How I can keep motor same power in the falling battery voltage?

Question

I have a machine project, which is going to work with a Lithium-ion battery. As you know, as a battery is used, its voltage drops with time, and I’m looking for a way to maintain constant power to the machine, and I'm asking for a method that will really work.

This machine uses a fan having a minimum required air flow speed of 3,000 RPM for cooling. But the dropping battery-voltage presents a big problem in maintaining the fan's minimum RPM. Do you think the method which I am going to describe now will meet this requirement?

To begin with, I found a DC motor having a wide working voltage range. For example, I found a DC motor having a 40 V maximum working voltage, which produces 3,000 RPM at 24 V, so I built an 8S Lithium-ion battery. When the battery is fully charged, it produces 8 * 4.2 V = 33.6 V, and the motor will be driven using 72 % duty-cycle PWM, which will produce an average voltage of 24 V, which will achieve the necessary 3,000 RPM. After a while, the battery voltage will drop, which will be sensed using a voltage divider circuit, yet we're going to produce the PWM modulation that will maintain the required 3,000 RPM (meaning that, with each voltage drop, the PWM will be slightly increased). Finally, when the battery voltage drops to 3 V, the microcontroller will produce a 100 % duty cycle PWM modulation. Using this method, a constant motor RPM will be maintained in spite of changing battery voltage.

Do you think this method will work? What is your recommendation?

Answer 1

Choose a motor working voltage either above the entire range of your battery voltage, or below the entire battery voltage range.

In the first case, use a boost DC-DC converter to supply the motor, in the second case use a buck DC-DC converter.

It's probably easier to source a suitable ready made buck or boost DC-DC converter module with an adjustable voltage output from one of the popular online retailers, than to build your own PWM solution.

What you have described, using PWM to achieve voltage stepdown into a motor load will also work, but you'll need to use a brushed PM motor.

Note that if you use a brushless motor, the ESC used to drive it will often accept a wide input voltage range directly from your batteries, effectively working as a buck converter, and can control the speed.

Answer 2

Do you think the method is correct, what would your recommendations be?

That pretty much describes a buck converter. A buck converter takes a higher input voltage and maintains a lower output voltage close to a regulated value until such time that the input voltage drops too low whereupon the buck converter can only supply 100% duty cycle (at best).

Or, you can use a boost converter to produce a higher regulated output voltage from a lower input voltage (again using PWM techniques) until such time that the input voltage drops below a lower working threshold for the circuit and battery recommendations.

Or, you can combine buck and boost into (wait for it...) a buck-boost converter that although more complex and slightly less efficient than either buck or boost individually, is used in many applications.