Is this circuit safe enough?

[Boncuk]

Hi All,

I'm planning to fit my self designed air condition in my house (under construction).

To distribute the cool air from the double ceiling I intend to use twin fans of 7" diameter each for each heat exchanger.

The fans however are high performance and high speed fans with a lot of annoying noise. To reduce the rpm (and the noise) I have tried to connect two of them in series which works properly.

If the temperature in a room rises above a comfortable level I want to have the option to switch both fans for parallel operation.

I made up a circuit using a DPCO relay. There is just a fact concerning safe operation of the circuit. One output terminal always carries Life potential which burries some kind of danger for an electric shock.

Of course using the switch and relay no fan will stop running. This is controlled by the air data computer.

Does anyone have an idea for improvement?

Regards

Hans

 

[ericgibbs]

hi Hans,
As you say, the fans are always running, either in series or parallel.

I would say if the wiring is well enclosed, I cant see a problem with the circuit, its nice and simple.

 

[Boncuk]

Thanks Eric, for your opinion.

Safety is always my concern if it goes to electric wiring of any kind. Nobody will be able to reach the switch box, because it will be located up the ceiling. To get there you'll need a ladder and remove the ceiling panels first.

Hans

 

[ericgibbs]

Thanks Eric, for your opinion.

Safety is always my concern if it goes to electric wiring of any kind. Nobody will be able to reach the switch box, because it will be located up the ceiling. To get there you'll need a ladder and remove the ceiling panels first.

Hans

hi Hans,
If necessary you could always fit a RCD [earth leakage current detector] unit into the mains supply to the fans.

Have you considered a triac speed control for the fans.?

 

[Hero999]

Is it safe?

That depends on whether you operate the components components within their maximum ratings and that you construct the circuit in a way that complies with the electrical safety regulations in your area.

We can't determine that you're doing either of the above, just by looking at a schematic of the circuit.

 

[Boncuk]

Is it safe?

That depends on whether you operate the components components within their maximum ratings and that you construct the circuit in a way that complies with the electrical safety regulations in your area.

Nice joke. Thailand is a paradise for non regulation regulations. Electric wiring is normally done twisting cables together and fixing them with insulating tape. I really wonder why there are so few electric fires.

Opening any electric household device made in Thailand brings about the same picture. Simply horrible!

The fans will of course be operated at their nominal power rating which is 0.65A per fan. The relay can stand 250VAC at 8A.

Hans

 

[Boncuk]

Hi Eric,

I guess I don't need to worry about earth protection. The entire electric system will be split into many subsystems each fitted with an "FI-switch". From there the circuit splits again for lighting and wall outlets for each individual room. Each half of the wall outlets will be fused individually, so no total power failure can happen in case of a short. The living room will have two identical subsystems.

PE is not supplied with the electric power. So I have to take care of proper earthing of the circuits which means another problem: There are no big copper blocks or bars available in Thailand. The only things you buy for earth connection are iron bars with a very thin electrolytic copper layer.

I also thought about using triac control, but those will be a bit costly. Each room will have two fans, the living room will have four of them. This means a total of 8 dimmers with a big disadvantage: The are adjustable from almost zero to full power. A 220VAC electric motor normally starts rotating at 85V to 100V. If somebody leaves the knob below that threshold the motor will of course draw current without moving, get hot and consequently burn up.

So I'm afraid I'll have to build the triac regulators myself.

Regards

Hans

 

[ericgibbs]

hi Hans,
IF you can get a short length of copper water pipe, say a metre, that makes a good earthing rod.

If you do design your own speed controller, you should include 'back emf' sensing. This could be used as a 'fan stalled' test.

 

[Boncuk]

Hi Eric,

I'm not familiar with the term stall in conjunction with motors. In aerodynamics a stall condition is present if the draft is higher than the lift, which makes an aircraft descend at a rate similar to that of a piano thrown out of the window.

Axial fans can stall as well, meaning there is no air movement along the longitudinal axis. This can be caused by a closed intake air duct or exhaust or just too high resistance (clogged up filters etc.) within an air system. If an axial fan stalls the probability loosing blades is very likely, because the vibrations move from the wingtip to the root.

Radial fans cannot stall, hence vacuum cleaners use radial fans. You can hear distinctively how it winds up to max rpm (no load) if you close the suction port.

May be you happen to have a link towards the mentioned stall sensor circuit.

Regards

Hans

 

[ericgibbs]

Hi Eric,

I'm not familiar with the term stall in conjunction with motors. In aerodynamics a stall condition is present if the draft is higher than the lift, which makes an aircraft descend at a rate similar to that of a piano thrown out of the window.

Axial fans can stall as well, meaning there is no air movement along the longitudinal axis. This can be caused by a closed intake air duct or exhaust or just too high resistance (clogged up filters etc.) within an air system. If an axial fan stalls the probability loosing blades is very likely, because the vibrations move from the wingtip to the root.

Radial fans cannot stall, hence vacuum cleaners use radial fans. You can hear distinctively how it winds up to max rpm (no load) if you close the suction port.

May be you happen to have a link towards the mentioned stall sensor circuit.

Regards

Hans

hi Hans,
If you used a simple speed controller, where say the speed had been adjusted to a low rate and then you switched off the supply.
If you reapplied the power with the setting still set for very low, there is a good chance the motor/fan would not turn, but just buzz, its stalled.

Also if you turned the speed lower and lower on a running fan you would get to a point where the fan would stop turning, but would still be drawing a current, its stalled.

What a 'back emf' system is designed to do, is to sense the stalling and apply the ac voltage for slightly longer period during the mains cycle, so that the motor has sufficient torque to turn the fan even at low speeds.

I forgot, as a flyer you would read the stall word as loss of lift...

 

[Boncuk]

Hi Eric,

this is understood. However, does that mean I'll have to measure the dynamic pressure in the fan?

Measuring static and dynamic pressure takes care of motor stall as well as air foil stall.

Hans

 

[ericgibbs]

Hi Eric,
However, does that mean I'll have to measure the dynamic pressure in the fan?
Measuring static and dynamic pressure takes care of motor stall as well as air foil stall.
Hans

hi Hans,
If you use a 'smart' back emf circuit, the back emf sensing/correction will be a function of the fan blade loading [dynamic/static/torque].

I did have such a circuit, but I cannot find it. I'll Google for something similar.

EDIT: a comprehensive databook for motor control.

https://books.google.co.uk/books?id...=X&oi=book_result&resnum=10&ct=result#PPR7,M1