A Drive System of PM Motor Using Energy Harvesting

Tomohiro Takahashi Student Member, IEEE

Shibaura Institute of Technology 3-7-5, Toyosu, Koto-Ku, Tokyo, 135-8548, Japan

ma12064@shibaura-it.ac.jp

Kan Akatsu Member, IEEE

Shibaura Institute of Technology 3-7-5, Toyosu, Koto-Ku, Tokyo, 135-8548, Japan

akatsu@sic.shibaura-it.ac.jp

Abstract— This paper describes a drive system of PM motor using Energy Harvesting. Thermoelectric elements are applied to PM Motor to generate small electric power that recovers the heat. Then, the heat of the motor is reduced, thermal demagnetization is resolved. This paper shows a proposed drive system and a drive circuit which is driven by generated small electric power. The system is evaluated by some experimental results. Keywords-Energy harvesting, Thermoelectric element, PM motor

I. INTRODUCTION A lot of induction motors are used in the industry

applications because these are low cost, toughness and there are possible to drive without inverter. However, since induction motors are low efficiency, it is necessary to replace the induction motor to the Permanent Magnet synchronous motor (PM motor) to save the electric consumption. However, PM motor is expensive because of recent expensive neodymium price. Adding that, PM motor needs the inverter, it leads to increase more space and cost. From these backgrounds, an integration motor [1] the inverter is integrated with the motor, has been studied for the purpose of downsizing and low cost. To integrate the inverter with the motor, off course the gate driving circuit should be integrated, a simple voltage supply is requested because the space for the gate driving circuit is not sufficient. Adding that, low heat and low vibration environments are requested to protect the inverter and its driving circuit. Therefore, this paper investigates to apply an energy harvesting technique to the integration motor.

Recently some large scale energy harvesting projects have been developed to save the electric consumption. However, there is less study that energy harvesting is applied to the motor because the generated electric power is too small to drive the motor. From this backgrounds, this study purposes to drive microcomputer and gate drive circuit by the electricity from thermoelectric generation as one of the energy harvesting. Thus, it is expected to realize power supply less and power saving, especially it is effective for the integration motor. Further more, since the thermoelectric device changes the heat to the electricity, the temperature of

the motor is decreased. This is one of the advantages because thermal demagnetization of the magnet is an important problem of PM motor.

In this paper, first the power consumption of the driving system, microcomputer, gate drive circuit, current sensor, position sensor and switch is measured to decide how much electric power should be generated from the thermoelectric devices. Next, the generation and control system by using thermoelectric device is considered. Since the device does not generate the electricity at the motor starting, this system uses rechargeable battery paralleled with the thermoelectric element. Third, a proto-type of the thermoelectric system for a motor is made and the system is verified by regulated DC power supply. Also the power generation has been experimentally confirmed by using thermoelectric element and generated electricity is measured. At the last, the total thermoelectric system with the motor is shown.

II. DRIVING POWER MEASUREMENT Usually the vector control by the three phase PWM voltage

source inverter is used for the PM motor drive to achieve high efficiency and silent drive. This system uses the following equipments adding the inverter power circuit, a microcomputer, the gate drive circuit, the current sensor and a position sensor. The power consumption of these equipment is measured to decide how much electricity should be generated from the thermoelectric device under the real driving condition. The voltage and current of equipments are measured by the oscilloscope at each voltage lines and a sum of each electric power is calculated. Table I shows a measurement result of power consumption.

Total power consumption of the SH-microcomputer, gate drive circuit, current sensor and position sensor of the motor drive is 3.45[W]. Note that the system requests multiple voltages 3.3, 5 and 15[V], adding that the generated voltage by the thermoelectric device is different from the load voltage. From these reasons, this system needs step-up DC-DC converter and step-down DC-DC converter also. In addition, 15[V] voltage line needs to be isolated from DC-DC converter. Therefore, considering the DC-DC converter loss, 4.9[W] is the required generation power.

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2012 IEEE 7th International Power Electronics and Motion Control Conference - ECCE Asia June 2-5, 2012, Harbin, China

978-1-4577-2088-8/11/$26.00 ©2012 IEEE

Table I Power of each part Lines Parts Power[mW]

3.3 Buffer 0.676 5 SH Microcomputer, Current

and Position Sensors, Switch 1250

15 Gate Drive Circuit 2190 DC-DC converter loss 1450

Total 4900

III. SYSTEM DESIGN Fig.1 shows a general motor drive system. A low DC

voltage source for the controller and the gate drive circuit and a main voltage source (mostly three-phase AC) for driving the motor are necessary. This paper proposes a system as shown in Fig. 2. The low DC voltage source is replaced by the voltage from the energy harvesting of thermoelectric elements. Where, the low DC voltage source supplies the voltage to microcomputer, buffer, current sensor, position sensor, switch and gate drive circuit. Since they are comparatively low power consumption, even low generated power by energy harvesting can drive these elements. However, because the thermoelectric device does not generate at the motor starting the rechargeable battery is needed, the parallel system of the thermoelectric device and the battery is required as shown in Fig. 3. In this parallel system, first, the motor is started to rotate by using the rechargeable battery, after the heat is increased and the thermoelectric device starts generate the voltage, the input is switched to the thermoelectric generation. In case the rechargeable battery voltage is decreased the voltage is charged by the thermoelectric generation. This system uses 10 sheets of thermoelectric elements. 5[V] line is generated by the DC-DC step- down converter, 15[V] line is generated by the isolation DC-DC step up converter and 3.3[V] line is generated by the regulator by 5[V] line.

