I’VE FINALLY
broken down and
developed a Web
site. I’m a novice at
Web design so it’s
nothing fancy, but I
hope it will be a
place I can point you
to for extra photos
and descriptions I
don’t have room for in the magazine.
There will be a section on the site for frequently asked questions,
etc., along with some article archives. I hope to also offer space for
readers’ photos. Check it out and be patient with me while I learn.
Unintentional Lie: In the October issue I wrote that in the next
column we’d look at how to determine motor constants if we didn’t
have them on a data sheet for a particular motor. After submitting that,
I decided I’d cover the electric simulation programs before the
constants because it would make more sense.
Visit Greg’s Web site!
April 2008 105
Electrics Greg Gimlick | [email protected]
Also included in this column:
• How to determine motor
constants
Testing for Kv and Io on an unknown brushless motor, the drill press
isn’t being used. The soft jaw vice, holding the motor and meters, is
mounted on the drill-press stand for convenience.
Greg holds the motor while the drill press spins it at 3,400 rpm
and the meter reads the voltage that the motor generates.
These tools are all you’ll need,
along with a drill press
or drill, to do basic
motor-constant
testing.
04sig4.QXD 2/25/08 10:29 AM Page 105
106 MODEL AVIATION
Greg is testing some Dualsky motors, and they look promising.
They are reasonably priced with a higher-than-expected quality
of internal construction.
This gear is required to determine the unknown brushless
motor’s Rm. The power is hooked up to any two wires from the
motor. Take readings quickly; do several to average.
I apologize to those of you who have been waiting and waiting.
The bimonthly schedule makes it drag out a bit when I make an error
and try to correct it.
Motor Constants: When we run the simulation programs, we’re
concentrating on three parameters that I defined in the last issue: Kv,
Io, and Rm. Most of our motors come with some designation that
usually refers to length and diameter of the armature and Kv. That
doesn’t help us when we want to model power setups, so we often
have to dig to find the missing numbers or come up with them
ourselves.
Since we’re not electrical engineers and we don’t have lab
equipment, our results aren’t going to be as accurate as the factory’s,
but they will be darn close and certainly accurate enough for our use.
There are some differences in how you can do things with brushed
motors compared to brushless, and I’ll point those out along the way,
but we’ll concentrate mostly on brushless since that is the trend.
You don’t need anything exotic to determine the parameters we’ll
be testing. If you’ve been around electrics for any time at all, you’ll
have them in your shop already. All you’ll need are a voltmeter,
ammeter, tachometer, drill or drill press, flashlight, servo wheel, tape,
battery, speed control, and radio for the brushless motor.
As you can see in the photos, there is nothing exotic. I use the soft
jaws on the drill-press vice to hold some of the motors and can set the
meter on the adjustable stand so I can see everything at once.
There’s nothing dangerous about what we’re going to do, but the
usual cautions about working around a drill press apply. If you have
wires near a spinning object, you must secure them to avoid
entanglement.
When spinning the brushed motor in the drill press, the motor can
or body must be prevented from spinning; I do that with my fingers.
Unless there is a problem with bad bearings or bushings, it won’t try to
spin anyway, but caution is the key. Moving the drill press’s soft jaws
to hold the motor would be even better.
We’re finally to why you came. The first parameter I determine is
the motor’s Kv. You can do this two ways if you’re testing a brushed
motor or one way if you’re testing a brushless motor at home.
1) This method drives the motor with a drill press at a
predetermined rpm and measures the voltage across the motor’s
terminals. I found that my drill-press speed table is way off from
reality, so be sure to tach your drill press and use the actual speed.
Mine is 400 rpm slower than the chart on the press shows. I like to use
an rpm that is higher than 3,000, and my press is set for 3,400 when I
test motors.
The rest is simple. Connect the clips from a voltmeter to the
terminals, spin the motor with the press (be sure to hold the motor),
and read the voltage on the meter.
If you’re testing a motor that has advanced timing, you need to spin
the motor in the opposite direction that it’s supposed to run a propeller.
This isn’t a problem since the drill presses I’ve used do this anyway. If
you didn’t run it the proper direction, the numbers you get would show
a motor with retarded timing and you’re projections would be way off.
The rest is simple math once you get your numbers. In my case you
can see that the motor voltage is 1.11 and my press’s rpm is 3,400.
Dividing the 3,400 rpm by the 1.11 voltage, I end up with a Kv of
3,063.
Since I had the data sheet for this version of Speed 400, I saw it was
a bit high and wondered about that. Then I remembered that I had
advanced the motor’s timing when it was in an airplane.
2) This method will work for brushed motors too, but it’s the only
one we can use for testing brushless motors. We’ll drive the motors
with a power source and measure the motor’s rpm with a tachometer.
