SCR Power Control Questions

[StarCat]

IN making a study of SCR Controls used in Process Resistance Heating applications, I need some clarification on the type of power the elements are seeing when using a 3 Phase Anti Parallel arangement Controller. I am clear that the control is able to use both positive and negative cycles of the AC waveform, but is the applied voltage actually DC, and can you read the output with a typical test meter for 208-480V usage? Any general use troubleshooting methods are useful.

 

[petersonra]

IN making a study of SCR Controls used in Process Resistance Heating applications, I need some clarification on the type of power the elements are seeing when using a 3 Phase Anti Parallel arangement Controller. I am clear that the control is able to use both positive and negative cycles of the AC waveform, but is the applied voltage actually DC, and can you read the output with a typical test meter for 208-480V usage? Any general use troubleshooting methods are useful.

SCRs by nature conduct only in one direction, so they are DC devices.

You would need a true RMS meter to read the waveform voltage.

 

[gar]

190311-1023 EST

StarCat:

I have no idea what you are talking about. What is a "3 Phase Anti Parallel arangement Controller"?

Start with what an SCR is. This is a device named Silicon Controlled Rectifier. What is a rectifier? It is a one way conductor. What does controlled mean? It means its turn on time can be controlled.

When biased in its reverse direction it is a very high resistance up to some breakdown voltage independent of of the state of its control element, the gate. You never want to apply a reverse voltage as great as its breakdown voltage. It should never conduct in its reverse direction.

In the forward direction it is a very high resistance until triggered. Once triggered it looks somewhat like a diode in its forward direction. The trigger is only required for a moment, then conduction in the forward direction continues until the forward current drops below a holding level. The trigger can be maintained or removed. If maintained, then conduction may continue below the drop out level, but not in the reverse direction.

In an AC application conduction starts with a positive anode voltage relative to the cathode and an applied trigger. Then conduction continues until the next current zero crossing. The current zero crossing may be different than a voltage zero crossing dependent upon the type of load. An inductive load produces a delayed turn off relative to the voltage zero crossing.

So a single SCR produces a pulsating DC output from an input AC waveform.

More than one SCR can produce a pulsating DC output or pulsating AC output. The result is a function of the circuit design.

You can use AC and DC meters to answer your question.

If a pair of SCRs are connected in parallel but reversed relative to each other, then a pulsating AC output is produced. There may be a DC component. This DC component will depend upon how close the source + and - half waves are balanced, and/or how close the + and - cycle trigger points are controlled.

.

 

[drcampbell]

... You would need a true RMS meter to read the waveform voltage.

An ordinary voltmeter is sufficient for troubleshooting. It won't provide an accurate measurement of an SCR-clipped non-linear waveform, but it will tell you if the voltage is going up when it should be going up and vice-versa.

I can't think of any good reason for using DC for resistance heaters, but it will only take a moment to figure out whether your controller is supplying AC or DC to them.

 

[Jraef]

With SCR heating controls, there are two basic TYPES of control used, and how you read the output will depend on which type. The two are Phase Angle Control and Zero Cross Proportional Time control (the 3rd method is to just use them as a solid state contactor, but that is not any different than any contactor).



In Phase Angle control, each SCR is pulsed to allow only a portion of EACH sine wave through and since you will have the other SCR on the other half of the sine wave, each phase will see an RMS voltage and current flow that can still be thought of as a sine wave. But it does mean that your meter must be able to read and display RMS values from that discontinuous flow. In old analog d'arsonval iron vane meters, this was inherent in the design but with digital meters it had to be done as an algorithm. That algorithm is collectively referred to in the meter industry as "True RMS" because it takes into account the non-linearity of the way current is actually flowing in the circuit. So if you have a good quality DMM with True RMS, you will read the output of a Phase Angle Controlled SCR controller as an AC signal, both for current and for voltage.

In Zero Cross Proportional Time control, each phase is controlled by each set of SCRs being fully on, but the number of cycles they are on for is varied by the desired EFFECT on the heater element in terms of POWER (watts), regardless of what the voltage / current waveforms look like. So for example if you want 50% watt output, you have the phases on for 30 cycles and then off for 30 cycles. If you want 25% output, you have them on for 10 and off for 40, etc. etc. When you go to measure V or I on that output with a DMM, you will see AC, but it will jump from fully on to fully off and depending on the speed of the meter and sampling rate, the values you see will be all over the place. Most common meters do not have the necessary capability to integrate the values correctly. Again though, an analog meter will be better for that because the effect this has on the magnetics in the meter is similar to what is happening in the resistive heater elements.

