Dimming a Powerhead?

[Astyanax]

Okay, so the heat tests on my brand-new 1.6 hex pico are running a little high. After running powerhead and light overnight, I get 85 degrees in this warmer weather...just barely enough to worry me! The powerhead is a Rio 50, and I'm thinking I can maybe squeeze a couple degrees off it. I had a few ideas, and could realy use your input!

 

(I don't have the space behind the tank for a HOB filter, nor do I have room for a closed loop system. Everything must be inside.)

 

1. Put an electrical dimmer/potentiometer on the powerhead. This would have the effect of allowing me to turn down gph by physically slowing the motor, not by restricting output flow. It runs about twice as fast (65 gph) as I really want anyway. Has anyone tried this? Is it safe? How many ohms should I look for? Would a resistor be better? If so, what kind?

 

2. Exchange the Rio 50 for a Microjet. Half the flow rate when cranked down, and I presume less heat. But I'm not sure, what do you think?

 

3. Don't worry about it. Is 85 degrees livable several hours a day for corals, assorted inverts and a single goby-style fish?

 

Really appreciate your input!

 

Astyanax

[theclearblue]

I wouldn't try changing the voltage on an AC motor. The details escape me at the moment, but basically it requires a device that would easily outweight the cost of a new powerhead. This is the same reason many are intrigued as to how Rio plans to create a controller for their SEIO powerheads.

 

Take a look at the watts that are used by each of the powerheads you're considering, this is more or less the easiest way to get an idea of how much heat they'll add to your system.

[Astyanax]

Ah, interesting. The Rio 50 runs at 5.3 watts, and it appears the Microjet runs at 6...

[Astyanax]

Going to try opening the rear part of the acrylic top, and running a small computer fan over it. Update coming soon.

[saltytoolman]

I'm pretty sure I have this right from my book learning, and from experience.

 

Varying voltage of an AC motor will only make the motor pull greater current (amps) to compensate for the lower voltage, this may make it run a little slower but not much. More than likely the increased amperage and related heat, yes heat, will cause your windings to burn up.

 

The device you are looking for is a vfd (variable frequency drive), wow wee thats technical sounding. This will vary the incoming ac frequency. The smallest units I have seen are about the size of a cassete player. The bad news is these get pretty pricey, in fact down right pricy, like say $400, look around maybe a good deal can be found on ebay. I work in manufacturing and see these devices occassionally in the scrap heap. But often these are usually intended for three phase motors and input. Your little powerhead is just a one phase.

 

Now for the maybe two nerdy section of my post, I wonder if anyone has thought of making a consequential pole powerhead that works like your ceiling fan with two or three fixed speeds? I wouldn't be that difficult for a powerhead manufacturer.

[theclearblue]

It's easier and more cost effective to change the amount of flow the powerhead can pull in (i.e. mini-jet, aquaclear PHs), although I'm still wondering how this affects heat generation with limited intake flow.

 

The Tunze pumps use a DC motor with electronically controlled current draw.

[neanderthalman]

Astyanax, I'd like to save you some trouble - I tried blowing air at the water surface of my 1.6 hex with a computer fan last summer, and it sounds like you're going to make the same mistake. With the fan running, I had about 1/2" of evaporation every 12 hours, until salt creep destroyed the fan. To avoid the evaporation and cool your tank, blow the air at the back glass of your tank. I did this in a panic last year with a desk fan, and it cooled it so much that my heater was always coming on. I'm planning to hang a small computer fan from the back of my tank this summer, but a small plastic desk fan will work too.

 

Clearblue, limiting the intake flow should INCREASE the amount of heat generated. Think of it like driving your car with the parking brake on - you're going to use more gas and your engine will run hotter.

 

Salty, you may be able to find schematics for one online - the co-op stdents we keep getting in at my work have to build them in school. You may need to look for something similar, called pulse-width modulation. It uses DC pulses to simulate an AC waveform. By changing the width of each DC pulse, it changes the effective frequency of the simulated AC wave. I'll try to post a diagram later if I'm bored and feel like drawing one up.

[Astyanax]

Thanks for all the info, guys. I decided that the powerhead is doing just fine as-is. Simply removing the acrylic top between light and water instantly helped. I'm finding that the higher the water level the warmer it gets (closer to the lights -- duh!).

 

So I ended up cutting a hole in the side of the hood and installing a computer fan in that. That way it blows across the lights when I want it to, and is out of the range of the water. But it seems that I will only need it in emergencies at this point. A slightly lower water level and a removal of the plastic cover was all I needed.

 

So far, so good!

 

Thanks,

 

Asty

[theclearblue]

I meant that more as a pondering how much it would increase. Seems strange that they allow you to do it with powerheads but advertise that it kills external pumps, probably because the powerheads can cool better.

[neanderthalman]

Kindof like driving your car with the parking brake on while UNDERWATER! That's right....you heard me.

 

I'd say you're right, the water volume would keep the motor coil much cooler in a submerged powerhead than an external filter. Reducing the input/output flow wouldn't help with an overheated tank though, it would make it worse.

 

Good luck with your tank Asty, don't forget pics