DC motor behaviour when stopped

[harrysmith]

I am making a hardware to control a DC motor using the PC parallel port. I want to stop when the brushless DC motor is stopped by for example an obstacle. Does anybody have a good solution to establish a bit to enter the PC? How many time can stay the motor under this condition? The current rise to a high value, Is it really true?

 

[ted]

I have no experience about the issue with brushless DC motor you are looking at, but could imagine a few approaches to develope some ideas how to solve the problem:

1) If you consider current increase to be a valid measure, you mght simply use a current meter and see what happens! If the current increase is suffciently big, then you need a current measurement circuit with threshold to detect it and convey the info to PC.

2) The most obvious way is to use some separate sensor for the motion detection. It could be for instance give pulses as function of rotation, and the PC software ccould "see" those. Again, some circuitry for interfacing is needed, depending on the sensor.

3) Very likely that motor has an AC component in the DC current consumption while running, and might not have that when stalled. I hope you have access to an oscilloscope. Then you might set a small resistor in series with the power supply and observe if there is some AC component depending on the motor speed. Resistor value and power rating are dependendent naturally on the DC current that motor draws.

In any case you need some circuitry to interface the PC parallel port to the "real world" analog signals. But what you need is depending what you find out!

Also: I guess nobody can answer directly to your question without knowing more about the particular motor you are using.

Good luck,
Ted

 

[JabaThaSlut]

Depending on the size of the motor used and the load it is attached to, motor current and the degree with which it changes under various load conditions can vary greatly. You would need to do some experimentation with the configuration you are working with. Place the motor and load under varying conditions and monitor current etc. Modern servo systems use a combination of current, voltage and speed measurement to determin the running condition of a motor. For breaking a motor under controlled conditions a number of methods are used. For AC motors, the most common methods are, resistive regeneration and DC injection.
On DC motors, resistive regeneration is commonly used.

If you inject DC to one or more of the windings of an AC motor, the end result is the cumulative magnetic field generated in the windings in which DC has been injected work against the AC field of the other windings.

Resistive regeneration is a system whereby you disconnect one of the windings of the motor from the supply, then connect this winding to a resistive load that acts as a current "dump". As the motor continues to spin
the winding connected to the resistive load is in effect a generating circuit. Again the resulting magnetic field associated with this winding works against the other fields and thus causes this breaking action.

I work with machinery that have 7Kw spindles running at upto 22,000 RPM.
Using the resistive regeneration techniques above, these can be brought to a complete halt within 1 to 2 seconds. All in a controlled manner.

Keep in mind these systems use high speed DSP processing in the control of the motors and are pretty much out of the question for the general hobbyist or low end user. As a matter of interest, the controllers use IGBT's for output switching, 3 modules, each costing around $2000. So $6000 just for transistor switching modules, you can see the cost of these units can be extremely high..


Jaba

 

[ahgu]

why does current increase when the load increase?

under same voltage? why does current goes up when the load goes up? does the resistance becomes smaller?

I understand current is proportional to the torque, but don't understand the physical reason for the incrase.

 

[ted]

To explain it in simple terms, all DC motors are actually AC motors when seen from the inside :!: You see, to make the motor rotate, the current has to continually change direction in at least one of the windings. The frequency is related to the rotaion speed. All windigs have some inductance, and thus the inductive reactance, and winding impedance, vary with the frequency = motor's speed.

Old brush DC motor had the AC current path in the rotor, whereas many of the brushless tiny motors may have a permanent magnet rotor, and the AC current is flowing in the stator winding. But the net result is always the same: Speed related AC current flowing in a winding, and thus the impedance is varying depending on the rotational speed.

Ted