Motor Capacitors

OBSERVED PROBLEM: Installing advanced DCC decoders with "BEMF" motor speed control and get problems getting good motor speed control assume there is something wrong with the decoder. They blame the decoder because when they put in a different decoder the engine works. This is unfortunate when there is a simple solution. Want to learn more about what BEMF is? Go here: Back-EMF (BEMF)].

ACTUAL PROBLEM: The problem is decoders with "BEMF" motor control will have problems operating correctly on any that places capacitors on the motor terminals or wires. The question is NOT if the BEMF decoder will work but HOW well it will work. There is NO requirement on behalf of the BEMF decoder manufacture that they must work with motor noise suppression circuits that contain the capacitors.


Motor make electrical noise. The motor noise contains lot or random high frequencies or static noise that are part of the radio frequency world. If the electrical noise gets out of the motor and on the wires that provide current to the motor, the wires act like a radio antenna transmitting the motor noise into the airwaves. Any interfering frequency getting into the airwaves is called Radio Frequency Interference (RFI). There is a circuit called a RFI filter who's job is to prevent the RFI noise from escaping the motor and getting into the airwaves.


The commutation action of the motor. The brushes of a DC motor are used to allow current from outside the motor to reach the coils mounted on the rotating armature of the motor. The brushes ride on top of what is called a commutator which has a contact surface for every number of coils or poles the motor has. A 5 pole motor has 5 coils. There is a insulation gap (material and/or air) between the contact surfaces so the contacts do not short to each other when no in contact with the brushes. When the motor spins, the brushes ride on top of the commutator under pressure from a spring to maintain contact. Due to vibrations and/or any uneveness in the commutator construction, the brushes literally vibrate up and down and side to side such that electrical contact can vary including losing complete contract with the commutator for a tiny fraction of a second. If you turn off the lights and look at the motor brushes while the motor is spinning at high speeds, you often can see a blue spark being generated. This is the electrical noise in visual form.


A capacitor is a device the stores and releases electrical energy. A capacitor can convert current into voltage (Charge) and voltage into current (Discharge). When it is in the voltage form, the energy is being stored is like the voltage on a battery. The conversion between current and voltage goes back and forth is instantaneous. How much current to voltage or voltage to current conversion happens depends on how fast the applied voltage to the capacitor terminals changes on the capacitor terminals and how big the capacitor is. The problem with BEMF is the capacitors store a voltage as a consequence of doing its job that will mislead or confuse the BEMF decoder. How much of a problem for the decoder has a lot of variables involved which is why the problem is shades of gray instead of a simple it works or does not work.

To learn more about capacitor basics, go here: Capacitance


Motor noise suppression circuits or RFI filters containing electrical parts called capacitors and inductors. With respect to BEMF, it is the Capacitor that cause the biggest problems.


The presence of a motor capacitor presents two problems.

1) It generates very high current spikes that passes through the decoder all the way back to the booster.

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2) It confuses BEMF decoder function preventing smooth motor speed regulation or control.

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Remove the capacitor. This involves cutting the capacitor off the motor terminals or off the PCB the connects to the motor.


The capacitor job is to short out the noise so to speak. Keep it contained right at the motor and not allow the RFI noise to get into the wires which act as a transmitting radio antenna. The capacitor impedance is very low at the high radio frequencies which allows the capacitor to appear to be a short circuit between it two terminals to radio frequencies but not with DC voltage/current which has no frequency. To learn more go here:Capacitor Impedance


Bachmann Engines


RFI noise is also know as EMI which standard for Electrical Magnetic Interference. The existence of this motor EMI/RFI noise filters depends on EMI/RFI noise limitations set by various country's standards. Engines that are sold around the world will often need these filters to be installed on the motor. The filter is designed to meet all of the standards. Limiting RFI is very important so much so the companies selling electrical based products must comply with country rules regarding RFI. A companies sale of a product that does not comply comes with strict penalties. This is vary important to companies that sell products world wide.

To learn more go here. RFI & Governments