amps, volts and top speed

I am thinking to build a new bike and decided to dive deeper this time regarding the motor and Wattage

I have read a few posts about volts/Amps and in my understanding volts are the pressure in the pipe and amps are the amount of "water" that are going via the exit surface aria of the pipe at any moment. I also read the more volts mean more speed and more amps mean more torque.

So if my statement is correct I have a few questions.

What is the physics beyond the association of volts with rpm and amps with torque ?

Can I say that volts determine the top speed of the motor and amps the how fast I will get from zero to top speed?

If I want to go faster on a a steep hill ( assuming I am no near my top speed on flat surface) I will need to add more amps (beside trying a smaller wheel)?

How do you know how much voltage and amps a motor can take and how do you find the sweet spot?

Thanks

I'll try and explain it in a simple way. A motor has a maximum speed at any given voltage. You get that info from the manufacturer. Let's say it's a 260 rpm 36v motor. Consider the example below, where I'm only referring to full power being applied by the controller.

At zero rpm, the controller gives it 36v, so that pushes the current through the motor. The motor has a low resistance, so a lot of current would flow. Therefore, the controller has to limit the current to say 20 amps. As the motor speeds up, it works as a generator. The voltage it produces is opposite to that from the battery. At 130 rpm, it will be generating 18v, so now the motor is only getting 18v (36-18). 18v might still be enough to push the maximum 20A through the motor, so you would still be getting the same torque. At 195 rpm, the motor will be generating 27v, so now the net result is 9v, which is not enough to push the maximum current through the motor, so the torque will be less. Finally, at 260 rpm, the net voltage is zero (36-36), so no current can flow, which is why the motor can't speed up anymore and why at somewhere around 50% maximum rpm, the torque gradually decreases.

Now consider the same motor at 48v with the same controller. The controller will still limit to 20A, but the net voltage will always be 12v more than before. If in the above example 18v pushed exactly 20A through the motor at 130 rpm, now, the net 18v (48-30v) comes at 216 rpm, which is the point at which the controller stops limiting the 20A. For the motor to generate 48v, it would be spinning at 347 rpm, which is its new top speed. The controller will now give 20A all the way to 216 rpm (up from 130 rpm) and then a linear decline to zero amps at 347 rpm. This means that the motor is getting more current above 130 rpm, which means more torque and power compared with the 36v version.

There's one more consideration. The controller is just a series of gates that blocks the battery voltage to the motor. It controls the current by PWM, which means it opens the gates thousands of times a second in short pulses. You can consider each pulse as a pulse of energy, which has a height of the voltage and a width of the time. When you use 48v, those pulses are 33% higher, so they carry 33% more energy. That means that the torque of the motor at the same limited current is 33% higher with 48v as it is with 36v during the rpm range where it's limited to 20A.

This is a very simplified version that explains in principle what's happening with voltage and current.

The torque is limited by the controller's maximum current, which can be changed by getting a different one, adjusting it or reprogramming it.

Although you get more torque and speed with a higher voltage, it also affects the efficiency of the motor. The motor's efficiency decreases rapidly below 50% maximum rpm, so in the above example, it was efficient above 130 rpm at 36v, but at 48v, it won't be efficient until it reaches 173 rpm.

Your sig. hardware indicates you are somewhat past electronics 101 but I will take a stab at this.
The water pipe analogy is ok, up to a point.
Volts is potential, the height the water source(lake, pond, etc.)is above the exit of the water pipe. In the water analogy, the force is gravity, the weight of the water.
Amps is the diameter of the pipe.
Watts is volume, the amount of water moving past a given point, which is a function of V X A.

The "water in a pipe" analogy can be carried a bit further to help conceptualize the movement of electricity.
When the electrons are thought of as marbles, "flow" comes about as one marble enters the pipe and
bumps into the marble that is already there, which, in turn, "bumps" into the next and so on til the last marble in the pipe, "pops" out.

The problem with all this is the concept of "torque" as applied the electric motors. It is somewhat more complicated and suttle than the Physics of "movement of force" , as applied to say, a gasoline engine(note: an Internal combustion device is termed an engine, whereas, the electrical device is termed a motor).

For the sake of this dicussion and to avoid getting too far into the "weeds", we will leave Amps discribed as torque for now(to really understand these concepts, this would be an area for further study).

In a "real world" application, the two most important items of consideration are;
1)Matching motor RPM @ a given Voltage(often incorrectly called the Wind)to projected road speed.
THis having a great impact on "motor efficiently".
2)Never allowing the motor speed to fall below one half of the "no load" speed, at which point, more heat is being produced than motive force.

A motor simulator, like the one @ Ebikes CA is a useful tool to understand how these concepts relate;
http://www.ebikes.ca/tools/simulator.html

A general rule of thumb is;
2 Times the motor rating should be the absolute max. and somewhat less than that for continuous usage.

Bigbikebob said:

I have read a few posts about volts/Amps and in my understanding volts are the pressure in the pipe and amps are the amount of "water" that are going via the exit surface aria of the pipe at any moment. I also read the more volts mean more speed and more amps mean more torque.

So if my statement is correct I have a few questions. Thanks

Click to expand...

Not totally correct. First consider VA=W (V x A=W). W=power. So increase either V , A, or both and power increases.Now every motor has a Kv rating, meaning how many rom it gets per volt. Increase the voltage for more speed and power. Increase the amperage for just more power.This is no load speeds. Under load just more amperage will also increase the speed because it will have more power to get towards it's Kv rating, but it will never exceed its max Kv rpm. If it reaches its max rpm (like going downhill) the motor will then act as a braking power. So the top speed will be limited. Increase the voltage and that braking force will not happen until the motors Kv rpm is reached which will be faster at the higher voltage. Now all motors have a saturation point where no matter how much more power you give them they won't turn any faster. At that point the coils heat up to dissipate the power and they're just wasting power. This is what burns motors up. Try to avoid it.

d8veh ,thanks for the very informative reply!

So to a point I understood right, the higher your volts rating the longer your maximum torque (20A in your description) will be applied until you hit the 50% speed mark. Upping your voltage will increase your speed only if there are enough torque to back that speed up. Basically upping the the amps to say 60A on a 36V motor won't make it spin faster then 260 rpm but it will give you the torque to 260 rpm on a steep hill

motomech

Never allowing the motor speed to fall below one half of the "no load" speed, at which point, more heat is being produced than motive force.

Click to expand...

can you elaborate?

wesnewell,
Thanks in potting it in layman terms.

Now all motors have a saturation point where no matter how much more power you give them they won't turn any faster. At that point the coils heat up to dissipate the power and they're just wasting power. This is what burns motors up. Try to avoid it.

Click to expand...

Doesn't sound fun

motomech said:

The water pipe analogy is ok, up to a point.
Volts is potential, the height the water source(lake, pond, etc.)is above the exit of the water pipe. In the water analogy, the force is gravity, the weight of the water.
Amps is the diameter of the pipe.
Watts is volume, the amount of water moving past a given point, which is a function of V X A.

Click to expand...

In the hydraulic analogy, resistance would be the inverse of the diameter of the pipe. Amps is flow rate, literally current in either case. Watts is indeed V*A, but that's not volume, because as the voltage/pressure rises, the current/flow rate must decrease to maintain the same wattage/power.

So volume (liters) would be charge (coulombs) under this analogy.

Bigbikebob said:

I am thinking to build a new bike and decided to dive deeper this time regarding the motor and Wattage

I have read a few posts about volts/Amps and in my understanding volts are the pressure in the pipe and amps are the amount of "water" that are going via the exit surface aria of the pipe at any moment. I also read the more volts mean more speed and more amps mean more torque.

So if my statement is correct I have a few questions.

What is the physics beyond the association of volts with rpm and amps with torque ?

Click to expand...

