Harry Chickpea said:
In the simplest form, DC motors tend to find a balancing point, where the load
on the motor and power usage are balanced. When the load increases, the speed
of the shaft decreases and the torque increases. When the load lightens, the
speed increases and the torque decreases. This type of self adjustment tends
to make the motor run at the ideal point in the power curve IF the motor is
matched properly to the load.
Such speed variation is known as 'speed regulation' and is usually
calculated given in percent of no-load speed. Most shunt wound motors have
a speed regulation less than 10% (5-7 is not unusual). Differentially-wound
compound motors can be designed such that the speed regulation is actually 0
or even slightly negative.
Only series wound motors have the kind of poor speed regulation you seem to
be thinking of. These motors can overspeed if unloaded and develop
tremendous torque when stalled. This type of motor sees use in applications
where high-torque but intermittent service are the norm. The classic
example is the starter motor on cars/trucks.
Even the earliest AC motors have the same behavior. They only develop as
much horsepower as required by the load to keep it running at the speed of
the motor. They do not draw rated kW from the line when only supplying a
fraction of their rated horsepower at the shaft.
In contrast, the common early AC motors tended to want to run at a fixed speed
determined by the frequency of the current and the windings, and were not as
efficiently responsive to changes in load as DC motors.
Even modern induction motors run at nearly a fixed speed. The amount of
torque they develop can vary from zero to full rating in as little as 3-5%
speed variation. The amount of power they draw from the supply varies
directly with the load on the shaft. Your statement 'not as efficiently
responsive to changes in load' makes absolutely no sense at all. The
response of even early AC motors is directly affected by shaft load.
AC *or* DC motors are very in-efficient at light load. By definition, they
are both 0% efficient at no-load. All motors have a particular load level
where they are most efficient. But change the load from that point and the
efficiency of AC or DC motors drops off. No inherent advantage (efficiency
wise) of either design.
DC motors, OTOH, could "run away" under low-load conditions, which made small
AC motors a safer choice for home appliances and tools. Most tinkerers have
played with washing machine motors and seen how they can stall out under load,
when a comparable DC motor would hunker down, pull more amps, and torque the
load into submission.
Only series-wound DC motors 'run away'. And they only do it when unloaded.
There are several types of DC motor windings, you seemed focused on only one
type. The series motor is used for 'traction motors' in locomotives because
it can develop high torques for starting.
Traction requirements are much different than other industrial settings.
With many industrial/residental loads, the torque required by the load
varies with speed squared. In pumping applications (and blowers), the
torque required at half-speed is one quarter that needed at full speed. So
the power required is one-eighth. Pumping power is proportional to speed
cubed. If traction were like that, it would be as if the train got
shorter/lighter as it slowed down.
Traction requirements, on the other hand, have a couple of different
components. First, rolling resistance varies with the speed of the train
and the sharpness of any curve. So the torque required to overcome that
varies with speed. But a *major* torque requirement is hauling a train up a
grade. The tractive effort needed to pull a long train up even a modest 1%
grade is sometimes 10 times the level-grade rolling resistance. This
tractive effort shows up as a torque requirement on the motor that is nearly
constant regardless of speed.
While it is true that an *overloaded* AC motor will stall, and an
*overloaded* DC motor will just slow down and burn up, that has little to do
with practical applications. Most people don't deliberately overload the
equipment.
Edison himself was a proponent of DC, and most early heavy usage motors were
DC. Tesla's AC motors were not as capable as the DC motors of the time, but
advances in switching technology and design have drastically reduced the gap
between the two types of motors.
Edison was a proponent of DC at many levels and waged a vicsous campaign to
discredit Westinghouse and Tesla. Fortunately, Westinghouse won that round.
Early polyphase AC motors were just as capable as DC motors, they just
developed a bit later. DC was first, but lost a lot of ground due to
maintenance, size and power transmission issues.
Ah ha!! Just as I suspected, you're thinking of DC series wound motors.
They are an excellent 'fit' for that application, but not many others. In
the case of A/C (the OP's question), series motors would *not* be the best
fit. A shunt, or compound motor with solid-state armature voltage control
would be better. But I believe they are actually variable-frequency driven
AC motors with no brushes (can't find much information about them).
So your comment of 'switching technology' is probably coming from the
background of AC motor drives. Newer locomotives have AC generators on the
diesel, supplying AC traction motors through a large bank of solid-state
switching. This varies the voltage and frequency applied to the motors to
suit the operating condition. The AC motor system is much less maintenance
than the older DC traction motors (which appeals to RR management).
IME, the whole "efficiency" idea doesn't distill down to a single percentage
rating. Different conditions may give one motor an advantage over another, and
any fixed rating only reflects a single set of conditions. If efficiency was
as cut and dried as some people think, there would not be the wide variety of
motor types that the marketplace supports.
Absolutely. Some applications require a constant power all the time. These
are the simplest to design for. A motor that is most efficient at that
speed and load is the best choice. But other applications present varying
load conditions. A pump whose flow is varied by some process needs varying
pumping power at different times. A variable speed drive might be the best
fit, but capital cost vs. the energy cost to run a fixed speed pump with
throttle valve have to be considered.
A locomotive requires high torque for prolonged periods of time while
starting and running at varying speeds. Each application presents unique
factors that make motor choice/design a compromise of several factors.
daestrom
