**3.2. Electric generator**

472 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

pressure drop (*Δp*) 4200 Pa head (*H*) 0.43 m nozzle jet speed (*c*) 2.17 m/s 2.78 m/s 2.84 m/s circ. runner speed (*u*) 1,65 m/s 1.33 m/s 1.33 m/s Runner diameter (*A*) 31 mm 25 mm 24 mm

**Hydraulic machines Turbines**

trade-off are reported in Tab. 4.

\*\*\* good, \*\* average, \* bad

are given in Tab. 5.

**Table 4.** Trade-off analysis between hydraulic machines.

with the generator and a limited risk of choking.

*3.1.1. Cross Flow or Banki turbine design* 

solutions.

**runner** 

and stator. Each of these aspects have been ranked in a three step scale. The results of this

Displacement (*V*) 2,5 cm3/turn

**Hydraulic machines Turbines Volumetric** 

 **Francis Pelton Banki Gear Pump** 

The Cross Flow or Banki turbine appears to be the best in almost all aspects examined. Its strengths are the simplicity of construction, the compact size, a good interfacing capability

Following the Banki water turbine theory reported in (Mockmore et al. 1959) the two main parts of the turbine, namely the nozzle and the runner, have been designed. The design drawings of the Banki turbine runner are shown in Fig. 3, and its characteristic parameters

Radial size \*\* \*\* \*\* \* Axial size \*\* \*\* \*\*\* \*\* Rotor complexity \* \* \*\*\* \*\*\* Stator complexity \* \*\* \*\*\* \*\* Electric machine interface \* \*\* \*\*\* \*\* Choking risk \* \*\* \*\* \*

**Reaction Action Axial Tangential**

**Table 3.** Preliminary design of hydraulic machines. Comparison of the main parameters of the different

 **Francis Pelton Banki Gear Pump** 

In order to convert the rotational mechanical energy from the turbine shaft into electrical energy which can be used to power the wireless spot and operate the valves of the system, a miniaturized electrical generator has been designed especially for this application. Two different configurations of generators have been investigated in order to obtain a clear perspective on the advantages and drawbacks of each one. The first configuration is based on a multiphase permanent magnet generator layout. The second is a single phase permanent magnet generator having claw pole structure. Fig. 4 shows the two different configurations describing the main components of the electrical machines, namely, rotating permanent magnet (1), generator's coil (2), and stator yoke (3).

Both cases consider permanent magnet excitation on the rotor. It is known that for reduced size applications such as the present one, it is better to use permanent magnet excitation instead of electrically excited magnetic systems. The electrical excitation is disadvantageous in these cases owing to unfavourable scaling of the currents (Arnold 2007).

**Figure 4.** Configurations of electrical machines studied during the trade-off analysis. a) Two phase generator; b) Claw pole generator.

A trade-off analysis is performed using virtual prototyping tools. The difficulties related to the mechanical layouts were studied using CAD models while the electrical and magnetic properties were analyzed using analytical and finite element (FE) models.


\*\*\* good, \*\* average, \* bad

**Table 6.** Comparison between multiphase and claw pole layouts.

From these models it is possible to obtain a relatively accurate perspective of the critical aspects related to the feasibility of each configuration of the generator. From the application point of view, the most important characteristics are compared in Tab. 6. Analyzing the table, it is easy to conclude that the claw pole configuration is more suitable for this application. Its layout makes it possible to obtain a simple and compact structure, and, since the output voltage must be rectified to supply the batteries, there is no advantage in having a multiphase winding, such as configuration 1. Furthermore, the single phase winding enables having a larger number of magnetic pole pairs, thus resulting in an increase of the frequency of the induced electromotive force (EMF), which is beneficial from the electronic point of view. Another aspect that cannot be neglected is the amplitude of the detent torque generated by the interaction between rotor's permanent magnets and stator's yoke (Lossec et al. 2010). A larger number of pole pairs tends to reduce the amplitude of the detent torque for the same rotor radius since the slot opening is reduced (Hendershot et al 1994). Moreover, the geometry of the teeth in the claw pole configuration can be adjusted in order to further reduce the cogging torque. On the other hand, the multiphase configuration creates problems in this aspect due to the difficulty in realizing yoke and windings having such small dimensions.

