Re: [CAD_CAM_EDM_DRO] Re: Declining motor torque with lower voltage.

Stephen Wille Padnos wrote:

>Jon Elson wrote:
>
>
>
>>[snip]
>>You can be sure this is what we are talking about by doing a power
>>calculation. If
>>the real resistance was 7 Ohms, the power loss would be I^2 * R = 18.5 *
>>18.5 * 7 =
>>2396 W or 3.2 HP. Since this equals the total power input to the motor
>>(130 V * 18.5 A = 2405W) there is no power left to come out the motor shaft.
>>Obviously a motor that absorbs 3.2 electrical HP and produces zero
>>mechanical
>>output is worthless.
>>
>>Jon
>>
>>
>>
>And using those formulas to calculate the resistance, voltage constant,
>etc.:
>Input power = 2405W
>Output power = 1865W (2.5 HP @ 746W per HP)
>Power wasted = 540W (ouch!)
>
>current = 18.5A, I^2 = 342.25
>R=P/I^2 = 540 / 342.25 = 1.58 ohms
>

This may be slightly high, as it is including windage, iron and bearing
losses
as if they were series resistance.

>voltage drop across the internal resistance = 1.58 * 18.5 = 29.23 volts
>voltage from back EMF = 130-29.23 = 100.77
>voltage / kRPM = 100.77/6.7 = 15.04
>
>So, going back to the calculations for what you'll get driving this
>motor with a Gecko:
>
>Max voltage: 130V
>Max speed: 6700RPM
>Torque constant: 20.16 oz-in/A (assuming 373 oz-in continuous torque)
>Continuous current: 18.5A
>

These numbers are all based on PEAK torque ratings, not the continuous
rating of the motor, which we just are not provided. There's no WAY even
a pretty big sealed motor could tolerate 540 W of losses continuously.
You are looking at something the size of a 3 HP AC induction motor with
fan cooling to handle this level of losses - almost 3/4 Hp of losses
internal
to the motor! In most cases, DC brush motors have MUCH poorer cooling of
the armature than an AC induction motor. There are some brush motors that
have big holes in the armature iron laminations, and a forced-air fan
run by a
separate motor to ram air through those holes to cool the armature windings
by conduction. There are also ironless-rotor motors that ram cooling
air through the
air gap, or even fill the air gap with thermal conductive liquids to get
the heat out.
You'll pay PLENTY for these exotic motors. These cooling problems are the
reason behind the big push to brushless motors. In these, the windings
are in the
STATOR, and the heat can be removed MUCH more efficiently.

On a typical fully-enclosed DC brush motor, the continuous rating might
be about
1/5 of the peak, although this can vary from 1/4 to 1/10 or so,
depending on the
magnets, winding insulation and a bunch of other factors.

Jon