Feb 29, 2008, 11:23 PM
Go ahead... Build it to crash!
Idea

# Crazy idea - variable Kv motor

Hi all,
I was driving one day (I drive alot for work 70,000 miles in 18 months) and had an idea, of the kind you can only have when you are alone in a car for several hours, and the mind starts to do wierd things. I have no idea if this could even have a chance to work, but I'm curious, so I'm going to post the question:

Is a variable Kv motor possible? Here's what my mind came up with. Imagine a motor, (I was thinking along the lines of vehicle applications) that has 40 winds of x guage wire. This gives us a particular Kv, depending on magnets, stators, wire thickness, as well as number of winds. If you change the number of winds, then all else staying the same, you would have a different kv motor. What if you had a way to change the number of winds on each stator pole on the fly, while still keeping full copper on the stator. Here's my thought, have each wind terminate into a switchboard or circuit or controller, or whatever you call it, so that you can switch between parrallel and series for the winds. If you have all in series, then you have 40 winds. If you parrallel pairs of winds, then you have 20 winds of lower resisitance, as they are in parrallel (at least as I understand it, parrallel connections = lower resistance) If you do 4 wires in parrallel per wind, then you have a 10 turn motor of even lower resisitance. If you do 8 in parrallel, then you have a 5 turn motor. 10 parrallel, gets you 4 turns, 20 in parrallel gets you 2 turns and then all in parrallel gets you 1 turn motor. So, in effect you would have 8 possible kv's for the motor corresponding to each situation. this would be like gears on a car (and is how I got on this train of thought, electric cars without the need for a gearbox) At high wind count, low Kv, you would have a lot of torque, so would be like 1st gear. As you got lower wind count, and higher kv, the motor would have lower torque but higher Rpm, as in higher gears. So a controller for this motor would have to control the pulses to spin it, but also be able to shift between Kv's as the need arose, based on the driving condition, just like an automatic transmission. I am thinking that this would make it possible to run the motor in it's most efficient configuration to meet the driving needs, excelleration, top speed, efficient highway cruising, in a way that a single motor would not be able to do without a gearbox. I hope this explanation is fairly clear of what I am thinking about.

Here's the questions I have about it. Whaddya think?? Is it crazy? Assuming you could fit efficient enough switches to handle the possible wiring configurations on each pole of the stator, would it even work? Do the winds have to be just like they are on our usual motors, or would passing through a circuit on their way around the wind completely make it not work as a motor i.e interfere with the electromagnetic field created by the winds? I am also wondering if it would be even possible to create that kind of switch setup, as I haven't exactly tried to draw it out on paper. This was purely an idea I had in the car. Assuming you could build it, would it work like I suggested? Or am I missing a big part of the picture on how motors work.
 Mar 01, 2008, 12:28 AM DHMO kills! Great concept. Maybe instead of trying to install switches on the windings have each set run to the controller separately. Then the switching would be performed outside the motor housing. Many ac motors have multiple winding terminations that allow for different speeds. I don't see how this should be much different in practice. You may want to do a patent search to see if someone has work something like this out yet. I really think that a brushless motor on each wheel in a vehicle would be the great. just think, all wheel drive with no complex mechanical load shifting. Drive assisted steering, you turn right the car puts more power into the left side to help the turn. You can loose 3 motors and still move the vehicle, talk about never getting stranded. Now 4 motors with variable Kv, very nice.
 Mar 01, 2008, 08:27 AM Go ahead... Build it to crash! Ok, good to know I'm not just crazy That was the thinking that got me on this subject. You wouldn't need a transmission, so there wouldn't be anything to service on the vehicle but tires, ac, wipers, brakes, and lights. It would also open up some interesting vehicle configurations if you don't have to have axles and a transmission. I might just do a search and see if it's being worked on. I hadn't done anything like that yet. I'm curious now though.
Mar 01, 2008, 09:41 AM
Registered User
Quote:
 Originally Posted by Fuegodeth Ok, good to know I'm not just crazy That was the thinking that got me on this subject. You wouldn't need a transmission, so there wouldn't be anything to service on the vehicle but tires, ac, wipers, brakes, and lights. It would also open up some interesting vehicle configurations if you don't have to have axles and a transmission. I might just do a search and see if it's being worked on. I hadn't done anything like that yet. I'm curious now though.
Torque depends on magnets count - more magnets = more torque but lower max rpm. You cannot change that by changing KV only.
You don't need variable KV if your motor is very very efficient at wide rpm range.

Best regards
 Mar 01, 2008, 10:19 AM Go ahead... Build it to crash! So... on 2 motors with the same magnets/poles and only the winds are different, if one wind is 500kv, and another is 2000kv, they would have the same torque for the same voltage applied? I thought lower Kv was able to swing larger props at lower rpm due to increased torque. Is that not the case? I'm just trying to understand
Mar 01, 2008, 11:18 AM
Registered User
Quote:
 Originally Posted by Fuegodeth So... on 2 motors with the same magnets/poles and only the winds are different, if one wind is 500kv, and another is 2000kv, they would have the same torque for the same voltage applied? I thought lower Kv was able to swing larger props at lower rpm due to increased torque. Is that not the case? I'm just trying to understand
Torque depentds on load at one side but also depends on the current on the other one.

