# How to select most efficient motor for generator?

First post. Please forgive me if this is a stupid question. This post can be broken down into two questions.

I am building a bicycle generator, but am new to the field of EE. I am trying to select the most efficient means of generating electricity, but have only been able to find suggestions in terms of selection (as opposed to a numbers-based approach). The goal for this phase of the project is to output the highest wattage possible (ultimately in the form of DC power).

1) What numbers and criteria should I use for deciding whether to use a motor or alternator for this project? I understand that alternators generate AC, and (many) motors generate DC.

2) Once I have decided on which approach to use, how I can select the most efficient component?

When searching online for the answer to this question, I came across some helpful links (examples here and here), but I did not see any quantitative methods for finding the most efficient solution.

Thanks.

EDIT:

I am building a generator designed to output 3-phase power for industrial applications. I am not sure whether to create AC (and use a variable frequency drive to convert it to 3-phase power) or use DC, and then convert it to 3-phase power. The idea is a ride a stationary bike (probably just a bike on a stand) to turn the generator shaft.

• If your goal is "output the highest wattage possible" then you will seriously slow the bike down. How about a more moderate goal? Motor or dynamo? Motors rotate when electricity is supplied. – Andy aka Jul 26 '14 at 17:23
• @Andyaka, my mistake. What would you suggest using in terms of efficiency? – Qu0rk Jul 26 '14 at 19:45
• I assume this is to make power while riding a mobile bike. Yes? As opposed to an excercise machine type stationery arrangement. – Russell McMahon Jul 27 '14 at 9:53
• More data ...! Tell us what you are trying to do. "3 phase generator" is almost an oxymoron. 3 phase alternator isn't, but it doe not tell us what you wish to achieve. 3 phase generator and dc power suggests 3 volotage levels out. Unlikely to be what you want. – Russell McMahon Jul 27 '14 at 21:28
• Sorry to be a negative nancy, but you're not going to be able to generate "industrial" scale power with a bicycle. Generating just 100W for more than a few moments is a struggle for most people. – whatsisname Jul 28 '14 at 2:16

Added at top as updated question modifies best response:

I am building a generator designed to output 3-phase power for industrial applications. ... The idea (best-case scenario) is to have multiple people riding bikes, and then convert the power into 3-phase electricity. ...
I am not sure whether to create AC (and use a variable frequency drive to convert it to 3-phase power) or use DC, and then convert it to 3-phase power. The idea is [to use] stationary bike[s] (probably just a bike on a stand) to turn the generator shaft.

My prior general comments below still apply but my specific answer is:

There are a number of ways to do this and none is 'best' as all are compromises, and the final configuration depends on what assumptions are made.
However, if you wanted the industrial norm of constant voltage constant frequency AC then you almost certainly need to store energy from the bikes and produce the AC from the energy store. As advised below, the most likely bike power producer would be a permanent magnet alternator producing multiple phase AC (usually 3 phase). Voltage and frequency and power level are immensely user dependant and the best method is likely to be to convert this output to DC, store it in a battery and then produce fixed voltage fixed frequency AC using a DC to AC converter - an off the shelf product.

A good way to handle the bike AC is to arrange for the alternator AC Voltage to be higher than the battery DC voltage at all useful power output and speed ranges, convert the AC to DC and then "buck convert"(= voltage down convert) the DC to battery level voltage. A charger-controller would handle the input from all bikes and manage battery charging. Depending on design requirements users may be requested to pedal at constant power or constant voltage (both of which can be enforced by a controller with feedback to the user) or be free to provide input as desired.

It would be possible to transfer energy directly from bike rectified DC via down converters to the DC to AC converter input directly without battery storage - and this is essentially what happens to most of the energy when bike user input is <= load, but completely batteryless operation would be difficult as the battery provides a stabilising influence and, in a properly designed system, an energy source that has no drop puts to below load requirements.

In a past lifetime I designed controllers for alternators used as loads for exercise machines so have a good feel for what is required to achieve this task. Realistic load levels for typical fit but non athlete users are.
50 Watts for say one hour with reasonable ease.
100 Watts for one hour for a very solid work out.
200 Watts - getting extremely strenuous.
500 Watts - I could do about 10 seconds :-).

I can answer specific questions if you have any.

Is this a real-world idea or an investigation of a concept or ...?

All considered, schemes like this would not prove economic relative to grid powered electricity at current grid prices.

"Generators" output DC directly by converting the alternating voltages within the machine to DC. This is typically done using a commutator and brushes - effectively a manual "synchronous rectifier". This arrangement has some drag, complex mechanical requirements, lower lifetimes and losses in the carbon to metal contact of the commutator.

