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I am proposing to run a nominal 12 volt DC motor using a LiPo battery providing a maximum voltage of 16.8 volts. The motor will be controlled by PWM so my understanding is that I do not need to regulate the input voltage to 12 volts provided I monitor the input voltage and set the maximum duty cycle to the equivalent of 12 volts. Is this correct?

I am also intending to monitor the input current. The motor has a 5 A stall current so I am intending to set a trigger voltage of say 3 volts where I will do something! The question is what to do. My hazy understanding of back EMF suggests that if I simply stop the motor or drastically reduce the duty cycle I could actually make the problem worse in the short term. What is the best action to take?

Thanks.

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  • \$\begingroup\$ No guarantee, but there is a good chance this will be OK. The higher your PWM frequency the better the inductance of the motor will mitigate voltage spikes. The lower your PWM frequency, the more similar it becomes to simply being ON at an unacceptable voltage for periods of time. Motors are often not particularly sensitive to instantaneous voltage, but there is still chance of a short or burnout. \$\endgroup\$
    – K H
    Feb 28, 2021 at 19:42
  • \$\begingroup\$ Worth noting even if it survives and functions well, you may significantly decrease the lifespan of anything electrical if you run it beyond its ratings. \$\endgroup\$
    – K H
    Feb 28, 2021 at 19:44

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PWM works to reduce speed because the mechanical inertia smooths out the rotation. But there is also electrical inertia: inductance which is much less, reacts much faster, and is therefore much more peaky.

This would actually work if there was enough inductance inside the motor windings relative to the switching frequency to smooth out the current so they aren't so peaky, but in reality this is usually not the case and you can almost never add more inductance because it's too would be too big, heavy, and expensive since that inductance would have to handle the same currents the motor can handle. You might as well just use a bigger motor where the coils of that extra inductor are inside the motor and can contribute to the work done by the motor.

The peaky currents from running in such a situatation is similar to knowing that you can withstand some number of punches, and hoping that also means you can withstand half as many punches twice as hard. We obviously know this isn't always the case.

You can reduce (or perhaps even eliminate) the extra inductance needed by running at higher PWM frequencies. This isn't an option for most people though because they have little control over the motor driver.

If this option is open to you, it does come with issues such as increased high frequency losses in the motor such as eddy currents, and more heating in in the motor driver due to increased switching losses. But significantly better than your initial plan of just turning down the duty cycle.

I am proposing to run a nominal 12 volt DC motor using a LiPo battery providing a maximum voltage of 16.8 volts. The motor will be controlled by PWM so my understanding is that I do not need to regulate the input voltage to 12 volts provided I monitor the input voltage and set the maximum duty cycle to the equivalent of 12 volts. Is this correct?

You also really do not want to regulate the motor voltage if you don't have to since regulating high power is tricky and the motor is often the highest power consumer in the system. You should design everything else around it and regulate the smaller loads, not the largest ones. Can't you just use a properly matched battery voltage?

But you may be in luck since max battery voltage doesn't last for very long as the cells discharge as long as your battery isn't massive mAh. It will spend most of its time around 14.4V which is just on the border of what I would be comfortable with (20%). If it always remained at 16.8V, I would not attempt it.

In this case, given the choice between regulating or just living with it, I would live with it. That's how bad regulating is.

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  • \$\begingroup\$ I like the analogy of the boxer. I can raise the PWM frequency so that is what I will do. \$\endgroup\$ Mar 1, 2021 at 10:02
  • \$\begingroup\$ @AlanJohnstone It helps if you can measure current to observe the results and/or inductance to do some calculations on the frequency required. \$\endgroup\$
    – DKNguyen
    Mar 1, 2021 at 16:38
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A lipo with maximum voltage of 16.8 V probably has an average discharge voltage of ~15 V. This is only 25% higher than 12 V. If the motor is designed for automotive use it will have to handle up to at least 14 V anyway, so you are not running on it much more voltage than it was designed for. The effect of the higher voltage will be ~25% higher rpm and stall current than at 12 V, and slightly higher no-load current. Most motors can handle this small overload with a small reduction in lifespan due to increased brushed wear.

The motor will be controlled by PWM so my understanding is that I do not need to regulate the input voltage to 12 volts provided I monitor the input voltage and set the maximum duty cycle to the equivalent of 12 volts. Is this correct?

Yes. Just make sure the PWM frequency is high enough for the winding inductance to significantly smooth out current flow. Iron cored motors generally need at least 3kHz, but 20kHz is better for reducing acoustic noise. Coreless motors generally need 32kHz or higher. If your motor 'cogs' when turned by hand then it has an iron core.

The motor has a 5 A stall current so I am intending to set a trigger voltage of say 3 volts where I will do something! The question is what to do.

If the motor may stall then include a current limit circuit to protect it from overheating. You may want to set the limit to less than 5A if the motor is not rated for continuous operation in stall (few are).

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  • \$\begingroup\$ Thank you for taking the time to answer in detail. I will increase the frequency and look out for puffs of smoke. \$\endgroup\$ Mar 1, 2021 at 10:04

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