Burgerman wrote:Less than 1K definitely!
For what its worth I bought some practically unused 8mph LINIX sunrise just like your ones on here for 320 a pair. Still boxed - looked new. Keep eyes open these things turn up! Obviously you would need to swap the hub for a 4 or 5 stud one from the old motors.
Using stronger magnets (or adding more magnets in the right), on a permanent magnet DC motor, gives essentially the same effect as gearing a motor lower (slower speed, more torque). Using weaker magnets (or adding more magnets in the right spot "upside down"), gives essentially the same effect as gearing a motor higher (higher speed, less torque).
It might seem counter intuitive, but it's the physics. Math first, but physical example is further down the post.
The no load speed of a permanent magnet DC motor is defined as:
speed = Vt*Kv - Where Vt is the voltage at the terminals of the motor, and Kv is motor speed constant (usually in terms of rpm per volt)
Everyone here is obviously very familiar with this concept however, it is actually more useful to use the back EMF constant (Ke) in the calculation, where the back EMF constant is simply 1/Kv:
speed = Vt/Ke
This still just applies to permanent magnet DC motors, but one can generalize it a bit to cover all DC motors.
speed = Vt/(Ka*phi) - Where Ka is the armature constant and phi is the magnetic flux from the field.
In a PMDC motor, phi is a constant (because it is made by the permanent magnets), so it is typically combined with Ka to give Ke (or Km), but on field wound DC motors, it is variable. Prior to fancy PWM schemes and such adjusting the field current (and thus the magnetic flux) was frequently a way one varied the speed of DC motors.
Anyway, that equation shows the inverse relationship between speed and flux. Less flux = more speed.
However the torque equation for a PMDC motor is as follows:
torque = Kt*Ia - where Kt is the motor torque constant, this time in terms of newton metres per ampere (N·m/A), and Ia is the armature current.
This should be familiar with anyone working on the motor compensation on their chair. It also can be generalized for all DC motors.
torque = Ka*phi*Ia - Where phi is still the flux, Ia is still the armature current, and Ka interestingly enough is the same armature constant as above. (and thus Kt=Ke=1/Kv, wikipedia goes into details of that relationship in the "Motors Constants" article if one really cares.)
This shows a direct relationship between torque and flux. More flux = More torque.
And it makes sense that it has to work this way. Torque * Speed = Power. You can't just keep increasing the magnetic flux and get more power. At some point you would be drawing more power out then you are putting in, and you would have just invented a perpetual motion machine.
-------------------Start reading here if you don't want to bother with the math----------------------
Here's another way to look at this. If you take a motor just starting to turn, and simplify it down to just a stationary wire carrying current in a magnetic field. It gets some force exerted on it proportional to the strength of the magnetic field and the amount of current. This should make sense to everyone. Lots a torque, zero speed. More magnetic field gives more torque. More current, more torque too.
Now consider the opposite extreme - a motor spinning at the no-load speed. This is a little harder to visualize. But imagine if the motor was spinning at the no-load speed because you were coasting down a hill - no battery hooked up to the motor (or resistors or anything else). At this point the motor would actually be acting as a generator.
So if it was a 24V motor, it'd be putting out about 24V as a generator, but zero current (and thus zero power). It wouldn't slow down the chair because you aren't pulling any of the mechanical energy out as electrical energy. If you then hooked up a 24V battery to it, nothing would change. It would continue to be in balance.
But say you made the hill a little bit steeper, now the motor (now acting as a generator) would be putting out 25V, and be charging the battery, and trying to slow the motor (this is regenerative braking). If you made the hill a little less steep, and the motor only generates 23V, then the battery would be pushing it along with just a little bit of energy. Makes sense so far? That voltage that the motor is generating? That's called the Back EMF.
Okay, now let's assume we are back on the slope where the motor was putting out 24V with no battery connected, and everything is the same except this time we put some weaker magnets in the motor. Actually let's make the Magnets exactly half as strong. If we do that the motor only puts out 12v as a generator. Makes sense right? Less magnetic flux, generator puts out less voltage. Great.
Now let's hook that 24V battery back up to the motor. Well, now there is a 12V difference between the back EMF and the battery voltage. That battery will be pushing that motor pretty hard. How hard? Well, it will push it to nearly twice the original speed. Counter intuitive huh? Less magnetic flux gives a faster speed.
But there are still good reasons to use neodymium magnets in some applications - for a given flux, they are both lighter and smaller than most other options. However, most neodymium magnets have a lower operating temperature than other types of magnets (about 80C). So one needs to ensure that they stay below that or they will loose their magnetism. All about choosing the right magnet for the application.
Burgerman wrote:Yennek wrote:Using stronger magnets (or adding more magnets in the right), on a permanent magnet DC motor, gives essentially the same effect as gearing a motor lower (slower speed, more torque). Using weaker magnets (or adding more magnets in the right spot "upside down"), gives essentially the same effect as gearing a motor higher (higher speed, less torque).
