Electric Power Conversion and Distribution

Abstract

The electrical system connects the electric power sources to the high- and low-voltage loads. It is shaped by the current and voltage demands of the various loads, the requirements for grounding, switching, and electrical overload protection imposed by good practice and race rules, the need for low weight, low power loss, reliability, rapid maintenance and repair, and low cost.

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6.1 Introduction

The electrical system connects the electric power sources to the high- and low-voltage loads. It is shaped by the current and voltage demands of the various loads, the requirements for grounding , switching, and electrical overload protection imposed by good practice and race rules, the need for low weight, low power loss, reliability, rapid maintenance and repair, and low cost.

The rules of solar-electric vehicle races differ. Therefore, to avoid having to consider many cases, the 2001 American Solar Challenge (ASC) rules (Chap. 16) will be applied herein. However, as previously mentioned, be sure to consult the current rules for the race you are considering.

6.2 Power Supply

Under ASC rules, once the race begins, the solar cell array and the battery are the only sources of power allowed, with three exceptions. Power for the radio, electronic panel meters, and telemetry equipment, etc., may come from “supplemental, replaceable” batteries.¹

6.3 Loads

Typical Loads

Table 6.1 lists the typical electric loads of a solar car electric system. The ventilation could be supplied by a single fan, as was done for the Sunraycer (MacCready et al. 1990). Also given are representative quantities for each kind of load and whether the load operates continuously (C) or intermittently (I).

Table 6.1 Electric loads

No headlights are shown in Table 6.1 because only daylight operation is required during a race.² All the loads must operate from the direct current (DC), otherwise an inverter , which converts DC to AC, must be employed. Inverters consume power and add mass.

Supplemental Loads

Powering the radio, some panel meters, and the telemetry equipment from the main storage batteries will reduce the energy available for propulsion and increase the mass of the car by the mass of the connecting wiring. Alternatively, powering these devices from supplemental batteries will increase the mass of the car by the mass of the supplemental batteries. The mass involved is not large (except possibly that of the telemetry equipment battery). Nevertheless, every kilogram increase should be thought through to see if it gives a net advantage. The trade-off question is: Will the reduction in range from powering the equipment from the main battery be greater than that from carrying the extra mass of supplemental batteries?

6.4 Basic Interconnections

Block Diagram

Figure 6.1 shows a simplified block diagram of a typical electrical system. The battery and solar array are connected in parallel with the motor controller so that either or both of these power sources can supply power to the motor controller, and excess power can be sent to the battery. The voltage of this main bus or battery bus is therefore applied equally to the solar array, the battery, and the motor controller.

Fig. 6.1

Electric system block diagram

Low-Voltage Loads

Fig. 6.1 shows that all the loads in the electrical system do not require the same voltage. Generally, there is a low-voltage subsystem (the “low-voltage taps,” typically 12 V, of Fig. 6.1) that powers loads such as, the turn signals, backup lights, horn , and the cockpit-ventilation fan. The DC–DC converters shown in Fig. 6.2 reduce the main bus voltage to the lower voltage required. They feed two branch circuits .

Fig. 6.2

Power system schematic

6.5 Efficiency and Voltage

Efficiency

The low power available to the solar racer makes it essential to pay close attention to minimizing electric power losses. Power losses in wiring are proportional to the product of the square of the current and the electric resistance (or to the product of the voltage drop over the wire and the current). For a fixed resistance, halving the current reduces this loss by a factor of four. Motor operation and battery charge and discharge are also more efficient at low current and high voltage. Therefore, the rule is to operate the electrical system at high voltage and low current.

Design Main Bus Voltage

According to the thumb rule discussed above, the design main bus voltage should be as high as practical.³ (“Design value” means the value of the bus voltage under specified design conditions, such as those of Chap. 16). Beside meeting this general goal, the design value must be an integral multiple of the battery module voltage, be within the allowable input-voltage range of the motor controller, and be within the allowable input-voltage range of the DC–DC converters supplying the low-voltage subsystem.

If the MPPTs are boost regulators , then the goal should be to have the array voltage always⁴ below the main bus voltage. For buck regulators , it should always be above the main bus voltage. The main bus voltage thus also influences the number of solar cells in the series-connected strings of the solar array .

6.6 Mass

Target

Chapter 11, Solar Racer: Construction, advises that the mass specified for the car be apportioned among the car’s systems. Thus the electrical system should have a mass target, most of which will be consumed by the batteries. Nevertheless, every opportunity to reduce the mass should be exploited; many small reductions add up to a big reduction. And a solar car has many small parts.

Current

Low current demand, in addition to improving efficiency, allows smaller wires, smaller fuses , and switches , which reduce the mass of the electric system. Wire size is selected by the amount of current to be carried, larger currents requiring larger wire, and mass is directly proportional to wire size. Size is indicated by the wire’s American wire gauge (AWG) size, with large AWG meaning small wire. The data in Table 6.2 for solid, bare copper wire show how weight and length are related (ARRL (1973)).

