News:


  • June 01, 2024, 10:50:26 AM

Login with username, password and session length

« previous next »
Pages: [1]   Go Down

Author Topic: RPM / VOLT?  (Read 460 times)

Offline John Rist

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2952
RPM / VOLT?
« on: October 27, 2010, 02:35:42 PM »
I am fairly new to serious electric power.  I have bench mounted a TURNIGY TR38-48-900.  I am running 11x5.5E prop.  Power source is a 12 cell Nicad (will be replaced with 4-5C Lipo).  the 12 cell Nicad should be around 14 volts. Speed controller is a MYSTERY 60A unit.

As I understand it the 900 rating is 900 RPM / Volt.  Is it true that this is the no-load rating?  At 14 V x 900 the rpm would be 12600.  Run test shows the motor to be turning the 11x5.5 prop at just over 7,000 rpm or around 56% of rated rpm.  My experience is with old brush motors.  50% rpm of no-load was the norm for them.  Does this hold true for brushless Out-Runners also?  If so I am about correct on the prop.

 ???   ???   :)
Logged
John Rist
AMA 56277

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12823
Re: RPM / VOLT?
« Reply #1 on: October 27, 2010, 02:58:22 PM »
50% of no-load RPM gives you -- more or less -- worse than 50% efficiency.  You can expect far better than that from a brushless motor.  Unless the mystery speed controller is throttling the thing down then something is wrong, even if it's your motor/prop combo.

What's getting hot?
Logged
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline John Rist

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2952
Re: RPM / VOLT?
« Reply #2 on: October 27, 2010, 03:13:27 PM »
I will make some more measurments,  It is old nicads.  My voltage may well be well below 14 volts.  Ran it very little but every thing was ice cold.
Logged
John Rist
AMA 56277

Offline Tim Wescott

  • 2016 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 12823
Re: RPM / VOLT?
« Reply #3 on: October 27, 2010, 03:21:15 PM »
Try running it with a little flat balsa board (think double-wide Popsicle stick) and tach that.  You really should see full speed out of the thing.
Logged
AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Igor Burger

  • Trade Count: (0)
  • Admiral
  • ******
  • Posts: 2166
Re: RPM / VOLT?
« Reply #4 on: October 27, 2010, 03:27:53 PM »
Motor RPM is Kv * effective voltage

while the effective voltage is battery voltage lowered by current * resistance (battery + ESC + winding resistance)

means motor RPM = Kv*(U-Ri*I)

so if your motor needs 40A and its internal resistance is 50mohm, then effective voltage is at least 2 volts lower. Means it makes 20% lower RPM on 10V battery

BTW it has exactly same rules like DC brushed motors

Logged

Offline John Rist

  • 24 supporter
  • Trade Count: (0)
  • Admiral
  • *
  • Posts: 2952
Re: RPM / VOLT?
« Reply #5 on: October 27, 2010, 04:54:08 PM »
Ok here is the real deal.  I think I am OK.  I recharged the old Ni-cad pack.  I re-ran the test with my volt/amp meter installed between the battery pack and the controller.  The readings were 10V, 19 amps and 7800 rpm.  At 900 V/rpm no load would be 9000 rpm.  Says it is running at 86%.  Of course it is probably higher than this because I am measuring the voltage at the battery.  The 10 volts does not allow for speed controller voltage drop and motor resistance drop.  Obviously the Ni-Cad pack is no good.  When the hobby budget recovers I will order the battery charger and 4S 3000 lipo batteries.  Looks like a good project for Santa.  LOL

                                      LL~

It will be awhile but I will post the results.
Logged
John Rist
AMA 56277

Advertise Here
Pages: [1]   Go Up
« previous next »
Tags:
 


Advertise Here