"Alternators" output AC = "alternating current" (and voltage) which is converted or "rectified" to DC outside the machine proper. Electronic conversion methods and components allow this conversion to be highly efficient.
Alternators come in two main "flavours" -

• Those which create the AC in the rotor and transfer it to the non rotating frame of reference (the one you are standing on) with slip rings, while the fixed stator is used to create the field that the rotor turns in to produce the AC voltages.

• Those where the AC is made in the stationary stator windings with the rotating part (rotor) providing a rotating field that interacts with the stationary output windings to provide the AC.
There are two main subsets of these stationary output winding machines.

• Wound rotor - the rotating magnetic field is produced by rotating windings which are fed DC field power via slip rings. Automotive alternators usually work like this. Advantages are that magnetics provided by wound copper coils are relatively cheap and the field magnitude can be controlled by varying the DC power which is fed to the winding. Disadvantages are mechanical complexity from slip ring feed and wound rotors.

• Permanent magnet rotor. Permanent magnets are sound to produce an alternating output voltage in the stator windings. Advantages are no need for DC feed to the rotor, relative ease of rotor construction, modern high strength rare earth magnets allow very energy dense alternators to be produced. Disadvantages are the inability to control the field strength.

There are variants such as AC induction motors used as generators but these are usually best used for specialist applications and can be difficult to control.

For your application where you require efficient energy conversion and probably low cost, low complexity and ease of "doing it" the best solutions are either a dedicated alternator OR a brushless DC motor (BLDCM) - sized to be of the wattage range desired in each case. Electrically these are essentially the same but one was produced with alternator roles in mind whereas the other (the BLDCM) was designed for motor use but will work very well as an alternator. Small dedicated alternators are rare but BLDCMs of the size range of interest are used 'everywhere'. These are typically found in computer printers, powered toys (especially flying ones), disk & DVD drives and much other equipment that uses small motors.

BLDCMs can be converted for alternator use or it may be practical to build your own alternator based on the same principles.

As above, when used as alternators, BLDCM's have permanent magnet rotors and generate AC in the stator with no mechanical connections (such as brushes or slip rings) from rotor to stator. The generated AC is converted to DC - usually with diodes. This is the overwhelmingly most common and sensible method to use in a very wide range of power levels and applications. There are exceptions but this is usually the best approach.

To decide how to proceed from here you need to know

• What order of power you require.

• Where and how you would like to mechanically power your device and why.
eg on a bicycle you may wish to use wheel rim , hub, pedal crank or chain drive. Or ...

• A concise but complete description of the application will help.

Tell us about power levels,application, more ,... .

• Thank you! I will add edits above (though I am trying not to make the question too localized for fear it will be demoted for not being a real question). – Qu0rk Jul 27 '14 at 4:12
• This is the most comprehensive answer, which is why I am marking it as the answer. I do not yet have the reputation to upvote, but I will do so when I can. – Qu0rk Jul 28 '14 at 21:43
• I'll upvote it for you then. And wish you good luck finding any useful 3-phase equipment consuming under 250 Watts ;) – John U Jul 29 '14 at 8:17

Sometimes the "most efficient" is the one you can get to work easiest for your application. This is especially true for motors because there are many parameters and unless you're building one from scratch you need to go with something off the shelf. Permanent magnet motors still hold a significant efficiency and package-size advantage for motors under 3 kW beating induction motors. For a bicycle you're looking at about 100W max.

You need high output at low RPMs for a bicycle. You need DC too. The easiest way to do this is go with a permanent magnetic motor. You get DC without any conversion and they are easier to find for low RPM applications.

This company sells several http://www.windbluepower.com/category_s/1.htm

Including a kit to modify a Delco automotive alternator to be a low RPM PM-motor. http://www.windbluepower.com/Permanent_Magnet_Alternator_Rotor_Fits_Delco_10SI_p/pma-rot.htm

• I like your answer! I marked the answer below as the answer because it was more in-depth. I will upvote your answer when I have the reputation to do so. – Qu0rk Jul 28 '14 at 21:46

I can ride at 200 watts steadily for approximately 2 hours, that's a moderately hard workout. 250 watts steady for about 45 minutes is my limit. 150 watts is essentially my indefinite output...i.e. several hours. Thought that may assist in estimating potential power production.

I'm building a system to generate power and will be able to assess the efficiency of that system...that is power expended (bicycle power meter) and power supplied to batteries measured by solar controller.

• Your figure of 150 W continuous supports the notion that "the amount of work you can get from a servant is 1 kWh per day". Rounding the numbers a little, 125 W x 8 h = 1000 Wh = 1 kWh. If paying a servant to haul water on Irish minimum wage this would be about €80 per day. If running a 150 W pump for his shift it would cost < €0.20. – Transistor Sep 20 '17 at 19:12