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Speed
Traps for the Beginner
by Mark E. Hazen
Are
you all excited about starting your EV conversion? Sure - so was
I. I've been involved helping beginners with their EV conversion
for a long time now. Along the way, I've gathered some pretty good
insights into the 'speed traps' that beginners get caught in. Speed
traps are those details in the building process and use of your vehicle
about which you are unaware - and before you know it, they've got
you! So, here I am flashing my lights, warning you to slow down
and learn. Take time to ask questions and assume nothing.
The following speed traps are not presented in any special order.
They are all important. Speed
Trap #1: Controller Installation Today's
pulse-width modulation (PWM) power controllers for electric vehicle
conversions are built well, but are not impossible to destroy.
It's important to slow down and learn about how to properly and safely
install the controller of your choice. Reverse
Polarity Blowout One
of the problems beginners run into is moving too fast to install the
controller and accidentally connecting it with the cables on the wrong
terminal. DC controllers have 3 main high-current terminals as
shown in the diagram below: (see
also the Wiring
page)
-
+B,
positive battery terminal (on the controller) to which the positive
cable coming from the heavy-duty contactor is connected
-
-B,
negative battery terminal (on the controller) to which the negative
cable coming from the negative terminal of the high-voltage battery
bank is connected
-
-M,
negative-side motor feed to which a short cable from the motor is
connected
If
the heavy cables are connected incorrectly to these terminals, the
controller will blow as soon as the heavy-duty contactor closes for the
first time. When a beginner gets on the blogs and talks about how
his controller blew with pieces of capacitors all over the place, I know
right away that he got in a hurry and connected the cables
incorrectly. Of course, he blames the manufacturer of the
controller. Nice!
Miles
of Separation
Another
problem beginners run into is assuming that there are other ways the
controller can be installed besides what is shown in the manual.
Controllers must be installed within a couple feet of the motor
to minimize voltage spikes induced in the cabling between the motor and
controller. These voltage spikes can destroy the controller.
A beginner that I know decided to mount his controller on the other end
of the vehicle from the motor, interconnecting the two with long
cables. Naturally, destruction followed.
I
don't want the positive cable connected there!
All
DC controller manuals, and any other wiring diagrams shown on the
Internet, clearly show that the positive cable from the heavy-duty
contactor must be connect directly to the +B terminal on the controller
ALONG WITH the positive cable that goes from controller to motor.
Some beginners assume this is optional and connect the positive cable
from the heavy-duty contactor directly to the motor, then a cable from
that motor terminal back to the controller. This is a very bad
mistake because now the controller is seeing a widely varying positive
voltage on its +B terminal caused by rapidly changing motor polarity,
which is an involved topic in itself.
Installation
Notes: (1) Some
people use a much smaller 250 A fuse instead of a 400 A or 600 A
fuse. That's fine as long as you don't experience nuisance
blowing. Basically, a fuse responds to average current,
which is usually much less than 250 A. (2)
The heavy-duty circuit breaker should be closer to the batteries
than it is to the contactor to protect more of the
cabling. I have mine behind the driver's seat where I can
still reach it. (3) The
heavy-duty contactor must be enclosed to keep dust out.
Dust will coat the moving linear armature and may impede its
free movement. Dust will also prevent solid closure of the
contacts. Gravity helps pull the contacts open when the
contactor is mounted vertically as shown. Some folks add a
second contactor at the negative terminal of the battery bank
for added safety in case of an accident - but, if a short does
occur, the fuse will blow regardless of a second contactor. (4)
All heavy cabling must be
installed so its insulation does not wear through and cause a
short. It should also be 'run' where it is less likely to be
pinched in an accident.
Speed
Trap #2: Motor, Controller and Batteries Abuse
Beginners
read things on the blogs and have no way of knowing if the information
is accurate or wise. Everyday, bloggers talk about connecting the
motor directly to the differential without using a transmission.
This is something like a super golf cart configuration. The
beginner thinks, 'Wow, this sounds good because I can eliminate the
transmission, which probably wastes power - and it's simpler!"