 

[gar]

190311-1118 EST

petersonra:

Why do you say a true RMS meter is required? What does that do for you?

What is a true RMS meter? Is a Fluke 87 a true RMS meter?

If I connect a diode in series with a resistor, feed this with a sine wave, use a Fluke 87 to read the AC voltage across the resistor, then from apparent "Vrms" of the Fluke 87 and R can I correctly calculate the power dissipation in the resistor?

A Fluke 87 meter is not a true RMS meter. Nor are most others that have the label True RMS. However, a Weston or other electrodynamometer meter is a True RMS meter within its bandwidth limits. It measures the RMS value of the AC and the DC components. A Fluke 87 and similar instruments strips the DC component.

Some rough measurements from an AC supply (120 V line), a diode, and load resistor.

120 V adjusted by a Variac.

DC across 230 ohm resistor. Simson 270 on 250 V range reads 54 V. Beckman 4410 (so called true RMS) reads 53.7 V. Note the Simpson is quite far down range, and also there is diode drop. Ideally the value read should be 120*1.414*0.318 = 53.9 . There is also waveform distortion.

Measurements on AC, diode, and resistor.

AC in series with diode across 230 ohm resistor. On AC range Beckman 4410 reads 65.1 V. Beckman has an input series capacitor that removes the DC component. Power from AC component = 65.1^2/230 = 18.4 W. Power from DC component = 53.7^2/230 = 12.5 W. Total power = 18.4 + 12.5 = 30.9 W. Calculated real true RMS = (30.9*230)^0.5 = 84.3 V, not 65.1 read on the Beckman.

Calculation of total power based on rectified half wave ignoring diode drop. Full sine wave power 120^2/230 = 62.6 W. One half is 31.3 W. Good correlation with 30.9 W. Kill-A-Watt measured wattage 30.6 W.

What this points out is that you really need to understand and know how your instruments work.

.

 

[rlundsrud]

My apologies for hijacking this thread but this is to closely related I couldn't resist.

Is an SCR heating control considered to be continuous or non-continous? By that I mean does having the SCR output switching off then on in short intervals allow it to be a non-continous load even if the switching occurs continuously for extended periods?

 

[gar]

190311-1453 EST

rlundsrud:

If it has the capability to be at full power continuously, then it is a continuous load, and any supply must be capable of powering it.

If this was supplied by a high reliability time clock that could only fail to an off state, and the on time was less than whatever criteria is specified by the supply, then it could be non-continuous. Time constants are of importance here. And obviously off time is also a factor. It boils down to power dissipation in the source, how that affects temperature rise, what happens to peak temperature, and what happens to the life of the source.

.

 

[petersonra]

My apologies for hijacking this thread but this is to closely related I couldn't resist.

Is an SCR heating control considered to be continuous or non-continous? By that I mean does having the SCR output switching off then on in short intervals allow it to be a non-continous load even if the switching occurs continuously for extended periods?

Pretty much anything on a thermostat or otherwise temperature controlled is not going to be continuous by the definition of continuous found in the code.

 

[StarCat]

Thanks for all responses

Thanks for all responses

Pretty much anything on a thermostat or otherwise temperature controlled is not going to be continuous by the definition of continuous found in the code.

The Anti parallel designation is taken right from the training pages on this type of control. It is said this type of arrangement allows one to take the negative half of the ac wave as well to power up the load.
For process heating of the type I am referring to, it looks like they typically run PID controls as TRIGGER ahead of the SCR and zero cross is preferred or adequate for liquid immersion type duty. As JR has noted you can set for percentage of power applied with is very useful.
I did figure the output was DC due to the Diodes, but none of the training material I have read in the last week says this directly. It does not quite compute.

