Limit start current of high power motor

Question

I'm working on a project that is clearly showing me that power electronics is a completely different beast.

We have a 5HP motor running on a 120Vdc battery. The motor is installed about 100ft (30m), away with big cables. At the moment the motor turns on it peaks at about 180A, then is rests at about 44A. I'm looking for a way to limit the startup peak at about 70A.

Following my first idea, I've chosen a 150A IGBT and a magnetic loop current sensor, like this:

Close to the motor, I have a high current freewheeling diode, and on the IGBT I have a RCD snubber with 1uF capacitance (seems small to me but here, I see that the rule of thumb is 1uF per 100A...).

My control is simply to turn on until the current sensor rises to a top limit then turn it off for a fixed amount of time and repeat... At some point it will work on forever without tripping the top limit. The motor should remain on for only a few seconds and then go off, so things should not get too hot, hopefully.

While projecting and layout-ing the board for this "simple" project I've realized that a lot of things can go wrong.

Does someone see something I'm missing? I do not have boards yet, so never tested the circuit, but, after seeing a lot of people having problems with IGBT dealing with much smaller currents I am already thinking that I should stop and study more before going ahead with this approach.

Another new idea is to turn on the motor with a high power resistor in series to limit the current for a few mili-seconds and then short the resistor with an SCR. But I feel I'm simply changing monster's name (is an SCR simpler to tame than an IGBT? I don't know...)

Also found the CS8312, IC. Maybe replace its serie resistor current sensor by my loop current sensor?

And, no! I'm not asking for a ready solution, just like to hear a more experienced opinion on what I should study to have a reliable solution.

Answer 1

You have the freewheel diode in the wrong place (or at least as greybeard points out in comments, you don't have a diode in the right place). One should be connected right at the IGBT and power supply, even if there's another one at the motor. The inductance of those leads to the motor could be significant. You want to protect the IGBT against current changing in the sum of the motor and lead inductance. The snubber is designed to protect the IGBT against current changes in only the incidental and unavoidable lengths of wire around it, that's why the capacitor is so small.

Note that the motor, and the leads to it, will maintain a more or less steady current, due to their inductance. When the IGBT is on, current flows through PSU, IGBT and their connecting wires into the long motor leads. When the IGBT is off, current flows through the freewheel diode into the long motor leads. It's the inductive voltage generated by the changing current in the PSU - IGBT- freewheelDiode - PSU loop that will damage your IGBT. First make this loop as small, or at least as low inductance, as possible. Second, put the snubber right on the IGBT to mop up the remaining voltage spike generated by this loop.

Answer 2

There are two different types of IGBTs, those with integrated intrinsic diode and those without it. From your link of snubbers, the used IGBTs do have the intrinsic diode, while yours is depicted as without it. Further, the diode is has a wrong polarization as from the link.

For driving such large IGBT you would need an appropriate IGBT driver and a chopper circuit with current feedback.

I don't know if the snubber is necessary, you can add it at later time.

The photo represents a brake chooper, used in large power VFD. The square area with a part named Concept is the isolated gate driver. Big holes are contacts for a brick IGBT. The left blue part is probably the snubber, the right one could be a current sensor. The ON/OFF command comes from main control circuit by use of fiber optic.

EDIT:

Now measuring the current through shut as from you schematics won't help too much, you have to measure the current on the motor. So the copper now becomes more complex. You could use an isolated IGBT driver with DESAT protection and place the IGBT as high switch:

^{simulate this circuit – Schematic created using CircuitLab}

TIDA-00638 :ISO IGBT driver with DESAT

UCC21750QDWEVM-025

SKM150GAR12T4

EDIT2:

^{simulate this circuit}

Answer 3

Use a pulse soft start with a pulse width that rises to an acceptable current then shut off and repeat.

Using a current sensor to can make it start with constant current by increasing the pulse repetition interval of acceleration to reach full speed at 100% duty cycle.