# CLL(Capacitor long life)capacitor filtering to reduce the current spikes

if we are using capacitor between the coomutation segments of brush dc moto then it reduces spark generation. as main function of commutation system is to reverse the current in the coil. by using capacitor filtering we can reduce the current spikes

i have basic understanging regarding commutation system its principle and fuction.

• Capacitors are used to filter out ac components in your dc rail. When you connect the motor to your power and try to run it noise can be caused due to the following reasons – Board-Man May 11 '15 at 11:49

1. In any DC motor/Gen. there are multiple coils generally equal to the number of commutator segments.
2. when a brush passes over a comm. segment the current is reversed in that coil from say +I to -I.
3. If inductance of that coil is say L henry then voltage induced in the coil= 2*L*dI/dt.
4. dI/dt is decided by the speed of the machine.
5. If say L=5 mH and current is 1 Amp. and current changed from +I to -I in 0.1 millisecond then the voltage induced=2*1*5*10^-3/0.1*10^-3=10000 Volts which would instaniously develop (figure are deiberately taken on the higher side!) and may break the air gap between the segment(now the segment is away brom the brush) and sparking will be seen between the segment and brush. This happens with every segment.
6. a capacitor forms a tank circuit between coil inductance and forms a voltage having a lower rate of rise across capacitor CVc=It means Vc=I.t/C 7.If C=1 micro Fd then Vc=1*0.2*10^-3/1*10^-6=200 volts. (p. note that all the values are illustrtive just to show he effect of capacitior.) vtingole
• thanks vijay ...it mean capacitor 200v will supressed to 10000 volt and then only induced voltage will 10000-200=9800volt..is it..? – ketan patil May 13 '15 at 5:54
• No. The current in the coil doesn't change instantly, only the voltage. The current obeys the usual rules. The sparking is because the inductor/coil is left open circuit for a while, resulting in a large voltage. – tomnexus May 13 '15 at 6:02
• A capacitor is an electronic component that will conduct only currents that are changing at a high frequencycz at high frequency capacitor offer low impedance and current flows....please can you explain what is this fequecy related to dc motor how it is calculated..?? – ketan patil May 13 '15 at 6:40

Capacitors are used to filter out ac components in your dc rail. When you connect the motor to your power and try to run it noise can be caused due to the following reasons - 1) Switching of MOSFETS. When MOSFETs switch there is a spike generation for a few micro or such seconds. These are due to an arcing effect akin to the effect of switching when we press a switch. 2) When motors start to run (brushless) they dont have a syncronised way to commutate and they tend to draw a lot of current from the dc source. This sometimes causes the dc line to fall down or reduce.

In all these cases capacitors filter out ac spikes to give a smooth dc level. But if you are using MOSFETs to switch PWMs then these spikes will exist and cannot be wished away. Another point is useage of type of capacitor. Please use low ESR (ideally) capacitors as they will not heat up during the constant dischargeing process.

10000 and 200 volts are solutions of two independent equations.. means if cap. is added the self induced 10000 voltage will reduce to 200 volts. current in the coil does not change instantly so it charges the said capacitor at at constant current as per (6). vtingole