I have a PM DC motor. I am controlling the motor speed using PWM. I wanted to know what is the relation between the torque vs dutycycle and speed vs dutycycle for the motor( given the fact that the peak amplitude of the PWM signal is Vm and frequency is f) . Characteristics of the motor in the datasheet are only stated relative to armature current. How do you relate armature current with the PWM voltage signal?
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2\$\begingroup\$ Maybe reading this helps understanding \$\endgroup\$– HuismanCommented Feb 25, 2019 at 14:41
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2 Answers
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Ideal PM DC motor speed (RPM) is proportional to the voltage applied. Armature current is proportional to load torque. (Unloaded ideal motor draws no current.)
Real motor can be modeled by adding series resistance R (wire DC resistance) and series inductance L (leackage inductance). Due to R, speed will lineary decrease with current (torque). L can be used as filtering inductance for PWM (if PWM freq is high enough). In this case effective voltage applied to the motor is proportional to duty cycle.
Summary:
- No-load speed is proportional to PWM duty cycle.
- Current is proportional to torque (controlled by mechanical load).
- Loaded speed decreases proportionally to current (torque).
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Armature current is a function of the applied voltage, the back EMF, and the armature impedance (primarily resistance and inductance). Back EMF is a function of speed, and speed is a function of torque and load.
All of these things are interrelated, and there is no simple mapping from duty cycle to any of these parameters.
If you want to control speed, your best bet is to use a speed sensor and a controller that uses the sensor output for feedback.
answered Feb 25, 2019 at 15:29
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\$\begingroup\$ Is there a way to find these experimentally? If so is it worth it? \$\endgroup\$– SohamCommented Feb 25, 2019 at 16:15