Table 6.2 Bare copper wire characteristics

Note that for a given length, decreasing the resistance by reducing the AWG increases the mass. However, the layout of the electrical system controls the length of the wire, and both the mass and the resistance are directly proportional to the length. Careful attention to the layout can reduce both mass and power loss by keeping wire runs short.

6.7 Wiring

Sizing Wire

The size of the wire affects not only the efficiency, but also safety . Undersized wire can overheat, resulting in damaged insulation, short circuits, or fires. The maximum possible current allowed in a particular wire (the ampacity) depends upon the wire size, the kind of insulation on the wire, and whether the wire is a single conductor in free air (better cooling), or bundled in a conduit or cable (poorer cooling). For example, Table 6.3, adapted from the National Electric Code (NEC) and presented in McCarney et al. (1987), shows that 14 AWG wire has an ampacity of 15 A when in a conduit or cable and covered with thermoplastic insulation, but has an ampacity of 20 A when in free air with the same insulation .

Table 6.3 NEC copper wire ampacity

Sizing Rule

A thumb rule recommended by McCarney et al. (1987) to size the wire is to allow a maximum of 2 % voltage drop in branch circuits (fed from load centers) and an overall maximum of 5 % voltage drop from the power source to the load (applied to runs between the array, the battery, or the motor and the main bus) . Smaller limits may be required by manufacturer’s instructions. Wider limits may be allowed, too. For example, the allowable input-voltage range to a commercial DC–DC converter producing 12 V might be 100–200 V, or the allowable input-voltage range to a motor controller might be 65–130 V .

For branch circuits, using the wire length to the farthest load and the peak current may result in too much mass. If so, divide the branch into segments and size the wire in each segment according to its length and peak current.

Wire Types

Copper is the preferred metal for use in wires because of its low electrical resistance. Wires may be solid or braided . Braided wires are more flexible and therefore, are preferred for large sizes such as those used for battery module interconnections. Wiring in solar electric cars must always be insulated. The insulation should be color coded to show the wire’s service assignment. As Table 6.3 implies, the type of insulation must be chosen for the temperature range and whether dry or wet conditions prevail. Summer operation dictates a high specified design temperature, at least 50 °C. Moisture inside the car should be expected .

Connections

To promote rapid maintenance and repair, modular construction and quick-release electrical connectors, and tie-downs should be employed, when possible. This allows rapid replacement of malfunctioning modules.

6.8 Switches and Fuses

Switches

Switches are used to connect power sources to the main bus, and to turn loads on and off. They are rated for a particular voltage, current type, and amount of current. Current type matters because DC current tends to arc (jump) across the contacts of a switch as the switch opens.⁵ Switches intended to interrupt DC current are designed for this. If the switch is not properly rated, it may burn out from repeated arcing, or in heavy current applications the contacts may weld together (McCarney et al. 1987) .

Fuses

Fuses are the means of overcurrent protection required by race rules; circuit breakers are not allowed. Fuses should always be placed in the positive wire; the grounded side of a circuit should never be switched or fused. If a fuse was placed in the negative wire, an overcurrent condition causing the fuse to open the circuit would disconnect the load from ground, not from the voltage supply. Thus, a person touching the load could be electrically shocked.

6.9 Grounding

Motivation and Definition

Grounding helps to prevent electric shock to people working on or touching the body or components of the solar car. In the case of vehicles, it does not refer to an actual connection to the earth. Instead, it means that a common point is provided in the electric system where the negative (return circuit) wires of the power and instrumentation circuits are electrically tied together. This point will be the common voltage reference for the car, the zero voltage point.

Grounded Frame

Sometimes solar cars are constructed using a “space frame” of metal tubing inside a streamlined, composite shell. The frame should be connected to the electrical ground point. Otherwise there will be an electric potential between the frame and any conductor in the car, even between the frame and the ground point. Also, an ungrounded frame can accumulate an electric charge, which when discharged can damage sensitive electrical devices. The frame should be connected to ground at only one location to prevent current flow between multiple ground points (ground loops) .

When the frame is connected to ground, electric shock hazard exists only at high-voltage points, such as the battery and solar array terminals or the main bus. Race rules require high voltage warning signs at such dangerous locations.

Electrical Noise

Electrical noise interferes with radio voice communications because it is picked up by radio receivers along with the transmitted voice signal, causing the voice to be less intelligible. Sources of electrical noise are sparking from propulsion motor and ventilation fan commutator brushes (if present), operating turn signals, and wheel and tire static. ARRL (1973) discusses solutions for these problems. The “receiver” for the electrical noise is the wire on board a solar car, which may be of considerable length. Consider using electrically-shielded wire with the shielding grounded. As usual, this must be balanced against the additional mass added to the car.

