Proper fusing of FC & RC boards

[livinlowe]

All-
I am starting to get my FC and RC boards mounted to something that will not lead to me accidentally burning a board up by having them flop around on a top or the floor. I am going with scary terry's idea of mounting them in an metal electrical junction box, then put real 14 gauge wire out to some outlets that I will also mount in an electrical box. I am going to screw these boxes onto a big plywood sheet, giving me lots of room for Prop-1's or what have you. As I started this project, it occured to me that I might want to protect my $90 investments by fusing the darn things so I don't burn up the boards accidentally. I calculated that:

120V/300 Watts = .4 amps maximum each channel

So, my question is: I would want to fuse the OUTPUT of each channel with, at most, a .4 amp fuse but probably a .3 amp fuse. Does this sound right? Also, would I want to fuse the INPUT to each board with a 1.6 amp (4X.4) fuse as well? Or would that be overkill? Do I want to fuse the RC boards as well? (Of course I would only need to fuse the outputs on those) Should I use a fast blow or slow blow fuse?

Thanks for any input!

[Caretaker.CCI]

Here are my thoughts on the matter...

1.   I fused the whole thing starting at the line in. The question you want to ask is, do you really want the circuit to continue to run if even one of the channels is having a problem? My thoughts were, shut the whole shooting match down until I've figured out what the problem is.
2.   Fusing the individual circuits is over kill in my opinion. Dove Systems manufactures a device they call the shoe box. It powers 4 channels but is only fused on the line in.
3.   If I understand the spec. sheet correctly, the 300W designation is for the whole board. If you want to average it... that means 75W for each channel (assuming each channel is loaded equally). I do not plan on running lights that are that bright (you usually don't want that much light for Halloween anyway) so I figured 60W max per channel (240W total). I want the fuse to go if we have exceeded that.
4.   I think you want a slow-blow fuse. If you size it conservatively then run it close to the max, you'd be replacing the fuse every time you get even a little spike in the system. Let's trust our calculations and give the system a moment to decide if there is really a problem worth blowing a fuse over.
5.   Remember Watts = amps x volts (I think wav) so 240W = 2 amps x 120V I think you're looking for a slow-blow fuse in the 2-amp range.


My rational for chiming in on this thread is that I would also like to know if my logic is faulty. If I'm in error in my thinking, the consequences could be disastrous. Thanks for posting this Shawn. If you take a look in this thread:

http://www.efx-tek.com/php/smf/index.php?topic=474.0


I have some pictures of some of the enclosures I have created. They might give you some ideas.

Greg

[Caretaker.CCI]

Two additional thoughts occurred to me as I re-read this post. First, I think it is a good idea to keep line voltage separate from any DC voltage. Having said that, it's probably a bad idea to mount your Prop-1's (or other devices) inside an enclosure with 120VAC. The temptation would (for me anyway) still be there to open the box and stick you hands inside. Second, fuse anything that has potentially dangerous voltage (RC-4's included).

Again, thanks for posting this thread. I'm looking forward to hearing what others have to say about our (my?) assumptions.

Greg

[JonnyMac]

The FC-4 is designed for 300W per channel, roughly 2.5 amps (I = P/E, not E/P).  You could go with a 3A fuse per output, or a 10A fuse for the input power. 

[Caretaker.CCI]

Man did I have that wrong. Glad I chimed in. Good to know though. That means you could run some pretty gnarly motors. Thanks Jon.

Greg

[Caretaker.CCI]

Oops! I'm thinking RC-4, there are some motors that would not like being run by the FC-4. Still, 300W per channel gives you a lot to play with.

[JonnyMac]

Yes, the FC-4 is for lights only -- do not try to run motors with it.

[jukingeo]

Oops! I'm thinking RC-4, there are some motors that would not like being run by the FC-4. Still, 300W per channel gives you a lot to play with.

Even with the RC-4 you have to be careful.  There is a thing called 'inrush' current and it is particularly bad with inductive loads.  So a motor can pull WAY more than it's normal running current upon startup.  To make matters worse, started a motor up on a heavy load will draw even more current.