#### *3.2.1. Finite element modeling*

474 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

generator; b) Claw pole generator.

\*\*\* good, \*\* average, \* bad

**Figure 4.** Configurations of electrical machines studied during the trade-off analysis. a) Two phase

properties were analyzed using analytical and finite element (FE) models.

 Multiphase Claw pole Rotor complexity \*\*\* \*\*\* Stator complexity \* \*\* Overall volume \* \*\* Number of pole pairs \* \*\*\* Winding complexity \*\* \*\*\* Detent torque \* \*\*

**Table 6.** Comparison between multiphase and claw pole layouts.

A trade-off analysis is performed using virtual prototyping tools. The difficulties related to the mechanical layouts were studied using CAD models while the electrical and magnetic

From these models it is possible to obtain a relatively accurate perspective of the critical aspects related to the feasibility of each configuration of the generator. From the application point of view, the most important characteristics are compared in Tab. 6. Analyzing the table, it is easy to conclude that the claw pole configuration is more suitable for this application. Its layout makes it possible to obtain a simple and compact structure, and, since the output voltage must be rectified to supply the batteries, there is no advantage in having a multiphase winding, such as configuration 1. Furthermore, the single phase winding enables having a larger number of magnetic pole pairs, thus resulting in an increase of the frequency of the induced electromotive force (EMF), which is beneficial from the electronic point of view. Another aspect that cannot be neglected is the amplitude of the detent torque generated by the interaction between rotor's permanent magnets and stator's yoke (Lossec et al. 2010). A larger number of pole pairs tends to reduce the amplitude of the detent torque for the same rotor radius since the slot opening is reduced (Hendershot et al 1994). Moreover, the geometry of the teeth in the claw pole configuration can be adjusted in order

Configuration 1 Configuration 2

The prediction of the generator's performance is developed by means of FE simulations. The simulations are performed using a stationary formulation without electric currents for one single pole pair of the electrical machine. Non-linear magnetic properties were considered in the iron parts of the structure. Fig. 5 shows the model used in the finite element modeling, evidencing the use of cyclic symmetry boundary conditions to improve the modeling quality with reduced computational cost. The analyses are conducted in order to calculate the flux linking the coil for different values of rotor angles. This information is then used to evaluate the induced voltage with respect to the rotor's spin speed. To this end, the problem is set to allow the rotor mesh to move with respect to the stator mesh, thus enabling the calculation of the magnetic quantities for different values of angular position between the two. Notice that the air surrounding the rotor and stator of the electrical machine is modeled, but is not shown in the illustration. Fig. 6 shows the results obtained from the FE model evidencing the path of flux lines inside the stator's yoke (Fig. 6b). Moreover, it can be noticed that the flux densities inside the iron are relatively low (Fig. 6a) resulting in very little or no saturation.

**Figure 5.** Settings of the FE model for the magnetic simulations.

The parameters characterizing the system described in the FE simulations are summarized in Tab. 7, and the flux linkage wave calculated with the FE model is illustrated in Fig. 7. Observing the graph it can be noticed that the flux linking the coil realizes one complete period every 45 mechanical degrees, evidencing the existence of eight magnetic pole pairs. Furthermore, the flux linkage is a sinusoidal function of the rotor angle and, since the generator operates at constant rotating speed during most of its operative life, the analysis can be performed in terms of RMS quantities.

The velocity constant *ke* can be obtained from the data plotted in Fig. 7 considering the flux *λ* linkage derivative with respect to the rotor angle *θ* as:

$$k\_e = \frac{1}{\sqrt{2}} \frac{\partial \mathcal{X}}{\partial \theta} \tag{6}$$

The induced electromotive force *e* can then be obtained as:

$$e = k\_e \alpha \tag{7}$$

where *ω* is the generator rotational speed expressed in radians per second.The value of the velocity constant obtained for the generator is reported in Tab. 7.