It gets... complicated. But understanding what voltage is the hardest bit.
Voltage can be thought of as Voltage pressure, Electromotive potential, or Electro Motive Force.
Simplified, voltage is an electron trying to move from a place where there are many to a place where there are few. Electrons move from one atom to another in a wire, and when an electron moves, it puts pressure on other electrons at a right angle to it's path in the wire. That pressure is the basis of magnetism. when an electron travels down a wire, the right angle pressure on other electrons proceeds around the outside of the wire, and looks like a wheel spinning around the outside with the wire being the spoke.The spinning always takes place in the same direction along the wire. If you coil that wire up like a hose, then all that spinning force combines to form the field of an electromagnet. The strength at which an electron is pushed down a wire defines the field of effect it has on other electrons, and defines the size of the magnetic field produced. The size of the magnetic field of a magnet defines how it will interact with other magnets in a motor, and sets the upper limit for the motor's RPMs. More voltage > larger field potential > greater interaction > greater RPM.

Understanding the physics of how Amps and watts fit in is easier. Once you grasp that Electro motive force governs how fast an electron can potentially push a motor in circles, then Amps is just more electrons doing more pushing. Watts is just a way to define how many electrons are pushing how hard in one easy concept.

Bigbikebob said:

Can I say that volts determine the top speed of the motor and amps the how fast I will get from zero to top speed?

Click to expand...

That's one way to put it, yes.

Bigbikebob said:

If I want to go faster on a a steep hill ( assuming I am no near my top speed on flat surface) I will need to add more amps (beside trying a smaller wheel)?

Click to expand...

Either Amps or volts. What you actually need to increase is the amount of watts available to your motor. You can increase that by either increasing the voltage or amperage.

Bigbikebob said:

How do you know how much voltage and amps a motor can take and how do you find the sweet spot?

Click to expand...

There are two ways:
1)With an advance degree in physics and engineering, analyze the design of the motor to calculate the efficiency, saturation, thermal dissipation, dielectric breakdown, and mechanical strengths of the motor. Then perform destructive testing on enough samples of the motor to establish a baseline.
Or
2) grab a BBQ thermometer, and run your motor until it starts to over heat, then back off the amps until it doesn't over heat.

Bigbikebob said:

The second way is a little easier.

Thanks

Click to expand...

Drunkskunk thanks for the physics explanation.

I wrote:

If I want to go faster on a a steep hill ( assuming I am no near my top speed on flat surface) I will need to add more amps (beside trying a smaller wheel)?

Click to expand...

and you answered

Either Amps or volts. What you actually need to increase is the amount of watts available to your motor. You can increase that by either increasing the voltage or amperage.

Click to expand...

I do understand that for me go faster up a hill I will have to increase my total wattage that equals VxA and by saying that you actually can say that you must increase total power. but what I am saying is that if your motor is doing 260 rpm on a flat surface for it to keep doing that while I make the surface more and more steep is to add more amps, adding more volts won't help. or am I wrong?

grab a BBQ thermometer, and run your motor until it starts to over heat, then back off the amps until it doesn't over heat.

Click to expand...

Sounds like a plan

Ebike motors can be efficient at almost all speeds, but with diminishing power capability and efficiency as they approach zero RPM. In practice, this means steep hills can be a problem for underpowered systems. You can demonstrate this for yourself by running partial throttle in the simulator on:
http://www.ebikes.ca/tools/simulator.html

To avoid confusion, always be clear whether you're considering battery volt/amps, or motor volt/amps.

Bigbikebob said:

I do understand that for me go faster up a hill I will have to increase my total wattage that equals VxA and by saying that you actually can say that you must increase total power. but what I am saying is that if your motor is doing 260 rpm on a flat surface for it to keep doing that while I make the surface more and more steep is to add more amps, adding more volts won't help. or am I wrong?

Click to expand...

Yep, you're wrong. Adding more volts would do the same thing as adding more amps: It would give you more power to get up the hill (watts).

The idea that more amps = more torque is true... sort of. It's only half the story, but for many ebike novice, it's the first lesson they learn when trying to increase performance. In many cases with an ebike, where the battery is expensive to replace and the controller can only handle so many volts, it's easier to increase the amp limit to allow more amps for more power. But there are also cases where it would be easier or better to increase the voltage instead of the amps to get the same desired power increase.

Power is what gets the work done of transporting you on your vehicle.

