# Electric Bikes - Watts vs. volts?

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View Full Version : Watts vs. volts?

pengyou
10-18-07, 03:51 PM
With a gasoline engine, an engine that has a larger bore is considered to be a "torquey" engine.

Electric motors are rated in watts and volts. Does more watts mean the motor has more torque? more volts=more torque? How does a change in either one of these affect the characteristics and performance of the motor? the efficiency?

To further explain my question, what is the difference between a 250 watt motor at 48 volts and a 350 watt motor at 36 volts?

Abneycat
10-18-07, 05:33 PM
My original post was lost to my cat. I have some things to confess: I suck at electrical mathematics. I'm studying environmental science, not electrical engineering. Honestly, I tried to remember everything you need to know, but its gone now!

http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

I think this should be useful though.

pengyou
10-18-07, 05:56 PM
Shame on the cat! Thanks! The site looks like it contains a lot of useful information....need time to process it.

woody113
10-18-07, 09:10 PM
Hi All

Rather than Watts vs VOlts it should be relationship of Watts to Volts.

Imagine a Water Tank. We all know that the higher it sits the greater it's Potential to release water. So this is Volts, the 'potential' of the system.

Now current is the flow rate. In a tank the larger the pipe the higher the flow rate can be, just like a copper wire.

Now in the takn if we take the potential (height) and multiple it by Pipe size we get force.

Electrically this is Volts * Current = Watts.

So to obtain a certain Power (watts) output you need to take the motor Voltage and mutilpe it by the motors current.

So simply speaking a 240W motor at 24V has the same power output as a 240W motor at 48V. The DIFFERENCE is in the current, for 48V you need 5A while for 24V you need 10A.

Now what comes in to play here is battery capacity (Ah) and more importantly here, its delivery rate (C).

Battery packs are made up of cells. Say for NiMh it is 1.2V. So a cell may be rated at 1.2V 10Ah. So a 24V pack will be 20 cells giving 10Ah or 240Wh. A 48V pack would be 40 cells at 10Ah or480Wh. But also keep in mind that the 48V pack is twice the size.

Now here is where the delivery rate comes into play. If the 24V pack has a rate of 2C then it can deliver its power at the equivalent of 20Ah for 30 mins (ideal). So would be suited to power a 240W motor easily.

But lets us say, for example, the 48V cells had a deleivery rate of 0.5C. That means they can not delivery at a rate greater than 5Ah and so would just be acceptable.

With eraly battery technology higher than 1C was not readily available and so a higher voltage was need to keep current draw down. Now with higher C rates it is less of an issue and comes down to cost, and more importantly size and weight.

It can be argued now that there is an advantage in using lower volatge systems now. A modern cell typicaly is a minimum of 2C so 24V 10Ah can handle 250W motors easy. Want more distance then get 2 packs. So this presents an option to keep cost and weight down. If you commute short distances then you can use a light weight 24V pack. For weekend trips just add a second pack. This is not an option with 48V. You have the size and weight all the time.

Having said that, once you get up to 500W or more you need to consider either a 24V 20Ah pack or 2* 24 10Ahr packs or a 48V 10Ahr pack. Either choice will be the same weight and size. But again with 2 * 24V 10Ah packs you can put on the bike in two locations to balance the bike, or make less obvious.

So for me I go the smaller option and then option up if needed.

Hope that helped explain Watts and Voltage system advantages.

woody113
10-18-07, 09:23 PM
Now Torque

Torque is a relationship of power AND speed. Imagine the same size gas engine. At lower RPMs via gearing it has more torque for hills. From below you can see that the lower the rpm the higher the torque FOR A GIVEN POWER OUTOUT.

Torque = (Constant * HP)/rpm

Converting Watts to Hp is 1HP = 746W.

Motors curves show that a motor has a certain torque at a certain speed. This is not a linear (straight line) relationship and normally you can consider max torque occurs at rated power output.

So a 240W motor will develop maximum torque at 240W. This then means also a GIVEN speed. So to change this ie; we gear the motor. So by lowering the speed, we increase the torque.

These relationship are direct. So without knowing anything about a motor you can state that is you put a 2:1 gear on the output you will double the torque (excluding system losses).

So dont get too lost in the maths. Just know that watts is watts, does not matter what the volts, unless also consdiered with the current ( play with this only to decide battery).

