**Abstract**

Electric vehicles are often designed in the same way as their conventional counterparts based on the internal combustion engine, they are heavy machines for comfort and safety reasons, and increasingly powerful. Under these conditions, in order to simplify the motor electrical supply system by reducing the current levels, the voltage chosen for the battery is very high and can go up to 700 V. However, for many applications where the power is relatively low (< 30 kW per motor), it can be more beneficial to size the system at very low voltage (< 60 V). This approach allows to overcome many constraining safety requirements and also to use off-theshelf components (motor controllers, connectors, etc.) that are more easily available on the market in this voltage range. There are also many regulatory provisions that may require to stay within this voltage limit. This article presents a variety of very low voltage motorisation solutions with a required power up to 100kW. They use two complementary approaches. The first is to implement an original permanent magnet synchronous machine technology with an optimised armature winding for low voltage operation. The second is based on power splitting where the electrical machine being designed to be driven by multiple controllers. Many examples of low-voltage motorised vehicles (sporty vehicle, tractor, re-motorised automobile, etc.) are illustrated in this article.

**Keywords:** Electric Vehicle, Very Low Voltage, Synchronous Machine, Permanent Magnet, Solid Bar Winding, Power Splitting

### **1. Introduction**

For the sake of safety, the standards and regulations limit the supply voltage level for the electric vehicles drive system. The standards dealing with the very low voltage systems (VLV) provide a general guidance. For example, the 2014/35/UE European Directive for the CE marking fixes the voltage level at 75 V DC, hence, in practice this could be the choice for the battery voltage.

Furthermore, in the automotive realm, the European Regulation R100 concerning the approval of vehicles with regard to specific requirements for the electric powertrain, has even reduced the maximum voltage level to 60 V (Class A). This range of voltage will therefore ease the design constraints and the operational maintenance of the vehicle. The 60 V is generally considered as the reference level of VLV for electric vehicles.

At this low voltage range, there is a large choice of commercially available offthe-shelf components for the power electronics needed to drive the electric motor, regardless its technology, DC motor, synchronous motor or induction motor, though knowing that the permanent magnet synchronous motors are emerging as one of the best candidate to dominate the market of powertrain electrification.

However, at low voltage, when the level of the required electrical power reaches a certain threshold, which is around 30 kW, the availability of the power electronics components becomes considerably limited given the high current level to be handled by the controller, which is greater than 500A at a battery voltage of 60 V. Indeed, this poses very challenging constraints on the design of the power modules where the high current gets closer to the switching capability limit of the transistors available on the market (very low voltage MOS technology). We will be detailing the challenges and the associated solutions related to this topic in a later section of the chapter.

The technology outlined in this chapter, where many validation prototypes are presented, brings some original solutions to the design of very low voltage electric powertrains, even at high power level. Many electric vehicles presented in this chapter involving a power as high as 100 kW.

First of all, we will discuss the design techniques of an electric motor being optimised to operate at very low voltage. Afterwards, several techniques of power distribution have been described, which enables the required total electrical power to be shared between several controllers. Finally, we present an overview of the limits of feasibility of the power electronics that would be required to drive electric motors at very low voltage, based on the current available technologies of the semiconductors components.

## **2. Very low voltage electric motor**

#### **2.1 Solid bar winding**

When an electric motor is operating at very low voltage, there is an opportunity to optimise its winding in order to significantly enhance its performance. Conventionally, the windings of electric motors are based on an enamelled round wire (loose random conductors), as illustrated in the **Figure 1b**. In this case, the copper

**Figure 1.** *(a) Solid bar winding vs. (b) Round wire winding.*

*High Power Very Low Voltage Electric Motor for Electric Vehicle DOI: http://dx.doi.org/10.5772/intechopen.99134*

fill factor inside the stator slot is very poor, where, unless relying on nonconventional manufacturing processes (segmentation, etc.), only around 40% can be achieved in the best case (pure copper CSA/naked slot area), it can be even less than 30% when considering very small size motors with tiny slots.

At very low voltage, the conductors inside the slot are connected in parallel where the number of turns is inherently very low. In the case of a winding design with one turn per slot, which is often the case at VLV, it appears to be more judicious to replace the multi-strand conductor with a single solid copper bar adjusted to the slot dimensions, as illustrated in the **Figure 1a**. In the latter case, the copper fill factor inside the slot can reach approximately 80%, which consequently doubles, even triples, the copper volume for a given motor size.

At a constant copper loss and a given slot cross sectional area, the relationship between the RMS current, Ib, in the solid bar conductor and the total RMS current, If, in the equivalent slot wound with multi-strand round conductor is as follows:

$$\mathbf{I\_b = I\_f.} \left(\sigma\_{\rm rb} / \sigma\_{\rm rf}\right)^{1/2} \tag{1}$$

The coefficients σrb and σrf represent the copper fill factor inside the slot with solid bar conductor and with multi-strand round conductor, respectively. With the 80% fill factor in the first case and 35% in the second one, the current carried by the solid bar conductor is 50% higher, and, consequently, the output torque of the motor increases in the same proportion.

The **Figure 2** illustrates how difficult it is to perform a high quality winding with loose round wire. It can be easily noticed that a non-negligible part of the copper is located outside the active part of the motor (i.e. stator). This bulky copper outside the stator slots increases the volume, the weight and the loss of the machine. All these drawbacks are addressed with the use of a solid bar conductor.

**Figure 2.** *Electric motor end-windings wound with loose round wires.*

**Figure 3.** *Solid bar winding, distributed winding.*

#### **Figure 4.** *Solid bar winding, wave concentrated winding.*

**Figures 3** and **4** illustrate some of our products made using a solid bar winding. It can be easily seen that the useless copper at the end-windings (overshooting the stator core pack) is less bulky and well controlled. These proposed winding techniques are most convenient for low voltage electrical machines.

The distributed winding shown in **Figure 3**, with one slot per pole and per phase, is well suited to medium range power machines (a few tens of kW) operating at few hundreds Hz electric frequency [1–3]. The structure shown in **Figure 4** is more original where the phases are wound around the tooth (wave concentered winding) and grouped in separate sectors [1, 4, 5], without phase overlaps at the end-windings of the machine. This structure is rather well suited for small electrical machines which can then operate at very high frequency (up to 2000 Hz), the resulting winding is very compact.

This technique is not commonly used in practice due to the fact that the solid bars are prone to very high AC copper loss (under alternating regime) which can be much higher than the DC ohmic loss.

Additional losses in massive conductors can be prohibitive, but a detailed study of these phenomena [1, 4] shows that the advantages of the approach largely outweigh the disadvantages if the winding is appropriately designed [1, 3, 4].

**Figure 5.** *Hairpin winding (courtesy of special machine tool company).*

Paradoxically, the concept can be perfectly applied, as we will see, to high pole count electric motors operating at high frequency, which is the case for all machines with high power density for embedded applications.

Many industrial motor manufacturers, especially for electric vehicles, are using the solid bar copper winding, in particular via the "hairpin" technique consisting in a "pin" forming that can ease the overlapping of conductors at the end-windings (cf. **Figure 5**), but the overall design approach of these machines remains conventional, especially because it uses several conductors per slot. The approach presented in this chapter is distinguished by the use of a single solid bar per slot (one turn per slot), which allows to optimise many parameters and to reach unmatched level of compactness, for high power electric motors operating at very low voltage.

In summary, the main pros in using solid bars are:


And the main cons are:

