Hello,in a small 12v drill motor the current without any load is 0.3A and as an overcurrent protection for the motor how much the current should be to save the motor from damage.Arbitrary i choose a ratio of 5,but i am not sure if higher current can be obtained.Is there any practical rule for this.

## Small motor current

you should also check what current the motor takes when it is blocked. and the current it draws when it first starts. these currents are much higher then the current drawn when just running free.

Heat is the killer here.

Block your motor so it can't turn, apply power and measure the current. Briefly. This is your "Stall current" and tells you the resistance of the windings. (R=E/I) This is the most current the motor can draw in your system. You don't want to apply this much current for very long or things will get hot, the magnets will weaken so you'll have no torque and eventually a winding will burn out and your motor will get a dead spot.

How much your motor heats and how hot it can get is something the engineers that designed it probably knew, but you likely aren't going to find out without a LOT of research.

I like to go by temperature. If the motor is too hot to touch, you're probably giving it too much current. If the plastic toy is deforming, you're giving it too much current :wink:

Locomotives are about the biggest electric motor abusers. They have big, very expensive, electric motors and want to keep them working a long time as well as get the most work out of them. They have huge blowers that blast air through the motors to cool them. The amp meters have 2 marks. One is, "maximum continuous current" and one is "maximum current for x minutes." To get a train moving, he runs up the engine to reach the max current for x minutes reading, and holds it there till he either reaches the desired speed, or x minutes, where he backs the power off. As the train accelerates, the current will decrease, so he can apply more power. (Assuming the train is already stretched out and not slack. He also will back it off if the wheels are slipping.) Sometimes, a coupler breaks when they're doing this. That causes and annoying delay as the conductor carries a spare knuckle (they're really heavy) to the break, hopes he has the right one, repairs the coupler, recouples the train and pumps up the break pipe. (In Europe, you don't have knuckles because you didn't have that dreadful "link & pin" coupler that hurt so many railroaders.)

Most likely the biggest effect your imposed current limit will have is on starting torque. All permanent magnet DC motors have a specification called "Torque Constant". It is expressed in Ft-lbs, In-Ozs, etc per Ampere. What this factor tells you is how much torque the motor shaft produces as you force one amp of current thru the motor winding.

If you have a spec sheet for the motor, the Torque Constant would be given in some form. If not, you can "easily" determine it yourself like this: Bolt the motor down or clamp it in a vise. Fix a torque wrench to the motor shaft with a suitable mechanical coupling. (I've used a standard set screw tubular shaft coupling with one end attached to the motor shaft and a short hex head bolt clamped into the other end. Then use a standard hex socket wrench to connect to the torque wrench's square drive head.)

Now holding the torque wrench good and steady so the shaft can't rotate, drive the motor from a suitable lab supply with a current meter. Bring the voltage up slowly until you get a one ampere reading on the meter, read the torque wrench. That's the torque constant of the motor - so many ft-lbs/amp. You can also run the current up to 2 or 3 amps, then divide the torque wrench reading by that much to get the torque per one amp (i.e. the Torque Constant).

Now, that tells you what torque the motor can produce. The next question is how much torque your application requires. This in turn depends on your application, which you have not yet revealed. In some applications the starting torque doesn't matter - e.g. simply rotating a flywheel oroptical disc, or similar. In other cases, like in a vehicular application, you may need to develop a certain minimum torque level simly to break the wheels loose (i.e. overcome the coefficeint of static friction).

Once you have numeric values for each side of the system (i.e. motor vs load), you'll be able to figure out how to sensibly size the current limit.

Is there a "practical rule" for this as you ask? No, each apllication is different.

The "motor" is just a drill used for pcb drilling,something cheap one.Today i finished it and works quite well.To measure the current without any load i run the motor for 6 seconds,sample the current and then multiplied with 5 to find the overcurrent value to stop it.The number 5 is an arbitrary constant,but i am thinking that is quite enough for high speed drills or angle grinders.But such a routine is great since is automatic adoptable for every voltage and current rate dc tool,12v or 15v or 24v or 36v and there is no need for any manual current adjustments.The next is to test it with several motors to see how it works.Thank to all of you.

In that situation, it likely doesn't matter as your motor is running at a pretty high speed and not a lot of torque. It probably has built in fan blades blowing air through it, so it can stand up to a lot if the rpm stays up.

My 40-year-old dremel has its air vents right where you want to hold it when doing close work. But, it still works after 40 years. Just remember to put a drop of oil now and then.