Bike Forums > Watts vs. volts?
 Register All Albums Elite Membership Forum Rules Calendar Search Today's Posts Mark Forums Read

 Electric Bikes Here's a place to discuss ebikes, from home grown to high-tech.

 10-18-07, 03:51 PM #1 pengyou Senior Member Thread Starter   Join Date: Jun 2007 Bikes: Posts: 316 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) Watts vs. volts? 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?
 10-18-07, 05:33 PM #2 Abneycat Hooligan     Join Date: Sep 2007 Location: Base of the Rocky Mountains, Canada. Wonderous things! Bikes: 2010 Cannondale Hooligan 3 Posts: 1,431 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) 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.
 10-18-07, 05:56 PM #3 pengyou Senior Member Thread Starter   Join Date: Jun 2007 Bikes: Posts: 316 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 1 Post(s) Shame on the cat! Thanks! The site looks like it contains a lot of useful information....need time to process it.
 10-18-07, 09:10 PM #4 woody113 Junior Member   Join Date: Aug 2007 Bikes: Posts: 11 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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.
 10-18-07, 09:23 PM #5 woody113 Junior Member   Join Date: Aug 2007 Bikes: Posts: 11 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) 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.
 10-18-07, 11:28 PM #6 mike-on-da-bike Senior Member   Join Date: Sep 2007 Bikes: Posts: 160 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) ohms law may help ya I=V/R electric current=voltage/resistance
10-20-07, 02:46 PM   #7
bikingbrit
Senior Member

Join Date: Oct 2007
Bikes:
Posts: 66
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
Quote:
 Originally Posted by pengyou 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.

10-20-07, 11:04 PM   #8
cerewa

Join Date: Jun 2003
Location: southeast pennsylvania
Bikes: a mountain bike with a cargo box on the back and aero bars on the front. an old well-worn dahon folding bike
Posts: 3,155
Mentioned: 0 Post(s)
Quoted: 1 Post(s)
Quote:
 a 100 lb woman standing on a 4' long wrench will generate 400 ft-lbs of torque, about the same as a Corvette engine.