BJT said:
Hi....
when the DC MOTOR start working, it will draws a huage current which may damage the motor. if we put a resistance in series with armature resistance, we can control the current.therfore,the armature current is constrained within 100 to 200 % of its rated value during the start-up.
my quastion is:
how can i discounct the resistance when the motor is starting up?
if i make the resistance as a swithces, how can i disconnecte them.
a piece of very thin wire can take a million amps, but for a very short period of time (in this case an extrealy short period of time).
A DC motor can handle this startup current, otherwise it would not of made it out of the design phase.
The reason this exists is becuase of
1) windage - effectivly wind resistance that needs to be gotten over
2) stiction - bearing sticking
3) inertia - the rotor's mass will wnat to keep it static
All these things are drastically reduced once the rotor is rotating (although windage still exists and gets worse with increase speed), but until that occurs these hinderences (whcih appear as a torque-load) MUST be overcome, to overcome torque, more current is needed.
YOu could switch in/out resistors BUT two probmes with this
1) an electrical machine is effectlivly an inductor (be it a variable inductor, but still an inductor), you
ust not open circuit an inductor when it has current flowing it it.
2) with a resistor to limit the startup current you will either drastically increase the time it takes to get to speed, or worst still it wont be able to start
and lets keep things civil