Why Capacitor with dc motors??

[ahmed osama]

Dear ALL

now i always read in many places that Capacitors prevent sparks so they sometime are connected to the inputs of DC motors to prevents the small sparks happend inside it??

is it true ?? and what is the theory beside that??

thanks all

 

[umery2k75]

I haven't heard anything like it in DC motor, where as in AC motor, it's used to kick start the direction of motor, either clock wise/anti clock wise.

 

[vomit]

The continous switching of coils inside the motor would produce all kinds of brief high-frequency pulse currents. The capacitor close to the motor will discharge and deliver some energy to satisfy these current pulses, so that they don't need to draw current through the long (inductive) cables, with all kinds of radiated noise as a result.

 

[banjo]

Remember that the voltage across a coil or inductor is
L* di/dt. Therefore, any rapid change is current through a coil will produce large voltage spikes. In a motor, the armature has many sets of windings, each set is energized in turn from the brushes. As the brushes switch between a previously energized pair and the next sequential un-energized one, a large voltage is generated at the energized one. This causes sparks at the brushes. These arcs have high frequency rings that can cause interference if radiated out the motor leads. The capacitor at the motor tends to suppress this.

You can experiment with an AM radio and a small motor to see the effect.

 

[umery2k75]

@banjo

You can experiment with an AM radio and a small motor to see the effect

What kind of experiment would this be? You means to say, AM radio waves would get disturbance near the motor, is that what you mean by...

 

[banjo]

Let me explain in a little more detail. AM radio uses, of course amplitude modulation. This is very susceptible to noise interference. Take an AM radio and tune to a quiet spot where no station is broadcasting. Now hold the motor close to the radio and turn it on. The motor noise will be heard over the radio. Move the motor away and the noise will fade. Attach the motor with long wire leads and noise will get worse.
The AM radio provides a crude auditory rf analyzer. This trick has been used for years as a simple test. If works for all kinds of RF that is within the radio's tuning range. The idea is to place the radio and motor at a fixed distance known to cause noise on the radio. Next add a capacitor at the motor and note whether the noise level changes. Vary the capacitor value to see where the large decrease occurs.

Another example, take your TV remote and place it next to an AM radio. Press a button on the remote and the radio will chirp. It is reacting to the RF field generated by the transmitting remote. Many times I used this trick to repair TV remotes without having the customer's TV in the shop. First, I verified that the remote flunked the AM radio test. Then I checked connections, cleaned the buttons, etc until it would pass the test.