Sophisticated braking methods are being incorporated into ever smaller
drives, thanks to their lower energy losses and harmonic distortion.
Indeed, it is predicted that in ten years, the only electrical braking
method permitted for continuous use will be the one based on inverter
supply units. But for now, there are plenty to choose from. Geoff Brown
helps us sort the wheat from the chaff.
In many applications, being able to stop safely and precisely is as
important as being able to start and accelerate quickly. Obvious examples
include cranes, elevators and ski lifts, but quick and precise stopping
is also necessary in machine tools, feed equipment and many other
processes. When choosing a braking method, a number of factors need to be
considered, not least being safety, frequency of braking, the security of
the supply, harmonics and the type of drive. Any scheme selected for
braking must be sized according to worst case braking torque and the
duration of braking. There are a number of braking methods, each suitable
for particular applications and braking needs.
Electromechanical
These may be either integrated brake motors or motors with separate drum
or disc brakes, of varying sophistication. This is the most basic
mechanical method that will stop the load quickly, though it may cause
unacceptable rates of change. It also provides holding torque at
standstill. This method is typically used in traction applications, as
well as in hoists and conveyors. The disadvantages are that stopping can
cause sharp jerking which can introduce instability into a system, and
that the brake linings will wear and need replacing. Brake lining wear
can be reduced by combining mechanical braking with any of the electrical
braking methods outlined below. Electromechanical brakes will also need a
power source to lift the brake, which may be taken from the main motor
connections when uncontrolled braking of a fixed speed motor is involved,
but which must be separately sourced in other cases. The use of
mechanical back up or holding brakes is often a necessary safety related
factor in the selection.
Counter-current or plug braking
This involves switching the motor stator to the opposite rotational
direction, using a specially designed starter. After deceleration to
standstill, the motor may restart in the opposite direction unless the
current is disconnected at the right moment. This method can create a
very high braking torque, resulting in a large amount of heat being
developed in the motor. Temperature monitors should always be used to
protect the windings.
DC injection braking
This is a widely used electrical method and can be performed with a fixed
or variable speed system. For a frequency converter fed system, a stop
command makes the frequency converter switch to supplying the motor with
direct current, developing a braking torque. The same effect can also be
achieved using suitable dc excitation equipment. This method can give a
considerably longer braking time than counter-current braking, and its
heat losses are relatively lower, so more frequent braking is possible.
Still, its use is confined to applications in which braking accounts for
a relatively small proportion of the running time, such as emergency
stopping for machine tools.
Flux braking
This relies on increasing the motor losses in a controlled way; inverters
with direct torque control (DTC) have this capability. When braking is
needed, the frequency is reduced and the flux in the motor is increased,
which in turn increases the motor's capability to brake. When braking is
not needed, DTC brings the motor flux down to its nominal value. Unlike
dc braking, the motor speed remains controlled during braking. It is
typically used where there is an infrequent need for small amounts of
braking.
Dynamic braking
A brake chopper and resistor can be used to achieve full braking