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pwm can be used to make a motor run more efficiently concerning power consuption,
can we use a similar technique on a AC heating resistor with the help of a triac?
i want to do what's exactly it is done with pwm drived motors...to make the heating element more efficient
by turning it on and off in intervals.
but i'm not sure if i understand correctly why pwm makes a motor run more efficiently... i guess it is
the inertia that helps it maintain it's spinning with an "interrupted" power supply.
PWM isn't ordinarily thought of as improving a motor's efficiency. In fact, since the motor would, essentially, lose speed and therefore torque during non-powered pulses, the energy needed to overcome lost inertia and return the motor to full speed/torque would decrease efficiency.
Rather, it's a means for greater and more precise control of the motor's speed or, in the case of a servo, it's position.
For a resistive load (a heater element, for instance) PWM would control the level of heat produced but, similar to the above case, would not increase efficiency.
Efficiency is increased/decreased by removing/adding parasitic loads.
So, with PWM control, you've also added the parasitic load(s) of the PWM control components.
<EDIT> Stupid typos from utter lack of touch typing ability...
1. You can get a much better lifetime using DC rather than AC for resistive heating. I have lots of experience with low voltage custom wound tantalum heaters that were initially run from a phase angle controlled triac
A DC supply actually turned out to be more cost effective for us because we had to make a panel with volt and current meters to keep an eye on the process. The AC voltages and currents were not correct and therefore power wasn't right either, but it did allow a metric for the process. we were also able to reduce the foot print to 1 RU for the power supply. Seven 1500 W power supplies and temperature controllers took up a fair amount of rack space. With our old system it was much more. We were also able to eventually change to a wide voltage power supply. Power distribution got clever, when someone added a fuse box that plugged into say a 50 A wall outlet.
2. PWM for motors allows you to lower the speed with the same amount of torque as the operating voltage of the motor.
3) Efficiency of PWM comes from the switching of the supply full on and full off, rather than use a linear element.
More efficient compared to what? Nothing is as efficient as connecting a heater directly to a voltage source... Do you think that putting pwm into the mix makes the heater more efficient?
DC powering is another matter which i'm not interested right now..
what i want to know is if i can make some sort of power saving by switching on and
off a heating resistor with a triac.
as far as i know triacs already have an non conductive state when at positive or negative period
which i could extend if i make the triac conduct only in one of the two periods, of course this
is not PWM, but my aim is power saving and efficiency.
@MikeMI i don't know...that is something i'm searching about.
by that mean can a heat reservoir maintain the same temperature as if it was
heated by a steady heat source??
meaning i would let's say... make a 500w heating resistor consume less energy but also offering less heat no matter if the off state
is very small so the resistor doesn't manage to loose noticeable temperature??
Suppose you have a 500W heater for a fish tank. Further suppose, after an initial warm-up it takes only 100W to keep the water in the tank warm. If you put in a simple thermostat, it would have to be on 1/5 of the time. Since the switch contacts in a thermostat produce no heat, the system is 100% efficient; in other words, you are paying the power company for an average of 100W, and all of 100W is going into keeping the tank warm. How are you going to get more efficient?
All the Phase-control brings to this party is that the decision to add heat to the tank is made cycle-by-cycle at the power line frequency, instead of in periods of several seconds-to-minutes at a time.
Maybe I'm missing something in this discussion, how do you PWM AC? Thought PWM was for DC. Only way I can see of using PWM on AC is to use it to pulse a SSR to control the AC, doesn't seem too efficient.
meaning i would let's say... make a 500w heating resistor consume less energy but also offering less heat no matter if the off state
is very small so the resistor doesn't manage to loose noticeable temperature??
Maybe I'm missing something in this discussion, how do you PWM AC? Thought PWM was for DC. Only way I can see of using PWM on AC is to use it to pulse a SSR to control the AC, doesn't seem too efficient.
What you are missing is that using a series resistor with the same resistance as the heater wastes as much power in the series resistor as in the load so the efficiency is only 50%. But PWM switches the switching device on so it has no voltage across it (therefore produces no heat) and the switching device switches off so it has no current (therefore produces no heat) so the switching device is almost 100% efficient when it reduces the power to the heater.
Why are we talking about a heater? It is usually controlled by slow switching on until it reaches the desired temperature then it is switched off until its temperature drops too low when it is switched on again. But a motor usually needs an accurate speed adjustment so a higher speed of the switching is used.
i thought that instead of using a continuous power supply a fast on off switching
created power efficiency, and i was wondering if similar thing could happen on a heating resistor, unfortunately i don't remember every physics law...
a little bit off topic but i would like to ask why then in a class D amplifier
pwm does makes a power efficiency???
I have built class D (PWM) audio amplifiers.
Back in post #11; 1/5 of the power is sent to the load (fish tank or speaker). I will use this as an example.
In a linear amplifier 1/5 of the power goes to the load and 4/5 of the power goes to the amplifiers heat sink and is wasted. not efficient
In a PWM amplifier; 1/5 of the time full power goes to the load and 4/5 of the time no power goes to the load. No power goes to the heat sink. There is no heat sink. Very little power is wasted. (switching must be very fast, faster than you can hear)
Per Ron's example, the gain in efficiency is between a linear system, and a switched system. In my thermostat example, it is already a switched system, so even though the time average power is only 100W into a 500W heater, no power is lost as heat. Making it a PWM or phase controlled system just speeds up the switching rate, but gains no more efficiency...
My comment is a long shot, but real controllers do use a control signal proportional to V^2, but not a lamp dimmer that doesn't have smarts. The application is usually heating. Some controllers can be bought specifically for tungsten heaters such as IR lamps. I do know that R changes by about a factor of 10 lower at cold than max. temperature. So, it's likely that it takes the temperature coefficient of R into account.
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