[lakedude]

Interesting question about the IMA motor.......

Is it AC or DC?

Are you sure?

It is said to be a "permanent-magnet type, brushless DC motor" here (just under the heading "Design"):

http://www.insightcentral.net/encyclopedia/enmotor.html

However if you look farther down the same page at the picture the hookup shows 3 wires and it says "3-phase cable connection".

I strongly suspect that the motor is 3-phase AC.

Who is with me?

 

[kapps]

I believe what it's saying is that the motor itself is DC. In a normal DC motor, a current will only turn the motor 180 degrees. One it gets past 180 degrees, it will push the motor in the opposite direction (slow it down). Because of this, DC motors must have a switch to reverse the current flow into the motor to continue pushing it another 180 degrees. Since the Insight's battery has no brushes or physical switch to create drag or something to wear out, they electronically switch it via the MDM. This is done by creating pseudo-AC current in 3 phase to and from the motor.

 

[lakedude]

I believe that the DC motor statement is in ERROR. I further believe that the error was caused by a misunderstanding in regard to the DC to DC function of the MDM. DC is not ever applied to the motor. You can call the voltage "psudo" AC if you like, but it is still AC. It may not be a pure sine wave because it it being produced by an inverter but it still alternates, making it AC.

I'm getting more sure of this the more I read.

 

[chrs]

Just semantics

The IMA motor/generator (aka "electric machine) is of a type most commonly called "brushless dc", but in an academic taxonomy of electric machines, this type would more properly be categorized as a permanent-magnet synchronous ac machine. So you are both right--it's a floor wax and a dessert topping!

Why this confusion? Well, the dc in a dc motor is really just in the wires coming out of it. The current in the windings inside the rotor is actually switched by the commutator brushes, and since it is switched to go one way, and then switched to go the other way, the rotor-winding current is actually ac. In a brushless dc motor, the commutation function (swapping current directions) is performed by semiconductor switches (e.g. MOSFETs). and the windings are on the stator instead of the rotor, so that there is no need for brushes to connect current to the rotor (which instead has only permanent magnets). So now there is DC coming to the MOSFETs, but then switched ac current going from the MOSFETs to the windings. That three-phase cable is between the MOSFETs and the windings.

So there really is ac in the wires going into the motor, and hence it "properly" should be called an ac machine. But for historical reasons, and because of the conceptual similarity to dc motors, it's called a dc brushless motor.

If you go buy something called a brushless dc motor, and something else called a permanent magnet (PM) ac synchronous motor, they won't be quite the same, because the brushless dc motor will (hopefully) be optimized for square switched waveforms whereas the PM ac synchronous motor should be optimized for sinusoidal waveforms. But that's a minor difference--either one could run on square or sinusoidal waveforms.

Charlie

ps--this is being discussed simultaneously in this thread:
http://www.insightcentral.net/forum/vie ... php?t=3080

 

[lakedude]

Thank you Chrs for your help in clearing this up. I've been doing some reading and have found the answer here:

http://www.insightcentral.net/encyclopedia/enmdm.html

The IMA motor is 3-phase AC, no 2 ways about it.

I suspected this from the beginning because of a hobby of mine. I fly electric planes which have DC batteries (of course) and either DC or AC motors depending on how much money you spend on the motor.

The DC motors are cheap and have brushes to provide the switching action internally to the motor. These motors can be hooked up directly to the battery.

The AC motors must have a controler and can not be hooked up straight to the battery. The "brushless" controlers have 2 wires with DC from the battery going in and 3 wires with 3 phase AC going to the brushless motor. The brushless controlers we use in planes look exactly like smaller versions of the MDM.

 

[Insightful Trekker]

Thanks Charlie for the link to the dual topic.

And since this is a better fit for Mod & Tech rather than General I am locking this thread and asking that follow-up's be posted in the Mod & Tech thread here:

http://www.insightcentral.net/forum/vie ... php?t=3080