Would it be possible to detect when a DC brush-type motor is stalled by applying a coil to the outside of the motor and detecting induced current or lack thereoff in the coil?

We will be using a 12 volt windshield wiper motor. It runs at about 6 amps most of the time but there are start and stop cycles that draw 25 amps for less than 100 milliseconds. We plan to use a microcontroller with a 10 bit (1024 levels) analog to digital input to detect whatever voltage can be generated by the external coil to determine if the motor is stalled or not. If it is stalled, then the 12 volt drive voltage will be removed.

It is desired to create the stall sensor with the most simple components possible. For example, a coil of wire, discrete resistors, common transistors, etc.

Sensors such as hall-effect sensors, rotation sensors, and torque sensors are not available.

 

What is wrong with a series resistor and measure the voltage drop across it..?

 

Sensing the motor current can help to detect a stall, but the current will also spike dramatically when the motor starts.
You can soft-start the motor with PWM to avoid this spike.

If you intend to try to reverse the motor after the stall, beware that the motor might not have enough power to un-jam the stall.

This phenomena occurs because the energy stored in the spinning rotor (inertia) gets dumped into the load when it stalls, this torque can be MUCH more than the motor can produce from a standing start.

 

What is wrong with a series resistor and measure the voltage drop across it..?

We want to deploy the system in areas where high-wattage resistors are not easily bought.

 

Sensing the motor current can help to detect a stall, but the current will also spike dramatically when the motor starts.
You can soft-start the motor with PWM to avoid this spike.

If you intend to try to reverse the motor after the stall, beware that the motor might not have enough power to un-jam the stall.

This phenomena occurs because the energy stored in the spinning rotor (inertia) gets dumped into the load when it stalls, this torque can be MUCH more than the motor can produce from a standing start.

It is possible that we can soft-start the motor but because I am not a motor expert, I wasn't sure if an external coil could be used to detect a stall. Thanks for your answer.

 

Hi
imho, it's possible. Unfortunatelly, the induced current will be not stable and greatly depends on motor shielding/case thickness ,motor speed , foreign magnetic fields, etc.
I guess, a much simpler and reliable is the optical sensor. Perforated disk, LED and photodiode , simplest amplifier and missing pulse detector built on 555' timer-it's all you need

 

We want to deploy the system in areas where high-wattage resistors are not easily bought.

How about 10 x 1R 1/2W resistors in parallel. 6A means 0.36W per resistor and voltage of 0.6V for processor to measure and over current should be brief.

 

Wrapping one of the motor supply wires around a reed switch might work. The amount of wrap (number of turns) needed would depend on the characteristics/sensitivity of the switch.

 

One more idea.
I saw this a few years ago, can't remember where I saw it.
The motor current was analyzed. When a DC motor is running, it's current is not very stable and a lot of noise ( because of sparking) and pulsation ( due to commutator) is seen on an oscilloscope. In general, motor current is a sum of DC current and AC current. A simple circuit was used to detect that AC noise and pulsations. When motor is stopped, no noise and no pulsations.
So, a current transformer can be used , and a simple amplifier/detector. Take in mind, DC is significant compared to AC, so transformer's core with air gap to be used

 

So, a current transformer can be used , and a simple amplifier/detector.

That only works if you don't need to differentiate between the motor being off and the motor being stalled.

 

It sounds like the OP does not have access to the typical choice of devices used for this application.
@hcdebey1 What is the application you are using the motor for?, mechanism etc?

 

It sounds like the OP does not have access to the typical choice of devices used for this application.
@hcdebey1 What is the application you are using the motor for?, mechanism etc?

You are very astute. Yes, I don’t “have access to the typical choice of devices used for this application”.

I am an American engineer working in Bangalore, India to make ventilators for use in low-income areas of the world. We are creating ventilator designs that can be:

• made virtually anywhere in the world,
• by anyone,
• from locally found materials, and
• used without the need for medical personnel.

The designs are super simple. The are made from:

• plywood (actually a cheaper version called OSB),
• PVC plumbing pipe,
• a car’s wiper motor,
• and other things that can be found in almost any part of the world.

To control the ventilator, we need a modest amount of electronics to run the wiper motor in one direction and then run it in another direction. The interest in this thread is to detect a motor stall condition and then disable power to the motor until the cause of the stall is resolved.

A fuse will do the job but having spent over 15 years in Africa and other low-income areas, it is likely that a fuse will be quickly changed out for a nail, a wad of steel wool, or a hefty chunk of wire. Hopefully, with a smart-fuse setup, we can minimize such modifications.

You may ask, now that COVID-19 is over, why are we working on ventilators. In about half of the world, people die each year from ventilator-treatable diseases at rates that are 6 to 7 times greater than the death rate from COVID-19.

The world needs heaps of ventilators that are cheap and easy to keep working.

 

To control the ventilator, we need a modest amount of electronics to run the wiper motor in one direction and then run it in another direction.

Can you not make use of the typical action of a wiper motor whereby the mechanics handle the reversing, the motor typically runs in one direction only.
It is possible in some original mechanisms to change the ratio mechanically. Arm length etc.
A stall will cause a over-current, so this could be a detected by electronic means.