Understanding power, current, voltage relationship

[dirtdad]

Time for a test for me. Here is what I understand, but I am not 100% sure. Feel free to correct my understanding.

Although I am not sure about this. People are so shy on this board and never seem to want to correct what other people are saying :wink:

1) Current draw is a function of the motor, amongst other things. i.e., a motor will only draw so much current (presuming the battery can deliver the current). It seems, for example, a Crystalyte 408 seems to be in the range of 20 amps. I see others reporting X5 series drawing 100 amps, and Eteks can draw around 300 amps.

2) Batteries are rated for max sustained and peak current. But if connected to a motor that is capable of higher current draw than the rating of the battery, the motor may still pull higher current levels (presuming there is no controller, bms, etc. protection) than the battery is rated for, which is very detrimental to battery lifetime. For example, an Etek motor may draw more than 20A from a battery with a max peak current rating of 20A, or it may continuously draw current at or near the peak rating of the battery. But the battery will not be able to deliver as many cycles that way. This is to be avoided and the perils of doing so vary with battery chemistry.

3) When you use batteries in series, you add their voltages, but amp rating stays the same (presume identical batteries, forget the controller again). If you put them in parallel, the voltage stays the same, but current draw is distributed between the two batteries.

4) The power you get from the motor is watts, which is amps*volts. Real world power is also diminished by efficiency of the motor, controller, drivetrain friction, and the earth's magnetic field (just seeing if you are paying attention with that last one). If you like HP, there are about 750 watts/hp

5) A motor can deliver more power by having more voltage available from the battery. Voltage does not vary with load like amps. As a battery discharges, the voltage drops. The voltage drop curve is a function of battery chemistry, and varies dramatically between battery chemistries.

6) Motors can have a voltage "sweet spot". For example, the Etek seems to be designed for 48V optimum. Running higher voltages has a diminishing return. If you want more power, it is better to deliver more current. Other motors seem to have different characteristics. The Crystalyte X5 seems to deliver more power linearly with increasing voltage up to 72V or more.

 

[TylerDurden]

I'll address #4:

Power is an expression of force. For ebikes, you have power-in from the electric system, and power-out at the wheel. The components of the electric system begin at the pack voltage and capacity, and that power is affected by the efficiency of the controller, motor and drivetrain, to result at the power delivered to the ground at the wheel.

 

[TylerDurden]

I'll address #6:

Motors are designed for typical or specific applications. They are 'rated' to deliver power in the manner most appropriate for those applications.

Better motors deliver the power more efficiently and will last longer, by using better materials and better design elements. Cost is always a design objective, to maximize value for the manufacturer and hopefully the user.

Any motor will have a point of peak power-out and (seperately) peak-efficiency. The motor will be designed to operate using a voltage (or range of voltages); changes in which, will change the peak points of P_out and Efficiency. ie: You can run a motor designed for 24V at 36V... the P_out will increase due to more P_in (volts*amps), but since higher voltage is less impeded by the motors wiring, more amps flow too... that can make more heat and reduce the efficiency. Higher voltage will make a motor rotate faster, as well as draw more current; current is proportional to torque at the motor output.

Summary: Motors have a designed 'rating' for power and efficiency, to be matched to the work (or load). You can use motors outside of their load-rating, but there will always be tradeoffs like wear, heat, weight, cost, etc.

 

[TylerDurden]

Disclaimer: I am neither an engineer nor a writer... I simply loiter here and hand out '40s.

 

[CGameProgrammer]

1) Current draw is a function of the motor, amongst other things. i.e., a motor will only draw so much current (presuming the battery can deliver the current). It seems, for example, a Crystalyte 408 seems to be in the range of 20 amps. I see others reporting X5 series drawing 100 amps, and Eteks can draw around 300 amps.

Not accurate -- current draw between battery and controller is not necessarily identical to current draw between controller and motor. An X5 40A controller will never draw more than around 40A from the battery, but it can deliver pulses of high current to the motor. You can use a 50A fuse in the wiring to the battery and it won't blow.

5) A motor can deliver more power by having more voltage available from the battery. Voltage does not vary with load like amps. As a battery discharges, the voltage drops. The voltage drop curve is a function of battery chemistry, and varies dramatically between battery chemistries.

Some errors here. Since power is volts * amps like you said, you can increase it by increasing either one. But power is meaningless anyway... speed and torque are more useful. With electric motors, speed increases with volts, and torque increases with amps. Usually this relationship is linear; e.g. 60 RPM per volt or 1 lb-ft of torque per amp. Also, battery voltage *does* lower under load due to battery resistance (I think) but current does not lower with load unless the controller lowers it on purpose, as far as I know.

