Question concerning Tormach 1100 Spindle Motor HP

[Eldon_Joh]

Keep in mind that the field weakens when the load is placed on the shaft because the current in the rotor is opposing the magnetic field produced by the stator magnets. The magnets aren't 3/4th inch thick for no reason!

Anyhow, you can't compare a 1 hp 3 phase induction motor with a 4MW motor, all motors get more efficient with size, for a reason that should be taught in college courses. for example, a 50MW transformer might weigh 50 tons, for a power density of 1KW per kilogram. the losses in the iron might be 2 watts per kilogram, and the transformers efficiency could be higher than 99%. a 1KW transformer might need to be >5Kilograms to get 95% efficiency, and 10kilograms to get to 97%. the reason is simply that the power capacity follows surface area of both the core cross sectional area and the winding cross sectional area, while the volume only follows the third power, with electrical losses proportional to volume, but power capacity of the transformer (or motor) to the 4th power of the base dimension.

For what its worth I hooked up one of my .75 hp 1750 rpm baldor motors to an amp meter, and rotated it by hand at about 60 rpm. it produced 2 amps of current. nameplate is 7 amps, 90volts.


Anyhow, permanent magnet motors are fundamentally more efficient than induction motors, because you don't have to keep paying for the I^2R losses to keep the magnetic field in the air gap alive. These losses are so high for fractional hp induction motors (60hz) that they cannot even supply their own losses if you plug them in (at nominal nameplate voltage and frequency), and drive them above the synchronous speed!

At some point the field strength limit of .4T for ceramic magnets makes induction motors more attractive for efficiency reasons, (due to weight, cost, size limitations) this threshold might be around 2-5HP.


If you can believe it, those 7MW direct drive wind turbines are using cinder block sized neodymium magnets. i don't think they would pay for them if they could produce an induction motor generator of similar torque and power density..

[jwatte]

not possible for a pmdc motor unless you can rotate the brushes relative to the magnets, or if you can weaken the magnets

Perhaps I wasn't clear enough. A brushed motor has a maximum RPM, where generated torque equals torque lost by friction and back EMF (and hopefully back EMF is the higher of those two!)
At that point, the maximum RPM, there is zero usable power available on the shaft.
As the motor slows down, it can generate more usable power. Or, alternatively, as you add load, the RPM will slow down because the power draw of the load will start dominating.
But, as a motor starts running slower, it gets to the point where power starts decaying because power equals torque times rotation speed, and rotation speed goes down.

That's all of the comment I wanted to make, and I'm pretty sure you and Zeptopan agree. (I also appreciate the significant detail you've both gone into, almost like a textbook!)

The original poster would probably see some MRR improvement from the Tormach 1100, from a basline stronger motor, and from a more rigid machine.

[Zetopan]

At some point the field strength limit of .4T for ceramic magnets makes induction motors more attractive for efficiency reasons, (due to weight, cost, size limitations) this threshold might be around 2-5HP.

Toshiba makes high efficiency 0.75HP 3 phase induction motors that are about 85% efficient (that would be a full load measurement) although I have no idea what the cost for one would be. Generally speaking low cost HP induction motors use aluminum shorting bars in the rotor while the more expensive high efficiency versions will use copper instead. Copper is more expensive and heavier but it also has much better conductivity. Silver would have even somewhat better conductivity but the cost and much lower tensile strength make that usually quite impractical.

If you can believe it, those 7MW direct drive wind turbines are using cinder block sized neodymium magnets. i don't think they would pay for them if they could produce an induction motor generator of similar torque and power density..

Using a 3 phase induction motor as a generator is generally a totally losing proposition since you would need an auxiliary power source to even get it to start generating. To run a 3 phase induction motor as a generator the stator field needs to lag rather than lead the rotor field (i.e. the slip factor must be negative), so that specific usage is virtually always confined to dynamic braking situations. Without a stator field to start with, the rotor cannot even generate a field through induction. Tesla does this in the 3 phase powered Model S and X automobiles, putting energy back into the battery when slowing down when regen is enabled (it can also be disabled to act more like an automatic transmission).

That is why 3 phase induction motors are not actually designed to be wind turbine generators since some type of auxiliary power is always required for it to even start generating. Of course the largest 3 phase generators do not use permanent magnets for multiple reasons. The curie point temperature for neodymium magnets is quite low, limiting their usage to situations where they would never see elevated temperatures. 700MW generators (note this is 2 orders of magnitude larger than the largest wind turbine generator) utilize DC current powered rotor windings (supplied through slip rings) to create the changing magnetic field in the stator field windings for power generation. Closed loop control of this DC current provides for rapid feedback to tightly control the generated output voltage. This is analogous to an automobile alternator minus the rectification diodes.

[Zetopan]

At that point, the maximum RPM, there is zero usable power available on the shaft. As the motor slows down, it can generate more usable power. Or, alternatively, as you add load, the RPM will slow down because the power draw of the load will start dominating. But, as a motor starts running slower, it gets to the point where power starts decaying because power equals torque times rotation speed, and rotation speed goes down.

For PM DC motors the maximum power output occurs quite close one half of the no load operating RPM (assuming a constant supply voltage). At higher or lower RPMs the output power drops and that is quite clearly shown in the plots that I previously provided for two different specific PMDC motors (the green curves on both).

The original poster would probably see some MRR improvement from the Tormach 1100, from a basline stronger motor, and from a more rigid machine.

I would suggest the word "probably" does not really belong in that sentence. There is insufficient data available for me to plot the characteristics of the Prolight spindle motor, but any Tormach PCNC 1100 owner who is willing to test the same conditions that the original poster was using should be able to demonstrate that there is actually a large difference (my supposition) between those two machines.

[skrubol]

For PM DC motors the maximum power output occurs quite close one half of the no load operating RPM (assuming a constant supply voltage). At higher or lower RPMs the output power drops and that is quite clearly shown in the plots that I previously provided for two different specific PMDC motors (the green curves on both).

That's the highest peak theoretical power output. Most DC motors have a current limit that limits the torque to less than half the stall current, at least for more than an instant.

I would suggest the word "probably" does not really belong in that sentence. There is insufficient data available for me to plot the characteristics of the Prolight spindle motor, but any Tormach PCNC 1100 owner who is willing to test the same conditions that the original poster was using should be able to demonstrate that there is actually a large difference (my supposition) between those two machines.

I agree with you there.
My original point was that the Tormach has more of its power usable over a broader range than the Prolight does (> .5HP from at least 500-5000RPM when using both ranges.)