current sense issue in BLDC motor

Discussion in 'Microcontrollers, Programming and IoT' started by pica, Aug 9, 2017.

1. pica

36
0
Jul 7, 2017
Hello All,

I am developing BLDC motor driver of 24V ,20A current.

The first situation is,
Suppose, motor is at full load and POT (Throttle) is at maximum and then motor is trying to start and somebody picks rotor by hand then motor is stopping. It should reach 20 A and then it should stop as 20A set as current protection range. But it is stopping at 1 A or 2 A of current.

The second situation is if motor is at full load i.e. at 19A (as 20A is shut down of motor). Then I stopped the supply. Then I again starts the supply, at that instance the motor is running and reaching to 19A.

The third situation is, if motor is running at 19A i.e full load and I stopped the supply and increased some more load using dynamo meter (lets say 25A) and then I started the supply at that time also it is running and once reach 20A it stopps.

The fourth situation is, if motor is running at 19A i.e full load and I stopped the supply and increased some more load using dynamo meter (lets say 30A) and then I started the supply at that time motor is stopping at 7-10A. But actually motor should stop at 20A.

The second and third situation is good. But at first and fourth situation, motor actually should run. Once it would reach 20A it should stop.

What could be the case?

2. kellys_eye

4,307
1,162
Jun 25, 2010
How are you measuring the current? Are you considering 'transients' in the current that could exceed - albeit briefly - the set cut-off levels?

3. pica

36
0
Jul 7, 2017
Hello,

i am measuring current using 5mohm current sense resistor.

The current sense resistor is connected to the conman point of all source of lower MOSFETS. The other terminal of resistor is connected to GND.

I am amplifying the voltage at current sense by using LM324. The amplified o/p is connected to opamp which is used as a comparator as an opamp.

Once o/p becomes 0 . The condition is of over current.

4. kellys_eye

4,307
1,162
Jun 25, 2010
That's not what I meant.

Have you 'scoped the supply (current) to see where the current overload comes from? There may be transients caused by poor supply filtering/regulation or poor decoupling in the sense circuit etc.

5. pica

36
0
Jul 7, 2017
Hello Kellys,

All you guys have great observation and great points that's why I always like your replies.

Let me tell you my observation,

Suppose I run motor without load with 24V and then start giving load from dynamo meter. At 20A my motor stops. Again I decrease the load lets say 19A and shut off the power supply. Now at 19A it should not stop. The load is 19A. Now I start the motor with 19A of load and lets say it is starting and the current is increasing like 1A,2A,3A. At 4A , I touched the motor by hand and apply little load. That time, it stops at 4A. I think it should go upto 20A and then should stop as it is current protection. All mentioned currents are from current of power supply.

What I am thinking, may be one spike is going and I am getting over current in software. How to resolve this if my thinking is right?

Please let me know your points.

6. Alec_t

3,058
834
Jul 7, 2015
You could change the software so that transients are ignored.

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