Motor comparison spreadsheet

[crossbreak]

Give me Kt or Kv, Rm, core loss info, inductance, and mass, then I've got everything I need.

Sure. But how do you compare different motors in a simple way? You can't integrate core loss into Km because it varies with the rotational velocity. We've included this as separate values. Maybe we could integrate it into the power side somehow?

If I²R loss is equal to core loss, efficiency is near maximum. This is a single characteristic operating point for a given machine. At this operating point, the ratio of core loss to Km² could be a value to compare different machines.

Km² has the unit Nm (or Watt), core loss has the unit Watt as well. Thus a ratio of both would be nondimensional

 

[Miles]

If I²R loss is equal to core loss, efficiency is near maximum. This is a single characteristic operating point for a given machine. At this operating point, the ratio of core loss to Km² could be a value to compare different machines.

How does that help, though? Core loss varies with rpm so, the torque value that matches it varies correspondingly....

 

[crossbreak]

this helps, since it kicks out the rpm dimension: Every one wants max efficiency, so a drive will be designed in a manner to gain good efficiency. This operating point is a constant for any machine. Thus the core loss at this point is a constant. rpm dependence does not matter anymore.

The usefulness of this value has to be evaluated, though. I'm just thinking out loudly

 

[Miles]

this helps, since it kicks out the rpm dimension:

But it doesn't.... There's a maximum efficiency that corresponds to every rpm. Peak efficiency will increase with speed unless windage becomes significant. Yes?

 

[crossbreak]

hm.. no i guess not. i have to more play with numbers. Any motor has an efficiency map with a peak efficiency point...not line as core loss rises with rpm, but i²r loss rises with torque. this can not be a "line of best efficiency" it must be a point for any BLDC machine

Just like a diesel engine:

source http://de.wikipedia.org/wiki/Verbrauchskennfeld

BLDC Motor, source http://www.ansys-blog.com/advanced-design-electric-motors/

nice discussion, but my weekend is just getting started

Have a nice evening miles!

 

[Miles]

Same to you!

I'll try and work out a better explanation.....

 

[John in CR]

Give me Kt or Kv, Rm, core loss info, inductance, and mass, then I've got everything I need.

Sure. But how do you compare different motors in a simple way?

If you need to get things down to one number then peak efficiency is the only real comparative measure of how good a motor is, because it includes everything. Then you compare motors in the weight and power range you need.

You can't integrate core loss into Km because it varies with the rotational velocity. We've included this as separate values. Maybe we could integrate it into the power side somehow?

That's why I say as a group we can come up with a better Motor Constant than Km. All the columns devoted to Km in the spreadsheet make it seem important, but they're really just trying to make Km a good number.

 

[Miles]

The problem is eliminating rpm dependency.... Increase in peak efficiency flattens off as rpm rises but there's no fixed point.....

 

[Punx0r]

Miles, Crossbeak, thank you for the explanations

Just to clarify a couple of things about Km:

1) It describes the torque output (presumably at stall, where BEMF = 0 and current is proportional to Rm), not power (which is dependent on rotational velocity)? Some sources I read earlier said torque, some said speed...

2) Km is torque output for (say) 1 Watt of winding resistance losses, not 1W input power? Again, some sources said one, some the other. Or does it not matter as long as you are consistent when comparing motors?

Lastly, I feel a bit of a dunce here, but Miles, when you say Km increases "quadratically (inverse)", how do you mean? I picture a graph with a downward curve described by a quadratic equation, but what is increase proportional to? Is this the doubling of core length not doubling Km issue?

 

[Miles]

Lastly, I feel a bit of a dunce here, but Miles, when you say Km increases "quadratically (inverse)", how do you mean?

Well don't! I'm not sure my definition is correct so...... Anyway, if you double the size of the motor or use two motors, Km will increase by √2 (1.4142...). Isn't that a parabolic curve?

 

[Miles]

Any motor has an efficiency map with a peak efficiency point...not line as core loss rises with rpm, but i²r loss rises with torque. this can not be a "line of best efficiency" it must be a point for any BLDC machine.......

Increase in peak efficiency flattens off as rpm rises but there's no fixed point.....

I did a simulation with my motor model, using 0.35mm cheap steel laminations : )

2250rpm - peak eta 94.60%
4500rpm - peak eta 95.37%
9000rpm - peak eta 95.81%
18000rpm - peak eta 95.83%

As eddy current losses increase there dominance over hysteresis losses, the increase in peak efficiency approaches a line.

Maybe, at some point, windage losses will dominate and peak efficiency will fall....

 

[liveforphysics]

Any motor has an efficiency map with a peak efficiency point...not line as core loss rises with rpm, but i²r loss rises with torque. this can not be a "line of best efficiency" it must be a point for any BLDC machine.......

Increase in peak efficiency flattens off as rpm rises but there's no fixed point.....

I did a simulation with my motor model, using 0.35mm cheap steel laminations : )

2250rpm - peak eta 94.60%
4500rpm - peak eta 95.37%
9000rpm - peak eta 95.81%
18000rpm - peak eta 95.83%

As eddy current losses increase there dominance over hysteresis losses, the increase in peak efficiency approaches a line.

Maybe, at some point, windage losses will dominate and peak efficiency will fall....

