Could I use brushless DC motor as a generator?

[medo79]

Dear all,
I would like to know if it is possible to use brushless DC motor as a generator.

 

[LionelHutz]

Yes, it should be possible. You will not be able to control the ouput of the motor. The no-load voltage output will basically be proportional to rpm and the current will then depend on the load and the motor internal impedance.

Quite a few small wind turbines use a permanent magnet generator (or in more proper terms a 3-phase alternator).

 

[medo79]

What do you mean by "You will not be able to control the ouput of the motor"?

 

[benta]

Should make a nice little three-phase generator
Your outputs won't be sinusoidal, though, so you'll probably choose to rectify and smooth to DC.
(if you choose a PMSM-type instead, you might get some nice sinewaves).

Benta.

 

[waross]

Although both terms are somewhat ambiguous and are at times used interchangeably, Generator often refers to DC and alternator often refers to AC.
Although many conventional DC motors will quite happily generate DC when driven, a brushless DC motor does not run on pure DC. When driven it will produce an alternating current and may be referred to as an alternator. It is also correct to call it a generator but it will not be a DC generator and will not produce DC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

The rotor has a fixed magnetic field using permanent magnets. So, the output is whatever voltage you get when you rotate it.

A typical alternator has a coil on the rotor and by varying the current in this coil, the rotor magnetic field is varied. This allows control of the output voltage.

 

[cswilson]

Can you generate electric power? Sure. Just spin the motor mechanically and put a load on the leads.

Can you generate electrical power cleanly and directly into a grid? Almost surely not. In these permanent-magnet brushless motors, voltage and frequency are not independent; they are almost directly proportional. So it is very difficult to keep it both the proper time-synchronization to the grid (frequency and phase) and the proper voltage levels.

Curt Wilson
Delta Tau Data Systems

 

[rockman7892]

LionelHutz

Are you referring to the rotor in your response as the armature? I thought in DC machines usually the field was what had the permenant magnets and it was the field that established the flux and therefore magnetic field.

 

[waross]

A brushless DC motor won't run on straight DC, rockman. The rotor has the permanent magnets and the stationary fields are fed by an inverter. The drive/motor package runs on DC but the motor alone will not run on DC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cswilson]

rockman: A "DC brushless" motor is actually an AC motor. That is, it requires AC into the armature leads to spin continuously. The term "DC brushless" is a marketing term invented to emphasize that you could replace a DC brush motor and its drive with this newfangled motor without brushes and its drive. The term has been the source of continued confusion ever since, not least in this forum

Curt Wilson
Delta Tau Data Systems

 

[OperaHouse]

I wonder if the question really asked was.....

If you connect a voltmeter to the power DC leads, do you get voltage out when spinning the motor?

Possibly the protect diodes will feed voltage out. My LG washer turns on when I spin the drum. A 12V computer fan doesn't

 

[TurbineGen]

Brushless DC motors are used for the generators in many motorcycles. It simply has a three phase rectifier. Voltage is controlled by a voltage reulator (sometimes switching, other times linear). Sometimes these are outrunners, sometimes they are inrunners. Either works fine.

------------------------------------------------------------------------
If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[roydm]

I have ridden motorcycle for a long time but I haven't seen brushless DC.
All the bike alternators I have seen have a spinning permanent magnet inside a 3 phase winding & 3 phase rectifier. My Guzzi just has a single phase winding. The voltage regulators I am familiar with short out the winding when the battery is up to voltage. I found that a bit puzzling at first but I expect you can only get so much power out of a permenent magnet alternator so shorting out doesn't cause excessive current.
Roy

 

[waross]

Hi roydm;
Brushless DC motors and motorcycle alternators are similar in that the rotor is composed of permanent magnets and there are windings on the stator.
There are differences in design compromises and size.
The motorcycle alternator is designed with a high resistance winding so that the voltage may be safely regulated by shunting. The excess power is wasted as heat in the windings. The alternator may be designed so that here is as much induction as possible. The inductive reaction will increase with frequency and aid in limiting the current at high RPMs.
The brushless DC motor will be designed for efficiency. The brushless DC motor may be constructed in much larger sizes that a motorcycle alternator. Using a brushless DC motor as an alternator with shunt regulation may cause the motor to fail at high speeds. Using a motorcycle alternator as a brushless DC motor may result in poor performance and overheating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

The term has been the source of continued confusion ever since, not least in this forum

I didn't think so, but it's now apparent to me the term is a big source of confusion.