The OP of this thread asks for a spark suppression on a brushed DC motor.
First of all, can a spark at brushes/the commutator better be described as a spark or as an arc? A spark is instantaneous, while an arc is sustained. At what duration do I say arc or spark?
Let's say the brushes ar connected to two poles of a multi-pole DC motor. When the brush leaves the commutator the distance is so small that immediatly a spark begins to appear between both. That means current flow continues. When the spark stops, the inductance of the winding that was just energized causes a voltage to appear on that winding. When this voltage is high enough sparking can occur again.
Is my reasoning correct so far?
The voltage on the winding is inaccessible since it is on the rotor. So a true spark suppression would only be possible on the winding not the stator, as this website seems to confirm.
A snubber on the stator would help to avoid propagation of transients on the power supply lines that's true, but what would it do to the length/intensity of the spark? There are often capacitors directly mounted on the motor terminals. How do they contribute to spark suppression? Do they at all? Or is their main function to suppress RF? Or to provide sustained current in high frequency PWM applications?
I understand that having a capacitor at the motor terminals shunts the high voltage that would come from the winding through the sparks to them. Without it there is still the motor power supply wire inductance in series until we get to the main DC-bus caps.
Informations on the web are contradictious. I tried to find a reliable source of information on this subject but failed. Something from a motor manufacturer maybe or a university.
THIS document describes very well forms of arc suppression and that these methods have no or little effect on the spark generation.
Can anybody shed some light on this? Some practical experience maybe? Did someone measure the length of sparks?
Btw, the answer to question 9 in AACs http://www.allaboutcircuits.com/worksheets/dcmotor.html does not sufficiently explain why commutating a winding would cause a spark.
First of all, can a spark at brushes/the commutator better be described as a spark or as an arc? A spark is instantaneous, while an arc is sustained. At what duration do I say arc or spark?
Let's say the brushes ar connected to two poles of a multi-pole DC motor. When the brush leaves the commutator the distance is so small that immediatly a spark begins to appear between both. That means current flow continues. When the spark stops, the inductance of the winding that was just energized causes a voltage to appear on that winding. When this voltage is high enough sparking can occur again.
Is my reasoning correct so far?
The voltage on the winding is inaccessible since it is on the rotor. So a true spark suppression would only be possible on the winding not the stator, as this website seems to confirm.
A snubber on the stator would help to avoid propagation of transients on the power supply lines that's true, but what would it do to the length/intensity of the spark? There are often capacitors directly mounted on the motor terminals. How do they contribute to spark suppression? Do they at all? Or is their main function to suppress RF? Or to provide sustained current in high frequency PWM applications?
I understand that having a capacitor at the motor terminals shunts the high voltage that would come from the winding through the sparks to them. Without it there is still the motor power supply wire inductance in series until we get to the main DC-bus caps.
Informations on the web are contradictious. I tried to find a reliable source of information on this subject but failed. Something from a motor manufacturer maybe or a university.
THIS document describes very well forms of arc suppression and that these methods have no or little effect on the spark generation.
Can anybody shed some light on this? Some practical experience maybe? Did someone measure the length of sparks?
Btw, the answer to question 9 in AACs http://www.allaboutcircuits.com/worksheets/dcmotor.html does not sufficiently explain why commutating a winding would cause a spark.