Question

Design a power supply to deliver 4 to 9 Vdc adjustable voltage to control DC motor speed. Motor power at maximum load is 20W at maximum speed (maximum voltage). Supply is 12V car battery with 70Ah capacity. How long the battery will last at maximum load and maximum speed? Add a 13.8Vdc communications power supply to continuously charge the battery at 500mA.

Answer 1

I have small doubt about power supply selection for a running motor. I have a small dc motor, which is rated for 12V , 3A(rated ). When the motor runs with a load 4000N, the current consumption is 1.5A.

So I have to choose a 12V, 3A = 12 * 3= 36W power supply to run the motor.This is because DC power supply can supply continous 3A current without any disturbance.

Now I wanted to run same motor on battery. I would like to know how much power should be supplied by the battery to run the motor theoretically.

When motor runs on battery, it takes full current from the battery; as per formula (e=ldidte=ldidt). It said that current required by motor = 3 ×× current required while running on starting.

When we run the motor on battery eventually battery voltage got dropped, taking more current.

I would like to know what is the percentage of current it takes.

If we consider above system 12V, 75Ah battery is it efficient?

Can someone explain with proper calculation here?

While running on battery; motor will turn on every 5 minutes and after that it will be turned off. The motor is made to run 11 hours on daily basis.

Before calculations you need to be clear about some fundamental concepts about motors. "It said that current required by motor = 3 × current required while running on starting." This statement can not be true, because motor draws more current at start up than it rotates in operational angular velocity.

"When we run the motor on battery eventually battery voltage got dropped, taking more current."

This statement is also false. Motor speed ----> Voltage, Motor torque -----> current.

My advice is as follows. Your application does not require detailed calculations with motor specs. What you need to do is measure the current when your motor rotates under the load, which you choose. Then compare it with battery capacity.

About voltage drop of battery, it does not draw more current when battery gets low on Voltage. Voltage is about speed(Theoretically). Thus, as long as your load is the same, theoretically, motor draws same current.

NOTE: Of course current change when the Armature voltage changes, but this is negligible.