For a given power output torque can only be increased by gearing and is a direct relation to gear ratio.

Again hope this helps.

mike-on-da-bike
10-18-07, 11:28 PM
ohms law may help ya I=V/R electric current=voltage/resistance

bikingbrit
10-20-07, 02:46 PM
With a gasoline engine, an engine that has a larger bore is considered to be a "torquey" engine.

Electric motors are rated in watts and volts. Does more watts mean the motor has more torque? more volts=more torque? How does a change in either one of these affect the characteristics and performance of the motor? the efficiency?

To further explain my question, what is the difference between a 250 watt motor at 48 volts and a 350 watt motor at 36 volts?

1. You seem to think that torque is the most important characteristic of a motor. Not true! A couple of examples:
a) a 150 lb cyclist standing on 6" cranks will generate 75 ft-lbs of torque. This is about the same as a Harley-Davidson engine. Now which would move a 700 lb vehicle faster? Obviously the Harley engine. The difference is the cyclist is only doing 100 or less rpm while the Harley is doing 3000-5000 rpm. Horse-power = (torque X rpm)/5252 so more rpm = more power and this means more speed.
b) a 100 lb woman standing on a 4' long wrench will generate 400 ft-lbs of torque, about the same as a Corvette engine. Which would move the Corvette faster? Again the difference is in the rpm. Power is more important than torque, contrary to the ignorant ramblings of many an automotive journalist.

2. Usually, more watts input power means more output horse-power. However, the efficiency of the motor gets into the equation. A 746 watt motor with 50% efficiency will output only 1/2 horse-power while a 500 watt motor with 90% efficiency will output 6/10 horse-power and so would propel the vehicle faster.

3. Voltage does not necessarily translate to power (or torque). A 6 volt starter motor from a 1948 Chevrolet has far more power/torque than a 12 volt mirror adjusting motor in a 2008 Acura. However a higher voltage makes it easier/more possible to generate higher power. This is why high power appliances like stoves and clothes dryers run off 220 volts.

4. Assuming equal efficiency, the 36 volt/350 watt motor will be more powerful/faster than the 48 volt/250 watt motor. The 48 volt motor can use thinner wires to connect to the battery, as the current will be lower.

cerewa
10-20-07, 11:04 PM
a 100 lb woman standing on a 4' long wrench will generate 400 ft-lbs of torque, about the same as a Corvette engine.

Every corvette is actually powered by a real, 100lb woman.

pengyou
10-23-07, 10:21 PM
Hi...wow! more good stuff to think about. So...if I understand what you folks are saying:

1. If I want to make my ebike go faster I can change the batteries (probably controller, etc also) to a higher amperage battery and it will make it a higher wattage motor, which will make it go faster. (Hopefully the motor was designed for this or it will burn out quickly)

2. If I start with motor A and increase the amperage there is a limit to the amperage/volt combo that motor A can handle without affecting it's life. At some point I should change to motor B which will have more windings, etc so that it can handle either more volts or more amps - or both.

3. The ability of a bike to accelerate quickly is related to how much torque it can generate. If I want higher torque to available at a broader range of speeds I need to use gearing.

4. More weight is going to require more watts (either more volts or amps) to achieve the same speed than the same bike with less weight? My guess is that because more weight, an additional rider or packs, will also increase wind resistance a bit that this relationship is not completely linear.

5. From other readings and postings...it seems that right now 36 volt batteries are more commonly available (could also mean cheaper) than 48v? I know that many bike's batteries are built up of 12 volt cells but I have read that lithium and newer technologies come in fixed arrangements, i.e. 12 volt, 18 volt, 36 volt, etc.

6. It was mentioned that higher voltage motors use thinner wires. Does this mean that they are lighter, i.e. power to weight ratio?

So....for example...if I am happy toodling around on my ebike with a 240w/36 volt motor and its speed but want to extend the range I can add another battery or two. If I want to make it go faster I can change the electrical system so that the batteries/controller, etc provide more amperage to the same motor. If I usually need to take a passenger or haul other goods and my bike does not provide satisfactory performance after maxing the amperage I need to upgrade the motor.

Which is lighter - given two systems with equal wattage, one has higher voltage, the second has higher amperage? which will have more range?

Am I close?