starting up the DC MOTOR

[BJT]

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.

 

[Someone Electro]

The windings can handle that curent peak whith no problem.An DC motor cant start on its no load runing curent.This peak is the same as the motors stall curent (It can work at that cureent for about 10s whith damage)

btw: READ THE RULES!!! :evil:

 

[Nigel Goodwin]

Thread moved from Electronic Projects forum.

Moderator.

 

[Someone Electro]

Thats better.

There are rules that only completed projects go in to the projects section.

 

[BJT]

The windings can handle that curent peak whith no problem.An DC motor cant start on its no load runing curent.This peak is the same as the motors stall curent (It can work at that cureent for about 10s whith damage)

btw: READ THE RULES!!! :evil:

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i think,

your replies should be more polite because i did not do any crime.

 

[Styx]

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

 

[BJT]

thanks styx,,

your answer is really good, i understood the main concept now.

i will try to do the analysis,,


i will ask u again

 

[Someone Electro]

Sory but you didnt read the rules for posting in the projects sectionand also violated them)
https://www.electro-tech-online.com/threads/very-strange.16180/

On any other froums your topic wod be deleted but since we have realy nice moderators they moved it.

 

[Phasor]

my question 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.

Normally, in Primary Resistance Starters, the resistance is not disconnected, but is rather, shorted out by a speed switch / contactor. Thus, there is not open-circuiting of the inductor.

Also, limitation of starting current is not normally for the purpose of preventing motor damage - it's to limit transients (spikes) and voltage sags to surrounding customers.

 

[BJT]

my question 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.

Normally, in Primary Resistance Starters, the resistance is not disconnected, but is rather, shorted out by a speed switch / contactor. Thus, there is not open-circuiting of the inductor.

Also, limitation of starting current is not normally for the purpose of preventing motor damage - it's to limit transients (spikes) and voltage sags to surrounding customers.

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hi........

yeah, you are right mr.phasor. we have to make our resistances to be controled by short circuit them one by one according to our design.So, we will control the armature current as we want.

is it right?

 

[ikalogic]

i think, the only thing you can worry about is your motor controller circuit, but the motor it self, i don't think you should worry about, DC motors are made to have a high starting torke, and hence start with high currents...

i never had a problem with one of my DC motors, even though i allways drive them with double their rated voltage....

 

[BJT]

i think, the only thing you can worry about is your motor controller circuit, but the motor it self, i don't think you should worry about, DC motors are made to have a high starting torke, and hence start with high currents...

i never had a problem with one of my DC motors, even though i allways drive them with double their rated voltage....

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Hi....

In startng u the dc motor.

i think we have to take care about the armatur cureent at the beganig because it will be high (armatuer resistance is too small).

but what i am thinking aout how can i make a conrtoler to control the thersistances that i will but them in series with armature,so, i need to vary them according to the cureent.

if th motor start, i need the resistance to be high. then, i will short circuit them one by one. So, the motor will ork in normal case.

my quastion:

how can do this controler (hardware)?
______________________________

Firiend in need is Friend indeeed
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[ikalogic]

well i know JFET (field efect transistors)can be somehow used as a variables resistance....

but you know, there is a relation between the velocity of the saft of the motor and the current, when velocity decrease, current increase, this is in any DC motor, and that's how they are supposed to work.

How ever, you can do it with relays to shortcircuit restistances, you can do that with 555 timer IC...as a timing circuit.... or for perfect results a PIC16f84 (programmable IC)...

why don't you do a search about PWM (pulse with modulation) it is a perfect way of controlling DC motors... you can control the everage current without resistances... but i think you will need a relatively expansive IC to do that .. the L293D, wich can contole motors using H bridge, and isolate the motor feedbacks from the rest of your circuit...

if you can't find.. tell me..

 

[Styx]

For brushed CD machines:
Speed is proportion to terminal voltage
Torque is proportional to winding current

The only reason that the current reduces at higher speeds is because the backEMF, whichh increases with speed, will oppose the voltage applied, thus the effective forcing voltage is reduced which reduces the winding current


An Electrical machine will draw whatever current it required.
You provide it with 50V it might only draw 1A, but increase the torque-load on the machine, then the amperage drawn will increase, but the speed (should) remain the same (if the machine is rated to provide constance speed at that torque level)

 

[JimB]

No body has asked the question:

How big is the motor which you are trying to start?

tiny, 1kW, 10kW, 100kW, 1MW?

It will make a difference to the answer.

JimB

 

[ikalogic]

yes .. JimB is right! it will make difference :lol: ! my last answer was based on motors ranging from 6 to 12 volts... and 2400 rpm...

 

[Someone Electro]

cod you imagine the sparks on the brushes if they made an 1MW DC motor.

Sparking on the brishes becomes an problem wen its an DC runing at more then 1kW.At such powers brushles motors are used.

 

[BJT]

No body has asked the question:

How big is the motor which you are trying to start?

tiny, 1kW, 10kW, 100kW, 1MW?

It will make a difference to the answer.

JimB

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A 10 kW, 100 V, 1000 rpm DC shunt motor.

requierments:

1) the starting resistance to be inserted to limit the starting current twice the full load value. in addition, the armature current should be constrained within 100 to 200 % of its rated value during the start-up.

____________________________________

friend in need is friend indeed

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[BJT]

hi all..........

i did the analysis of finding the needed value of resistances.

the problem is:

how can i make a feed back,so, as the cureent decrses the resistance should be decreases also.

Mr. Jamal gave me an idea of using the JFET as a variable resistance.

i am tring to think how can i use it :?:

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easy come --- :arrow: easy go :wink:
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[Nigel Goodwin]

hi all..........

i did the analysis of finding the needed value of resistances.

the problem is:

how can i make a feed back,so, as the cureent decrses the resistance should be decreases also.

Mr. Jamal gave me an idea of using the JFET as a variable resistance.

i am tring to think how can i use it :?:

Well you've never mentioned the size of the motor before!, and that makes a HUGE difference. I would suggest using contactors (seriously big relays) to short out the starting resistors as speed increases.

One BIG problem of course is that the motor might not start at all!, by serverely limiting the startup current it won't have much torque to start under load.