**Re: [EVDL] Overdiven 10hp AC**
>

> Hi Jeff,

>

> You seem to propagate a common misconception here

>

I am not propagating any misconceptions here.

> about poles and torque. It is my opinion that pole

> count increase does not increase torque density in

> these double air gap motors. Torque is a function of

> the total air gap area, flux density and current

> density.

>

For a given motor that can run as 4 or 8 pole you are agreeing that the

rpm drops in half with double the poles

And I think you also agree that hp = (rpm*torque)/5252

So the mathematics of this tend to show that if two motors have the same

HP, the product of the rpm and the torque must be the same.

1/2 the rpm means twice the torque.

> Let's stick with 60 Hz induction motors. If you have

> a 5 hp rated 4 pole motor, it has a speed of 1800 RPM.

> Let's neglect slip for simplicity. So it would have

> a rated torque of 14.6 lb.ft. And a 5 hp rated 8 pole

> motor would be 900 RPM and have a rated torque of 29.2

> lb.ft. It would appear that doubling the pole count

> has doubled the torque. Right?

>

OK, you do agree.

> But take a look at a major motor manufacturer's

> catalog. The 5 hp, 4 pole motor is a 213 frame size,

> 115 lbs. The 5 hp, 8 pole motor is a 256 frame size,

> 211 lbs. Both TENV. The mass has nearly doubled for

> twice the torque. The torque density is about the

> same.

>

I wasn't talking about density and I assume what is going on here is

that the 4 pole can get by with lighter construction because it is not

made for the torque.

for example, a Nema 213 has a 1.125 or 1.375 and a 256 has a 1.375 to

1.625 (U or T designation)

> I'm sure you can find some examples which contradict

> this example.

A lot I am afraid, as the primary equation determines this

But i think we can think it through also. common sense

ok, for a given pole pair they each apply a tug on the rotor 120 times a

second at 60hz, once for the positive swing of the arc and once for the

negative.

Sure the size of the poles makes a difference, but I am talking about

the given volume of motor. it still produces the same HP

but instead of energizing 1 pole pair, now we energize a second pair in

parallel, the torque doubles but the rotor now takes 2 cycles to turn a

revolution

> Nameplate ratings can be dangerous.

> So, with particular cooling schemes, the power ratings

> for motors with different pole counts may be in the

> same frame size. This is due to the fact that the

> rating is strictly dependent on the losses.

And here is the point, if it is a wound rotor motor, the excess energy

(that would result in the heat that limits the amount of amps you can

put in for the amount of cooling you have), is brought out of the rotor.

> So if the

> motors are of equal efficiency, the power rating will

> be the same.

Power the same agreed.

> But the breakdown torque will also be

> the same. You haven't increased the torque with more

> poles, you're just rating it at a lower speed.

>

Sorry, I disagree. I could run a math proof, but I promised myself when

I graduated i wouldn't do any more of those.

> Regards,

>

> Jeff M

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