# I need to build an electromagnet to provide 10mN eddy current brake

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1. Jul 17, 2015

### Anoop Varghese

Hi I am building a motor tester, for that I need to build a variable load. For which I was planning to use eddy current brake with electromagnet.

Please help me to build a electro magnet which can provide more than 10mNm load to a DC motor with rotating disc of 75mm in a RPM of 6500.

I tried with trial and error methods, as I am not good at solving mathematical expressions.
case 1. When I tried with a large rod core with 2100turns and 500mA current I was able to load only upto 3mNm. Case 2. when I change the system to two electromagent place at the both side of the disc (1400 turns each and 300mA cureent through both coil) I was able to load upto 9mNm. However as it need to be placed both side I have a space contrain.

Thanks & regards,
Anoop

2. Jul 17, 2015

### Dr. Courtney

What mathematical expressions are you trying to use?

It is not clear what you are trying to do.

3. Jul 17, 2015

### Staff: Mentor

Can you post a photo of your experiment?

4. Jul 17, 2015

### DrZoidberg

The magnetic flux should go in a circle with air gaps being as small as possible. A U shaped magnet could work.

5. Jul 17, 2015

### jim hardy

Dr Z is right.

My 2 cents(overpriced at that)
Experiment with a C-clamp (iron one of course)
gives you a magnetic loop with adjustable air gap.
Tighten clamp and tack weld that wobbly tip to the screw so it won't flop around.

Last edited: Jul 17, 2015
6. Jul 17, 2015

### Hesch

As for a motor-tester I don't think it's a good idea: The rotating disc will be heated up by 680W Eddy current losses and will become hot - very hot.
The specific resistance of the disc will change due to change in temperature, so the load will not be constant.

Also the torque will depend on rotational speed.

Another idea for motor testing is to connect two (identical) motors mechanically, the one is used as motor, the other is used as generator. Then you let the generator provide power to the motor ( perpetual motion machine ). Of course that will not work, but if you insert some power supply in the electrical connection between generator and motor in series, adding some small voltage to the generator voltage, it will work. The power supply will just have to provide power, corresponding to losses in the motor/generator.

EDIT: Sorry, only 6.8W is lost in the rotating disc.

Last edited: Jul 17, 2015
7. Jul 17, 2015

### DrZoidberg

108.3 rotations/second * 2 * π * 1m * 0.01N = 6.8W

8. Jul 17, 2015

### Baluncore

So why not use a DC PM motor as a generator with a resistive load? That will give you eddy current braking.
If you load a DC generator, (motor), with a constant current network you will get close to constant torque over a wide range of RPM.

9. Jul 20, 2015

### Anoop Varghese

The tester need to support 50 motors need to be tested together- it is used for the endurance testing
And the user must be able to connect all their different type and sizes of motors in the system. So we thought of using the eddy current braking in the system with the disc and magnet. We have made the system with permanent magnet, however now customer need configurable load, so we are planning replace the permanent magnet with electromagnet.

10. Jul 20, 2015

### Hesch

Which type of DC-motors: Mechanical/electronic commutation?

I have tested synchronous motors by driving them with a very poor commutation angle. Thus they were heated up while running, though they had no mechanical load, so that edurance due to heat and rotational wear were tested at the same time, but I'm not sure if the same can be done with a BLDC motor, which has trapezoidal commutation?

Last edited: Jul 20, 2015
11. Jul 20, 2015

### Anoop Varghese

Hi Hesch,

Thanks for your suggestions, however we are testing normal DC motors. Not BLDC motors, the shaft and Disc is isolated with nylon bush so that the heat is not transferred to the motor from the disc.

For larger motors which is up to 1Nm torque we are using permanent magnet for loading. For smaller motor customer wants electromagent loading for simulating the load profile performance over the operational area.

Regards,
Anoop