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Controlling the speed of a DC motor

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engineheat:
Hi,

I have a low speed, low torque application that I want to use a DC motor for. Here is a motor that satisfy my size requirement:



It appears to be a brushed DC motor. But the rpm at around 20,000 and that's way too high. I need about 500 to 1000 rpm. Again, the load is very light (close to nothing). To bring down the rpm, I can either use a gear train, or use PWM. I have a few questions:

1. A gear train trades rpm for torque, so if I get 1000 rpm at the output, I get a corresponding multiplier for torque. Is this also true for PWM speed control approach?

2. Is it even possible to bring down the rpm from 20,000 to something like 500 using PWM? That's a super small duty cycle.

3. Can you recommend a general approach for my problem? FYI, I also want to reverse the spin periodically (I guess a H bridge is for this?)

I have been playing around with the 28byj-48 stepper motor lately. I guess I can use a stepper motor but I heard it's not as efficient.

Thanks

phil from seattle:
I would definitely use a reduction gear. You can reduce the size of the motor if you do that. PWM does not increase torque.

Yes, HBridge for reversing. gear reduction to get you close to your speed range. PWM for fine control.  I'd also suggest and encoder fro feedback to control the exact speed.  PID is a good way to control speed, btw.

Nusa:
The generic answer is change the voltage to change the speed. However, you have to have enough voltage to start the motor in question.

2) Doubtful. You're clearly way outside the limits of that motors design.

3) Find a motor that's intended to operate in the range you desire.

oldway:
What's the power supply voltage ? I guess 5V, is it right ?
Choose a 24V motor instead of a 3.6V motor.

What's the precision and stability of the speed you need ?
For high stability, you should use a feedback.
Speed information can be obtained from another dc brushed motor coupled on the same shaft or from an optical or magnetic pulse generator.

engineheat:

--- Quote from: phil from seattle on January 12, 2018, 07:29:05 pm --- I'd also suggest and encoder fro feedback to control the exact speed.  PID is a good way to control speed, btw.

--- End quote ---

By “encoder” do you mean a rotary encoder? with the shaft of the rotary encoder coupled to the motor output shaft?

thanks

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