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Text version of the page
<Ę> 00
Mechanical commutation
Graphite brushes
In combination with copper commutators for the
most rigorous applications.
More than 10 million cycles were attained in
different applications.
Graphite brushes are typically used:
- In larger motors
- With high current loads
- In start/stop operation
- In reverse operation
- While controlling at
pulsed power stage (PWM)
The special properties of graphite brushes
can cause so-called spikes. They are visible in
the commutation pattern. Despite the high-
frequency interference caused by the spikes,
these motors have become popular in applica-
tions with electronic controls. Please note, that
the contact resistance of the graphite brushes
changes dependent on load.
Precious metal brushes and commutator
Our precious metal combinations ensure a
highly constant and low contact resistance,
even attera prolonged standstill time. The
motors work with very low starting voltages
and electromagnetic interferences.
Precious metal brushes are typically used:
- In small motors
- In continuous operation
- With small current loads
- With battery operation
- In DC tachometers
The commutation pattern is uniform and free
of spikes, as opposed to that of other motors.
The combination of precious metal brushes
and maxon rotor system results in minimum of
high-frequency interference, which otherwise
leads to major problems in electronical circuits.
The motors need practically no interference
suppression.
CLL concept
With precious metal commutation, the wear on
commutators and brushes is caused mainly by
sparks. The CLL concept suppresses spark
generation to a large extent, thus greatly ex-
tending service life.
When driven with a pulsed power stage (PWM)
higher no-load currents occur and an unwanted
motor heating can result.
For further explanations, please see page 49
or "The selection of high-precision microdrives"
by Dr. Urs Kafader.
Commutation pattern with
graphite brushes
|
1111 r |
|||||||||
|
11111 |
11111 â– |
11111 k |
11111 |
JIM : |
1 1 1 1 1 1 |
1 1 1 1 1 |
1 1 1 11 |
..... (lij |
|
|
a |
V |
TT |
Tj |
r |
Hf |
1 |
J. |
' i |
|
|
+++++ |
11111 |
11111 |
11111 |
-H-H+ |
JIM |
1 1 1 1 1 |
1 1 1 1 1 |
..... |
-H+H- |
May 2011 edition / subject to change
|
Commutation pattern with precious metal brushes |
||||||||||
|
11111 |
-H+H- -t |
1 11 1 1 |
O inn |
++++^-H-j-H |
11111 |
|||||
|
inn; |
JIM |
|||||||||
|
1 |
n |
Ěli |
||||||||
|
11111 |
IlII J |
-H-H- |
1 11 1 1 |
inn |
-H+H--H+H |
11111 |
||||
|
inn; |
JIM |
|||||||||
Commutation pattern
The commutation pattern shows the current
pattern of a maxon DC motor over one motor
revolution.
Please place a low-ohm series resistor in series
with the motor (approx. 50 times smaller than
the motor resistance). Observe the voltage drop
over the resistor on the oscilloscope.
Legend
O Ripple, actual peak-to-peak ripple
© Modulation, attributable mainly to
asymmetry in the magnetic field and
In the winding.
© Signal pattern within a revolution
(number of peaks = twice the number of
commutator segments)
Technology - short and to the point 25