**Table 7.** Claw pole generator nominal parameters.

**Figure 6.** Results of the finite element simulations on the claw pole generator. a) Magnetic flux density in the stator in teslas (T); b) magnetic flux lines inside the iron.

**Figure 7.** Flux linking the generator's windings for different values of rotor angle.

#### **3.3. Energy storage unit**

In order to properly manage the energy coming out from the generator, it is necessary to consider that the maximum amount of power is generated when the impedance of the load is nearly equal to the impedance of the generator. Due to this limitation it is important to design a control system shown in Fig.8 that can monitor, manage and store the energy in order to increase the efficiency of the whole system. For this reason the energy converter can be divided into several subsystem:

• rectifier

476 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

can be performed in terms of RMS quantities.

**Table 7.** Claw pole generator nominal parameters.

in the stator in teslas (T); b) magnetic flux lines inside the iron.

linkage derivative with respect to the rotor angle *θ* as:

The induced electromotive force *e* can then be obtained as:

velocity constant obtained for the generator is reported in Tab. 7.

Furthermore, the flux linkage is a sinusoidal function of the rotor angle and, since the generator operates at constant rotating speed during most of its operative life, the analysis

The velocity constant *ke* can be obtained from the data plotted in Fig. 7 considering the flux *λ*

1 <sup>2</sup> *<sup>e</sup> <sup>k</sup>*

> *<sup>e</sup> e k* = ω

Parameter Symbol Value Unit Stator's outer diameter *De* 31.5 mm Stator's inner diameter *Di* 20 mm Rotor's outer diameter *de* 19 mm Active length *la* 12.5 mm Air gap length *t* 0.5 mm Number of pole pairs *p* 8 - Coil turns *N* 1200 - Permanent magnet induction *Br* 0.42 T Velocity constant *ke* 9.41 V/krpm

**Figure 6.** Results of the finite element simulations on the claw pole generator. a) Magnetic flux density

where *ω* is the generator rotational speed expressed in radians per second.The value of the

λ

<sup>∂</sup> <sup>=</sup> <sup>∂</sup> (6)

(7)

θ


The main idea was to develop each single subsystem in order to have more degree of freedom for each subsystem: active rectifiers to reduce energy losses and perform a Power Factor Corrector (PFC) regulation, independent DC/DC regulator with different Maximum Power Point Tracker (MPPT) according to the instantaneous situation, charge controller to ensure a good storage reduce as much as possible the memory effects in the battery, and a unique controller to manage properly all the interaction and the functions of these systems.

Due to the limited time for testing and to simplify the construction of the first prototype, an integrated solution shown in Fig. 9 is preferable to reduce cost and to obtain a suitable industrial solution. For these reason some solution from Linear Technologies (LT) turned out to be useful because they include into a single chip the DC/DC regulator, the charge controller and the main control system, therefore reducing cost and implementation time. Only few additional components have been selected and added to the integrated chip to obtain the final requested solution.

**Figure 8.** Architecture component interaction and interconnection.

**Figure 9.** Block diagram of the integrated solution with LT components

The electric generator produces a sine wave with an electric frequency proportional to the mechanical velocity of the hydraulic turbine *n* expressed in rpm with the following relation:

$$f\_e = \frac{m}{60}p\tag{8}$$

where *p* is the number of the pole pairs as reported in Tab. 7.

The easiest way to convert this sinusoidal voltage into continuous voltage is using a rectifier bridge; the ideal solution is based on active rectifier to obtain a voltage drop as lower as possible, but for this application, during the first tests, the results were not so different using a traditional passive rectifier instead. For this reason the first prototype was developed using a simple single phase rectifier composed by four Schottky diodes to reduce as much as possible the power losses. Using BAT54 diodes the voltage drop is about 250÷400 mV, equal to 500÷800 mV for each stage of conversion according to the current flow. A 10uF capacitor is enough to keep constant the voltage with a low output ripple (<1% of the maximum peak voltage).

