# AC motors

1651 Views 11 Replies 5 Participants Last post by  brian_
Hey guys, I have a question, so I've read that any AC motor can be turned into a generator, say I have a motor with an input voltage of let's say 70v...does that means that the output voltage, when used as a generator would be also 70v?
1 - 12 of 12 Posts

#### kennybobby

· Registered
Joined
·
1,970 Posts
To run at a variable speed an AC motor needs some sort of motor driver, also known as an inverter, to create a 3-phase alternating current waveform from the DC Buss Voltage. This is used to excite the motor stator windings in the necessary sequence and frequency to induce or create magnetic fields in the rotor and cause it to run as a motor.

When coasting or braking is desired, this same inverter can extract some of the energy from the spinning magnetic field. In this case, current is induced into the stator windings and the inverter conttrols the phase timing in order to pump it back onto the DC Buss to recharge the batteries. This is called regeneration braking since it will slow the car down depending upon the rate of current extraction.

There is always losses in the switching and conversion process such that you can never recover as much energy as was put in.

#### Lord Conqueror

· Registered
Joined
·
8 Posts
Discussion Starter · ·
OK thank you so much, so is there a way to calculate how much you will get back from regen braking?

#### kennybobby

· Registered
Joined
·
1,970 Posts
Are you needing an exact formula or equation, or you looking for general rule of thumb type estimates based upon general practice and experiences in actual operation?

We would need to know more about what it is you are trying to do to know how detailed an answer you are needing. e.g. design, engineering, shade tree mechanic, curiousity, etc.

#### Lord Conqueror

· Registered
Joined
·
8 Posts
Discussion Starter · ·
Thank you for your kind response, I'm looking for a formular as I'm looking to just explore different ac motors to use as generators, cause the current generators I'm finding either produce low volts or they just have a very low power output, so I thought motors would work well in this

So basically I had finished developing a charging system using wind energy so now I'm changing it, trying to make it much better, as it had windzilla generators with an power output of 50w and a voltage output of 24v, I figured 50w would not work well... Don't worry, I've already eliminated the excess drag issue and I'm want to build a company with this.

#### kennybobby

· Registered
Joined
·
1,970 Posts
Okay glad i asked. In the context of diy electric cars, the drive inverter is used to create regen braking to slow the vehicle by converting the kinetic energy of motion into either wasted heat or into potential energy of electrical charges stored in the battery pack.

i know nothing or less about windmill controllers. Just from the physics involved i would guess that if you know the inertia of the blades and rotating masses, then you could calculate the kinetic energy, torque and power from the wind while it's turning at a certain RPM. The controller would need to synchronize with this frequency and then operate in "regen" mode to harvest some energy. The "raw" harvest product would be a variable-frequency 3-phase AC with a variable voltage amplitude proportional to the windspeed; depending upon objectives and the electronics used, this could be converted to whatever electrical power was desired for an output.

#### Lord Conqueror

· Registered
Joined
·
8 Posts
Discussion Starter · ·
Thank you so much, you have been a great help, I'd like to ask something else, do you think it would be possible to create AC motors with an electromagnet instead of the conventional system?

#### Hollie Maea

· Registered
Joined
·
1,021 Posts
Thank you so much, you have been a great help, I'd like to ask something else, do you think it would be possible to create AC motors with an electromagnet instead of the conventional system?
Yes, AC motors that utilize electromagnets are not uncommon. One variant uses brushes to excite coils in the rotor (though much lower power than the brushes on DC motors since they're only powering the field and not the armature). But the most common version is the induction motor, which uses relative motion (slip) to induce current in the field windings (or more commonly a squirrel cage).

#### kennybobby

· Registered
Joined
·
1,970 Posts
Thank you so much, you have been a great help, I'd like to ask something else, do you think it would be possible to create AC motors with an electromagnet instead of the conventional system?
i'm curious about whatever it was that you were referring to as an AC motor.

i was considering an AC motor as an electromagnetic induction motor, what type of AC motor were you originally thinking about?

Did it have permanent magnets? did it have brushes?

Do you have a picture or part number?

#### Lord Conqueror

· Registered
Joined
·
8 Posts
Discussion Starter · ·
Well I'm thinking of a wheel that generates electricity but I wanted it to generate electricity at specific times, like I wanted the magnetism to be turned on and off, making it operate like a regular wheel at some times and make it operate like a generator also at some times

#### remy_martian

· Registered
Joined
·
5,395 Posts
Hey guys, I have a question, so I've read that any AC motor can be turned into a generator, say I have a motor with an input voltage of let's say 70v...does that means that the output voltage, when used as a generator would be also 70v?
More or less correct.

The thing I don't understand is your obsession with voltage when what you are after is power.

To get any power out of an AC motor, you have to excite the windings, whereas a brushless DC motor (because it has 3 phases, people like to call it "AC", but to me an "AC" motor is an induction motor) has magnets that induce current into the windings.

Your vague description of first a wheel, then a generator smells to me like kooky overunity stuff. If you take the cloak of mystery off, you'll get better answers or people assessing whether you're wasting your time.

#### brian_

· Registered
Joined
·
8,519 Posts
... a brushless DC motor (because it has 3 phases, people like to call it "AC", but to me an "AC" motor is an induction motor) ...
A synchronous AC permanent magnet motor is called an AC motor because it runs on AC (alternating current) power... not just three phases of switched DC, but 3-phase sinusoidal AC. It's okay if you are only familiar with asynchronous (induction) motors, but that doesn't make all other AC motors (which includes nearly every EV motor other than those on the Tesla Model S/X) anything other than AC.

Among AC motors, both asynchronous (induction) and synchronous (permanent magnet or separately excited) types can run as generators.

1 - 12 of 12 Posts