Unipolar Versus Bipolar

Doing a lot of VFD programming and it got me thinking. What method of direction control do you guys use and why. I usually use unipolar logic control, but I was wondering if there is a benefit to using bipolar control.


Your Thoughts?

If you want to control both speed and direction without having to switch discrete direction inputs and/or cause a hesitation, then bipolar makes sense.

We have used unipolar with direction selection on every application so far. We have not avoided bipolar, we just have not installed one.

Our applications are moving product (ore, brine, dry product) from here to there.

The conveyors that do reverse, do so in specific situations. A separate signal for reverse is seen as a positive.

We have screw conveyors that move product in 2 directions normally. They coast to a stop forward before allowing reverse, coast to a stop reverse before allowing forward. Again, uni-polar is seen as a positive.

We have a rotary plow that allows for reverse when jammed. It is bi-polar and we have actually wired it to a uni-polar output and used an output to drive a DPDT relay ... to make it uni-polar forward or unipolar reverse.

One problem that can come up with bipolar speed and direction is that it is hard to get a true zero speed. The slightest offset voltage or even noise can cause the shaft to creep forward or backward. Sometimes, on high dynamic response systems, noise at zero voltages can cause the motor shaft to "jiggle" in tune with the noise. With unipolar signaling, you can set up a deadband around zero if noise or offset voltage becomes a problem.

It is true that you can set up a deadband around zero with bipolar on some brands of drives but it is usually harder than on unipolar.