There
are many things here that the beginner doesn't realize. First, knowledgeable
people who do this, understand that the differential gear ratio must be
changed to be in the range of 1:6 to 1:5 to give the motor 'some'
mechanical advantage. Otherwise, the motor is working too hard to
accelerate the vehicle. Golf carts have similar gear ratios. Also,
the practice of 'direct drive' is more prevalent in racing applications
in which components are beefed-up and cost much more than street
components. Racers spend time on blogs discussing topics that are
totally out of focus for the beginner who's wanting to convert a street
vehicle for practical transportation, adding to the beginner's confusion.
Motor
Strain
If
the motor is not able to accelerate quickly without a lot of laboring,
the life of the motor will be shortened. Heavy mechanical loading of the
motor causes much higher currents to flow through the motor and its
brushes. This causes both motor windings and brushes to heat more
quickly to a higher temperature. As a result, the life of the
brushes and the windings insulation is shortened.
Controller
Strain
In
addition, the controller is faced with this exceptionally high current
level. The controller must handle a higher average current over
longer periods of time. This causes the controller to heat up more
quickly and to a higher temperature. At some point, the controller
may fail because the internal MOSFETs or IGBTs can't handle as much
current at higher temperatures. This can occur even if the
controller has over-temperature cut-out. Also, if the controller
does not have excellent peak current limiting, the high peak
motor currents can destroy the controller. Peak motor
current can be higher than 10,000 A (amps) for very brief
durations during the first part of acceleration and pedal
travel. (For more on this, see Electric
Vehicle Controller Operation.)
Another
type of controller strain is poor ventilation around the
controller and lack of heat sinking to allow the controller to
get rid of its heat. Beginners often overlook this and
it's very important. Air-cooled controllers require air
and lots of it. They also require a heatsink to spread the
heat out, exposing it to cooler air. The larger the
heatsink, the better. Spreading the heat out over a larger
surface lowers the temperature. Even mounting the
controller on a 1' by 1' aluminum plate that is 1/4" thick
is a big help when there is good airflow, although an actual
finned heatsink is better.
Battery
Strain
The
batteries also suffer when the motor is being overloaded. The chemical
reaction in the batteries is producing this high current with increased
deterioration of the battery plates. Also, the constantly high
current can cause shorting in battery cells and terminal meltdowns.
This
overloading of the motor also happens when an EV owner, who's vehicle
does have a transmission, decides not to use the 'low' gears and always
starts out in third gear or higher. Not good!
All
of the above adds up to high costs for the beginner and a lot of
discouragement.
Speed
Trap #3: Over-revving
If
direct drive is used, as discussed above, and a reasonable gear ratio in
the differential is chosen to allow for rapid acceleration and minimal
motor loading, there is still a major problem that will turn out to be a
teaching moment for the beginner - over-revving.
Without
a Transmission - Direct Drive
Over-revving
means that the motor is forced to operate at speeds (RPMs) that are above the maximum rating for the motor.
If adhered to, this maximum RPM rating limits the safe top speed of the vehicle, given the gear ratio in
the differential. Consequently, beginners force the motor to a
higher RPM to gain more speed. In so doing, the centrifugal force
eventually pulls the motor windings out enough to touch the casing and
stators. Of course, this causes the motor to fail, and maybe the
controller too..
With
a Transmission
Even
when the vehicle has a transmission, over-revving also occurs when the
beginner decides to only use 1st or 2nd gear and ignore the higher
gears. I recently received an email from a beginner who blew two
motors and wanted to blame the company and the guy who rebuilt his first
motor. When he described to me how he drove his vehicle, using no
gear higher than 2nd, I immediately sided with the manufacturer and the
motor rebuilder. As a beginner, he did not understand what over-revving
was, but he was sure that he wasn't doing it.
Speed
Trap #4: Bad Batteries
Beginners
often get batteries donated to them or get a few here and
a few there to build up a battery bank. The result is a very 'soft'
battery bank whose voltage bounces up and down over a wide range
opposite the current being drawn by the motor. In other
words, as the current drain increases, the drop in voltage
increases. As the
batteries discharge during use, the voltage during acceleration
drops lower and lower and as the batteries age, the problem
becomes worse. The result is a drastic loss of power and
it may jeopardize the health of the controller, depending on
controller design.
Don't
Mix'em
There
are many things that beginners don't understand about batteries.