The second stage is the regulator used to increase or reduce the input voltage to provide an output voltage around 3÷3.3 V to properly supply any kind of microprocessor or actuator. Using a buck-boost converter it is easy to satisfy this requirement but, due to the lower amount of energy generated by the turbine, it is necessary to implement also the MPPT algorithm. The Maximum Power Point Tracker is an algorithm normally used in photovoltaic cells; its role is to constantly check the input voltage and current to know exactly how much power is available and to limit the current absorption from the generator so as to keep always the condition of maximum production (see Fig. 10). To perform this operation it is necessary to use two different feedbacks: the first is used to check the output voltage to keep it constant with a low ripple; the second one checks the input current, limiting its absorption by varying the duty cycle of the DC/DC converter. In this way even if a very heavy load, alike a completely discharged battery, is connected, the converter can make the generator work in the most efficient condition. This algorithm needs to be included into the general control system alike an independent microprocessor or an integrated chip. The use of a microprocessor allows to modify these conversion algorithms without hardware changes, but only varying the internal control parameters.

478 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**Figure 8.** Architecture component interaction and interconnection.

**Figure 9.** Block diagram of the integrated solution with LT components

where *p* is the number of the pole pairs as reported in Tab. 7.

voltage).

The electric generator produces a sine wave with an electric frequency proportional to the mechanical velocity of the hydraulic turbine *n* expressed in rpm with the following relation:

60 *<sup>e</sup>*

The easiest way to convert this sinusoidal voltage into continuous voltage is using a rectifier bridge; the ideal solution is based on active rectifier to obtain a voltage drop as lower as possible, but for this application, during the first tests, the results were not so different using a traditional passive rectifier instead. For this reason the first prototype was developed using a simple single phase rectifier composed by four Schottky diodes to reduce as much as possible the power losses. Using BAT54 diodes the voltage drop is about 250÷400 mV, equal to 500÷800 mV for each stage of conversion according to the current flow. A 10uF capacitor is enough to keep constant the voltage with a low output ripple (<1% of the maximum peak

*<sup>n</sup> f p* <sup>=</sup> (8)

**Figure 10.** Current adjustment from MPPT to achieve maximum efficiency

In this system, the recovered energy can be used directly by the loads or can be partially stored for future use. The storage system can be divided in two main components based on different technologies:


If the load is composed by a combination of continuous small absorptions with occasional high requests of energy, it can be useful to combine the above storage technologies in order to reduce energy losses.

The first prototype was not developed using an independent control system and energy converter, but using an integrated solution were all the three elements are included into one single chip. Two different boards were developed using two chips from Linear Technologies. The first solution used the LTC3108, a buck-boost converter without MPPT algorithm but capable of converting input voltages lower than 200mV. This solution was adopted due to the extremely low power coming out from the generator in the first prototype developed. Increasing the generator production, it was possible to move to the LTC3105 regulator, that is capable of converting input voltages higher than 500mV till to 5V, including an internal MPPT control in order to adapt the load absorption according to the generator production.

**Figure 11.** DC/DC converter: LTC3108 controller (1), output voltage selectors (2), input terminal, switching inductance and filters (3), regulated output 2.3V÷5V (4), 5.25V storage battery or capacitor (5)

In the test board two different topologies were tested. The first one used an output voltage set to 2.2÷2.3V connected to a 1F 2.3V super-capacitor (see Fig. 11). This solution was adopted to supply a very light load like a microcontroller (PIC16F886) that run a simple code to only switch on and off one led. The second solution used an output voltage of 4V to supply directly the load and to recharge the 3.6V 220mA battery; specifically, when the converter was on, the remaining current that was not used by the load was employed to recharge the battery, whereas when the input generator was off, the OUT pin was disconnected from the converter so that all the energy required was provided by the battery. A custom external controller is necessary to ensure battery protection from deep discharges (< 3V) during switch-off period to prevent irreversible damage to the cells.
