Calculation of power

[jsmith]

Hi Bruce,

Congratulations on the new board, it looks like a huge improvement, I just hope that there is still a way of accessing the old information.

The school I'm teaching at has entered the Formula Schools UK - which is basically a competition for radio controlled nitro cars, designed and built by the students.

I want to build a dyno so that they can run and test components and engine modifications. From what I understand it should be quite easy to build an eddy current dyno (is this the best dyno to build for this application?). However I don't know where to start. I would greatly appreciate any help that anyone here can give me.

Thanks in advance.

[84-1074663779]

The simplest and most straightforward way to make yourself an eddy current power absorber, is to just use an ordinary ac induction motor. By feeding a dc current through the external field winding, eddy currents flow in the solid metal (squirrel cage) rotor creating a suitable braking torque.

Any ac induction motor will work, from a tiny kitchen exhaust fan motor, up to a multi horsepower three phase monster. The power you can absorb has nothing to do with the original power rating of the motor either. How much power you can absorb, and for how long, only really depends on the allowable temperature rise in the solid metal rotor.

The rotor being solid, does not dissipate heat well, so you might need to allow lengthy cool down periods between power runs. For a do it yourself effort though, it can be quite a simple, low cost, practical solution.

Ideally the motor should have a double ended shaft, that is an open shaft should project from each end of the motor. You then use external bearings at each end to support the motor on some sort of static frame or bed.

The rotor then spins on these external bearings, and the motor outer casing is also free to rotate through a small angle on the original internal motor bearings. You just fit a torque measuring arm onto the motor housing resting on a weighing scale.

You can then set the braking torque by powering the field winding from a low voltage laboratory power supply, or something similar. Maximum dc current should not go much above the original ac current rating for that motor. There should be a rating plate to tell you what that is.

A hypothetical motor might be rated at 240 (110) volts ac 1 amp RMS, and the winding resistance might be for example 4 ohms. For the dyno you only require 4v at 1 amp dc, not high voltage ac.

You can try this on the workbench by just connecting any old induction motor up to a low voltage dc power supply and while turning the shaft by hand, wind up the current and see.

Even a small motor has a surprisingly high braking torque, and if you then try to turn it at very high RPM, the power absorbed can be ORDERS OF MAGNITUDE higher than the original motor power rating.

So sinking enough torque is hardly ever going to be a problem, but huge internal temperature rise at high applied power will be. An external air blower is a good idea, and for a small table top dyno, a vacuum cleaner might be sufficient to pull some cold air through it. If you get really serious, the whole thing could be immersed in oil with a circulating pump and cooling system.

The only other problems might be speed related due to the bearings and centrifugal force on the copper or aluminium squirrel cage. It might fly apart, particularly if it gets red hot ! But the explosion should be fully contained within the motor casing. Best to be conservative with the speed though, as torque sinking is no problem, best to gear it down a bit if in doubt.

These things also work quite well on exercise bikes, hehe.

For something a bit larger, a gigantic and ancient three phase induction motor from a machinery junk yard should go pretty cheap. Just connect your dc power supply to two terminals, and ignore the third. It will work fine.

Have fun.......

[jsmith]

[color=#000000]Thanks Tony,

I might become a bit of a pain with this as I am not too hot on electonics.

Is this the type of motor I need? The shaft only sticks out of 1 end - I don't recall seeing any motors where the rotating shaft protrudes from both ends but if you can point me in the right direction.................



"The IM series single phase, permanent capacitor, shaded pole and three phase AC induction motors provide performance characteristics suited to applications ranging from blowers, fans, hand dryers and office equipment, to grass cutting machines and reclining chairs. With ratings at continuous full load operation up to 200 watts, this family of small induction motors has the scope to meet an number of AC motor requirements.

Single phase 50Hz or 60Hz
Wide range international operating voltages
Shaded pole or permanent split capacitor
Single rotation either direction
Dual rotation
Class B insulation
Totally enclosed or ventilated
Continuous operation (IEC S1 duty)
Short duration and cyclic ratings
Aluminium end shields
Ball bearings
Operating ambients -25

[84-1074663779]

There are a few applications that readily come to mind that use a double ended motor, the common bench grinder is one, and every domestic airconditioner is another. The airconditioner has a motor with two long projecting shafts, with an independent fan on each end. One fan (usually a propeller type fan) pushes outside air through the condenser, the other, (usually centrifugal) inside air through the evaporator.

Many electric motors also have a fan mounted on the back covered by a shroud. Cooling air blows over the fins on the outside of the motor casing. If you remove the shroud and plastic fan, there will be a short shaft stub perhaps an inch long at the rear of the motor that can be used to mount an external support bearing.

An ideal place to find a really nice motor is the local tip !

[jsmith]

Ok,

So far so good.....

However, how will I measure the engine RPM? Ideally I would like to rig up a system that would give me a digital output and was fairly accurate.....

Thanks again.

[98-1074649673]

Setup a trigger wheel to read some kinda sensor ie inductance coil or something else and feed it to a digital readout meter. Thats how I'm doing it on my dyno.

I have a toothed wheel (sprocket with the teeth machined down some) and read the teeth with an inductance coil sensor, feed this to an RPM meter I got from ebay. This gives u accurate crankshaft rpm. If u run it 2:1 or 3:1 reduction then just set this in your meter to make the correction. Programable RPM panel meters on ebay go cheap!

[Guest]

I work for Fasco Motors and Blowers in Cassville, MO. We have double Blowers that have a double ended shaft motor.

[Shawn]

try this place for the motor your looking for-

i love this site!!
Shawn

[Sandra]

I have a rolling road but I am not sure how I should calculate.

This is how I have calculated: (torque*RPM)/5252= Bhp.
My problem is that I am not sure how to calculate the torque. I have a loadcell that is assembled 30 cm from the cnter of the brake. Do I have to consider the diameter on the rolls for the calculation? The diameter is 22,5 cm.

Can I do it like this: kg(on the cell)*(0,3+11,25)*9,8=Nm?
9,8 is the number to get it from kg to Nm.

And finelly should I use the RPM on the rolls or on the wheel?

[Tony]

Roller diameter makes absolutely no difference.

Larger rollers turn more slowly, but with higher developed torque. Because you multiply torque by Rpm in the calculation, roller diameter makes absolutely no difference to the final power reading.

[Sandra]

[color=#000000]But if I want to calculate the power on the surface of the wheel do I not then have to consider the diameter of the rolls? Bigger rolls gives higher torque so shouldn

[bruce]

What type of absorber are you using?