So if you apply the same voltage to different kv motors each with the same prop size - higher rated one will tend to higher rpm at higher torq loaded.

But loaded torq never be higher than that what motor is able to give us at given rpm. That max motor torque is limited by stator diameter and height, poles number and magnets strenght.
Stronger magnets allow for higher torque max.

If you add extra winds to lower kv for given motor you don't get higher torque limit for that motor.
You only change the usefull voltage range for that motor but rpm range would be the same.

Higher diameter motors have usually lower Kv but it is quite different story. They are able to give us more torq because they have larger diameters, more poles etc. Lower Kv at that case is not a reason (of higher torque ability) it is a consequence of given turns count for given voltage range and rpm range.

Higher diameter motors are not able to spin at so high rpm as their smaller diameter counterparts.
So they have to be winded for lower kv for the same voltage range. If they were winded for the same kv they will must operate at lower voltage range and higher current range. So they must be winded with very thick wires or many parallel wires what is not convenient in any way ( your supply cables should be thick as cables for welding machine).
So that is the reason why they usually have low or very low kv.

I hope that help.

Best regards
 Mar 01, 2008, 11:47 AM Go ahead... Build it to crash! Ok, so If I understand, the total potential torque of the motor would not change, but if you have a lower KV, it will turn slower for the same voltage, which may keep it from getting too much current if you were trying to accelerate from a stop. Where as a higher kv would pull more current and fry itself in the same situation? So it wouldn't be a difference it torque, it would be a difference in the efficient rpm range of the motor? Which still seems it might be an advantage to have. Once the car is cruising down the highway at higher rpm's, there would be less load on the motor so less current would be pulled at the higher rpms, or you could maybe cruise at higher speeds by applying less voltage to keep the rpm up? I'm just trying to wrap my mind around the real world effects or applications of differing kv's if all else is constant. Thanks for taking the time to explain.
Mar 01, 2008, 12:59 PM
Registered User
Quote:
 Originally Posted by Fuegodeth Ok, so If I understand, the total potential torque of the motor would not change, but if you have a lower KV, it will turn slower for the same voltage, which may keep it from getting too much current if you were trying to accelerate from a stop.
Yes it is.

Quote:
 Originally Posted by Fuegodeth Where as a higher kv would pull more current and fry itself in the same situation? So it wouldn't be a difference it torque, it would be a difference in the efficient rpm range of the motor?
No, efficient rpm range would be still the same, only voltage range will be different (higher one for lower kv one).

Quote:
 Originally Posted by Fuegodeth Which still seems it might be an advantage to have. Once the car is cruising down the highway at higher rpm's, there would be less load on the motor so less current would be pulled at the higher rpms, or you could maybe cruise at higher speeds by applying less voltage to keep the rpm up?
The last is the true.
At lower kv motor is able to withstand higher voltage, but rpm is still in the same range as its geometry (diameter, height) and poles number are still the same.
Higher kv is usefull in case if you have limited voltage supply which is unable to cover all efficient rpm range at lower kv.

But if you are not limited by voltage the lower kv is usually better as motor operate at lower currents range for the same power.

Ask if something still isnt a clear.

Regards
 Mar 01, 2008, 01:29 PM Go ahead... Build it to crash! Thanks again, I think I'm slowly getting it. I was thinking just now about the graphs you sometimes see for cars, that show horsepower and torque for each gear and the speed, so it basically shows the full profile for all the gears. In the case we are talking about, it's actually very similar to a car. The peak torque does not change for the engine, but where the torque peak occurs in relation to the vehicles speed is determined by the gears. The same goes for horsepower. I am thinking this would be a similar set of graphs for our imaginary motor. The peak horsepower (watts) and torque would not change. But at what rpm the peaks occur would change with the kv of the motor. for low kv, the torque peak would be at lower rpm, so would result in better acceleration. At high kv, the peaks would occur at higher rpm given same source voltage max, as it would be on a battery powered car. If peaks occur at 5000 rpm vs 500 rpm, then that would be better for high speed driving, wouldn't it? Same power is consumed, and same torque is put out, but the rpm's are higher, and the current draw is determined by the load on the motor. So at the higher kv, if the load is lower, such as a car at cruising speed that is not accelerating, then the current draw for a given voltage would be lower. So effectively what would change would be the torque/rpm value and the power/rpm values. So if there is a limiting voltage (source battery), then variable kv's would act like gears on a car. Is that right? I think it pretty much agrees with your last post: Higher kv is usefull in case if you have limited voltage supply which is unable to cover all efficient rpm range at lower kv. Which might be the case on the car if you want a high top speed.... right? But if you are not limited by voltage the lower kv is usually better as motor operate at lower currents range for the same power. So, if voltage is a limiting factor, then the lower kv might not be adequate for the full desired operating rpm range, in which case being able to bump up to higher kv would help??? So, thinking further, you would have to design the motor to be able to run at the max rpm desired based on the number of poles/magnets, and diameter of the motor. Otherwise, when you bump up kv, it wouldn't actually be able to turn any faster because of the physical limitations. However, if you lower the kv, then it would be able to generate the peak torque and peak power at a much lower rpm than the overall peak rpm of the motor, thus allowing the motor to work within it's operating range and give the most desirable output at the rpm we want for what the car is doing. Does this make sense? Or did I miss something. I think I'm getting it though. Thanks again for your help.
Mar 01, 2008, 02:16 PM
Registered User
Quote:
 Originally Posted by Fuegodeth So, thinking further, you would have to design the motor to be able to run at the max rpm desired based on the number of poles/magnets, and diameter of the motor. Otherwise, when you bump up kv, it wouldn't actually be able to turn any faster because of the physical limitations. However, if you lower the kv, then it would be able to generate the peak torque and peak power at a much lower rpm than the overall peak rpm of the motor, thus allowing the motor to work within it's operating range and give the most desirable output at the rpm we want for what the car is doing. Does this make sense? Or did I miss something. I think I'm getting it though. Thanks again for your help.
You miss one important thing: you are able to lower kv by the two different ways.
A) by gears
B) by turn numbers.