What Gar has said here may be what I was trying to get my mind around with the negative cycle of the AC waveform. I do not know a lot about how DC behaves as read on an osilliscope , but recall seeing DC as shown as square wave or similar above the x axis strictly. So I Guess my question is how does that negative have of the AC wave also become DC?
"If a pair of SCRs are connected in parallel but reversed relative to each other, then a pulsating AC output is produced. There may be a DC component. This DC component will depend upon how close the source + and - half waves are balanced, and/or how close the + and - cycle trigger points are controlled.." [GAR]
Thanks for all responses. They are helpful

 

[StarCat]

Reference

Reference

To achieve full wave conduction two SCR’s must be used.
They must be connected in parallel but opposite directions (Figure 9). This circuit configuration is known as anti parallel or “back to back configuration."

https://www.chromalox.com/-/media/files/training-manuals/en-us/tm-pk501-scr-power.pdf

 

[Jraef]

...
I did figure the output was DC due to the Diodes, but none of the training material I have read in the last week says this directly. It does not quite compute. ...

What is AC? It is DC that can't make up its mind which way to go...

The point is, technically because you are switching on only 1/2 of each sine wave with each SCR, the output of a single SCR is going to be pulsating DC. But you are never really dealing with only one SCR in this case, you are dealing with PAIRS of them, both connected to the same phase. So those two pulsating DC outputs comprise an AC output for all intents and purposes.

 

[gar]

190311-2046 EST

StarCat:

The Chromalox discussion is quite good and accurate, except for the gross error that voltage going to zero turns off the SCR. Certainly the voltage being at zero with no gate drive means that the SCR is off, but it is the current going below the holding current with the gate off that turns off the SCR. Big distinction. It is a long time since GE put out its first SCR manual, about 1960. But if you can find one it would be worth your study.

What is a DC output? You need to talk about average DC voltage or current, and the ripple on the DC component. An absolutely constant DC signal will have its instantaneous voltage or current at any instant of time identical to its average DC value. Any other nonzero average DC value will consist of two components, a constant DC value equal to its average DC reading and a superimposed AC component, ripple.

A back to back SCR pair or a Triac in a practical circuit will likely have some small average DC component. The size of this DC component can be of importance when driving an inductive ferromagnetic load because it can cause core saturation, and destruction of the load, or SCRs (Triac). A series capacitor can eliminate this DC component.

.

 

[StarCat]

VOM Setting

VOM Setting

Gar you should let them know this.
JR Excellent. So do I set my meter for AC or DC if I want to read the Power Output?
Or try both?
Thanks guys very useful comments.

 

[Jraef]

You set your meter to AC. But again, if it’s a Zero Cross Proportional control, what you read will look erratic.

gar,
I read that again and they say the SCR switches off when CURRENT passes through zero, not voltage, which is correct.

 

[steve66]

To achieve full wave conduction two SCR’s must be used.
They must be connected in parallel but opposite directions (Figure 9). This circuit configuration is known as anti parallel or “back to back configuration."

https://www.chromalox.com/-/media/files/training-manuals/en-us/tm-pk501-scr-power.pdf

Isn't that (meaning 2 back to back SCR's) basically just a TRIAC?

​

 

[Jraef]

Isn't that (meaning 2 back to back SCR's) basically just a TRIAC?

​

Close, but not exactly the same. In a triac there is one "gate" that turns the whole circuit on, like closing a contact. With back-to-back SCRs, each SCR has its own gate circuit and can be controlled independently. So for simple single phase circuits, you can use triacs more easily but for 3 phase circuits, triacs can essentially only be solid state contactors; all on or all off. When you want to do more complex patterns of current control, as you do in phase controlled firing, you have to control the timing of positive and negative cycles between phases, so you have to have the gate circuits separated. So for example to accomplish this, you have 6 SCRs, 2 in each of the phases A, B and C, Positive and negative and you fire in a pattern that looks like A+ and B- together, then B+ and C- together, then C+ and A- together, etc. etc. etc. You can't do that with a triac.

 

[gar]

190312-1950 EST

Jraef:

From the section on "Zero Crossing Firing" ---

It should be noted that once an SCR is turned on, it will remain on until the conducted current falls to zero. This occurs naturally every ½ cycle in an AC waveform. An SCR always turns off at a zero voltage crossover (assuming zero phase shift.)

This is not precisely correct. Falls below a holding current would be correct. The comment on zero voltage crossover is not a clear statement. It would be better left out, and only talk about current falling below the holding current.

