# Replace a brushed motor with a Resistor (+ Diode)?

My first question at this forum was put at hold because initially was bad formulated and never get to normal in spite I had edited the question, so hope more luck now!

Once I am a noobie at electronics I only know to formulate the question in generics terms and because of this, I just can expect some generic help!

We have a Hubsan quadcopter/drone that has 4 brushed motors (BDC) and we want replace them with Brushless Motors (BLDC).

One of us that is more advanced at this conversion project has connected the 4 ESC (necessary for (BLDC motors) to the correspondent Mosfet. The “signal wire” of our ESC is a twisted par with white and a black wires, so the PWM Signal/White wire is connected to the Gate and the Signal/Ground to Source of Mosfet.

With these connections the BLDC are working, so the quadcopter is flying! The problem is for that happen we must keep the brushed motors connected to the Flight Controller/Receiver (integrated) and spinning at same time as BLDC! Each BDC is connected by two wires to the correspondent + and - of the FC/Receiver (see picture)!

One of us has thought that a Resistor could replace the BDC but the truth is that after applying the Ohm's Law to calculate Resistor specifications and connect it to + and – where the BDC were connected, the BLDC don´t work anymore!
The question is: if the resistor alone do not replace the BDC (or we are applying incorrectly the law) can you give us some tips to further search?

Note - brushed motors draw 1V & 0.05A at idle and 6.5V & 1A at full throttle!

the motors are inductive with series R but also generate back EMF Voltage rising with RPM until prop with wind loading draws more current, so impedance at full load is 6.5V/1A= 6.5Ω equivalent, but nonlinear.

## Nonlinear Z

DCR of motor may be < 1Ω so if full throttle is applied at idle, the surge current may be as much as >6.5V/1Ω = 6.5A and reduces to 1A proportional to RPM which depends on ESC, prop load , weight , climb rate, so Amps will increase, with these factors.

Normally series R to drop 75 mV max senses output current for feedback.

• Thank you @Tony by your clear explanation! I understand now that Ohm's law cannot be used linearly here but I have difficulties with the last paragraph, because I am not english native and not an expert at electronics. To be honest I am not understanding the whole sentence! Can you say it in another words what are you suggesting? Note - only you and another expert use the abbreviation "Series R" at SE, so a technical doubt also! Commented Jul 30, 2017 at 18:45
• all coils including motors are rated in DCR the "series R" at DC. This is a fact. It is similar to the effective "series R" or ESR in dielectrics like Caps and Batteries except ESR can change from DC to some freq, f for batteries. I was listing some load factors for motors that control current at full RPM, like prop load. No Worries but don't doubt me. Commented Jul 31, 2017 at 2:16
• As a noob @Tony I could not doubt you at all! I only wanted understand the meaning of Series R once I googled a lot and had 2 answers. Series Resistor and found also a Series R specific type (Brand?) of resistor! I think I understood now. But can we think at a Resistor to replace the motor or not? Thks a lot! Commented Jul 31, 2017 at 7:47
• if I understood well the "load factors" as props and wind are not present here once the brushed motors are inside the body only with the shaft as you can see at the picture! As I tried to explain at first post, the shafts are spinning at the same time as BLDC, otherwise these do not run! Commented Jul 31, 2017 at 11:15
• Motor generates Back EMF and draws power in energy conversion by external torque but , Resistor just converts energy to heat loss.. We say R is series or parallel. In this case "in series " Commented Jul 31, 2017 at 16:34

The solution is not a resistor so I will follow another track based at a comment of expert Spehro Pefhany that says about a way to decrease RPM of a brushed motor "I think you could do simple negative source resistance (also called IR compensation) with an op-amp driving the reference terminal of a linear regulator, which might be appropriate for a small motor" Special thanks to Tony Stewart.