You make some incredible motors my friend.

 

[John in CR]

Any motor has an efficiency map with a peak efficiency point...not line as core loss rises with rpm, but i²r loss rises with torque. this can not be a "line of best efficiency" it must be a point for any BLDC machine.......

Increase in peak efficiency flattens off as rpm rises but there's no fixed point.....

I did a simulation with my motor model, using 0.35mm cheap steel laminations : )

2250rpm - peak eta 94.60%
4500rpm - peak eta 95.37%
9000rpm - peak eta 95.81%
18000rpm - peak eta 95.83%

As eddy current losses increase there dominance over hysteresis losses, the increase in peak efficiency approaches a line.

Maybe, at some point, windage losses will dominate and peak efficiency will fall....

You make some incredible motors my friend.

+1 . I'm itching for the chance to buy a stockpile Milesmotors at cost plus regardless of how much assembly I have to perform. 8) . Keep the catchy name as my meager contribution to the effort.

 

[crossbreak]

there seems to be a certain rpm, from which Eff does not rise much anymore. 2250 in your case. Power at peak eff stays the same, too. So we could define this point as "point of best efficiency": The one with the lowest rpm, at which efficiency off PeakEff -1% is archived

Edit:

Power at peak eff stays the same, too

No! absolutely not the case. It rises almost quadratic

 

[Miles]

there seems to be a certain rpm, from which Eff does not rise much anymore. 2250 in your case. Power at peak eff stays the same, too. So we could define this point as "point of best efficiency": The one with the lowest rpm, at which efficiency off PeakEff -1% is archived

Nothing like keeping it simple....

 

[crossbreak]

hard to tell if there is a way to keep it simple. In the real world, there will certainly be a point of peak eff.

Maybe there is no way to keep it really simple..

 

[Miles]

I think we should stick with variable rpm and somehow integrate the core loss into the "power coefficient". Any thoughts on that?

Or, we could simply compute peak efficiency for the chosen rpm...

 

[Miles]

there seems to be a certain rpm, from which Eff does not rise much anymore. 2250 in your case. Power at peak eff stays the same, too. So we could define this point as "point of best efficiency": The one with the lowest rpm, at which efficiency off PeakEff -1% is archived

1% difference is quite a lot at around the 95% mark......... 95% to 96% is 20% less heat.....

 

[crossbreak]

i'm still thinking

 

[speedmd]

Some thoughts :? Torque and power don't tell you enough without some gauge of efficiency and heat dissipation abilities of the motor. Some measure of work / time may be key to understanding what a motor can really do. If we know the power, torque and efficiency, we would need to know the heat loss capability of the motor to get a duty rating to better gauge what type of work cycle it is suitable for. Thinking some sort of energy dissipation value/rate is key. Take two near identical motors with one of them able to dissipate two or three times the heat. You have a completely different real world work ability in the two motors. Some sort of measured thermal dissipation value (Td) would tell us loads from a comparison stand point.

 

[Miles]

If you really want to define a point where peak eta is at the maximum achievable, I guess you could use the proportionality between hysteresis losses and eddy current losses...? After all, this is what makes the difference..

 

[John in CR]

Absolute max would be an interesting box. A useful practical alternative is peak at a given voltage, and display for all motors as you vary pack voltage.

 

[Punx0r]

Lastly, I feel a bit of a dunce here, but Miles, when you say Km increases "quadratically (inverse)", how do you mean?

Well don't! I'm not sure my definition is correct so...... Anyway, if you double the size of the motor or use two motors, Km will increase by √2 (1.4142...). Isn't that a parabolic curve?

I tried a couple of graphs in Excel but only got a linear result - one gradually diverging from Km x 2, Km x 3. So I want to calculate Km properly. To clarify, when the stack length doubles, Km should also double? What is the effect on Kv, Kt and Rm? To show willing, I figure Kv stays the same, Kt and Rm doubles (ignoring end-turns)?

 

[Miles]

I figure Kv stays the same, Kt and Rm doubles (ignoring end-turns)?

If you're doubling the stack length, Kv halves; Kt doubles; Rm doubles.

 

[John in CR]

I figure Kv stays the same, Kt and Rm doubles (ignoring end-turns)?

Then you've broken the first law....

Kv halves; Kt doubles, Rm doubles.

Unfortunately Rm goes up about 4X. Double the turns comes with half the thickness of wire. Thank goodness HubMonster has about 1/6 the Rm of common high power hubbies with half the Kv. That's just the advantage during acceleration when currents are high. Far fewer poles and top quality steel give it a big advantage at cruise due to a significant advantage in iron losses.

Heat dissipation is a big "?". While we come up with a way to measure that relatively easily, an entry box for heat dissipation could get us to power info for motor comparisons, at least between similar motors. For sealed hubbies, I use 1kw as a baseline for heat rejection, and for my external radial blade ventilation I conservatively double that to 2kw . I think Justin's testing put it at about 800W for the 9C he used. I spin them significantly faster, which sheds heat better, but I'm less tolerant of high temperature so I used a lower delta T in my own calcs.

For power calcs don't forget we'd need to add a significant % to rm to take into account operating temps. Many run their motors at temps that increase the resistance of copper by 30% or more.