Fig.1 General motor control system

Fig.2 Proposed motor control system with energy harvesting

Fig3. A parallel voltage generation system

IV. SYSTEM CONFIGURATION DETAILS Here explains about a detail of the thermoelectric system

described above section. Fig. 4 shows a hand made circuit. This circuit is consists of a PIC microcomputer, DC-DC converters, the capacitor and a relay. The PIC microcomputer works to select the power source, the DC-DC converter is required to change the output thermoelectric element voltage to the rechargeable battery voltage. The capacitor is needed to reduce the voltage fluctuation which is occurred by the converter switching. The relay is required to change the power source. This relay can work by lower voltage than the threshold voltage of semiconductor device.

The circuit is verified by the quasi loads, 15[V] output connects to 100[Ω] for load 2.25[W], 5[V] output connect to 20[Ω] for load 1.25[W]. Fig. 5 shows a connection diagram and Table II shows a list of the measured voltage. Note that a DC power supply is used for one of the power source to check the circuit verification. Fig. 6 shows an experimental result of operating measured voltage waveform of this system. Purple line is the input voltage by the DC power supply, blue line is relay signal outputted from PIC microcomputer. In case the input voltage is over 12[V], the input source is changed from the rechargeable battery to the input power supply, the output 5[V] and 15[V] are kept as constant.

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Fig.4 Prototype voltage control System

Table Ⅱ List of the measured system

Measurement equipment

Connection

Oscilloscope CH1 Thermoelectric element connection terminal

Oscilloscope CH2 5[V] Output terminal Oscilloscope CH3 15[V] Output terminal Oscilloscope CH4 Relay signal

Regulated DC Power Supply

Thermoelectric element connection terminal

Fig.5 Connection diagram

Fig.6 Experimental results of the output voltages

V. BASIC STUDY OF THERMOELECTRIC ELEMENTS Thermoelectric device generates the electric power by the

temperature difference. The overview of the thermoelectric element which is used in this experiment is shown in Fig. 7. N-type semiconductors and P-type semiconductors and metals make the π-type connection. The specification of the thermoelectric device is shown in Table III.

A basic formula of the output voltage is shown in equation (1).

V = α ΔT (1)

In (1), α is Seebeck coefficient [V/K], ΔT is a temperature difference. An equivalent circuit of the thermoelectric device is shown in Fig. 8. In Fig.8, Re is the internal resistance and RL is the load resistance. In case the internal resistance (Re) matches to the load resistance (RL), this circuit can generate the maximum power. The formula of the maximum power output is shown in (2) [2].

P = ( α ΔT ) 2 / ( 4 Re ) (2)

By using the device which is shown in Fig. 7 the generated voltage is measured by putting the device on a hotplate. A diagram of the experiment is shown in Fig. 9. The device is put on the hot plate and the heat sink is put on a top of the device for cooling which is cooled by a fan to make the temperature difference. Experimental result that the open voltage is measured is shown in Fig. 10. In Fig. 10, the output power 1[W] is measured at 90[deg.] temperature difference. However, the theoretical value is 1.5[W]. This is because the thermal resistance especially between the device and the heat sink is high. Therefore, it needs five thermoelectric devices to output satisfied electric power, 4.9[W], as shown in the above measurement.

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Fig.7 A picture of the thermoelectric device

Table III Spec of thermoelectric element

Spec Value Maximum Open

Voltage 8.6[V] (230K)

Max Voltage 4.2[V] (230K) Max Current 1.4[A] (230K)

Internal Resistance 3.14[Ω] Size 40[mm]×40[mm]×3[mm]

Seebeck electromotive force

α

0.0478[V/K]

Fig.8 Equivalent circuit of the device

Fig.9 Output voltage measurement by using a hotplate

Fig.10 Experimental result of the output electric power

VI. DEVICE INTEGRATION WITH THE MOTOR Here discusses the place of the thermoelectric device with

an IPM motor [3]. Motor loss is the copper loss and the iron loss [4], in case to use the copper loss, the coil end is suitable to set the device. On the other hand, in case to use the iron loss, the stator is suitable. Although it is depend on the motor characteristics, since this motor does not rotate high speed, a larger heat is generated from the coil end. Then the device is put close to the coil end to utilize the temperature difference between the coil end and the outside air. IPM motor with the thermoelectric device is shown in Fig. 11, and a simulation model is shown in Fig. 12. From the simulation result, almost 120[deg.] temperature difference is confirmed as shown in Fig.13, the sufficient condition is obtained by set the device at the coil end.

