Motor getting hot after about 10 seconds of unloaded, full throttle...?

[lt130th]

I'm curious as to how you can control the flow of CA to get into the tighter gap of where it sits and not in any gaps of the bearing cover. I've never used glue to hold a bearing in place before, usually the races keep my bearings in place without it.

Until it's load tested I'm afraid we are all guessing.

Very nervously, believe me, LOL! I used a sharp end of a toothpick to let just a small drop flow around that little ridge. I don't know why they are glued in either, but I had to do a lot of reading before I could even figure out how to properly get the motor open. The front cover is threaded into the can and it is glued in with CA glue. Dont know why they couldn't just use screws or press the covers into the can like just about every other brushless motor I've seen. I will get some more help with this one locally. I have lots more to learn.

Thanks again, guys.

[Rumdog]

Never run a motor unloaded. Especially at full tilt for prolonged periods. Yes. It will get hot.

[Fluid]

.... Hadn't found any good reads or vid's on bench-testing.

That's because there is no good reason to "bench test" an electric boat motor. An electric motor will take the energy from the battery as fed to it through the ESC. The more you load the motor the more energy it consumes, that's how we test our setups on the water. On the bench the motor is not loaded, so most of the energy it consumes is turned directly into heat. Running a motor at full throttle without a load is potentially damaging! Run it too long and it WILL overheat, and 20 seconds is just too long. Not only that, but the motor can over-speed without a load, risking bearing damage or throwing a magnet. When the latter happens you usually toss the motor in the trash.

Forget about bench testing, since you are testing nothing of value. A quick blip of the throttle (<1/4 second at part throttle) to let you know the motor works is plenty. The important testing is with the boat on the water with different props, strut settings, CG placement, etc. That has huge value - but of course it requires water.


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[lt130th]

That's because there is no good reason to "bench test" an electric boat motor. An electric motor will take the energy from the battery as fed to it through the ESC. The more you load the motor the more energy it consumes, that's how we test our setups on the water. On the bench the motor is not loaded, so most of the energy it consumes is turned directly into heat. Running a motor at full throttle without a load is potentially damaging! Run it too long and it WILL overheat, and 20 seconds is just too long. Not only that, but the motor can over-speed without a load, risking bearing damage or throwing a magnet. When the latter happens you usually toss the motor in the trash.

Forget about bench testing, since you are testing nothing of value. A quick blip of the throttle (<1/4 second at part throttle) to let you know the motor works is plenty. The important testing is with the boat on the water with different props, strut settings, CG placement, etc. That has huge value - but of course it requires water.


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OH!! Wow, I wasn't aware of any of that, but it seems 100% logical. Thank you for the knowledge.

[fwlbp]

Oh really? Is a loaded motor able to run at WOT more efficiently? I'm used to my cars where you can hold full throttle for a long time with the wheels not touching the ground and the can won't even get warm.

Dude, your boat motor requires water cooling under WOT, your car uses convection cooling via air and is OK. On a bench you have zero water flow over the motor and heat ramps up rapidly at WOT.

[Rumdog]

^ that is not why it gets hot on the bench. Read Fluid's response.

[fwlbp]

^ that is not why it gets hot on the bench. Read Fluid's response.

OK. Out of curiosity, how quickly does a boat motor overheat in a loaded, but uncooled state? Say if your water inlet was plugged?

[Rumdog]

That depends on the load on the motor

[fwlbp]

Was just thinking in general terms at WOT. Say a basic 30", 1800 kv, 4S at WOT. Are we talking a few seconds (as is the consensus for unloaded) or minutes?

[tlandauer]

What is your definition of a few seconds, the OP thought 20 seconds were short bursts but it is not. For me a few seconds is no longer than 5 sec. When I check motor rotation or connections in general, 1 or 2 sec. is what I do.
As for your second question, if the water is blocked, how many minutes it takes for the motor to over heat, I can assure you this is a scenario most of us are trying to avoid, to get an exact number might just be difficult. It can take more than a few seconds, but depends on whose definition it is based on.

[785boats]

I was googling for some info on motor timing & came across this. The third paragraph goes some way to explaining why motors get hot when running with no load.
Just thought it might be of some help. It helped me understand the philosophy of it all a bit better.
It's also a good read about timing too.

Q. How does Timing effect the power of an electric motor?
A. Tom Cimato, MaxCim Motors

In general terms this describes motor action:-) But (bet you knew that was coming) more specifically you need to first visualize that the permanent magnets create a fixed field in magnitude and position (for this discussion). This field has a shape with a maximum intensity exactly between the magnets (in a 2 pole motor - most DC motors - directly opposite each other). To generate the maximum torque from the motor, the electric field generated by applying current to the windings must be located 90 ELECTRICAL degrees from the center of the magnet field (90 physical deg. in a 2 pole motor). This is defined as the max. Kt (torque constant - oz. .in./A)

What happens in a real motor under load is that the electric field lags the magnet field due to inductance effects in the windings and possibly some saturation effects in the steel parts (magnetic iron). The amount of lag varies with load (and rpm), so is a complex variable.

We try to preset the motor timing (brush position) so that at a selected load current, the fields are at the 90 E deg point for max. Kt., and max. magnetic power conversion efficiency. At no-load (without a prop) this will be an advanced position and since it is ahead of the 90 E Deg. point, will have a reduced Kt (take the SIN of the angle x KT90, e.g. SIN108 x 1 = .951 x Kt), which requires more current to turn the shaft (at no-load. The current will pull into phase at the selected load current. This will actually look like the current decreasing as the load increases, up to a point.

In a brushless motor the same thing is happening, but we advance the hall sensors for this effect. The big difference is that we have to visualize the analogy while sitting on the rotor (stationary frame of reference) and look at the stator field.

This is explained in most texts of ac and dc electric machines. Check your university book store:-)

It's a lot more complex than this, so I hope I've given you some insight as to the goings on in that little bundle of wire and magnets.