Best device for low power DC current generator

[mgagne]

Hi,
I would like to know what is the best device to use to convert mechanical power to DC current (low voltage 12 V, low power, about 60 to 100 Watts).
I want to know if a brushless DC motor is the right choice?
Is it more efficient to use a 12 VDC brushless motor or a higher voltage motor (90 or 180 VDC) with a DC/DC converter to have a 12 VDC?

Martin Gagne

 

[Heydave]

A "brushless" motor will not work. The replacement for brushes used to commutate the armature current are solid state switches designed only to switch current into the motor. The power level and voltage you mention is approximately that of an automotive alternator or generator. The former uses diodes to rectify the generated AC while the latter uses brushes to do the same thing. If you remove the switching circuits from a brushless motor with permanent magnets and replace them with diode bridges, you will get an unregulated DC. The automotive alternator uses a variable armature current to regulate the magnetic field and thus the voltage out. It may need battery voltage to start producing power.

 

[jbartos]

Suggestions to the original posting marked ///\\I would like to know what is the best device to use to convert mechanical power to DC current (low voltage 12 V, low power, about 60 to 100 Watts).
///A suitable small dynamo might be used.\\\
I want to know if a brushless DC motor is the right choice?
///A motor is normally used for electrical energy conversion into mechanical energy.\\Is it more efficient to use a 12 VDC brushless motor or a higher voltage motor (90 or 180 VDC) with a DC/DC converter to have a 12 VDC?
///It is better to have the lower parts count, if possible, to have a high reliability, low maintenance and operation costs.\\\

 

[CarlPugh]

I have used a brushless DC motor to convert mechanical power to DC with good results. A three phrase full wave bridge was used to convert the motor(generator) output to DC.

If the speed of the power source varies, the output voltage will vary. (This may or may not be a problem)

The output voltage may be much lower than the rated motor voltage.

 

[nbucska]

How much voltage variation can you have and what is the current and RPM range ?

The BRUSHLESS MOTOR is a 2 -- 3 phase AC motor which can
be used as a generator. With diodes it generates
rectified AC with considerable ripple. Peak voltage is
proportional to speed.


<nbucska@pcperipherals.com>

 

[aolalde]

The &quot;brushless motor&quot; cited by CarlPugh is in reality an AC alternator with a DC rectifier.

My opinion is that the less expensive solution will be an automobile alternator. A lot of them are designed to charge 12 Volts batteries.

If the required voltage has to be exactly 12 VDC, you should adapt a voltage regulator sensing the DC output.

 

[GOTWW]

The DC wind generators have all gone to perminent magnet alternators with rectifiers and regulation. This has proved the most reliable under the punishment these things take.

Using Car alternators are problematic as these require a field current to be supplied. I had a home made wind generator self destruct because the car alternator lost the field power. It was a lousy design, the field was initially generated using a bicycle type generator riding on a belt.

hey, by the way, what about the good od bicycle PM generators or several?

 

[jbartos]

Suggestion: The original posting needs clarified &quot;best&quot;. E.g. If the generator voltage is increased and DC/DC converter added, this will increase parts count and negatively affect the system reliability.

 

[mgagne]

thanks for the answers.
I would like to clarify my question.
What I want to do is convert mechanical power to electric power. Since I don't have a lot of power (between 60 to 100W) I want to have the more efficient solution.
I have found a 10A permanent magnet DC generator that can do the job. I think to use a 12VDC voltage since its more standard for batteries and lights I think and I will have to charge a battery with the generated current.
I would like to know if a brushless permanent magnet DC motor can be used as DC generator? If yes, is it more efficient than the DC generator? What about the shaft RPM?

Thanks for your help,

Martin Gagne

 

[Laplacian]

One minor detail:

If you must charge a standard automotive 12VDC battery, you need to generate ~14VDC to recharge it.

 

[mgagne]

I know that. I will be able to adjust the load on the generator in order to set the voltage.

Martin

 

[jbartos]

Suggestion to mgagne (Mechanical) Feb 26, 2004 marked ///\\\

I would like to know if a brushless permanent magnet DC motor can be used as DC generator?
///Yes. DC commutator motors tend to have the larger diameter of the commutator than DC Generator to negotiate larger currents during starts/stops, load variations, etc. Also, the DC motor can be designed structurally stronger because of the load side impact and more severe duty than the DC generator.\\ If yes, is it more efficient than the DC generator?
///No.\\ What about the shaft RPM?
///Optimal RMPs are preferred for DC Generator\\\

 

[aolalde]

A regular brushless DC motor has permanent magnets in the rotor, three phase stator winding, rotor positioning sensors, encoder sensors and an electronic commutator control.

To make it work as a generator the electronic commutator control should be replaced with a three phase rectifier bridge identifying the output 3 phase power leads from the stator.

With permanent magnets, power electronic regulation of the voltage output is required. Other way the voltage will change proportional to the rotational speed and the load.

This machine is MORE EFFICIENT than the traditional DC machine because it has no field winding losses. The optimum speed should be that in the nameplate data.