Haha I feel kind of foolish. I was talking about Tesla the car which I heard inverts DC from the battery to an AC motor. Why would they use AC?

 

I'm building a mobile robot which is quite heavy and needs some serious HP. For this and other reasons I want to mount 4 induction motors to each wheel (for a total output of about 8kW) and power them via batteries. However, those motors need 3-phase current and I obviously can't plug them directly to the batteries. This is why I need a variable frequency drive but without the prepended rectifier (input current is already DC). I have been looking all around but couldn't find any devices that seem to be able to do just this. Can anyone with a bit more oversight maybe lead me into the right direction?

May I suggest b.less DC motors (140-170kV Inrunners or hubs) and dedicated drivers ?

 

Did you ever get this figured out?

Ive just done something similar and it worked out great. Ive got a 1/2 HP 230 volt 3-Phase induction gear motor with a 50:1 reduction running from VFD powered by a 17.2 volt 20ah lithium ion pack. Controlling the speed and direction through the analog and digital inputs is trivial with a microcontroller or through a switch and a pot.

There is obviously some power conversion loss in this setup but it is way better than supplying 500vdc to run the vfd inverter from the DC bus. And I still get longer battery life and more output power than the DC motor with speed controller that this replaced. I don't know why more people are not doing this.

If you are still trying to work on this project and need parts feel free to hit me up and I can share with you the parts I used.

 

There are several sub-families of AC motors. Tesla didn’t use the common induction AC motor for a host of reasons, way too long to list, but suffice to say that the HP/lb ratio is not good, nor its efficiency.

Rather, they went for synchronous AC with powerful rare-earth permanent magnets. These motors are also called, IMHO erroneously, BLDC.
An external DC to 3-phase inverter applies a variable frequency, variable voltage to the stator. The electrical phase angle must closely track the mechanical phase angle. In other words the field rotation follows the mechanical rotation. To determine the angle, this is done with quadrature hall sensors which accurately determine the rotor’s position.

The drive electronics are not simple, but because this motor is both the most efficient and compact, many semiconductor houses have developed all sorts of IC solutions, from DSPs to IGBT drivers to current sensors.

 

Here is a system we often deploy with lead acid or lithium battery packs.

https://www.curtisinstruments.com/products/motor-controllers


Pretty sure Tesla uses a reluctance motor with magnets embedded into stator slotting to increase efficiency and modify torque bands.