Making a two speed motor - TRIAC and PWM question

Question

I would like to create a dual speed blower which goes into HI when an SSR from a PID is activated. I would like to be able to control the speed of each of the high and low settings. When the SSR is not activated the blower would be in low.

For an AC motor, using TRIACs: If I set the low speed controller to my desired setting and use this to power the fan and then connect another TRIAC speed controller in parallel to the first, where the second on is activated via the SSR, will these operate correctly in parallel and "boost" the blower?

Or would there be some sort of feedback? If I set each to controller to say 50%, in low would we be operating at 50% fan speed then in high we would have full speed or are these not additive?

Second part of this would be if I were to use a DC motor and PWMs. Would a solution to this be to have the SSR switch between the two PWMs so that only one is operating at a time? And if so, what would be a good approach to this - would I use two SSRs and a NOT to switch between the two? Something like:

I would really appreciate any thoughts, or if you have better ideas please let me know.

Thanks for any input.

[edited based on comment below]

Motor contenders: I'm not really sure what type to get - whatever is most easily controllable, and I'm not that familiar with the motor types. Some contenders were a 115V Dayton shaded pole blower (e.g. dayton-model-1tdp7-blower-146-cfm-3100-rpm-115v-60-50hz-4c446 (sorry can't post full links because I don't have a reputation)), but these say "not recommended for speed control" so that doesn't sound too promising. The 12V version (very similar) also says not "recommended for speed control".

Finally there is: www.spalautomotive.com/files/centrifughi/catalogues/004-A41-28S.pdf Which is interesting because it has 3 resistors on it for high, med and low. So I think it is a different type of motor from the Dayton. I submitted a question to the manuf about acceptable speed control, but I haven't heard back.

Note that the speed difference I am looking to implement would probably be about 10% different while running close to the max speed (I'd choke off the input to reduce flow). I believe speed control issues are usually related to running too slowly and causing overheating so that might be the reason for the Dayton warning.

Any recommendations about the type of motor would be appreciated also. I need something relatively high pressure and CFM (1-2", 150CFM).

Thanks.

Answer 1

I would recommend to use only one control element. Only one SSR, only one PWM. You can adjust this single controller to have two speeds, by changing its input.

Answer 2

Triac dimmers are a really old circuit, used in dimmer switches etc. They're pretty simple. The first thing to understand is that you decide when to turn the power on and it automatically cuts off at the end of the cycle - when the voltage crosses zero. So you get a kind of sharks-fin shape.
So how do you make a circuit to turn the power on a certain amount before it's going to go through zero? You use inductance or capacitance to mess with the phase of the AC.
So to get 2 different speeds you want 2 different 'turn on' signals which you can feed into the triac, one for fast (turn on early in the cycle so you get a long period of on) and one for slow (turn the power on just before it's gonna get cut off in each cycle).
Alternatively you can have just one signal and have it adjust - so perhaps switch in some extra resistance somewhere in the circuit to change the phase delay. Googling for triac dimmer will give you an idea.
This kind of dimming is a bit crude, noisy, etc so it is less common than it once was - but it's good enough for millions of domestic dimmer switches all over the world. If your blower is massive then you probably need something more sophisticated.