# Why does increasing torque on DC motor in circuit affect the speed and functioning of another DC motor in the circuit?

I am working on a project in which there are system components listed below as module:

1. 8 ultrasonic sensors 12-30 V (Sensor module)
2. 3 DC motors (Drive module)
3. 35 V to 5 V buck converter and a 35 V - 5 V isolated DC-DC converter (Power module)
4. 1 Microcontroller (Controller module)
5. 2 Encoder (Encoder module)

Let me explain the required functioning of the module. I am using a LiFePo4 battery (26.5 V, 18 Ah) for powering the PCB and I have isolated the sensor module and drive module using optocouplers for the safety of my microcontroller.

The motor module optocoupler uses 5 V logic and the sensor module optocoupler uses 3.3 V, so the sensors gives signals to the controller through an optocoupler and then the controller gives a signal to the motor driver through an optocoupler and the motor runs accordingly.

I am using a buck converter circuit to convert incoming power from the battery to 5 V that it can deliver to the motor side optocouplers and I am using another isolated DC/DC converter circuit to convert the battery voltage to isolated 5 V to power up the microcontroller.

My microcontroller can source 3.3 V to the sensor optocoupler circuit and direct power from the battery is going to the power sensors and the motor driver; there is no regulation or any other circuitry.

Let me attach details of components:

1. Buck converter IC 35 V - 5 V MC34063
2. Isolated DC-DC converter Hi-link 18~36 V to 5 V
4. 2 low power Motor driver DRV8872
5. 1 High power motor driver Cytron MD10C

I don't have datasheets of the sensors, motors, and encoder right now but I remember some current ratings:

• Each sensor's current output is 100-200 mA; operating volatge is 12-30 V
• High power motor continuous current which I have noticed is 2-3 A but at some places when it require more torque it reaches 4-4.5 A and the operating voltage is 20-30 V
• Low power motor of which are are two each normally take 500 mA; at some places when they require more torque current reaches to 900 mA and the operating voltage is 20-30 V

My question: when I test the whole system on the bench, and there is almost no torque compared to normal operation, when I then try to put torque on the high power motor, why do both low power motors start behaving abnormally?

They start running with a jerky motion. I have placed a LED on the input from the controller to the driver, that LED also flickers so it seems the motor driver gets a similar signal from the controller I put torque on the high power motor. I want to know why it is happening even though the circuit is isolated (24 V <> 5 V).

Let me share the circuit diagram:

• No these regulators are only to to give power to the controller and optocoupler for motor and sensors and I am giving direct power from battery input which is Vin for sensor and VM for motor driver Dec 5, 2021 at 18:22
• You should check what happens to the LiFePo4 battery (26.5 V, 18 Ah) voltage when one or two motors draw a current of 4.5 A.
– Uwe
Dec 5, 2021 at 18:30

Why does increasing torque on DC motor in circuit affect the speed and functioning of another DC motor in the circuit?

It looks like all three motors share the same power rail (VM) so, if one motor is drawing a lot of current the VM voltage may sag and reduce the speed on the other motors. If you want to avoid this then, make VM more powerful or, regulate VM with a sufficiently rated regulator to avoid droop. You can of course use individual regulators for each motors to help avoiding "crosstalk". Maybe you have some form of feedback that you can use for adjusting the control signals to the motors that are labouring on the diminishing VM supply?

• In the circuit all three motor driver using same power rail is it a bad practice ? how I can make VM more Powerful Dec 15, 2021 at 7:31
• It's only a problem if VM sags and causes the other motor speeds to be affected. It certainly isn't generically bad practice. I can't tell you how to make VM more powerful until you disclose information about it. Dec 15, 2021 at 7:40
• What Information do yo need Dec 15, 2021 at 7:42
• Look at the info you have already put in your question and see how you can improve it by adding links to data sheets and a link to the place you bought it. However, it's beyond the scope of this current question to make an amendment to it so, I suggest you leave links in a comment below this one. Dec 15, 2021 at 7:45
• OK, just checked and there appears to be a link but please identify which one for voltage VM. Dec 15, 2021 at 7:48

If the behaviour is also replicated on the LEDs output to the controller there is probably something wrong with the controller when powering the motor at high power.

It would be a good idea to check whether the controller is 'happy' enough to supply the high-power motor with that current.

If not, i.e. feeding the low-power motors, does something similar happen to the high-power one?