Speed loss constant Ks (watt/krpm)? 3

[Clyde38]

I have a customer specification for a "speed loss constant Ks" with units of watts/krpm. Are any of you familiar with this. For example a value of .5 watts/krpm. I'm not sure how this is determined. I would not think that this could be a linear relationship. Any ideas?

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[waross]

Ask the customer for a technical reference so that you may verify that you are both on the same page.

Bill
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"Why not the best?"
Jimmy Carter

 

[Clyde38]

Waross,
Thanks, I have done that and I'm waiting for a reply. The spec is from my customer's customer, so the response is slow. I just thought maybe some of the readers might have been exposed to this.

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[LionelHutz]

Is this a variable speed (VFD) application?

I doubt it would be a constant either. Things like windage losses and the cooling fan load are not linear with speed.

 

[Clyde38]

LionelHutz,
This is a Brushless PM motor and it operates over a wide speed range (0-35000 r/min). I agree with you, I see no way that this is a constant nor will the loss be linear. Still no response from the customer's customer. Thanks for you response.

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[7anoter4]

The mechanical losses depend on rotor velocity but the dependence is not linearly.
The mechanical losses Pmech=Pair-gap+Pfan+Pbearing
Pair-gap= kair-gap*rpm^3
Pfan= kfan*rpm^2
Pbearing= kbearing*rpm
So it is not possible to state ks=Pmec/rpm as a constant. The manufacturer could provide may be kair-gap, kfan and kbearing as ks1,ks2 and ks3.Usually Pmec=1/3*noload losses[could be 2-3% of rated power].

 

[Clyde38]

Thanks 7anoter4,
I still haven’t heard from the customer. The best I’m able to come up with is that they are referencing a damping factor which is normally expressed in units of torque/speed i.e. Nm/krpm or oz-in/krpm. This is the multiplier factor defining the torque losses proportional to speed. This torque is due to the viscous friction of the ball bearings as well as to the Eddy currents in the stator originated by the rotating magnetic field of the magnet. This normally represents losses proportional to the rotational speed.


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[cswilson]

If you look in a catalog for brushless PM (servo) motors, and analyze the torque-vs-speed capabilility curves, you will see on many plots that the continuous torque rating gradually diminishes in a linear or approximately linear fashion as the speed increases from zero. This is for speeds below those that are voltage-limited.

If the copper resistive losses were the only losses, the continuous (i.e. thermally limited) torque rating would be constant up until voltage-limited speeds. This rating is based on the idea that the motor is able to dissipate a certain number of watts continually (but read the fine print for the conditions under which it can do this).

The downward slope of this rating as speed increases therefore represents the watts/krpm lost due to other mechanisms. To the extent this curve is really linear, this value is a constant. What I don't know is how well motor manufacturers really characterize this relationship, or if they just draw a straight line between the zero-speed case and a high-speed case -- which would be a conservative approximation.

Curt Wilson
Delta Tau Data Systems