50 / 84
For most applications involving
a DC or BLDC (brushless DC)
motor, it is advisable to use a
motor controller, in fact if you
are using a brushless (electrically
commutated “EC”) motor, then
you have to use a controller to fire
the correct phase winding at the
right time.
For all DC motor brushed types here are
the common reasons why:
Motor protection: Most modern
controllers have the following protections;
under-voltage, over-voltage, short circuit
protection, current limit protection,
thermal protection and voltage
transients. Without these protections the
motor is “exposed” to threats that will
possibly result in permanent electrical or
mechanical damage.
Speed control: All DC motors will lose
speed as they are loaded and increase
in speed when they are unloaded, in
a linear fashion, according to their
speed/torque gradient. For applications
where a specific speed is required, with
an unknown load (so a final speed
cannot be calculated), or a fluctuating
load (conveyor belt, pump, grinding
tool, reel / converter, Cam) a controller
is a must.
In the cam application in particular,
where the motor is operating as
a “motor”, for half the cycle and a
“generator” for the other half, as the
load “pushes” the motor a four quadrant
drive must be used. This provides
dynamic breaking control, to ensure
the motor remains under control and
at the constant speed. It is not possible
to achieve this with a simple supply or
a simple single quadrant controller.
The controller must also cope with
the varying load, yet maintain the
motor at a constant speed, by varying
the voltage to the motor as the load
changes to compensate.
In fan and pump applications the power
curve follows the “square law” i.e. for
an incremental increase of the speed
the power increase is to the second
power. Using a speed controller to
lower the speed of the motor to meet
the demands of a system, in place of
constricting the flow from the pump or
fan whilst running the motor at full speed
will reduce the power consumption.
That said there are losses across the
controller, typically 1-5%, but these are
minimal in comparison to the motor or
the mechanics it is linked to.
Torque control: A DC motor’s current
is proportional to the torque delivery.
Having control over the current to the
motor will govern its torque delivery.
Without control of the DC motor
torque/current, the motor is allowed to
pull large currents that can often result
in torques delivered in excess of what
is mechanically viable for the system,
leading to imminent failure (especially
when gear with a large reduction ratio
is used). Also failure can be in the form
of burn out from stalling the motor,
for instance when the mechanics are
jammed. In this condition, unless the
current is limited, the motor pulls the
stall current. As you can see from the
diagram, the stall current is far from
the operating area of the motor and
will cause a thermal failure in normally
few seconds.
Summary
The benefits of using a controller.
•
Electrical protection of the motor and
subsequently the mechanics.
•
Maintains constant speed, even when
loads are changing.
•
Dynamic response to changing system
demands, even in a braking condition
with 4 quadrant drive.
•
Monitoring to evaluate machine
performance / diagnostics.
•
Energy saving.
•
Accurate speed control.
Why Use a DC Motor Controller?
Maxon
New-Tech Magazine Europe l 50