Fig.11 IPM motor with the thermoelectric device at the coil end

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Fig.12 Simulation model to calculate the temperature difference

Fig.13 Simulation results of IPM motor heat

VII. EXPERIMENTAL SETUP OF THE MOTOR From the above discussion, the device should set close to

the coil end. However, in the real condition, it is difficult to set the device close to the coil end because of some problems about the casing and the wiring. Then in this experiment, the device is put on the motor case, the output voltage is evaluated by adding the current to the motor.

Thermoelectric device and the PM motor are installed as shown in Fig. 14. Usually the motor case is circular shape and this motor has 12[cm] diameter. However the thermoelectric device shape is rectangle. Then, if they are installed together, there is the air gap as shown in Fig. 15. To fill the gap, the heat transfer sheet is sandwiched between the case and the device. The thermal conductivity of this sheet is 6.5[W/m・K], and the thickness is 1[mm]. In order to prevent the decrease in efficiency due to the gap, the device is put with the pressure on the upper heat sink by carrying the weight of 2[kg].

Next, the experimental method is described. First, DC 50[A] is applied to the coil, the coil is heated. The resistance of the coil is 0.03[Ω]. Therefore, it generates heat 75[W]. Thermoelectric device is attached on the PM motor, the device generates the power. The open voltage and

temperature in various locations are measured. The output power is estimated from the open voltage and an internal resistance. Table IV summarizes the experimental equipments.

Table IV Experimental equipments Name Model Number Specification

PM motor N/A 3φ SPM motor Concentrated windings

Thermoelectric device

Nihon tecmo TEP1-1264-1.5

Refer to Table III.

Regulated DC Power Supplies (Parallel drive)

KIKUSUI PAT250-32T

Power capacity : 8[kW] Max output voltage :

250[V] Max output current :

32[A] Data Logger Graphtec

GL900 8 CH input

Sampling time : 100[ms]

Heat Sink CPU Cooler Brushless DC motor Input voltage : 12[V]

Fig.14 PM motor and thermoelectric device

Fig.15 PM motor and thermoelectric device and air gap.

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VIII. EXPERIMENTAL RESULTS Experimental results are shown in Fig. 16, Fig. 17 and Fig.

18. Fig. 16 shows the temperature variation of each measured point which is shown in Fig. 15, K-type thermocouple is installed close to the coil, the thermoelectric device hot side and the thermoelectric cold side. When the temperature of the coil exceeds 190[deg.], the experiment was stopped. Fig. 17 shows open voltage output and Fig. 18 shows the output power. According to Fig. 16, it is observed that, the coil temperature is rapidly raised as the current is added. The hot side temperature (stator temperature) is increased as the coil temperature rises, however the cold side temperature slowly rises because of the heat sink. Therefore, the temperature difference is appeared and the output power is generated. The output voltage is proportion to the temperature difference as shown in Fig. 17. Therefore, the output power is also proportion to the temperature difference as shown in Fig. 18. However, even when the coil temperature is 190[deg], the hot side temperature is only 95[deg]. The reason of this is considered that the thermal resistance between the coil and thermoelectric device is high. Then the thermoelectric device should be directly contacted to the coil. The final output power from one device is 0.12[W]. This is lower power than the experimental result by using the hot plate. The reason is that the contact area is small, temperature difference is small and the heat resistance is high between the device and the coil. However since only one device with the poor contact situation generates 0.1[W], the full contact situation that means the case is rebuild to fit the device will generate enough power to drive the circuit. This is a future work.

Fig.16 Time – Temperature waveform

Fig.17 Time – Open voltage waveform

Fig.18 Time – Output power waveform

IX. CONCLUSION This paper proposed the driving system of PM motor by

using the energy harvesting. The thermoelectric device was used to supply the electric power to the driving circuit, the voltage supply circuit configuration which was the parallel source with the battery was discussed and the output voltage was measured. A single thermoelectric device output power was measured by the experiment, the output electric power was sufficient by using five devices. Also the place to set the device to the motor was discussed, the coil end temperature was simulated. Only one device was put on the stator case, the output power was measured, 0.12[W] was confirmed with the worst contact situation. From these results the enough output power to drive the circuit will be obtained.

REFERENCES [1] H. Hijikata, K. Akatsu, "A Basic Study of MATRIX Motor" , IEEE

Energy Conversion Congress & Exposition, (ECCE), Phoenix, Arizona, US 2011, pp. 3285-3290.

[2] M. Ishizawa, Y. Nozaki, N. Nozaki, M. Yamamoto, "Thermoelectric Generation System using Fuel-cell Exhaust Heat" IEEJ Trans on IA, Vol.119-B, No.2, pp.223-229 (1999)

[3] Y. Takeda, S. Morimoto, K. Ohyama, A. Yamagiwa, "Conparison of Control Characteristics of Permanent Magnet Synchronous Motors with Several Rotor Configurations" IEEJ Trans on IA, Vol.114-D, No.6, pp.662-667 (1994)

[4] K. Ohnishi, "Output Evaluation of Non-Lap Winding Permanent Magnet Motors Based on Copper and Iron Losses" IEEJ Trans on IA, Vol.119-D, No.3, pp.399-404 (1994)

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