 

[TylerDurden]

Let's refine on CGP's reply.

re #1: Motor current increases with load, maxing at stall. It may increase a bit more (at stall) as heat increases (metals usually increase in resistivity with temperature rise), until the motor fries and/or the wires fry and/or the pack fries. Fuses are a good thing. As stated in above: higher voltage allows higher current, as the copper wires' resistance is overcome by higher voltage.

Voltage drop under load is a function of batteries and should be addressed in response to #2.

 

[TylerDurden]

re #2:

Batteries are rated for their voltage, capacity, and charge/discharge rates.

Voltage: chemistry determines the voltage of a single cell, cells in series determine the battery voltage (summed cells).

Capacity: larger cells can contain more chemicals and therefore more electrical capacity. Expressed in Ah (Amp-hours). i.e A 3Ah battery might deliver 1A for three hours or 3A for one hour.

Discharge Rate: usually expressed in proportion the the Capacity or 'C'. A 3Ah battery that has a discharge rate of 3C can deliver 9A. If it had a discharge rate of .03C, it could only deliver 1A.

Charge Rate: Like above, but for putting the energy back. Some chemistries can discharge fast, but charge slowwwww. (SLA)

Battery longevity is affected by age of battery, storage conditions, depth of discharge (expressed in percent of capacity, "% DoD"), rate of discharge (which can build internal heat and long term internal resistance) Rate of charge (which can overheat chemistry). More battery capacity can permit lower DoD and lower rate of discharge, for longer battery life. Tradeoffs are weight, size, cost. Lifespan may be expressed in both calendar life and number of discharge/charge cycles.

Voltage drop, or sag: different chemistries react differently to discharge rate... SLA batteries exhibit voltage sag (or "Peukert Effect") greatly when delivering higher current. Lithium-based batteries not so much. It is harder to tell when a lithium battery is about to quit, since the voltage does not drop much until the battery is depleted.

 

[CGameProgrammer]

re #2:
Discharge Rate: usually expressed in proportion the the Capacity or 'C'. A 3Ah battery that has a discharge rate of 3C can deliver 9A. If it had a discharge rate of .03C, it could only deliver 1A.

No, if it had a discharge rate of 0.03C than that's 3Ah * 0.03 = 0.09 amps, but I think you meant to say 0.3C which is 0.9 amps.

 

[TylerDurden]

... thanks.

Did I mention I am a mathemagician?

 

[safe]

Describing the Elephant...

Sometimes you are better off looking at the visual representation and getting past words... words have a way of being misinterpreted, but images tend to have more lasting value.

The "powerband" of a typical electric motor looks like:

...once you study this enough and it "sinks in" then the motor related issues are resolved.

The other "big issue" is the way a PWM controller "chops" the battery into pulses and the weird ways that the resultant energy gets used. (the so called "current multiplication") I'll leave that for later though.

 

[Toshi]

1) Current draw is a function of the motor, amongst other things. i.e., a motor will only draw so much current (presuming the battery can deliver the current). It seems, for example, a Crystalyte 408 seems to be in the range of 20 amps.

This is not quite correct. As TD noted current draw increases as you near stall. At low speeds (and low efficiencies, sadly) a 408 can draw well over 20A at 48V.

To illustrate we can use the ebikes.ca simulator. Plug in a 26" wheel, a 48V 18Ah NiMH battery, a 408 motor, 100% throttle, and a 50A controller. Follow the x axis to 5 mph or its metric equivalent. Note that the motor is putting about 430W to the ground. Also note that it is quite a bit under 25% efficient -- let's call it 22%. 430W / 0.22 / 48V ~= 40A being drawn off of the battery. At 10 mph it's still drawing ~37A.

 

[EMF]

That's right Toshi. A motor will pull more amps if it is given more load - until the poor thing is shut down via a protection in the circuit - or it simply melts the insulation off the windings, giving off a sweet smell of death.

First thing I do, when I suspect a motor is overloaded is feel how hot it is with my hand*. Volts x amps = watts which is heat.

* Yes, I have burned my hand this way. I now s l o w l y move my hand towards the motor checking for radiant heat.

 

[Robert2]

Hi from St. Catherines, Ontario. Looking for advise on efficiency...need a controller for a Hienzmann 400W 24V hub motor.(1999 ebike chassis, IEC VG 300 controller)). So...should I go to a different topic post ? Want to run bike for distance...efficient...half trottle...2001 NASA reports (Goddard). I have library internet access...once or twice a day. Thanks, Robert

 

[TylerDurden]

Hi robert2,

The very, very efficient bikes use aerodynamics extensively. If you get/make a recumbent bike, you will get much more range and have a lot of fun.

ES member swordman published a nice guide for a DIY 'bent. search for his posts.