Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Amperage draw of DC motor

Status
Not open for further replies.

doh

New Member
Sorry if this sounds too basic but what determines the amperage draw of a small DC motor? I am trying to do a small project with my son that involves a 12V dc motor, waterproofed, that we attach a propeller and run underwater to create thrust. I first used a small boat bilge pump motor that is rated at 1.5 Amps, attached an RC airplane propeller, and placed a switched 12V battery (8 AH UB1260 ) source. The resultant thrust was too little.
I then took the motor out of an old cordless 12V drill, which was the exact physical size of the previous motor (~1.5x3 inches) connected the same propeller and battery source. This created tremendous thrust, heated the 14 gauge wire rapidly and drew over 20Amps (higher than my ammeter registers).

What is the determinant of how much Amp draw there is in two physically similar motors. Is it the number of windings? the gauge?

We are trying to basically create a very small trolling motor with a simple on/off switch and that can create ~10 lbs thrust. The smallest trolling motor(Seyvelor) creates 35 lbs. thrust. What is the best type of motor/ lightweight/ torque combination? I know nothing about brushless DC motors but would they have a use in this project.

Any advice would be greatly appreciated.
 
Sorry if this sounds too basic but what determines the amperage draw of a small DC motor? I am trying to do a small project with my son that involves a 12V dc motor, waterproofed, that we attach a propeller and run underwater to create thrust. I first used a small boat bilge pump motor that is rated at 1.5 Amps, attached an RC airplane propeller, and placed a switched 12V battery (8 AH UB1260 ) source. The resultant thrust was too little.
I then took the motor out of an old cordless 12V drill, which was the exact physical size of the previous motor (~1.5x3 inches) connected the same propeller and battery source. This created tremendous thrust, heated the 14 gauge wire rapidly and drew over 20Amps (higher than my ammeter registers).

What is the determinant of how much Amp draw there is in two physically similar motors. Is it the number of windings? the gauge?

We are trying to basically create a very small trolling motor with a simple on/off switch and that can create ~10 lbs thrust. The smallest trolling motor(Seyvelor) creates 35 lbs. thrust. What is the best type of motor/ lightweight/ torque combination? I know nothing about brushless DC motors but would they have a use in this project.

Any advice would be greatly appreciated.

the current drain is the load plus the losses and there is no way to tell from the information that you supplied. aren't there throttles on the trolling motors?

you have two problems with what you are trying to do... boat propellers are drastically different from airplane propellers and they are meant to rotate MUCH slower.
 
All trolling motors have multiple forward/reverse speed controls. I am looking to create a small lightweight submersible motor and used the trolling motor as an example. The propeller we will probably use is from an RC boat if we can find one. The real confusion I am having is understanding how two exact same size of the "can" motors can have such drastic differences in Amp draw. The over 20Amp draw motor (cordless drill) has thicker gauge windings wire, does this cause it to draw more Amp under the same load? When it was in the drill, why didn't it draw 20Amps under heavy load?
 
As UG6.3 noted, the main factor determining the current draw of a motor is the load and the motor HP rating, up to it's rated voltage.

Did you measure how much the drill motor took when it was in the drill?
 
I did not check the current draw in the drill so I don't have a reference. I may purchase another one for testing, they are pretty inexpensive.

I think I worded my question poorly in that I want to know what determines the HP rating of two similar sized motors. Is it the number of windings, strength of magnet?
 
All the motors you are talking about are called "three pole" and have the same number of windings (3). But the number of turns and the thickness of the wire will be different.
At this point in the discussion, the thickness of the wire is not important.
It is the number of turns that makes the difference in current taken by them.
You will find that one motor has 4 times the number of turns of the other motor.
The next important factor is the RPM.
If one motor revolves at 50% higher RPM, the back voltage (EMF) will be higher for each turn on each pole of the armature.
These are the two major factors.
Now, the size of the wire.
You can wind all the motors with thin wire and they will behave in a similar way to your experience. But if you have a few turns on a pole, it will not take up all the available space. This means you can use thicker wire. The thicker wire will have a lower resistance and it will not heat up as much and at the same time it will allow a higher current to flow. But the first two factors are the major contributors.
From here you can go on to all sorts of discussions such as PWM, adjusting the voltage, adding a gearbox, etc etc etc.
The way to see how much current and how much torque is required and delivered by a motor is to hold it steady in a vice and either add a coupling or propeller to the shaft or provide a friction device and watch the current as the motor is slowed down. The full RPM tells you nothing.
 
Last edited:
  • Like
Reactions: doh
Thank you colin55 for the excellent reply. It clears up a few basics on what determines HP of a motor.
Since I possess the two extremes in motors, 1.5A rated (too weak) and one with great thrust but over 20A draw. Are there simple electronics to limit the amp draw without wasting too much battery charge as heat or even better a small motor with low rpm and high torque (I understand that they are usually inversely related)?
Do you have any recommendations where I can find a relatively small (~3.5"x2"), or lightweight motor that is relatively low RPM to power a propeller to create about 10 lbs. thrust, or what companies I can contact, or what products I can remove a motor from (ie cordless drill)?

I really want my kids project to be created and would gladly pay a few hundred devalued american dollars for motors and advice.
 
This is where PWM comes in. It provides very good starting torque and yet is can reduce the current taken by a motor when it is under load and thus determine the RPM. The advantage of PWM is the low losses it produces.
You will find PWM on the drill you have taken apart.
It's that easy.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top