6.10 Wiring Diagram

Figure. 6.2 ⁶ shows one way of connecting the power sources to the loads. The figure shows no instrumentation to measure the voltage and current in different parts of the system. Instrumentation will be added in Chap. 7, Instrumentation. The two power sources, the motor controller, and Auxiliary Bus 2 incorporate a fuse and a disconnect switch in the positive wire. Auxiliary Bus 1, which powers the battery fan, has no disconnect switch nor fuse but is energized whenever the battery or the solar array is connected to the main bus, as required by race rules.

6.11 Example 6.1

Size the wire, using the sizing thumb rule given above, fuses , DC–DC convertors, and circuit breakers for the solar car electric system of Fig. 6.2. Estimate the weight of the wiring. Add allowances for insulation, connectors, and uncertainty in wire run length.

Loads (main bus at 120 V):

• 1. A 2-hp DC motor with an overload capability of 4 hp for 15 min (30-A overload).

• 2. 12 V, 1 A for the electronics, motor controller, and battery fan.

• 3. 12 V, 9.25 A for the turn signals, running lights, cockpit fun, and horn.

Power sources:

• 1. Solar cell array giving 9 A at 120 V, (under standard testing conditions).

• 2. 20, series-connected, 6-V, Ag–Zn batteries.

Solution

Figure 6.3 shows the approximate distances for the wire runs and the locations of the components. The main bus is a terminal strip housed in a waterproof, plastic junction box.

Fig. 6.3

Wire runs for example 6.1

The motor controller is located near the motor in the rear of the car close to the driven wheel. The two DC–DC converter connectors are near the main bus box. The battery box is mounted just forward of the main bus box.

The data in Table 6.1 were used to size the wires. The codes in the table of results, below, are those next to the component names in Fig. 6.2. Somewhat larger wires were chosen to allow for uncertainties in wire run lengths.

Wire run	Max. volts	Max. amps	One-way length (f)	Loss (%)	AWG	Type	Wte. (lb)
A-MPPT	120	3	1	2	18	Solid	0.01
MPPT-MB	120	3	2.5	2	18	Solid	0.05
B-MB	120	35	2	2	14	Braid	0.05
MB-MC	120	30	12	5	12	Braid	0.48
MB-AB1	120	1	1	2	18	Solid	0.01
AB1-1	12	0.25	12	2	22	Solid	0.03
AB1-2	12	0.15	2	2	22	Solid	0.005
AB1-3	12	0.2	2	2	22	Solid	0.005
AB1-4	12	0.2	2	2	22	Solid	0.005
AB1-5	12	0.2	2	2	22	Solid	0.005
MB-AB2	120	2	1	2	18	Solid	0.01
AB2-6	12	5	5	2	16	Solid	0.08
AB2-7	12	1.0	10	2	18	Solid	0.1
AB2-8	12	1.0	10	2	18	Solid	0.1
AB2-9	12	1.0	12	2	18	Solid	0.12
AB2-10	12	1.0	12	2	18	Solid	0.12
AB2-11	12	0.25	5	2	22	Solid	0.012
B-B	120	35	15 × 0.33 ft	Small	2	Braid	1.005
						Total	2.20

The calculation for branch AB2-9 was as follows. The maximum two-way length for a particular AWG and peak current should yield less than the specified percentage voltage drop

$$ \frac{\Delta V}{V}>\frac{{{I}_{peak}}{R}'\,2L}{V} $$

(6.1)

where R′ is the resistance per foot from Table 6.1, L is the one-way wire run length, and ΔV/V is the fractional voltage drop allowed over 2 L. Substituting the numbers for the example branch,

$$ 0.02>\frac{1A\times 6.510({{10}^{-3}})\frac{\Omega }{ft}\times 2L}{12V} $$

or, L < 18.4 ft. Because the run is 12 ft, this gives a 6.4 ft, one-way allowance for run length uncertainty, using #18 wire.

The battery interconnects (B–B) were made much larger than necessary, following the advice of McCarney et al. (1987).

Allowing 20 % for connectors and insulation brings the total from 2.20 to 2.64 lbf.

All switches and fuses are sized for 150 % of the peak current and the relevant DC voltage. This will satisfy the sample race rules which restrict the battery fuse to not more than 200 % of the maximum expected current draw. Thus, in our example, the battery fuse must be rated for 53 amps. S₂, the battery disconnect, must be rated for at least this current.

The DC–DC converter for Auxiliary Bus 1 must supply 12 V at 1 A, or 12 W. The converter for Auxiliary Bus 2 must supply 12 V at 9.25 A, or 111 W. The manufacturerʼs data sheets show that the converters will weigh about 0.5 lbf each, bringing the system weight to 3.64 lbf.

6.12 Final Thought

When planning the wiring of the car, viewing the actual interior space may inspire ideas for improving the layout, that is, reducing the wiring weight and power loss. A vehicle mock-up, in addition to the uses suggested in Chap. 11, is very handy for planning wiring layouts.