No matter of it comes from a 10V 100A source or 100V 10A source, it's capable of exactly the same efficiency and torque and speed and heating etc provided the motors kV scales to the voltage and the copper fill% in the motor remains the same (and the 100A system needs a few feet of thicker wire between battery/controller/motor).

At some point also, if you want more power, you add more copper and more magnet.

That way, you can more or less ignore the BBQ thermometer, because you can have some pretty crazy power, for a bike, without saturating the motor.

So whether you ever really understand it, ( I don't) you at least can run 2000-3000w without ever thinking about overheating problems with a bigger motor. At 2000w, you can maintain 15 mph up some pretty big mountains, on roads that go to 10% or so, with total weights of bike and cargo up to 450 pounds. Since you don't stall the motor,( too low rpm with full power applied) you don't overheat. It's plenty perky riding around town with 2000w as well. 48v 40 amps will do, unless you really want to go faster than 30 mph.

This is direct drive motors with 35 mm wide magnets, or at least not 28 mm. They won't saturate with 3000w, if you want 40 mph.

I have ridden a 10,000w bike. Honestly, I just found it hard to ride. Wheelies are fun, to be sure. But I just had to keep lunging at the bars to be able to steer. IMO, the bike would have been more fun to ride with 5000w.

I often say, "get a motorcycle for that" and last month I finally took my advice. It's a lame 400cc scooter, so no wheelies. But it takes Harleys from a light easy, since I don't spend any time shifting it. Now when I want to go fast, I hop on the scoot and do 75 mph. Heading out today to ride a 300 mile loop, 200 miles of it twisty mountain roads.

Yep, you're wrong.

Click to expand...

But there are also cases where it would be easier or better to increase the voltage instead of the amps to get the same desired power increase

Click to expand...

This exactly what is not clear to me.
if you are saying that I can get more torque using more volts then basically a 100V motor with 10A will be able to climb a hill as good as 10V with 100A?

From what I have read (in this posts and others) that is not the case.

liveforphysics

No matter of it comes from a 10V 100A source or 100V 10A source, it's capable of exactly the same efficiency and torque and speed and heating etc

Click to expand...

I understand that W=V*A
Are you saying both configurations will give me the same acceleration, and both of them will provide for the same top speed on a very steep hill?

dogman dan,

I often say, "get a motorcycle for that"

Click to expand...

I hear you and I am not looking to drive a 10Kw on a frame that is not meant to handle so much.
I asking the amps / volts relations because If I can get torque using volts I prefer dong so because it means less heat and less thick wires.



is more amps = more torque = more acceleration?

The fastest way to learn about motors is to use the simulator. It shows what happens when you change voltage, current, etc.

http://www.ebikes.ca/tools/simulator.html

10V x 100A = 1000W. 100V x 10A = 1000W. With the same 10Kv motor, 10V x 100A would give the best acceleration up to a 100rpm max speed. But if you used a 1Kv motor and 100V x 10A, they would both accelerate at the same speed to 100rpm. But the latter could be done with 20g wire, where the first would require 6g wire. This is what LFP was saying. It's all a matter of compromise of what you want. Personally I went to higher voltage with a 20% increase in amps because it requires less strain on the wiring and mainly the battery pack, and I mainly wanted more speed anyway. And i still get better acceleration.

What about cables. I understand that if they are thinner that you need you'll melt them. But what about the opposite, when you use a lot more thicker than you need?


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juanfeli said:

What about cables. I understand that if they are thinner that you need you'll melt them. But what about the opposite, when you use a lot more thicker than you need?

Click to expand...

That's fine. The only bad thing is it weighs and costs more. Better too heavy than too thin. The cost and weight are generally not real significant.

fechter said:

juanfeli said:

What about cables. I understand that if they are thinner that you need you'll melt them. But what about the opposite, when you use a lot more thicker than you need?

Click to expand...

That's fine. The only bad thing is it weighs and costs more. Better too heavy than too thin. The cost and weight are generally not real significant.

Click to expand...

A few feet of heavier wire is generally always cheaper than more BMS channels for a higher voltage pack.

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