Selecting low-power dc motors
Selecting a dc motor could involve complex calculations to assess
heating effects, start/stop angles, the apparent reflected inertia, and
so on. But the calculations are there to give confidence that the first
choice is as near as possible the right one. The initial choice is based
on some very basic parameters, but where to start?
A motor is merely a device for converting electrical energy, which is
easy to transport, into mechanical energy, which is not. How it does this
depends on the motor construction, but all rely on magnetic fields,
usually two of them, the one trying to align itself with the other.
Expending energy means doing work, moving a force through a distance,
linear or rotary. The rate of doing work is power, and it is here that
the choice of motor starts.
A motor must be capable of developing power at least equal to the rate of
work that is required of it. If you need two Watts of power, you need at
least a two-Watt motor. Nothing works at 100% efficiency - at least not
yet - so a larger than 2-Watt motor will be needed. If we take it that we
want to start with the smallest motors possible, so as to minimise size
and cost, then we need to operate the motor at either maximum efficiency
or maximum power. From theory, maximum power happens at a maximum
efficiency of 50%. Maximum efficiency may well be above this, but the
power delivered will be less. At this point the specifier must decide
which is more important.
Work is made up of two main components: force and the distance the force
moves through. The force, in turn, can be made up of more than one
component: friction, inertia, and gravity. Friction again can have more
than one component: mechanical surface-to-surface, air and other viscous
fluids. So these all need to be assessed before the power can be
calculated and the selection process started.
The distance the force moves has a critical parameter affecting it, and
that is time. Power is the rate of doing work, so the time taken to do
the work needed will have a controlling impact on the selection process.
Given long enough, a man could move a ship under ideal conditions, but it
takes many thousands of horsepower to move the ship at speed. A 50cc
moped will get you down a drag strip, but it takes a 6,000hp dragster to
do it in less than five seconds.
Once force and speed are decided, the power can be calculated, and the
way in which these are generated can then be decided. To further narrow
the field, it is necessary to decide on the level of control needed. If
the motor can simply be switched on to run at a speed set by the power
supply and winding, then a very simple system can be adopted. More
complex control, such as four-quadrant, will require additional feedback,
electronics and software.
Defining the full specification is the hardest part of all. Once the
target price and potential demand has been defined, the designer can get
to the right quality/quantity/cost mix that offers the optimum solution
or, put another way, the best compromise. These should be matched against
the published specifications from the suppliers who offer motors in the
relevant size, quality and price range. But everything about a product
and its application cannot be put on a datasheet, so the supplier's
knowledge and experience should be tapped. Samples should then be tested
to prove that the product meets the specification, and that the
specification itself is complete and correct.
If compromise is not in your design philosophy, then there are now a
growing number of companies who can offer a custom design to meet your
exact needs. Much will depend on the annual volume, but there is also a
broad spectrum of manufacturers to offer small medium and large volume
solutions.
Electro Mechanical Systems (EMS) offers a broad range of small dc
motors, ac and dc linear actuators, brushless dc motors, stepper motors,
encoders, and gearheads from leading European and Asian manufacturers.
EMS can also provide a unique custom design from its Poole design,
development and manufacturing facility.
Electro Mechanical Systems Ltd
Calleva Industrial Park, Aldermaston, Reading RG7 8LN
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