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Designing Electronic starter for DC motor

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sharpuee

New Member
I have to develop an electronic starter for 230 V, 2HP DC motor as my B.Tech project. After rectification of initial power, I am using MOSFET to control the armature voltage through PWM technique. The whole problem is to control armature current by applying gradually increasing voltage at the time of starting. I don't know which technique would be best for this purpose. For MOSFET control, Some advice to use 555 ramp generator along with comparator, some say to use op-amps only and some say to go for digital timers. Is PWM best technique for this or some better technique is there ?? Will these controllers be able to drive MOSFET connected in 207 V DC power circuit???? Please help me with the power and controller circuit design ..
 

tcmtech

Banned
Most Helpful Member
What? Do you need a slow ramp up time? or is there a power source limitation?
Just use a regular motor contactor onthe AC side of the rectifieres. a 2 hp is not that big.
I would use a full wave diode bridge with SCR's for two of the four diodes. this will allow you to use phase angle control to bring up the speed slowly.
 

Ubergeek63

Well-Known Member
first off, you are better off if you need good control you should start with a switcher chip.

second, the motor voltage is motor speed divided by max speed times rated voltage plus motor resistance times motor current.

you compare a command voltage to the motor voltage and motor current.

Dan
 

sharpuee

New Member
how to implement that ?

thank u tcmtech for response,
plz, can u throw some more light on ur suggestion or any cicuit diagram if u can ... I would like to know which one of the two would be better :
  1. first rectifying the AC to DC and then using MOSFET and driver circuit to produce variable duty cycle, so that we can have gradually increasing voltage at the time of starting
  2. As you said, to control phase angle on the AC side itself at the time of starting. In this case what i guess the automatic control circuit for firing angle will be required. How to do that ??

Of both the cases which one is more efficient and protective. Plz reply with detail or diagram. I am just in bachelors.

Thank u in advance..

@ DAN
I got ur equaltion but didn't get ur idea. Are u suggesting to have speed sensors to calculate immediate speed??
plz elaborate
 

Ubergeek63

Well-Known Member
not at all... the armature voltage is proportional to speed and current. it is easy to control the speed well with out a speed sensor as long as you do not need to know the absolute speed.

set the command voltage an calibrate the controller. the trick is that you typically have to calibrate the IR compensation (speed is proportional to current times armature resistance). All motor analog motor controlers need to have manual trims for this since motors do not all have the same armature resistance.

Dan
 
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