# open loop dc motor control

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#### seyyah

open loop motor control

In an open loop pwm control of a dc motor what makes the starting torque be enough for the load to begin its movement?
For example which choice is good for this purpose
1000 rpm 24V 300W motor? or
1400 rpm 24V 300W motor? or
1400 rpm 24V 200W motor? or
1000 rpm 24V 200W motor?
I have also choices of using a gear turns ratio of 5 to 10.
I tried 1/10 turns ratio and 200W 1400rpm motor. It has problems at starting and top speed is near enough (it musnt be lower than this speed)

Is 1000rpm 300W motor with a gear ratio of 1/5 helps me?
What other suggestions can you propose?

#### jamesyang1209

##### Full Member level 1
static friction in dc motor

I think you got confused about friction, watt and gear ratio.
If your system has no friction, you only need to calculate the gear ratio and motor rpm, you will know the output rpm. You don't need to worry about the Watt of motor.

But, in fact, the friction comes.
So, you need to care about is torque.
I mean, more watt and less rpm motor will give you more torque to overcome the starting friction (static friction).
And, more gear ratio, the more output torque you get. That is, 1/10 will output more torque than 1/5.

Yes, it's hard to calculate, since many variables. And the friction is hard to measure.
I sugged you to check the motor's torque first.
And re-calculate this problem.
Try error is another method.

#### CMOS

As far as I know DC motors are generally not controlled in open loop since speed variation due to change in supply voltage is much larger than ac motors. In fact ac motor's speed depend very much on frequency rather than voltage.

#### jamesyang1209

##### Full Member level 1
CMOS said:
As far as I know DC motors are generally not controlled in open loop since speed variation due to change in supply voltage is much larger than ac motors. In fact ac motor's speed depend very much on frequency rather than voltage.

No.
In manufacture, lots of motor controled only in open loop with limit switch.

#### river78

##### Member level 1
Hi!
I think the torque of motor depend on power of motor. But in your case, i think u should have an current loop control , i will improve the start torque motor.
Good luck!

P = T * w

#### ricky_wcy

##### Junior Member level 3
Agree, in manfacturing, open loop control is used in driving dc motor for limited switches

Any circuit design for such condition ?

#### Mr.MEB

##### Member level 3
In process plants DC motors are controlled both in Torque and speed.
Look at paper mills you will see plenty of them.

#### ricky_wcy

##### Junior Member level 3
sorry, what is paper mills ?

#### Mr.MEB

##### Member level 3
A paper mill is a factory devoted to making paper from wood pulp and other ingredients
https://en.wikipedia.org/wiki/Paper_mill

It's a good example for applying continious systems control theory.

#### yangxh

##### Member level 5
jamesyang1209 said:
CMOS said:
As far as I know DC motors are generally not controlled in open loop since speed variation due to change in supply voltage is much larger than ac motors. In fact ac motor's speed depend very much on frequency rather than voltage.

No.
In manufacture, lots of motor controled only in open loop with limit switch.

you mean PWM?

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