On 2/7/2013 1:44 PM, Ds2inc wrote:
Also if I read this correctly with pm motors there is no need to run
them in parallel to achieve higher hp I just need more voltage in
each parallel? Am I missing then concept here?
Mechanical power (horsepower) is torque x RPM. Electrical power (watts)
is volts x amps. With a PM DC motor, volts = RPM, and amps = torque. The
relationships aren't perfect (they assume the motor is 100% efficient).
But in practice, a motor can easily be 90% efficient, so this
relationship is accurate within about 10%.
A PM motor tries as hard as it can to run at the RPM demanded by the
voltage. With the numbers you gave (72V 6.75HP 4200 RPM), you can expect
something like:
no load: 72v, 10 amps, 0hp, 4300rpm
full load: 72v, 80 amps, 6.75hp, 4200rpm
double load: 72v, 160amps, 13.5hp, 4100rpm
If you increased the load, the motor would just keep trying. The RPM
would barely fall, but the current would go up to supply the horsepower
demanded, until the motor burned up from excessive current.
Two motors in parallel would supply exactly the same RPM. But each motor
draws half the current and supplies half the torque (thus half the power
in, and power out). The main advantage of two motors is that they can
supply power for twice as long before burning up.
With PM motors, RPM is *directly* controlled by voltage. Double the
voltage, double the RPM, etc. It doesn't matter how many motors you
parallel; they *all* run at the same RPM.
You said your motors produce 4200 RPM at 72v, and your gear ratio is
4:1. That lets us precisely calculate your maximum speed. Your wheel
will be turning at 4200/4 = 1050 RPM. A 24" diameter tire moves 3.14 x
24" = 75.36" per revolution. 75.36"/rev x 1050 RPM = 79128"/minute =
6594 feet/minute = 1.249 miles/minute = 75 miles an hour. If your motors
were connected straight to a 72v pack, the car would go 75 mph, uphill
or downhill, drawing as much current from the batteries, or regenerating
as much current into the batteries as it could to stay at this speed, as
if you had a "cruise control" engaged.
--
The principal defect in a storage battery is its modesty. It does not
spark, creak, groan, nor slow down under overload. It does not rotate.
It works where it is, and will silently work up to the point of
destruction without making any audible or visible signs of distress.
-- Electrical Review, 1902
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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