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        Help! Huge jump in no load current

#1 leybold12 Mar 12, 2009 06:13 PM

Huge jump in no load current
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Okay, I know this isn't RC exactly, but I am experimenting with RC brushless motors as a substitute in industrial applications. This might apply to motor construction.

I have a Turnigy SK 63-64-230kv outrunner running off 37 volts. I am driving it with a Sentilon HV-100A esc. Without load, I draw approximately 2 amps as my Io/no load current. I mount the motor in an adapter so the motor can mounted inside a tool. I made the adapter out of copper pipe (cheap, easy to machine, good heat transfer, easy to braze). See photo below.

Now when I turn on the motor, the no load current at full rpm is close to 5 amps. When I shut off the throttle, the motor now stops like the brake is on (it is off). In 15 seconds the copper adapter becomes too hot to touch. The copper around the rotor (about 0.1 inch air gap) gets hot very quickly as if the copper is being heated by induction. I had thought that I would not have inductive heating since I used copper.

I tried it with an Towerpro 5330-10T. The no load current jumped from 2.6 amps to 5.8 amps. I tried it with a Scorpion HK4035-560 which leaves a 0.5 inch air gap between the rotor and the copper and there was no increase in no load current.

Any idea what is going on?


#2 leybold12 Mar 12, 2009 06:16 PM

Quick update. I had a piece of copper pipe that left about a 0.1" gap around the Scorpion outrunner motor. When I ran the Scorpion motor inside the pipe, the was no increase in no load current.

I also got a second piece of pipe that fit around the SK 63-64-230kv motor that did not have the cross piece for mounting the motor. Holding it around the motor while running increased the no load current to near 5 amps. The copper pipe got hot fast even though it was not in physical contact with the motor.

#3 DavidB. Mar 12, 2009 06:57 PM

RC motors can leak a lot of flux...you're just getting major eddy current losses in that copper. I would make that can from PVC, Delrin or something similar.

#4 leybold12 Mar 12, 2009 07:13 PM

If I cut the cylinder in half along the axis and reattached the two halves with an insulator and epoxy, would that stop the eddy currents? I am just trying to visualize the direction of the current flow.

The copper pipe fits perfectly and is mechanically rigid so I would prefer to use it in some form.

#5 Ron van Sommeren Mar 12, 2009 07:22 PM

Or beef up the rotor-ring to keep the flux were it belongs. This will lower Kv though...
Or build the ring up from thin metal 'washers'.
Vriendelijke groeten ;) Ron

#6 Ron van Sommeren Mar 12, 2009 07:25 PM

Lamination thickness versus eddy currents

Nice eddy current demos, notice the braking effect:

Vriendelijke groeten ;) Ron

#7 IBCrazy Mar 13, 2009 07:55 AM

Look up Lenz law. The leakage flux from the rotating motor induces a current in the surrounding copper. You can limit this effect by cutting a slice down the copper sleeve and placing an insulating strip in there. This will break the electric circuit in the copper ring.

Another solution is to use a less conductive material. Ron's solution of increasing the thickness of the flux ring is the best solution, but that may not be practical in your case. It might be easier just to buy a Scorpion motor.


#8 tclark Mar 13, 2009 11:24 AM

Since the motor has many alternating poles around the rotor, a single axial cut in the copper tube will only yield a small improvement in losses. Lamination in the direction Ron suggests would be electrically better, but not practical to make. As others have suggested, contain the flux with a thicker flux ring and/or use a non-conductive material for your mount.


#9 rich smith Mar 13, 2009 01:17 PM

I'm confused. How come aluminum mounts don't cause heat?

#10 Ron van Sommeren Mar 13, 2009 02:26 PM

I take it you are referring to the stator-carier? Hardly/no flux (changes) in them.

Prettig weekend ;) Ron

#11 Ron van Sommeren Mar 13, 2009 02:32 PM


Originally Posted by Ron
... Or beef up the stator-ring to keep the flux were it belongs. This will lower Kv though...

Ooops I meant rotor-ring of course.
I corrected that post.


Originally Posted by Ron
... Or beef up the stator-ring to keep the flux were it belongs. This will lower Kv though...

Or put thin iron strips between the magnetpoles, either on the in- or outside of the flux-ring.

Vriendelijke groeten Ron

#12 Bruce Abbott Mar 14, 2009 06:20 AM


Originally Posted by rich smith
I'm confused. How come aluminum mounts don't cause heat?

They do (in a poorly designed motor that doesn't have enough back-iron).


#13 cyclops2 Mar 14, 2009 07:57 AM

The type of metal AROUND the motor when it is mounted in the machine can affect the magnetic fields of the motor. It may be a easily magnetized metal.
Test it with any magnet. Post back the results.

#14 cyclops2 Mar 14, 2009 07:59 AM

Do you have a picture of the motor in the machine ?

#15 leybold12 Mar 14, 2009 10:25 AM

The copper pipe adapter slides snugly into a a plastic housing (replacing an 120v series wound motor). You would see even less with the motor mounted into the housing.

So from your guys description, there is magnetic field leaking out of the rotor can. If I were to run the motor power leads close to the rotor, I might induce enough emf to confuse the sensorless controller (they were going to run just outside the copper pipe which would have shielded the wires).

I just order some fiberglass laminate sheet to form an fiberglass tube. I will have to think about moving power wires futher away from the motor.


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