dovepa via EV wrote:
You left out the fact that series motors get too hot when operated at highway
speed for extended periods of time. With the advance in battery technology this
is causing some people issues.
Series motors intended for on-road EVs have built-in fans. I've never
needed to provide extra cooling for them in any of my EVs. The internal
fan works *better* at high speeds -- most actually have more cooling
than they need at high RPM. The internal fan is also an advantage they
have over AC motors, which generally require a complex liquid cooling
system.
Now if you're talking about a fork lift motor, some of them *are*
sealed. They're for light-duty use (such as to run the hydraulic pumps
for steering and lifting), or for operation in dirty or hazardous
environments where a spark might set of an explosion.
EVDL Administrator wrote:
I'm not a motor expert, but I suspect that the total system cost is low
because the controller is relatively simple.
It's the combination of many things. Based on the generally available
examples, DC motor advantages:
- Old tech: Simpler to understand, design, and use.
- Same RPM range as an ICE makes conversion easier.
- Torque-speed curves match an ICE+transmission. Maximum torque
at lowest speeds, even without a transmission.
- Easy to get: Mass produced, untold millions already in circulation.
- Cheaper: No rare earth magnets, fewer expensive power transistors,
microcontrollers and software not even needed.
- Rugged and durable. Can produce extremely high peak horsepower
without failing (which is why racers and railroad locomotives
use them).
- Very simple controllers. Can even be little more than contactors.
- Cooling usually built-in.
- Adaptable: Don't need to carefully match motors and controllers.
- Long life: Plenty of examples 50-100 years old.
On the other hand, the usual drawbacks of DC motors include...
- Boring old tech: Not fashionable.
- Lower efficiency: A consequence of being designed to be cheap
rather than efficient.
- Common examples don't do regen. It takes extra parts, so it's
usually left out to keep it simple and save money.
- Brush maintenance. Often exaggerated; brushes last at least 10x
longer than ICE oil changes.
- Hard to liquid-cool a DC motor due to the commutator. Air
cooled motors are noisier (though far less so than ICEs).
You can't fireball the commutator of an AC induction motor because it
doesn't have one.
No; but you can fireball the AC motor's controller. :-)
In these perennial AC vs. DC debates, it's important to keep in mind
that they are fraternal twins. One boy, one girl; but otherwise nearly
identical. Every generality like "boys are stronger" or "girls are
smarter" will fail, because you can always find examples where the
opposite is true.
All motors are really AC devices. The commutator is just a simple crude
example of an electronic inverter. Anything you can do to optimize some
aspect of an AC motor can also be done to a DC motor (and vice versa).
So, you can compare a *specific* AC motor to a *specific* DC motor. But
you can't generalize from this that all motors of that type will have
these same characteristics.
Series motor redline RPM is much lower than induction, for the same reason.
Commutators have a tendency to fly apart if they overspeed.
That is a consequence of the cheap plastic commutators in common use.
There are banded and steel commutators that work fine at very high RPM.
In automotive applications, you don't really want high RPM as it
requires an excessive amount of gear reduction to get it down to wheel
speed.
A series motor will overspeed if it's run at full voltage with no load...
You also need safeguards to detect controller failure, because the typical
failure mode for a series motor controller is FULL ON.
Yes. These are consequences of making the controller *too* simple. As
Einstein said, "Things should be kept as simple as possible. But not
*too* simple."
Complex systems also have these failure modes, as Audi and Toyota have
found to their dismay (unintended acceleration).
Regenerative braking is a major challenge with a series motor. This is not
the same as the plug braking available on some Curtis controllers, which is
generally not recommended for use in a road EV.
True; but a consequence of not designing regen into the controller to
begin with. Since every motor (AC or DC) is intrinsically a generator as
well, there are lots of DC motor systems that *do* have effective
regen... when it was designed in to begin with.
A lot of good and expensive silicon has become smoke trying to handle regen.
Very few commercial EV controllers today try to do it. Zapi is the only
brand I know of, but there may be others.
Zapi, Sevcon, GE, Cableform, and more... Even Curtis has a regen
controller (the 1221R).
While I'm not in the market for a $150k high performance sports car,
if I were, I wouldn't buy one with a DC series motor.
I think that's exactly it. DC is not flashy and high-tech enough for an
expensive luxury sports car. You want cutting-edge style, you want
flash and glamor; you want to make a statement.
--
Knowledge is better than belief. Belief is when someone else does
your thinking. -- anonymous
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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