For the most part if you are starting up small shaded pole or or brush AC motor you should be OK (The RC-4 switches AC only, so you probably need a rectifier and a filter cap to use small hobby type DC motors).  That is if your loads are fairly light.

When you get to larger motors such as those on power drills or those large capacitor start motors...the relays on the RC-4 will probably not handle those.

However, all is not lost.  There is always a tool for the task at hand.  Doing it properly, you could control very large motors with something called a motor starter.   Basically this is a fairly large relay which is referred to as a contactor in electrician's parlance, but there is more to it than that.  Motor starters are different in that they are rated and protected with overload protection devices.  These are not really fuses, but similar.  What they do is cut the current to the circuit if the inrush exceeds what the motor is rated for (or in the event the motor gets overloaded, it prevents it from overheating).  So not only do you protect your motor, but also your circuit.  You should still use fuses on the motor circuit and also the controlling side of the motor starter.

Like a relay, a motor starter has a lower current (and sometimes lower voltage) control line (usually it is 120volts).  THIS control line you CAN hook up to the RC-4.   In essence you are letting a small relay control a very large one.

How big of a motor could you use with a motor starter?  Think big...very big.  Motors so huge you need a crane to pick them up! Think huge, amusement park ride, 3 phase 30HP (+) 440volts big.

[Caretaker.CCI]

Really good stuff... thanks for the input. I'm familiar with contactors but have never used one.

If I am trying to decide whether to use a fuse or go with a contactor, can the inrush feature on a fluke be trusted? Has anyone had experience with this? I'm not sure I have ever seen a motor's spec sheet with inrush current listed. Besides, as you said, if you put a load on it then everything changes. It would seem the best route to go is to test what you're working with.

Greg

[livinlowe]

The FC-4 is designed for 300W per channel, roughly 2.5 amps (I = P/E, not E/P).  You could go with a 3A fuse per output, or a 10A fuse for the input power. 

:P Man, it's appearently been awhile since I've done any electrical calculations! Thanks for that catch Jon. I went to radio shack and they have a 4 circuit fuse holder, but I want to see how much cheaper it is on mouser or Jameco.

I am glad so many people helped with this thread, as I'm sure, mistakes can happen when we all feel the "2 days to go till the holiday....AAAAAHHHH!" rush!  ;D

[jukingeo]

Really good stuff... thanks for the input. I'm familiar with contactors but have never used one.

If I am trying to decide whether to use a fuse or go with a contactor, can the inrush feature on a fluke be trusted? Has anyone had experience with this? I'm not sure I have ever seen a motor's spec sheet with inrush current listed. Besides, as you said, if you put a load on it then everything changes. It would seem the best route to go is to test what you're working with.

Greg

Hello Greg, 

You need to still use a fuse (or circuit breaker) with a contactor.  However, a contactor is for lighting or other AC devices that have a low inrush.  If you are going to use large AC motors, particularly the capacitor start type motors, you will need a motor starter.  The big difference between a standard contactor and a motor starter is the presences of the overload protectors.

First and foremost, what you need to do is determine the size of your motor for your task.   When you determine what horsepower you need, then you need to buy the motor BEFORE you buy the motor starter.   Because there are alot of variables.  For one, the number of phases.  Is the motor a single phase or three phase? What voltage and what current the motor runs on is also important.   THEN once you know the voltage, current, and horsepower, you can determine what type of motor starter you need.

Now again, I am going nuts here because usually anything under 1/8 HP you probably can by with on a regular contactor and this type of motor is still very powerful to operate most large props.   But if you can find a 1/8hp motor starter with protection, that is all the better.  So far the smallest I seen was 1/4 horsepower and up.   Large 2 story web (continuous roll)  printing presses and amusement park rides can hit the 200hp mark.    But as laughable as it may sound, the programmable control circuitry for these large devices are not much different than the Prop-1 and Prop-2 controllers we are programming here.

Now you can take this further and read up on what are called 'soft start' motor circuits.  What this will do is purposely limit the inrush current and gradually increase the current on the motor (and your load) so that it is not slammed with the full inrush.   When you get to motors as large as that 200hp job above, YOU MUST have inrush limiting, otherwise you will constantly be frying your electrical panel and/or you can even damage the motor too.