Since we can’t tach the motor’s shaft, we need something to help us
that won’t put a load on the motor. I use a big, flat servo wheel I’ve
drilled the center out of so it fits the propeller shaft and placed a piece
of white tape across the wheel for the tach to see. This does put a small
load on the motor since we’re working in the real world, but it’s close
enough for our purpose.
This is also where the flashlight comes in unless you’re working in
an extremely bright shop with no lights on. If you have lights on when
you use the tach, you’ll pick up their frequency instead of the motor’s
rpm. The DC-powered flashlight will prevent that or you can do it
outdoors.
If you’re testing a brushed motor, you can hook it to a power
supply with a known voltage or a battery pack that is at least half the
motor’s rated running voltage. If you have a 6-volt motor, you should
use at least 4 volts to give a good reading.
Hook the power to the motor and read the tach. Do several tests to
get a good average. Divide the rpm by the voltage and you have the
Kv.
With a brushless motor you’ll need to hook up your radio and
speed control. The same applies to using a battery that is more than
half the motor’s rated capacity. You must run the motor at full throttle
or you’ll get faulty readings.
Tach the motor at full throttle and do several readings to get a good
average. In the case of my unknown brushless motor I averaged 31,800
rpm at 12.24 volts, giving me a Kv of 2,598.
The Io, or no-load current, is an easy parameter to test. If you’re
using method 2 to determine your motor’s Kv, you can test the Io at
the same time.
You can see in the setup for my brushless motor where it’s clamped
in the drill-press vice that I can also see the wattmeter on the shelf to
read the no-load current while I’m taching the running motor. This lets
me get both readings at the same time. Since I’m doing several runs
and averaging them, it helps to have an assistant record the numbers.
The method is simple; connect the motor to the power source and
read the current on the wattmeter. This should be done at full throttle,
04sig4.QXD 2/25/08 10:10 AM Page 106
and if you’re using a brushless motor/ESC the
meter will have to be between the controller
and battery. Do this quickly; you don’t want
to do continued long runs with no load. My
unknown brushless motor’s Io was 3.07
amps.
Rm, or armature resistance, causes most
people concern because things happen
quickly and you’re stalling a motor with
current going through it. If you follow these
steps and do it quickly, you will have no
problems and you won’t harm anything. The
methods are basically the same for brushed
and brushless motors, but I will separate them
for clarity.
• Brushed Motor: It must be kept from
spinning, so if you have your servo wheel
mounted to the motor you can use several
layers of tape across that to hold it to the
motor case. It won’t take too much to stall the
motor, but you must use some method to keep
the shaft from turning during the test.
I use a battery clamp from West Mountain
Radio to hold a single D alkaline cell as my
power source. I connect my wattmeter to that
so I can read voltage and current at the same
time. Since the wattmeter requires more
voltage than a single cell to power it, I
connect a 4.8-volt receiver pack to the
wattmeter. When you’re ready, connect the
lead from the wattmeter to the motor and take
the reading.
This is where it gets busy because the
numbers will change quickly. A video camera
or helper is handy to have. Take the reading
quickly and disconnect the power. Repeat this
a couple times and average the results. You
don’t want to leave it hooked up too long or
you’ll damage the motor.
• Brushless Motor: This method is the same
except you won’t need to secure the motor
shaft. Since you won’t be using a speed
control, you can connect the power leads to
any two of the motor wires and take your
readings the same way you did for the
brushed motor.
For my unknown brushless motor I
averaged 11.58 amps and .28 volt, so by
dividing the voltage by the current I end up
with an armature resistance of .024 ohm. The
simulation programs use milliohms, so
multiply that by 1,000 to move the decimal
108 MODEL AVIATION
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three places and you end up with an Io of 24
milliohms.
Now you have all three constants to plug
into your simulation programs or with which
to compare your motor to the data sheet.
Chances are that if you have the data sheets
you won’t need to do any testing to get
numbers. In other cases you may wonder why
your model isn’t running the way you think it
should and want to pursue the problem.
I test many motors, and some are
prototypes that come with no data other than
being told to run so many cells and a certain
propeller. How did my results for the unknown
brushless motor compare to what the
manufacturer finally told me? Not bad and
close enough to show that our easy methods
are more than accurate enough for sport use.
My tests came up with the following
results.
• Kv = 2,598 and the factory specs were 2,400
• Io = 3.07 amps and the factory specs were
2.50
• Rm = 28 milliohms and the factory specs
were 20 milliohms
Final Approach: I’m using these methods
to test new motors I got from Vampower
Pro.com, and they have been very close to
the factory specs. I’ll be doing more with
these Dualsky motors in the future, but
right now they look promising and are
reasonably priced.
So much for testing and shop work. See
you at the field! MA
Sources:
Greg’s Web site
www.gimlick.com
VampowerPro.com
(850) 974-2511
http://vampowerpro.com/store/
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