design of the electrical power system for a rc-toy car

Bogdan DOROBANŢU
DESIGN OF THE ELECTRICAL
POWER SYSTEM FOR A RC-TOY CAR
Abstract: This paper presents the process of designing the electrical propulsion system for a radio
controlled toy car by analyzing the resistant forces and the necessary power to overcome these forces.
Keywords: design, propulsion, car, modeling, electrical motor.
1. INTRODUCTION
The systems of a toy car are as complicated as a real
electrical car because it has an electric motor, differential
and/or gearbor, control unit, batteries, transmitter and
receiver for control, power steering unit and suspension.
All these systems are interconnected because of the
voltage used by the electrical engine, ex: an electrical
motor that uses 7,4 V needs a battery to supply the 7,4 V
and a control unit to manage the power to the motor.
f[-]
2. ELECTRICAL MOTOR
To determine what electrical motor we need for the rc
car we must calculate the power necessary to overcome
the power losses by the forces that act on the car, like: air
drag, rolling resistance of the tires, transmission
efficiency, sloping rolling resistence (because all roads
have a slope, even if it’s very small).
Speed [km/h]
Fig. 1 Dependence of the rolling resistance factor
with the speed
2.2 Determing the maximum cross-sectional area of
the vehicle
2.1 Determing the rolling resistance of the tires
Rolling resistance depends on many factors like tire
design, speed, tire pressure, road or surface, weight.
The rolling factor or coefficient is determined by
experimental way based on the results obtained from
different empirical formulas, the simplest one based on
the speed of the car :
f= + ∙V + ∙ There are 2 ways to determine the maximum crosssectional area of the vehicle:
• Using the formula:
A= ∙ ∙ ∙ ∙ [m²]
(1)
•
where:
represents the rolling resistance factor at low speed •
[h/km] and [h2/km2] influence factors of the speed,
picked from standardized tables.
For calculating the rolling resistance we need to know
the tires of the car which are 26 mm in radius with a very
low section height.
And so for our kind of tire the rolling resistance
factors are stated below:
=1.6115∙ 10
=-9.9130∙ 10
[h/km]
=2.3214∙ 10 [h2/km2].
The chart (Fig.1) shows that the rolling resistance
values increases with the speed and so for high speed the
rolling resistance is higher than for low speed.
(2)
where:
=1 is a shape factor,
=0,14 m is the width of the vehicle (including mirrors)
- =0,07 m is the height,
-h =0,015 m is the ground clearence (height from the
road to the lowest part of the bumper) ,
- N =2 is the number of the tires on the back axle,
- B! =0,017 m is the tire section width.
A=1 ∙ 0,14 ∙ 0,07 0,015 2 ∙ 0,015 ∙ 0,017 '
=0,00821 m²
• Using the contour of the front view of the car
To calculate the cross-section area you can use
AutoCAD with the command “area” and it calculated for
our contour of the car an area of A=0,00754 m², value
that will be used to determine the wind resistance or
wind drag.
JOURNAL OF INDUSTRIAL DESIGN AND ENGINEERING GRAPHICS
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Design of the electrical propulsion system for a RC-toy car
Ga weight of the vehicle is expressed in daN;
3 is the slope angle of the moving vehicle.
To calculate the power required to overcome rolling
resistance is used the relation:
140mm
4-. '
70mm
5∙67 ∙89: ;< ∙5
=
(4)
>?@A
To calculate slope resistance we used the formula:
,B ' /C ∗ sin 3 [daN]
15mm
Fig. 2 Cross section of the car
Power required to overcome this resistance is calculated
as follows:
2.3 Determing the wind drag factor
Given that the shape of the car body is designed to be
very close to that of the real model, we can agree that
they have a value of air resistance coefficient close to
that of the model, and so : C) =0,25.
This value was chosen in the range of [0.25, 0.35]
taking into account both its value of the similar model,
and the value of the cross-section area.
2.4 Determing the transmission efficiency
Power developed by the engine is transmitted to the
wheels through the transmission to propel the vehicle.
This phenomenon always occurs frictional losses in the
transmission, losses are characterized by η* - transmission
efficiency. For a car with a main cone transmission and
fixed ratio transmission is adopted a value of η* =0,97
from the interval (η=0,96 – 0,98).