I disagree. Having tested this on my bench with smaller hobby motors. You gain a lot of rpm if driving a propellor. Because it increases torque. If free running you also gain a lot of RPM. That is easy to test too.
Stall torque was the same when limited to my bench power supply to 2A. But if set to a high non limiting figure stall torque also uncreses as you can no longer hold the shaft still with fingers!
Burgerman wrote:Yennek wrote:And it makes sense that it has to work this way. Torque * Speed = Power. You can't just keep increasing the magnetic flux and get more power. At some point you would be drawing more power out then you are putting in, and you would have just invented a perpetual motion machine.
But nobody claimed you were getting something for nothing. Current goes up! Across the board. My power supply shows this.
Burgerman wrote:And both torque and rpm definitely do increase! At least under load. Even when that load is just some motor bearings and gearbox A free running powerchair motor takes 5 to 7A. Due to windage/grease/bearings. And about 20A flat out on a road or pavement. So the extra magnets speed it up.Yennek wrote:
-------------------Start reading here if you don't want to bother with the math----------------------
Here's another way to look at this. If you take a motor just starting to turn, and simplify it down to just a stationary wire carrying current in a magnetic field. It gets some force exerted on it proportional to the strength of the magnetic field and the amount of current. This should make sense to everyone. Lots a torque, zero speed. More magnetic field gives more torque. More current, more torque too.
Agreed.Yennek wrote:Now consider the opposite extreme - a motor spinning at the no-load speed. This is a little harder to visualize. But imagine if the motor was spinning at the no-load speed because you were coasting down a hill - no battery hooked up to the motor (or resistors or anything else). At this point the motor would actually be acting as a generator.
Agreed.Yennek wrote:So if it was a 24V motor, it'd be putting out about 24V as a generator, but zero current (and thus zero power). It wouldn't slow down the chair because you aren't pulling any of the mechanical energy out as electrical energy. If you then hooked up a 24V battery to it, nothing would change. It would continue to be in balance.
True. But I was not refering to driving down a hill. But on the flat.
Burgerman wrote:That takes power. At full speed you will be moving at say 80% of the motors free running wheels off deck speed. Andas you say, adding stronger magnets is the same as a ower impedance motor. So now the thing draws more current and the motor goes 90% of the no load speed. Theres no time other than wnen running downhill that the motor load is zero. Typically you are using 150 to 200 watts.Yennek wrote:But say you made the hill a little bit steeper, now the motor (now acting as a generator) would be putting out 25V, and be charging the battery, and trying to slow the motor (this is regenerative braking). If you made the hill a little less steep, and the motor only generates 23V, then the battery would be pushing it along with just a little bit of energy. Makes sense so far? That voltage that the motor is generating? That's called the Back EMF.
Absolute sense.Yennek wrote:Okay, now let's assume we are back on the slope where the motor was putting out 24V with no battery connected, and everything is the same except this time we put some weaker magnets in the motor. Actually let's make the Magnets exactly half as strong. If we do that the motor only puts out 12v as a generator. Makes sense right? Less magnetic flux, generator puts out less voltage. Great.
I dont know why we are going downhill but OK we will carry on! No I dont think the motor will produce 12V it will produce the same 24V but behaves as a higher impedance motor. But not 100 percent sure.
Burgerman wrote:Yennek wrote:Now let's hook that 24V battery back up to the motor. Well, now there is a 12V difference between the back EMF and the battery voltage. That battery will be pushing that motor pretty hard. How hard? Well, it will push it to nearly twice the original speed. Counter intuitive huh? Less magnetic flux gives a faster speed.
No. I disagree. The motor will make the same 24V (not sure) and behave as a higher impedance motor, and so will charge the battery less than with the strong magnets.
The magnetic feild doesent determine the open circuit voltage the armature rpm does I think. But it does under load.
Burgerman wrote:When I swap out ferric magnets for rare earth magnets in my RC truck 550 motor for eg (I swapped the motor can over for a better one) I get more torque, and I get more speed. And I get a shorter run time so greater Amps. So that cannot be the case.
Burgerman wrote:Yennek wrote:But there are still good reasons to use neodymium magnets in some applications - for a given flux, they are both lighter and smaller than most other options. However, most neodymium magnets have a lower operating temperature than other types of magnets (about 80C). So one needs to ensure that they stay below that or they will loose their magnetism. All about choosing the right magnet for the application.
My hobby motors get super hot. Its not really an issue. The motor coils and armature burn up long before the motors casing/magnets get super hot on a powerchair. The real reason is cost.
Burgerman wrote:You may be right about them generating less volts with seaker magnets. I dont understand your formulas. When off this bed I will do some actual tests to find out for sure.
Burgerman wrote:This is a 700 watt rated motor that I regularly run at 1200 watts or more. It turns at up to 54k rpms. And is loud. Its got brushless 2 turn Super low impedance. Runs at 120A and is 26mm diameter. Magnets? If sticking 1200 watts, 120A through these doesent mess up they magnets I dont think a powerchair motor would even tickle them.
Yennek wrote:... but you have 10x the resistance...
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