On 12/04/11 02:48, John Green wrote:
> Is it correct that any DC motor generally has the speed controlled by
> lowering the volts?
In general yes.
> Surely the 'given power' will be less, as the purpose of the lower volts
> is to reduce the speed,
Yes Power = volts x amps, lower volts, lower power.
> therefore the amps might well remain at the same
> level?
Not necessary. This is where it gets 'interesting'.
More or less depending on the motor type, the amps drawn is not
related directly to the voltage but to the load on the motor.
In the case of my perm magnet motor - the shaft speed is directly
proportional to the voltage. The motor will draw what ever amps it needs
to produce the torque needed to obtain that shaft speed. If it wasn't
for current limiting in the controller, if the prop was snagged on
something, the motor would quite happily destroy itself trying to get to
it's desired speed by drawing insane amounts of current to get enough
power to break past the obstruction and then burning up as result.
> Can I assume that the numbers on this are not proportional, that the
> speed will reduce more than 50% if the volts are halved?
There are proportion factors, but they are based on load as well, so
it's not a nice simple formula.
If you drop the voltage on a perm magnet motor, it spins slower.
Normally this would mean a lower load on the prop, lower load on the
motor, so the current drops as well as not as much power is needed to
maintain the slower speed.
If you hooked the motor up to some sort of rig that applied a constant
retarding torque to the motor, no matter the speed, you probably would
get a perfectly inverse relationship between applied voltage and
current. As the voltage went down the current would go up to enable the
motor to maintain the required power to maintain it's speed against the
constant load.
On my boat if I turn into the wind, the current on my motor will go up
by a few amps with out me touching the throttle at all - the windage
puts more load on the motor by making it harder to maintain speed. The
motor doesn't want to slow down, so it draws more power (via amps as the
voltage is fixed) to compensate - with out me or the controller doing
anything.
Series DC motors have a slightly different characteristic, but are
similar. The PMAC and other AC motors have some what more complex load
behaviour - in part governed by the algorithm implemented in their
controllers. I imagine most are designed to behave as close as possible
to the ideal of 'do what ever it takes to maintain selected speed'.
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