# AC brushless motor as generator

How can i use an AC brushless motor to charge a battery? If I need to do it with a rectifier then what kind of rectifier should I use?? My motor is 1250kv and another one is 500kv. Can I use an Esc instead of a rectifier?

• are you sure it's a brushless AC motor? Most AC motors are brushless since 100+ years ago. Also 1250kV? Right now state-of-the-art power transmission voltage is "only" 1000kV and we are talking about cross country power grids. – user3528438 Nov 1 '17 at 19:04
• he mean PMAC machine... Also that 1250kv is not kilo-volts but the voltage constant of the motor. 1250V per some velocity unit – JonRB Nov 1 '17 at 19:09
• @user3528438: I made the same mistake some time back. See learningrc.com/motor-kv for more on Kv. – Transistor Nov 1 '17 at 19:11
• @user167195: Welcome to EE.SE, but you are asking for someone to provide you with a complete design when you have given no sign of any research of your own. Please edit your question to explain what your research has shown and what aspects of it you don't understand. – Transistor Nov 1 '17 at 19:13
• This is actually a potentially complex topic. Can you provide full details of the motor (power, rated speed, max current as well as Kv) and explain what force will be driving the motors, and also let us know the battery voltage? Some ESC's are capable of regenerative braking. So it may be possible to just connect up an ESC and constantly brake the motor. This will recharge the battery. But how fast depends on details you have not provided. – mkeith Nov 2 '17 at 1:52

You can not use an ESC. The best thing to use is a charge controller that rectifies the output of the generator and increases or decreases the DC output voltage to the level that is best for the battery. It might be possible to just connect a rectifier between the generator and battery, but generator voltage would need to be fairly close to the voltage required charge the battery. A transformer could be used to convert it to the proper range. The speed of the generator would then need to be controlled to some degree in order to avoid charging the battery with excessive current. If a three-phase motor is being used as a generator, a three-phase rectifier is required. If it is not a three-phase motor, some more research is required.

Note that I have assumed that by "brushless AC motor" you mean a permanent magnet motor that might be called a brushless DC motor or a permanent-magnet, synchronous motor. Induction motors are also brushless except for the rarely-seen, wound-rotor slipring-motor. It is best to state exactly what type of motor you intend to use.

With a permanent-magnet, synchronous motor, touring into an un-controlled rectifier, DC power is easily generated simply by turning the motor shaft. The generated voltage is unregulated and determined by the speed at which the generator is driven. However it should be relatively easy to get enough regulated power from that to control a voltage boosting charger.

With an ESC of any kind, considerable modification of the design would be required to convert it to a battery charge converter.

The only answer to a question of this type that is reasonable to provide here is to outline a general concept that is a reasonable approach to the problem. In order to proceed, you must first completely define the characteristics of the generator and the prime mover including the expected operating speed range and controllability of the prime mover. That dictates the requirements of the generator power processing system.

The second step is to define the power processing approach and determine the parameters of its basic elements.

• There's no firmware for it, bur hardware wise an ESC has everything needed for synchronous rectification. After all, generating is just like driving a motor, except that you set the effective motor voltage (which is controlled via PWM) below the back-EMF induced within the motor. – jms Nov 2 '17 at 2:21
• @jms how do I set it? – user167195 Nov 2 '17 at 3:32
• @jms Yes. Why don't you post that as an answer and explain how it works? – Charles Cowie Nov 2 '17 at 4:03
• @user167195 While conceptually simple, it's way too complicated in practice, unless someone has done it already (and shares the code). I just pointed out that it isn't impossible. Normally the throttle input controls the PWM duty cycle, but you would need a feedback loop where you adjust the PWM duty cycle in response to the rectified output voltage (which ESCs can sense for LVC) instead, so that the output stays within a narrow range (say 12 V) instead of swinging wildly with RPM. You also need to get rid of the motor startup routine, and modify a bunch of other details. – jms Nov 2 '17 at 9:34
• @jms The value of your original comment is then what? – Charles Cowie Nov 2 '17 at 11:50

Unless you're talking about fans or steppers, note that Brushless motor = AC induction motor. No magnets, just a stator coil/core and a squirrel-cage iron rotor.

Search for: DIY induction generators. The trick is to put a large-value capacitor across your motor, chosen to resonate at the motor's output frequency (e.g. at 60Hz and 1800RPM for a 2-pole AC induction motor, or larger capacitor if you're spinning your motor at below rated RPM.)

Your goal is to form an electromechanical oscillator, a resonant tank circuit (capacitor uF and motor henries) where the AC voltage constantly rises as the leading-angle generator-phase is injecting energy into the circuit. Then, add a load of appropriate horsepower/wattage so that the output voltage stabilizes at a desired value (example 120VAC.)

• Correction: a 2 pole motor would rotate at 3600 rpm at 60 Hz - slip. A 4 pole motor is 1800 base rpm at 60 Hz again, - slip. – R Drast Nov 2 '17 at 10:31
• Considering that op is referring to the velocity constant (rpm/V) as "KV", the motors in question are brushless DC motors for radio controlled models AKA permanent magnet synchronous AC motors. Not induction motors. – jms Nov 3 '17 at 2:18