4 '
VOLUME 8 │ ISSUE 1 │ JUNE 2013
(6)
>?@A
, '
J∙K∙5L M
=
(7)
>NCA
where:
-Ra is air resistance, expressed in daN;
-k=0,06125·Cx , is the drag coefficient;
-A is the cross-sectional area of the vehicle;
Vx=V+Vv [km/h] is the vehicle speed relative to the air.
where: - V is the vehicle speed,
- Vv is the wind speed. In this case it is considered
Vv=0 km/h
Power required to overcome air resistance can be
calculated with the formula:
J∙K∙5L M ∙5
O
P
(8)
>?@A
In Fig 3 are centralized all powers necessary to
overcome the resistance values and their calculated when
the vehicle is moving, no wind, on tier.
0,03
Prul
P [KW]
0,03
0,02
Pa
0,02
∑P
0,01
0,01
0,00
0,00
0
50
100
V[km/h]
Rolling resistance using the formula:
14
=
4 '
Forward resistance forces are the forces resulting
from the interaction of the vehicle with the environment,
as opposed to its advancing.
There are several types of resistance : rolling
resistance, air resistance and slope resistance. Inertial
forces appearing in moving vehicles are considered a
drag resistance and is called resistance start or resistance
acceleration. In the following calculations it does not
appear in the balance of power to the wheel as it
considers a motion evenly (without acceleration) and
acceleration at maximum speed is 0.
We will calculate the drag resistance for two
situations:
- tier movement (αp=0)
- slope movement pmax=8% with αp=arctg(0,08)=4,57⁰;
To calculate these forces, were used the relantions
presented below:
where :
,-. is the rolling resistance;
f is the rolling resistance coefficient;
67 ∙GHI ;< ∙5
Air resistance is calculated by the following formula:
2.5 Determing the power loss of the vehicle
,-. ' ∙ / ∙ cos 3 [daN]
(5)
(3)
Fig. 3 Resistance powers an tier.
In Fig 4 are centralized all powers necessary to
overcome the resistance values and their calculated when
the vehicle is moving, no wind, on a slope.
P [KW]
Design of the electrical propulsion system for a RC-toy car
0,05
0,05
0,04
0,04
0,03
0,03
0,02
0,02
0,01
0,01
0,00
Prul
Pa
Pp
∑P
0
50
V[km/h]
100
Fig. 4 Resistance powers on a slope
Using the formulas presented we calculated that the
power required to overcome rolling resistance and air
resistance for the car is 0,044 KW to achieve 100 km/h.
Total power developed by the electric motor must be
greater than that calculated to account for power losses in
transmission (ηt = 0.97). [1][2]
4IQR '
S
ηT
'
,OO
,U
' 0.045?@
pack it can get up to 40mph with just the standard
gearing. With up to 8 NiMH cells or a 3S LiPo (11.1v)
pack and higher gearing (using a smaller spur gear and
larger pinion), it can get up to 60mph, perfect for
massive track or insane speed runs. When geared
correctly, the Warp 5700 is equivalent to a high quality
9-turn or 10-turn brushed motor using 6 NiMH cells - a
very powerful motor.
The Warp 5700 features:
-powerful, high-speed brushless motor
-excellent pairing of huge power and extraordinary
efficiency
-same size as standard 540-size motor
-sleek black-anodized aluminium casing
-zero maintenance design
-external solder tabs for easy wire replacement
-oversized precision ball bearings for long life
-high Torque/ High Temperature Neodymium rotor
-slotless stator design delivers smooth, linear torque
-easy 4-point mounting for convenient installation
-longer run times than a comparable brushed motor
-easily rebuildable with bearings and rotor being userreplaceable
-compatible with any sensorless ESC
(9)
2.6 Chosing the electric motor
The electric motor needs to have more power than it
is needed to propel the car. The most important
parameters for these motors are Kvs, volts and rpm.
KV as we use it refers to the rpm constant of a motor
- it is the number of revolutions per minute that the motor
will turn when 1V (one Volt) is applied with no load
attached to the motor. In summary, we call it revs per
volt - but do not think you will obtain those revs when
you attach a gearwheel; obviously the revs will be
reduced because of the load.
What does the KV tell us? Well it is related to the
power out from a motor, or more usefully the torque
level of a motor. It is determined by the number of winds
on the armature (or turns) and the strength of the
magnets. So KV allows us to get a handle on the torque
we
can
expect
from
a
particular
motor.