If you change the kv by gears you are able to change output rpm range - native rpm range is still the same.

At case B you are not able to change rpm range - max torq will be the same for given rpm for any kv.
So at case B you are still in the same rpm range for any kv, except you are limited by insuficient voltage. But in that case you need only change kv once for higher kv for which your voltage is able to cover all rpm you need (within the range which is constant ).
So still you no need variable kv at that case. (You need higher one but not variable).

I don't answer this time for all questions for better clarity.
You have to understand what i wrote above and all other questions will be adressed too i hope.

Best regards
 Mar 01, 2008, 02:55 PM Go ahead... Build it to crash! Hmmm, I'm not sure if I'm not understanding the answer or if I'm not asking the question in the right way. So basically you are saying there would be no real advantage to it, and that the reason such a motor is not in use is not because it's not possible, but because it is impractical? As it would surely cost more to produce than a conventional motor, but a conventional motor can do everything this would be able to? Maybe Kv isn't what we need to make variable to make a variable duty motor. I'll think on this some more and post back when I come up with something else to ask about. I need to try and work some numbers to help me understand how it all really works. Thanks again for all your input.
Mar 01, 2008, 03:26 PM
Registered User
Quote:
 Originally Posted by Fuegodeth Hmmm, I'm not sure if I'm not understanding the answer or if I'm not asking the question in the right way. So basically you are saying there would be no real advantage to it, and that the reason such a motor is not in use is not because it's not possible, but because it is impractical? As it would surely cost more to produce than a conventional motor, but a conventional motor can do everything this would be able to?
Yes and yes again

Quote:
 Originally Posted by Fuegodeth Maybe Kv isn't what we need to make variable to make a variable duty motor. I'll think on this some more and post back when I come up with something else to ask about. I need to try and work some numbers to help me understand how it all really works. Thanks again for all your input.
High efficient motors have wide efficient rpm range and are able to work efficiently at variable duty within that wide range.
However if you want keep costs low as much as possible maybe better way is to use two cheaper decent efficiency motors to combine theirs differents rpm ranges in one wide range.
Electric motors have much wider rpm range than ICE (internal combustion) counterparts.
So two motors together are able to complement each other in very wide rpm range. So much wide that ICE needs a few gears to cover such wide range.

Best regards
 Mar 02, 2008, 06:14 AM Registered User Another way todo the same thing but is easier todo and probably more efficient. Instead of 8 different terminations to 40winds you would have 40 batteries and 8 different combinations of connecting them. Just for examples sake 40 time 1Volt 1Amp batteries would give you a 40Ampx1V battery, 20Ax2V, 10Ax4v, 8Ax5v, 5Ax8V, 4Ax10V, 2Ax20V and a 1Ax40v battery. Considering larger EV's at least, could use motors 1000v or more just fine (although so far people seem to be opting for very low voltage motors) I don't know why people are, I think it is because motors and batteries in these power ranges are available from other applications but we need to start making custom parts for the application. Lets take an array of 100x 3.7V 10,000mah lipo batteries. 370 has factors 1, 2, 5, 10 37, 74, 185, 370. At 30c burst rate this battery would be able to output 30,000Amps in 1cell mode which is 111000Watts.
Mar 02, 2008, 07:48 AM
Registered User
Quote:
 Originally Posted by Rufe0 Lets take an array of 100x 3.7V 10,000mah lipo batteries. 370 has factors 1, 2, 5, 10 37, 74, 185, 370. At 30c burst rate this battery would be able to output 30,000Amps in 1cell mode which is 111000Watts.
Who want the car for two minutes at 30C rate load ?
 Mar 02, 2008, 11:29 AM X-Era Motors FYI, HAAS CNC uses a dual drive motor in their CNC milling machines. It switches from Delta to Wye on the fly so to speak. This is important to keep current draw under control at slow speeds.