Max output power = half of constant electrical power rating?

[swbluto]

Just curious if I'm getting my information straight. I like the idea of "6000 watt" motors and all and I would really like all the power I can get for steep hills, but it occurred to me that if 6000 watts is the input rating and the max output power of a motor is exactly when motor the efficiency is 50%, would that suggest the max continuous output power of a "6000 watt" motor is really 3000 watts?

Just trying to get a "fix" on things. Of course, for "relatively short" hills, I think one could get away with 4000-5000 output watts or more.

 

[liveforphysics]

This depends entirely on when parts start to saturate. Efficiency on some motors slopes sharply upward, then very slowly downward with increased current. On other motors, it looks more like a little hill than a gentle flat slope downward.

 

[Miles]

Maximum continuous rating is for output Watts, and is limited by heat dissipation.

Maximum power output is at 50% of no load speed with efficiency of 50%, as you said.

 

[voicecoils]

Maximum continuous rating is for output Watts, and is limited by heat dissipation.

Maximum power output is at 50% of no load speed with efficiency of 50%, as you said.

So...

If you take a motor, apply a load which drops it's speed to half that of no-load it will produce max power (x watts) and an equal amount of heat (x watts).

Is there anyway of predicting whether the motor will heat up so much that it dies in that situation, other then testing it?

Cheers!

 

[Miles]

Is there anyway of predicting whether the motor will heat up so much that it dies in that situation, other then testing it?

Usually, it's a question of how long before it dies, not if.... I guess you could get a reasonable idea of the thermal inertia from a sophisticated model but testing seems more practical, if you have the motor....

 

[gogo]

I seem to remember a thread where some heat dissipation formulas were discussed. What I came away with was that the ratio of surface area to power input/output was better for a smaller motor, and that two smaller motors had heat dissipating advantages over one bigger motor.

 

[voicecoils]

Is there anyway of predicting whether the motor will heat up so much that it dies in that situation, other then testing it?

Usually, it's a question of how long before it dies, not if.... I guess you could get a reasonable idea of the thermal inertia from a sophisticated model but testing seems more practical, if you have the motor....

Thanks Miles.

It just seems a shame to buy 2 Astro's just to do destructive testing one one of them to try to figure out how long the motor will output max power before failing.

I suppose practical alternatives would be to either run conservatively (below max power) or to add active cooling to increase the rate of heat transfer away from the motor.

 

[Miles]

The testing doesn't have to be destructive... the ratings for the insulation and magnets are known...

Why not go by the figure of 500 ounce inches of torque for 1 minute, given in the 3210 spec.?

The max. continuous torque rating is also given....

The coil insulation will die before the magnets so, if you really want to run it to the limit, add a thermal sensor there and limit the current with a feedback loop...

 

[Tiberius]

Is there anyway of predicting whether the motor will heat up so much that it dies in that situation, other then testing it?

Cheers!

Yes, that's quite easy. Regardless of what the motor is doing, or the controller or the voltage, the heat comes from I^2R. For a given motor there is max safe continuous current. In many cases the datasheet will either tell you the limit or give you some clues to work it out. For instance, I think Astro give a recommended limit.

Nick