In summary, a low KV motor has more winds of thinner
wire - it will carry more volts at less amps, produce
higher torque and swing a bigger gearwheel. That may
sound confusing, but compare it with a high KV motor
which has less winds of thicker wire which will carry
more amps at less volts and spin a smaller gearwheel at
high revs.
For example, a motor of Kv, 5,700 RPM/V, supplied
with 11.1 V, will run at a nominal 63,270 RPM
(5,700 RPM/V × 11.1 V).
The Flux Warp 5700Kv Brushless Motor seems to be
a good choice for this project. The Warp 5700 is the top
choice for nearly all 1/10th scale electric touring car,
buggy and trucks. It can go super-fast on the track and
get those blazing speed runs with the same motor! With a
standard 6-cell NiMH battery pack or 2S LiPo (7.4 volt)
Fig. 5 Flux Warp electric motor
The Flux Brushless System is HPI's answer to
hobbyists and racers who want a powerful, versatile and
affordable brushless motor system. The Warp motors are
extremely powerful, very durable and highly efficient to
get you going on the road to victory. HPI Warp motors
are sensorless type motors, so there are fewer wires to
worry about, and less hassle for you.
Flux Warp motors must be used with a brushless
electronic speed control that is suitable for sensorless
motors. [3]
3. BATTERIES
For this kind of motor it is used the new Plazma
battery pack (47,4 Wh):
-voltage: 7.4V
-capacity: 6500mAh
-current rating: 95C
-case type: rectangular LiPo case, sealed plastic
-dimensions: 138.6 x 45.8 x 25.1mm 25.1mm)
-plug:DeansUltraPlug for extreme high-power
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Design of the electrical propulsion system for a RC-toy car
PlazmaPro competition range features two high-end
packs designed purely for use on the race track! This
pack fits within all accepted maximum pack sizes,
allowing it to fit in any competition touring car, buggy,
short course truck and other types of racing car. The 95C
power rating will give plenty of acceleration while the
6500mAh rating will allow to fully utilise the power of
the pack throughout a 5-minute race. It also features a
real Dean's connector and a universal balancing plug for
simple maintenance.[3]
Fig. 8 Electronic speed controller
5. CONCLUSION
Fig. 6 Battery pack
4. ELECTRONIC SPEED CONTROLLER
Engine control system is called ESC (Electronic
Speed Controller) and its role is to change the speed of
the electric motor, change the direction of rotation or to
serve as a dynamic brake by using the electric motor as a
generator.
Through a process called PWM (Pulse Width
Modulation Figure 7), the controller varies the speed by
changing the motor voltage very quick simulating a
medium voltage. This switching frequency can reach up
to 20 kHz.
Analyzing structural variations of similar patterns and
trends in the field it is found that the rc-car will be
designed to run on flat surfaces, having independent
suspension with oil shocks and springs.
To overcome the encountered resistances like: rolling
resistance, air resistance, slope resistance and the power
loses from the transmission resulted from calculations
that the car needs a electric motor capable to deliver 45W
to achieve the speed of 100 km/h.
This speed can be delivered by the Flux Warp
5700Kv brushless motor in conjunction with a 3S LiPo
(11.1v) battery pack and a higher gearing transmission.
But for this project I chosen a smaller battery pack, the
PlazmaPro that delivers 6,5 Ah at 7,4 V and 47,4 Wh.
To control the engine speed will be used a type ESC
Pulse With Modulation. The power will be transmitted
from the engine to the wheels through a gear, differential
and drive shafts.
Fig. 9 Chassis modeling in CATIA V5
Fig. 7 PWM
The Flux Blur speedo is the most powerful brushless
speed controller we could find. With the ability to handle
twin 3S LiPo batteries (that's a total of 6S LiPo power! it
can deal with anything powerful brushless motors.
The 6mm bullet connectors secure the motor wires
for loss-free connection to the 12-gauge wires, and to
connect to each battery pack has attached real Dean's
connectors for zero power loss and total efficiency.
Topping off the versatility of the Flux Blur speedo is an
integrated cooling fan, which allows the speedo to
operate at its ideal temperature.[3]
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VOLUME 8 │ ISSUE 1 │ JUNE 2013
REFERENCES
[1] C. Andreescu – “Vehicle dinamics”
[2] G. Danciu –“Electric and hibrid vehicles”
[3] www.hpiracing.com
Author: Bogdan DOROBANTU, Master Student,
University Politehnica of Bucharest, Department of
Engineering Graphics and Industrial Design
E-mail: [email protected]