Following is a fairly good description --- https://www.electronics-notes.com/a...nts/scr/how-does-thyristor-work-operation.php
And this helps a little more --- https://electronics.stackexchange.com/questions/225737/why-do-we-bother-with-scrs/225742
The wiki discussion is not very good --- https://en.wikipedia.org/wiki/Silicon_controlled_rectifier

On your comments on SCR vs Triac. Both can do much the same thing and it has nothing to do with 3 phase vs 1 phase. A major difference is turn off commutation, and that the Triac can be controlled with a single gate, whereas two SCRs generally require two gate drive circuits. SCRs can handle much greater power levels.

Depending upon on the gate drive circuit for a Triac it would be possible to have different turn on phase angles for the two half cycles.

.

 

[RumRunner]

190312-1950 EST


From the section on "Zero Crossing Firing" ---
This is not precisely correct. Falls below a holding current would be correct. The comment on zero voltage crossover is not a clear statement. It would be better left out, and only talk about current falling below the holding current.

Following is a fairly good description --- https://www.electronics-notes.com/a...nts/scr/how-does-thyristor-work-operation.php
And this helps a little more --- https://electronics.stackexchange.com/questions/225737/why-do-we-bother-with-scrs/225742
The wiki discussion is not very good --- https://en.wikipedia.org/wiki/Silicon_controlled_rectifier

On your comments on SCR vs Triac. Both can do much the same thing and it has nothing to do with 3 phase vs 1 phase. A major difference is turn off commutation, and that the Triac can be controlled with a single gate, whereas two SCRs generally require two gate drive circuits. SCRs can handle much greater power levels.

Depending upon on the gate drive circuit for a Triac it would be possible to have different turn on phase angles for the two half cycles.

.

Ah

The voltage crossover [zero shift] is still relevant due to the fact we are talking about AC.

It would be different if this were DC since we won't be dealing with wave forms when taking about circuits from a DC source like a battery.

The sinusoidal wave form is continuous as long as the circuit is energized. . . therefore the zero to peak are ongoing events.

The SCR's role is to provide a controlled output. . . a controlled rectifier, hence-- silicon controlled rectifier—unlike a run-of-the-mill diode.

The timing-- when to trigger the gate of the SCR is the name of the game so to speak.

You trigger it only once [which you know already] while the sinusoidal wave form is on its way to its peak on the half cycle per JR's block diagram.

It continues to conduct until the sine reaches the zero junction. Since this is negative (below the axis) the SCR will not conduct and therefore turn off.

This is the gist of JR's post.

The double SCR arrangement [back to back] is not taken into consideration at this point because we are analyzing the “molecular” level of SCR's “mechanism” and its functionality as applied in the circuit.

The earlier you trigger, the less amplitude. . . the later you trigger the higher the amplitude.

The gate triggering signal is made possible by the carrier frequency pulses. The timing depends on the setting and consequently interpreted by the PID. [proportional- integral- derivative]

The timing is carried out by oscillator [usually IC 555] timer that is the one being adjusted to control the output voltage. The on and off state of the timer is the function that results in the carrier frequency that is also being used to trigger the SCR gate.

This is what is displayed on the operator console.

I talk a lot sometimes, probably more gabbing than necessary.

Cheers

 

[gar]

190313-1519 EST

myspark:

What I am trying to get people to understand is that an SCR is turned off by the forward current from anode to cathode dropping below a holding current level that exists in this forward direction, a characteristic of the SCR. When this opening occurs there is still a small positive voltage drop from anode to cathode of the SCR. If you try to turn off the SCR very fast, then charge carriers in the SCR may cause conduction to last longer. Not a problem at 60 Hz.

The reference I provided has an equivalent circuit for an SCR using two transistors. This is a positive feedback loop and once triggered remains locked in one state, on, until the loop gain goes below unity at low current when it goes into non-conduction.

This latching characteristic of the SCR is the same whether AC or DC is the supply. If the SCR is switching a load resistor in a DC supplied circuit, then once turned on it will remain on until you do something to lower the SCR current to less than its holding current. There are methods to do this, probably dating to the late 1950s.

Other than for high frequencies the voltage drop across the SCR is non-zero and positive when it is conducting.

.