I won't presume to answer questions directed at Mike, but as a 30 year practicing electrical engineer, I can assure you that torque and inertia are only distantly related quantities.
Member
I won't presume to answer questions directed at Mike, but as a 30 year practicing electrical engineer, I can assure you that torque and inertia are only distantly related quantities.
Contributor
So why do we not use RI motors in woodworking equipment? Here are some of the reasons: Comparisons are made to equal HP motors.
1. They are significantly more expensive than capacitor start or three phase induction motors.
2. They are physically larger than standard induction motors.
3. They have brushes and moving parts that are not in standard induction motors. Essentially all motors are going to brushless solutions for reliability.
4. The brushes require maintenance. Standard induction motors only have bearings that require maintenance - as do RI motors.
5. There are no three phase RI motors, only single phase.
6. When operated in the motor's normal operating range, there is no advantage to the RI motor. It is just an induction motor like the standard induction motor. The repulsion mechanism is only used to get the motor up to speed.
The only advantage of an RI motor is that it has higher starting torque which is of value when the load is attached at startup. This is generally not true for woodworking tools.
What Phil is trying to put forward as an advantage is that if the motor is significantly under-powered for the load, the motor will get a "kick" as the RPMs decrease, as the expense of significantly excess current flow through the motor. In reality, the motor should not be operated in such a fashion - its HP should be matched to the load so that it never stalls in that fashion. It's trying to take a problem and turn it into a "feature".
Mike
Last edited by Mike Henderson; 07-20-2015 at 8:11 PM.
Go into the world and do well. But more importantly, go into the world and do good.
Guest
Feature is your word, not mine. I'm just pointing out functional differences in the motors.Originally Posted by Mike Henderson
My point is, those old bullet motors can be pushed to their limits and once you hear them losing RPM's you don't have a fraction of a second before they stall (as you would with a modern induction motor). You have some time to adjust your feed rate and finish your cut.
I'm not suggesting they be overloaded on purpose. OTOH, it has to have been the single best-selling cabinet saw in history, with the great majority of them being used by employees in a commercial setting. Point being, they were put in environments where they were likely routinely pushed to extremes and yet held up very well.
You can try twisting this however you like, history is on my side.
Last edited by Phil Thien; 07-20-2015 at 8:07 PM.
Contributor
Ah, yes, and the facts are on my side.
Mike
Go into the world and do well. But more importantly, go into the world and do good.
Guest
Oh and #5 is sort of silly, don't you think? That is like me implying that a lack of any starting caps on modern three-phase induction motors somehow makes cap-start motors inferior.
Contributor
Yes, because of the cap, a capacitor start induction motor is inferior to a three phase induction motor. The cap is a point of failure that the three phase motor does not have.
The only reason we use capacitor start induction motors is because we don't have three phase. If we had three phase, we'd use three phase motors. More reliable.
Three phase motors have good starting torque, also.
Mike
Go into the world and do well. But more importantly, go into the world and do good.
Guest
Yeah but we are not discussing three phase and yet you listed the fact that there are no three phase RI motors as some sort of proof that RI motors are unsuitable for woodworking.
Again your position is that a Unisaw with 1.5-hp RI motor is inadequate and everyone should have at least a three hp motor.
Last edited by Phil Thien; 07-20-2015 at 8:48 PM.
Contributor
Phil & Mike -
You are both highly respected, and highly knowledgeable resources. We all look forward to reading your thoughts and advice on many topics.
However, at this moment, on this topic, you are in a death match, determined to wrestle each other to the ground. And you are wrestling in an arena that very few of us understand.
Please keep up the contributions to the Creek.
And, consider taking this match to PM or a local bar. If there is a local bar, I will join in, buy a couple rounds, just for the pleasure of watching.
Very best regards to both of you.
Kent
When I started woodworking, I didn't know squat. I have progressed in 30 years - now I do know squat.
Contributor
Okay, Kent. But it was interesting.
Mike
[@Alan Speers - I'm sorry I didn't address your posting. It happened to fall at the end of one page on my computer and I didn't see it. I'd have to think about the number of poles and whether that might affect the torque. Off the top of my head, I don't think so. I don't think that the rotor is turning between poles, at any time, due to inertia - there is always force on the rotor. If that were not true, there would be rotor positions where the motor would not start.
Also, while a 3450 RPM motor only has two poles, each pole covers close to half of the stator. The poles are just bigger than the poles in a four or eight pole motor. Look inside a 3450 RPM motor and you won't see much empty space on the stator. Remember, in an induction motor, the field is rotating and the rotor is following the field (a bit slower than the field in an induction motor - the rotor is chasing the field and the difference in their RPMs is known as "slip"). In a four pole motor, the field just rotates at half the line Hertz.
But whatever the torque, it will get factored into the HP since HP and torque are related.
And don't get confused by torque and HP. When cutting wood, it is HP that's important, not torque. For the same HP, a slower RPM motor will have greater torque but it will do exactly the same work as a higher RPM motor.
And if you'll permit me, I made a mistake in my comments in post #13. I did the calculations based on a 1HP motor and assumed that the recovery would only have to generate 1 HP at 75% of the rated RPM. However, induction motors will not stall at 100% of the rated HP. They will provide greater than 100% of the rated HP, albeit at excess current flow. Service Factor captures this. So if you have a service factor of 1.15 on a 1HP motor, it means that the motor will provide 1.15 HP but the increased current flow will cause the temperature of the motor to increase by 10 degrees C, and operation at that level will reduce the life of the motor by approximately half.
The calculations should have been based on a recovery HP of about 1.20 HP (the assumed stall HP level), so the torque required at 75% of rated RPM is about 160% of rated operating torque, which would require current significantly above rated, with the consequent heat. If the motor designers had intended for the starting circuit to be used that way, they would have not designed a disconnect into the system - they would have left it in all the time.]
Last edited by Mike Henderson; 07-21-2015 at 11:23 AM.
Go into the world and do well. But more importantly, go into the world and do good.
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