What is CLL and what to consider with PWM?

What is CLL?

CLL stands for capacitor long life and is used to extend the operating life of DC motors that have precious metal commutation. CLL suppresses brush sparking using the principal of an RC filter.

In the commutation process with brushes, contacts are continuously closed and re-opened in the presence of the inductivity of the winding segments. The induction arcing that occurs when a contact is opened damages the metal brushes and commutator segments by electro erosion. The result is a reduced life expectancy of the commutation system. This electro erosive effect is more severe, the higher the current load and coil inductance.

In the CLL concept, an additional RC filter is interposed between neighboring commutator segments, i.e. parallel to the opening contact. The inductance (L) of the coil segment that is connected induces a spike voltage that is diverted into the capacitance (C) in the form of a damped oscillation in the RLC resonant circuit formed by the coil segment and the CLL. In the ideal case, the maximum voltage that occurs remains below 12 V, the minimum voltage required for discharges between commutator and brush.

CLL principle for the suppression of brush arcing. a) schematic diagram using the example of a 7-segment commutator, b) schematic voltage curve (general) when the contact between the brush and the departing commutator segment is opened, c) rotor with CLL disc.

CLL is used only with precious metal brushes that are sensitive to arcing. With graphite brushes, they have no advantage in terms of operating life. The effect of suppressing this brush fire also has a positive influence on reducing the electromagnetic emissions.

CLL and PWM

However, the use of CLL also means that the motor has an increased terminal capacitance value, which must be taken into consideration with a pulse-width modulated actuation (PWM). The terminal capacitance acts as a short circuit for the fast voltage change, which can result in an elevated no-load current and excessive motor heating. Additionally, motor life is affected.

The following picture shows the current peaks after the PWM switching. The current reaction exhibits the fast response due to the RC term of the CLL and the slower current increase of the RL term of the winding.

There is the possibility to find a replacement circuit as shown in the following example:

Motor chokes (= additional inductances) in use as external components or integrated in a motor controller's power stage can reduce the current peaks. Nevertheless, the use of CLL in combination with a modern motor controller with a PWM power stage is normally not recommended.

Conclusion:

CLL can be useful in the following applications:

• When using brushed motors powered directly from a power supply or battery (i.e. without a motor controller with a PWM power stage in between).
• When using a so-called linear power stage in use (instead of a PWM power stage).
  Remark:
  Linear power stages apply a variable DC voltage to the motor. The motor voltage is not generated by a pulse width modulated (= PWM) voltage, but via a kind of variable electronic resistor. Due to the poor dynamics and the low efficiency (= high power loss) depending on the motor operating point, linear output stages are not very common nowadays. Linear power stages are only rarely found in the delivery program. maxon's older motor controller "LSC" (= Linear Servo Controller) still has such a linear power output stage, but is just available on request.

If possible, CLL should not be used when using a motor controller with a PWM power stage. All up-to-date maxon controllers (e.g. the ESCON and EPOS product line) have PWM output stages, i.e. these should not be combined with DC motors with CLL. If this is nevertheless the case, the necessity of additional motor chokes must be checked.