver and motor will be used with ceramic or magnetic bearings to increase the speed and stored energy
REFERENCES
[1] M. R. Patel, Spacecraft Power Systems , Florida, USA: CRC Press,
2005.
[2] B. H. Kenny, and P. E. Kascak, “Sensorless Control of Permanent
Magnet Machine for NASA Flywheel Technology Development,” 3 7 th
Intersociety Energy Conversion Engineering Conference , July 2002.
[3] K. L. McLallin, R. H. Jansen, J. Fausz, and R. D. Bauer, “Aerospace
Flywheel Technology Development for IPACS Applications,” 36 th
Intersociety Energy Conversion Engineering Conference , Aug. 2001.
[4] Ahmed, T.; Nakaoka, M.; Tanaka, T.; Nishida, K.; “Advanced control of a boost AC-DC PWM rectifier for variable-speed induction generator”, Twenty-First Annual Applied Power Electronics
Conference and Exposition , 19-23 March 2006, Page(s):7 pp
[5] Zitao Wang; Liuchen Chang; “PWM AC/DC boost converter system for induction generator in variable-speed wind turbines”, Canadian
Conference on Electrical and Computer Engineering , 1-4 May 2005
Page(s):591 - 594
[6] M. H. Rashid, Power Electronics Handbook , Canada: Academic Press,
2001.
[7] J. W. Dixon, “Feedback Control Strategies for Boost Type PWM
Rectifiers,” IEEE , pp. 193–198, 1990.
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