3-Phase AC Motor Control ???

[fechter]

Take a look at these:
http://www.curtisinst.com/index.cfm?fuseaction=cProducts.dspProductCategory&catID=8

Curtis makes several AC induction motor controllers, but most of them are lower voltage than your motor is likely wound for. If you rewound the motor, it may work nicely with one of these.

I have seen some industrial motor speed controls on eBay for cheap in the past. These are usually called VFAC drives. These may be rated for the voltages the motor is designed for. They typically use a 480vac input, some are made for 240 or 120 vac input. Like a switching power supply, I susupect many of these would work fine on DC input if the voltage is in the right range.

Most induction motor controllers would also require a shaft encoder of some sort to send rpm data back to the controller.

 

[Chris]

Frank,

you don't need a midpoint connection since there's no need to hook up the machine's star point (if it has one at all). Even more - it might hurt since a fixed star point cuts off some percentage of the achievable output voltage. Think of the three phase voltage system as a regular "Y" (i.e. angles of 120 degree between all arms). Put a pin through the center and stitch it to the wall. Start turning the "Y" and draw a horizontal line through the highest point that can be reached by the tips of either arm. Repeat for the lowest point.

The distance of these parallel lines stands for the total DC voltage from your batteries.

While turning the "Y" you notice that you only touch one limit at a time (top or bottom). With the "Y" standing in its natural position there's a gap between the upper arms and the top line (the single lower arm is touching the bottom line). Rotate by 180 degree and find the same situation reversed (gap to the bottom line, top line is touched).

Having a connection between the DC midpoint junction (at half of the total battery voltage) and the star point is exactly like using a pin to fix the "Y". You don't make full use of your DC voltage.

Now, instead of changing the DC voltage (i.e. moving top and bottom line closer to each other) just look for a way to increase your output voltage (i.e. using a "Y" with wider arms). Try yourself - it works without hurting the limits, but only if you remove the pin and slightly shift the center up and down while rotating the "Y".

As said, by doing so you gain some percentage of output voltage.

Hope that helps (and no one's rushing in while you're drawing lines to the wallpaper ... otherwise just hide the pencil and blame the kids ).