Magmotor S28-400 failure, replace or different motor?

Hi Guys,

I'm not a robot builder but the mods have very kindly activated my account as I hope someone with experience of the S28-400 Magmotor can help me, as I know that model was aimed at robot builders.

I have a WR27-1 jet engine that I built a starting system for using the Magmotor (http://www.youtube.com/watch?v=WLUoDoNkNgE). It worked fine over about a dozen starts and then seemed to lose speed, the motor was still drawing the same current as normal except the speed was about half what it should have been. A friend who claimed to know high-power brushed motors removed the brushes and filed them trapezoid instead of the flat profile they came with. The motor then seemed to work normally again, for about eight seconds and then seized solid with a rather terrible noise. Disassembly shows that the commutator seems to have exploded, many of the bars have separated from the green expoxy-like substance that made up the commutator (which has itself come away from the shaft and shattered) and wrapped round each other.

The motor was being run as follows:

48V with roughly 1 Ohm in series until the engine was spinning at a sensible speed and then directly connected to the batteries until the max speed of about 10K RPM was reached.

I was seeing about 200A peak which died down to about 70A when the motor reached top speed. As you can see in the video the entire start is only about 20 seconds.

A sprag clutch in the engine's gearbox disconnects the motor once the engine accelerates past 23K RPM although I always disconnected the power after the engine starts to audibly accelerate under its own steam anyway.


I know I was operating the motor over the recommended voltage of 24V (I needed at least 10K RPM) but my peak and average power outputs were not above what the sites selling this motor claimed it could handle.
Magmotor have refused to honour any warranty, so it's either import another S28-400 or swap to a different motor.

Any thoughts and suggestions are gladly received!

Jon

No experience with the s28 but i do know they dont like 48v. If its like the c40 300 mag then it sounds like you have over volted the s28 and possibly burnt out the brush springs which could result in arcing and serious damage. taking out the brush and filing it has worsened the problem and bang goes your motor..........i say all this as a complete guess. without looking at the motor its hard to give a solution to whats happened.

Personal advice would be to not use a mag motor and use something that will give you the rpm at the right voltage....maybee a little more but not double.

Others are more knowledgable in this field and i am sure will post.

regards

Shane

The mags don't like being over volted.

Would it be possible to put some gearing in your design to get the desired rpm? It sounds as though you were running the motor at 48V purely to get the extra rpm. At 48V you are asking the motor to provide 4 times the power so around the 18Hp mark.

Obviously by gearing the motor up you are taking the torque down. Would this be an issue?

You would have been better off using a gear ratio to achieve the higher rpm then using a higher voltage. one of the reasons motors are limited to a certain voltage is that the commutator bars will come appart due to centrifugal forces. You were probably damaging the bearings too, depending on the type, and seal i suspect all the grease may have come out of them causing them to not allow the armature to rotate freely.

I have heard a lot about mag motors falling appart. Try the baby lynch (130mm) at its max recommended voltage, and use a ratio to achieve your 10,000rpm. You will be pulling a serious current but for 20secs i'm sure the motor can handle it. Maybe a fan to aid cooling.

I dont know of any DC motor that will do 10,000, usually thats AC motor territory.

Thanks a lot for the replies.

Yep, the only reason for running at 48 volts was for the extra speed, as it was I was only just reaching the required speed at 48V. If the motor hadn't failed I was thinking about trying another 12V to see if I could get the start temperatures down.

Another thing I didn't mention is that the motor has to sit between the pumps, a huge mass of piping and the load compressor air intake so the available diameter is a tiny bit larger than three inches. Which was another reason I chose the Magmotor.

I honestly don't know what the required torque is, the Magmotor didn't seem to have any trouble spinning the engine up so I guess I could get away with less.

If I read the Wiki page on torque and horsepower right I think I can calculate the steady-state HP as: 70A * 48V = 3360W, 3360W / 746 = 4.5HP.
And then the Torque as: 4.5HP/(10000RPM/5252) = 2.36 Pound-Feet

I don't know if that calculation is right but that is only 453.12 Inch-Ounces so I guess gearing it wouldn't be a problem if I could design some sort of vertical gearbox that fits in a 3 diameter space.

Drop the voltage to 24V and use a planetary gearbox like these

http://www.slidesandballscrews.com/gear ... 93764ccc8b

These wont be an exact match. Only reason I know about them is because I'm building a CNC plasma cutter. Others will be available but I don't know of anyone that makes on for the magmotor.

If you have the facilities you could machine your own

Thanks for the info supernova, I've found the Lynch motor website and am having a look now.

A pancake motor would be a decent choice, I could mount the motor on a frame below the engine, use a toothed belt to perform the reduction and have a long drive-shaft to connect down the space where the motor is now.

Thanks for the info on planetary gearboxes typhoon_driver, do you happen to know if they can be driven backwards instead of as a reducer?

I shouldn't have believed the website that said overvolting the Magmotor wasn't a problem, direct-drive was nice and simple though :(

As a rule of thumb we only overvolt to 150%. So in the case of a 24V motor we up the voltage to 36V max. It is also dependent on the runtime of course. If you have sufficient torque left you might get away with a gear gear ratio that increases the end speed instead of reducing it.

sorry if someones said this above , but brushless motors tend to ave higher rpm's , so you could try a lrage brushless inrunner as this would get what you need. As for the engine itself , will it start on compressed air ? you might want to assist a brushed dc motor with a leaf blower as this should get it started , or try and start it on propane ! I dont know mcuh about small jets but there my 2 p :P.

Hi muchalucha, thanks for the suggestion about brushless motors. I have looked at the Neu Motor 2230 (http://www.neumotors.com/Site/2200_series.html) series which will do a 5KW continuous at 35K RPM. Annoyingly they won't respond to my emails about torque curves or provide me with the Kt constant so I have no idea if it has enough torque.

My engine isn't a model one, it's an APU (produces 5KVA 400Hz from it's generator and starting air for the main engine from it's load compressor) out of a full-size military aircraft and is about the size of a small table. It doesn't have impingement start and due to the massive volume of compressed air required that's usually pretty impracticable anyway.
It originally had a very nice bent-axis hydraulic motor but the previous bloke who owned the engine cut it in half. I have tried to get a working one but I've had no success to date.

Again, thanks everyone for all the really helpful information you've given me.

I'm fairly sure that this is the formula for calc brushless stall torque in N-m

Stall torque = ((1352/Kv)*(Voltage/Resitance in ohms))/141.69


Regards Woody

although mag motor are by there nature very very torquey , id imagine a brushless motor would be more so , gram for gram , there a hell of a lot more torquey than a normal dc brushed motor, mag motors may be different :shock: .