You can't mix them in terms of make, model or age, unless you have a
distributed charging system (more on this in Speed Trap #5). Series connected
batteries are like the chain that is only as strong as the weakest
link. The least capable, or least healthy, battery in the chain will determine the amount of
charge all of the batteries receive and will limit your overall driving
range.
Don't
Take'em
Donated
batteries, that have been sitting around in someone's garage, are most
surely heavily sulfated and will not provide adequate service to the
beginner who doesn't know what to do with them. The best advice
here for the beginner, or anyone, is to start with a new fresh set of
batteries.
Don't
have too few
An
insufficient number of batteries to meet the vehicle's power needs is
also a beginner's trap. What beginners don't realize is that
the fewer batteries you have, the more current the motor will demand
from them for a given amount of power (horsepower). Electrical
power (watts) is translated to mechanical horsepower by the motor.
Electrical power is battery bank voltage times the average current
delivered to the motor (P = V X A). So, if the bank voltage is
low, the current must be higher for a given amount of acceleration
power. Consequently, the lower the bank voltage the shorter the
driving range and the shorter the life of the batteries.
Speed
Trap #5: Bad Charging
Most
beginners know almost nothing about battery chargers and battery
charging. In fact, many beginners give little thought to the
charger until they are actually trying to drive the vehicle. A
recent letter from a beginner revealed that he only had a 24-V charger
to charge two of his 12V batteries at a time. With 10 batteries
total, that's five days of charging to use the vehicle one day.
Yikes! Do you think he will wait 5 days to drive his
vehicle? Nope!
Battery
Charger Most Important!
From
my engineering background and experience, I can easily argue that a good
charging system is the most important component of the entire
system. A good charger will deliver full charge and long life for
the batteries and top performance for the vehicle.
Charger
Short Course
For
flooded lead-acid batteries and sealed absorbed glass mat (AGM)
lead-acid batteries, the charger must deliver between 10 and 30 amps of
charging current during the 'constant-current' stage of charging (first
stage). At least two charging stages are required -
constant-current then absorption during which the voltage is fairly
constant and the current drops as the batteries charge. A three-stage charger is best,
which has a finishing stage that is referred to as the float stage or
soak-in stage. Three-stage chargers can be left on for days
without harm to the batteries. (See also Battery
Service Life.)
Two
Charging System Topologies
There
are two primary charging system topologies, or configurations, used to
charge electric vehicle battery banks: Bulk and Distributed. A bulk
charger is a single high-voltage charger that charges the entire battery
bank of series-connected batteries. A distributed charging system
is one in which there are many chargers that distribute charge directly
to each battery or pair of batteries.
Distributed
charging systems are best for many reasons:
-
All
batteries, or battery pairs, receive a direct charge unhindered by any other batteries
in the chain.
-
All
batteries receive their maximum charge.
-
Individual
batteries, or pairs, can be replaced in the chain without affecting the other
batteries negatively.
-
Each
battery, or pair of batteries, can be easily monitored for health if
each charger has an ammeter or there is a common ammeter that is
switch selected. A common switched voltmeter is also helpful.
Some
distributed charging systems use many 12V chargers. In cases where
the batteries are 6V, each charger is connected to a battery pair.
In my case, I have twenty-four 6V golf cart batteries in series for a
total bank voltage of about 154 V (144V nominal). I have one 12V
charger for every two batteries. Note:
A 12V charger actually puts out around 14.4 V during the 2nd stage of
charging.
By
the way, if you are a beginner and want to make your own
distributed charging system, make sure that the chargers you
choose are 'isolated'. That means their low-voltage output
is electrically insolated from the high-voltage AC input.
In other words, the output is 'floating'. Only isolated
chargers can be wired in series. I have added an article
to the Library
that shows you how to build your own distributed charging
system.
Well
that's it for now. I'll add to this over time and eventually move
it to the Library
page.
Don't
be a victim as a beginner. Open yourself up to the experience of
others, many others so you can test the validity of what each is
saying. Don't depend solely on blogs either. Often times
things that are stated on blogs are very unwise and go unchallenged.
If
you have any questions about this article, please write to me
at: mail@evhelp.com
Enjoy
the ride, but be careful out there. Look out for those speed
traps!
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