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Posted by David Sherman [24.32.202.83] on Friday, February 05, 2010 at 00:54:29 :
In Reply to: You may need 100 hp to accelerate... posted by Chriscase [76.212.169.61] on Thursday, February 04, 2010 at 23:51:50 :
DC Electric motors with wound fields can be constant torque, constant horsepower, constant speed, or somewhere in between all those conditions depending on whether they're series wound, shunt wound, or some combination. Torque is goes up as armature current or field current goes up. Speed goes up as armature current goes up or field current goes down. For a given armature current, increasing the field current makes the motor slow down and torque up. Loading down a motor mechanically makes it draw more armature current. Thus, if you put the armature in series with the field (series wound), when you load the motor more heavily and it starts to draw more armature current, it will draw that current through the field, which will increase the field current and make it slow down, but also make the torque go up. This can be a "constant HP" setup. Series motors are used when you need to get lots of torque at low speeds and then spin as fast as possible once things are moving. A starter motor is a perfect example of a series motor. A series motor with no mechanical load on it will theoretically run away with itself, so there need to be second-order effects, or a small shunt winding, to limit the speed. A shunt motor, on the other hand, has the same field voltage and the same field current regardless of the armature current and the load. As you load it down, it draws more armature current, which makes the field current look lighter by comparison, and will actually make the motor speed up with increasing load. Most DC motors are "compound wound" in order to get the right torque/speed characteristic for the application. By 100 years ago, the engineers had figured out all the tricks of compounding, plus tricks of where exactly to place the brushes and and how exactly to put the windings in the slots in the iron in order to optimize the motors for particular applications.
So, DC motors can be constant torque if you want them to be, but they aren't necessarily. Likewise wound-field (synchronous or universal) AC motors can be set up that way. The common squirrel cage AC motors are definitely not anywhere near constant torque, and generally develop very little torque until they're up near synchronous speed.
As far as what size motor puts out 100 ft-lbs, any HP of motor theoretically could, but smaller ones would only do it at slow speeds. Just like with a gas engine, you have to specify the speed at which you want those ft-lbs. A given amount of HP at high speeds represents less ft-lbs than at low speeds.
I think the big problem for anyone building their own hybrid PW is that high voltage high power wound field DC motors are hard to find. They haven't been widely used in 50 years and are pretty much special-order items even for those big industrial plants that still need them today. What you have to choose from in mass-produced DC motors are starter motors and fractional-horsepower universal motors such as are used in power tools and vacuum cleaners.
By the way, for switching polarity, you would probably use a small switch on the dash and a box of high-power relays mounted close to the motor, just like they do on electric winches. Remember, if we're running at 250 volts, the amount of amps needed for a given power is only 1/20th of that at 12 volts, so the switching is reasonable. For the kinds of power levels and batteries we're talking about here, it might be feasible when braking to just dump all energy into the batteries without worrying about over-charging them. If not, some very simple ana.log electronics (no computer to go berzerk) or even just some adjustable voltage and current-sensitive relays would suffice to avoid damaging the batteries.
Again, this is just a rough guesstimate, but I think it's reasonable that putting together a hybrid system using 1930s technology would probably be around half as efficient as they get with today's fancy computer controls, rare earth permanent magnet motors, lithium batteries, and so on. Maybe the figure is 30% or maybe it's 70%, but it's not going to be 90% as good, and it's also not going to be only 10% as good. You'd basically be building something along the lines of the electric streetcars, subways, and railroads of that era, only with batteries added.
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