Do anyone know that what is the purpose for a resistor parallel to DC brush motor?

resistor form of dynamic braking called resistive braking. load on windings of motor spinning freely when power (current or voltage) not applied external source.

since motor has permanent magnets, magnets induce current windings , current must bled off of voltage rise , jump across commutation elements.

if commutation performed electronically (as today) mosfets or igbts controlling commutation , voltage rising motor spinning fets off rise above tolerance of fets , destroy them.

if there batteries sourcing power motor drive (such in electric vehicle), commutation fets can left on , redirected battery thru regulator rechage batteries. called regenerative braking. regulator has alot of capacitance , super capacitors being used support better means of regenerative braking.

resistive braking energy being lost heat, unless heat useful, more wasted energy. resistor brake can hot fast , ususally needs big heatsink , large wattage value.

1 final note brake resistor must higher value (> 10x) of coil impedance or motor may burn out. problem if high, not dissipate enough energy fast enough. typically values brake resistors between 0.1 ~ 100 ohms.

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