Solving SCR Line Voltage Notching

Sep 1, 2000 12:00 PM, By John A. Houdek, MTE Corp.

To solve 3-phase voltage notching problems, use a 3% impedance reactor.

When silicon-controlled rectifiers (SCRs) are used in electrical controls, we frequently experience line voltage distortion in the form of “notches” in the waveform. The types of equipment that frequently use SCR control schemes (and thus experience notching) include DC motor speed controls and induction heating equipment.

Line notches are an irregularity in the voltage waveform that appear as a notch. They’re typically present in the waveform during SCR commutation or when someone turns a one-phase SCR off and the next one on. During this small amount of time, we actually experience a short circuit between the two phases. This results in the current going high and the voltage going low. In the most severe cases, the notch touches the zero voltage axis. This typically causes the most problems.

Zero voltage crossing. In a normal cycle of sinusoidal voltage, the voltage crosses the x-axis, or zero, at 0° and again at 180°. Under normal operating conditions, there are two zero crossings in each cycle. Some manufacturers design electronic equipment to be triggered on the zero crossing or when the voltage is zero, allowing equipment to activate without the surge currents or inrush currents that would be present if you switched it on while voltage was present. Some equipment, like digital clocks, use the zero crossing for an internal timing signal.

When notches exist, particularly in 3-phase equipment, you can experience extra zero crossings. Instead of two zero crossings in each cycle of voltage, you can actually experience four notches. Think about it. There are now four signals in each cycle, which tells other equipment to “turn on.” That means the equipment will turn on twice as fast, run twice as fast, or turn on at the wrong time—putting your equipment at risk for damage.

Solution. The solution to solving SCR line voltage notching is quite economical and easy to install. If you consider the DC drive or other SCR controller as a source of “notch voltage,” it’s easy to understand how voltage is impressed onto the impedances in the circuit and back to the source.

You must understand where other sensitive equipment connects to the same voltage source. To protect the sensitive equipment, you must reduce the notches before they get to the equipment, by using a simple voltage divider network.

If you add impedance (in the form of inductive reactance) in series with the SCR controller [between the controller and the point where the other equipment connects, then the notch voltage will distribute itself across the new impedance (reactance) and the pre-existing line to source impedance. If the added impedance is half as much as the amount already present, then one-third of the notch voltage drops across the new impedance. Two-thirds still remains at the point of common connection with the other equipment.

If the new impedance equals the existing input impedance, then the notch distributes equally across both impedances. Half of the original notch voltage is now present at the point of common connection.

Experience has shown a 3% impedance reactor solves most 3-phase voltage notching problems. It’s typically not recommended to use a 5% impedance reactor with SCR circuits because the reactor not only reduces the depth of the notch, but it also increases the notch width. Excess impedance could increase the notch width (time) too much, causing problems in the controller itself.

If you add the new impedance anywhere else but between the SCR controller and the sensitive equipment, it will have a minimal impact on the notch voltage. Placing the reactance on the opposite side of point B will not improve your notching problem.

Reducing the notch voltage at the point of common connection with other sensitive equipment by approximately 50% or less of its initial value (depth) is normally sufficient. This eliminates the multiple zero crossings and typically solves the interference problems with neighboring equipment.

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