Time based modulation

Derrick Ellefson

Are they using somthing called a SDR relay? It is a solid state relay with no moving parts. I know Leibert uses them. Also Thermolec and laing boilers.

Tom Hopkins

Dristeem has a resistive electric humidifier (commercial), that uses something they call "time based modulation". It basically turns the heater on and off for a percentage of every minute. Yes... every minute. This would beat the crap out of the contactor, would it not? Please give me your thoughts.

BTW this is a commercial sized unit... 50#/hr steam.

Tom Hopkins

No, they are literally turning the contactor on/off. 25% modulation would be on for 15 sec. off for 45. Seems corny to me. I guess I could consider the solid state relay version. I think they have that too. Thanks for the reply

jp_2

i worked with

a machine that would cycle 370VAC 80amp contactors on for 15 seconds every 2-3 minutes 5 days a week up to 3 shifts. In my 10 years as a service tech, never replaced one of these contactors.

Tom Hopkins

OK. Just seemed like a corny way to control the thing. Thanks for the response.

Brad White_187

Pulsed Width Modulation

or PWM is what it sounds like to me. This type of control varies the on-time as you note, in proportion to the deviation from setpoint. The further from setpoint, the longer the on time. The closer, the shorter.

A common control strategy for a number of applications, but is not really "modulation" because at it's heart, it is on-off control.

John Ketterman

If you are using a solid state relay (or any electronic non-mechanical circuit), once a minute is unnecessarily slow. The fact that it is only once a minute means they are using a mechanical relay.

You can modulate resistive heating with variable-width square pulses (pulse width modulation) with a frequency of 60Hz or something like that. This is technically on/off, but because it is so fast it is in effect true modulation. This method works even better for things like electric motors, which are very nonlinear, so half the voltage doesn't mean half the speed (the motor may not even run at half voltage). But if you use square pulses to turn the full voltage on and off 60 times a second, you can get any speed you want depending on the pulse width.

Brad White_187

I like

your explanation better, JC!