DC motor speed control using SCR

Question

I'm trying to design a DC motor speed control circuit using an SCR, where I want to control the speed of a DC motor like the way electric fans work by changing the fan speed.

In this project we are required to use SCRs. What I'm trying to do in the circuit is controlling the gate voltage using a voltage regulator IC, but I realized that once the SCR is triggered, changing the gate voltage will not change the voltage and current passing through DC motor load and SCR's cathode - or am I wrong? Is there a way to reconfigure the circuit in the way to do?

Answer 1

There is a way to do this, but it is probably (hopefully?) well beyond the scope of what you were asked to do. I agree with comments that it was probably a miscommunication.

They also might've meant a universal motor, which as the name suggests, can be run on DC, but isn't exclusive to DC. These are common targets for AC speed control, as their speed is poorly regulated otherwise. Handheld high-speed rotary tools are a classic example of this design: both the series-wound / universal motor, and the thyristor-based speed control.

Anyway, I won't go into detail of a complete DC control scheme, just drop the hint that it is possible to use SCRs on DC. The key is finding some way to force an SCR to turn off.

In this example:

^{Source: my website, https://www.seventransistorlabs.com/Images/SCR_Inverter.png}

A series pair of SCRs fire alternately, connected to the load. The load is wired between Output and GND, and the supplies are bipolar (in this test it was just ±17V from a bench supply). This is an AC output; but it is supplied by DC, so it accomplishes the key step of turning off SCRs alternately.

The key is the 9µH + 0.47µF series resonant circuit: when one SCR turns on, the other one is still on, so together they short out the supply. The power source is delayed from absolute shorting-out by the series inductance (the 2 x 160µH), and when this resonant circuit goes from ~nominal supply voltage to suddenly a short circuit (a step voltage change across its terminals), it resonates, current rising to a peak, then reversing, and when this reverse current exceeds the load current, and stays reversed for longer than t_q, the SCRs turn off (both of them!) and the MUR2020 is left holding the resonant current. After completing one full cycle, that diode turns off as well, and voltage returns to nominal supply. (We could turn off completely at this point, and some clamping would be needed to absorb flyback from the 160µH, and then we'd be done.) To switch on again, gate drive is maintained to one side, and thus load current flows, in this case alternately.

Thus, we have a half-bridge inverter circuit, with shorting-mode commutation. Ordinarily, such as with MOSFETs and IGBTs, we prevent shoot-through by leaving dead time between the gate drive signals. But SCRs are, in some respects, the inverse of those components, and we can harness their latching ability with appropriate circuit design (the inductor and resonant circuit), and thus we actually use shoot-through to our advantage.

The left-side circuitry is just to make a variable time delay between the two gate triggers, so that PWM output can be generated -- the output here actually has a mean DC component, so can in fact drive a DC motor, in this case it would be reversible even (as PWM is variable above and below 50%). Additional clamp diodes would be needed to handle regenerative current (and the inductive load) of a real motor, among various other improvements; this was just a test circuit I made years ago, far from a practical, reliable circuit.

Answer 2

During a repair of a 40 year old forklift truck I discovered a solution like this. At the time of construction SCRs were the only switchable semiconductors, that could carry more than 100 A continuous current.

The PWM generator provided two short pulses, one to turn on the SCR and one to turn on Q1 for a short time to turn the SCR off again.

The component values are just a guess from looking at their physical size and their function.

There is a minimum on time for the SCR until C1 has been charged enough to provide the polarity reversion energy across the SCR.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

but I realized that once the SCR is triggered, changing the gate voltage will not change the voltage and current passing through DC motor load and SCR's cathode... or am I wrong. Is there a way to reconfigure the circuit in the way to do?

Once the SCR is triggered, it will remain triggered until the motor is disconnected or burns or the battery/supply is removed. SCRs and DC do not work together well unless there is some natural commutation of device current.

DC Motor Speed Control using SCR

I would use a MOSFET with PWM if I were you.

Answer 4

In this project we are required to use SCRs.

That's odd.

You (or your project manager) must have confused the SCR's role in an AC motor control circuit which is normally something like this:

^{simulate this circuit – Schematic created using CircuitLab}

Is there a way to reconfigure the circuit in the way to do?

No. Andy's answer is more than clear about SCR's usage with DC. Read it again along with the circuit above. The anode voltage crosses zero periodically so the SCR stops by itself with no extra effort. With the adjustment circuit you adjust the point the SCR triggers at along the sinewave, which basically means speed control.

Answer 5

"I want to control the speed of a DC motor like the way electric fans work by changing the fan speed." Hi, I think question is a very good & very clear also... look, you have almost half answer by yourself, everything seems good,, what I modify in this diagram is the sellector-switch-thing 'S4' to include a simple -switch-thing like 'S1' in a way every single time you switch 'speed' also 'jump' S1 even for a second or so... you can use a rotating-switch-kind-of, main idea is that every time is switching it unplug IC regulator or 'unenergized' status. sure it works, did it in the past.. good luck on your job.