Control of DC motor with current limit

Hi,

I need to run a PMDC motor rated at 6V. The stall current however is ~ 6 A. I need to run the motor using a 6V supply and draw <2A during stall. How can i achieve this ?
I need to limit the current through the motor to be <2A at stall condition.

One solution which I tried is as follows.
Using the output of a 5V Voltage regulator and pot , drive the gate of a n channel mosfet (sts12nh3ll). This circuit works for I<0.5A however I am unable to limit the current above this value.
May be I need to choose a lower rating N Channel mosfet. Will it work ?

what other options are available to limit the current of the motor (basically torque control is required).

rgds
mike

Hi,

It's possible.
For power dissipating reasons (heating) I'd use a step down switch in regulator.

One problem I see is the current limit.
If I understand you right, then you want 2 limit at stall, but a 6A limit when the motor runs normally.

How to handle this?
You could use the motor voltage to increase the current limit.

But when the motor starts, there will be low voltage and thus only 2A.
Is this enough to start the motor in any case?

Or do you need something more intelligent, that recognises the start and sets current limit for a fixed time to 6A even when voltage is low.

Klaus

mike, the method you are describing is very well known, it is called foldback current limit. Google the term, there are thousands of circuits and descriptions.

Foldback is very useful to reduce a linear regulator's dissipation during short circuit (stalled motor) conditions. However, it has some pitfalls.

A DC motor's torque is directly proportional (to a first degree approximation) to its armature current.
If the motor stalls, and you foldback the current to 1/3 of its running current, the motor's torque may not overcome the load's inertia.

Also, during actual operation, if the load current for whatever the reason exceeds -even briefly- the maximum, the current will fold back, the torque will be reduced, and the motor will stall.

What exactly are you planning to drive?

It does not sound to me that he ever wants to hit 6A. I think he just wants a 6vdc supply with a 2A current limit.

thanks guys for the replies,

here's what Im trying to do overall.

I have a Lithium Polymer battery thats at 7.4V, 10C discharge capacity and say 1000mah. That makes it ~10A discharge / burst current.
The DC motor (MOTOR 1) is motor is 6V rated, Im running it at 7.5 V (from Lipo battery), Continuous current of motor (under No load) =~0.5A at 6V, Stall current ~ >6A after testing it at a power supply, starting current I have no clue .
I want to run a RC car with this set up. However, at stall the motor draws >6A, which i want to avoid to increase endurance of the system. So i wanted to regulate the motor current to ~ = 2A or 3A during stall of the motor. AND, if there's a short circuit i don't want SMOKE !

Also I observed the following with my circuit (image in url)...

https://obrazki.elektroda.pl/6223290100_1444890216.jpg

Instead of a battery im running it of a power supply. With the regulator's voltage divider input to the gate , and a lower rated motor (MOTOR 2: 6V, stall current ~0.5A) present , I am able to limit the current from ~0.005 to 0.5A by adjusting the pot , thus altering the torque.

However, if i try the same with MOTOR 1, the current through the motor begins to increase (trickling) and reaches ~6A and the power supply trips.
what is the reason ?

I am checking the "foldback current circuits" too.

How else can I limit the current ? .


rgds
Help greatly appreciated




rgds
mike

Hi,

I recommend to use a resistor from source to gnd.
About 0.2 ... 0.47 ohms and at least 2W (for 2A), maybe a 5W is good.

Use a mosfet with low V_gs_th. (Here your Mosfet is good).
Use a Mosfet with at least 10W rated power dissipation (here your Mosfet is not good).

You circuit dissipates a lot of heat, especially when in current limit mode. It will become very hot.

Klaus

Like KlausST, I think a resistor of R = .8V/Iout is required between source and ground. So for 2A this is .4 ohms (at 2W). So if you then take the source to the base of a NPN transistor. The emitter going to earth, and the collector going to the gate. Then as the voltage rises on the new .4 resistor the transistor turns on and shorts out the gate voltage so the FET turns off. This will results in the FET dissipating 2A at the supply volts so it needs a heat sink.
Frank

you should detect stall by using micro to look at motor voltage, because it is low when stalled.....so when your micro detects stall, it switches in the 2a current limit.......ie changes the reference into the buck converter so that it only doles out 2A to the "bad" stalled motor.

The motor is stalled when it begins to run, then it draws 6A. if you limit the current to only 2A then it might not start or it might start running very slowly. Is that what you want?

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treez said:

you should detect stall by using micro to look at motor voltage, because it is low when stalled.

Click to expand...

No.
The power supply voltage feeds the motor. The power supply voltage should not be low.
Are you talking about when the power supply becomes overloaded and "trips"?

I was presuming that the power supply is current limited, and that the stalled motor, with no back emf, will end up with more current being drawn, and thus it would pull down the power rail due to the extra current draw.....
As you say, software would have to be written to allow it to get started.

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the way to stop the dc motor stalling at low supply voltage is to make the supply voltage a pwm supply voltage...ie do not give it 6v as such, but give it 24v pwm'd with 0.25 duty cycle...that is what the camping toilet /shower pumps do....look at whale gulper pumps.

I agree that PWM gives full power pulses so the torque is high but the average current can be fairly low.

yes the whale gulper uses a mains transformer, and they just simply pwm it into the pump...works great...and of course, mains transformers can be overloaded and it doesn't break them , unlike their bad-boy switch mode alternative...the bad boy switch mode alternative is also more expensive at powers of some 40W, and often bigger, and also more expense to develop...try making an offline smps that is 40w average but 100w peak......much easier and cheaper with a mains transformer.