I will say that if you are using small motors (anything under 1/8th hp) you don't have to go nuts, but it is still a wise course of action to use a contactor to protect the relays on the RC-4.

It all boils down to how much motor power you need.  If you need the big stuff, you can look through any electrical contractors supply house catelog.   Allen Bradley, Bryant, General Electric....all of these companies make contactors and motor starters.

As for calculating for load, on larger motors (over 1/8hp) all the work is pretty much done for you.   For example, I helped someone a few years ago install a new motor on a carousel.  The load specified requested a 10 to 12HP motor.    So we got a 220v three phase 10hp motor.  All we really had to do was get a 10hp motor starter rated for the voltage and current of the motor.  We DID hook it up to a soft start circuit.    The key figure here is the horsepower and your load.

Once you know how much power you need to drive your load (in terms of HP), then you can easily calculate the inrush and sustained motor drive current.  If you feel that your inrush is too much, then get a soft start circuit.

Incidentally, what is it that you are trying to control?  From the sound of your original post, it sounded like you want to control something big.   If that is the case I do recommend a cool reference book for you.  It is an electrician's standard called "Ugly's Electrical References".   If you are doing any kind of electrical work, this book is indispensible.  It is like a little electrical encyclopedia that is small enough to fit in your pocket, yet has all the electrical formulas you could ever want or need.  There is a WHOLE section on motor control as well.  It is a small book and it is very inexpensive.

Here is the link on Amazon:

http://www.amazon.com/Uglys-Electrical-References-Revised-2005/dp/0962322970/ref=pd_bbs_2?ie=UTF8&s=books&qid=1203792352&sr=8-2

Buy it...You will not regret it.  Most of the information I have told you about here I gleened from this book AND this also helped me out with the carousel project I talked about above.

Oh!  And remember, you CAN control DC motors with the RC-4, but you need a rectifier and a filter cap to change the AC to DC and filter out the ripple voltage.

Hope that all helps!

GEo

[Caretaker.CCI]

Wow! Thanks for all the great information. I know that took some time to write all that down (not to mention the years to accumulate the knowledge). I ordered the book and can't wait for it to get here.

As for the running anything big... probably not, but you never know what I might get myself into. The more I learn the more comfortable I feel tackling projects that come my way. The purpose of the question is because I usually just buy whatever I find at the surplus stores (nice and cheap) and you never know what you're going to run across. Besides, I might have a fairly large-scale motor sitting around that could find a new home as a Halloween project. My thoughts were to arm myself with as much information as possible. Safer for me and the people that help me.

Again, thanks for the information. I've learned a lot here. Hopefully Shawn got what he needed as well.

Greg

[jukingeo]

Wow! Thanks for all the great information. I know that took some time to write all that down (not to mention the years to accumulate the knowledge). I ordered the book and can't wait for it to get here.

As for the running anything big... probably not, but you never know what I might get myself into. The more I learn the more comfortable I feel tackling projects that come my way. The purpose of the question is because I usually just buy whatever I find at the surplus stores (nice and cheap) and you never know what you're going to run across. Besides, I might have a fairly large-scale motor sitting around that could find a new home as a Halloween project. My thoughts were to arm myself with as much information as possible. Safer for me and the people that help me.

Again, thanks for the information. I've learned a lot here. Hopefully Shawn got what he needed as well.

Greg

Hello Greg,

I am not an electrician, but I am a technician and I always been facinated with control based systems, hence one of the reasons why I made my way here to EFX-TEK.  I have taken courses in motor control for the aforementioned carousel project and I worked with licensed electricians on that project.  So I do know how to set up a commercial motor control system.

More than likely for haunted house use, you will come across medium current draw type DC motors, which will pose a challenge to solid state relays which are usually AC controlled.   I believe that Crydom does sell DC variants of their solid state relays, but you can't beat the deal that EFX-TEK is offering for both the board AND the relays...that is why I snatched up a couple myself.   Since solid state relays are expensive, you may be better off with a DC conversion.  What you will need to do is if you have four motors, you need an ac transformer that will step the 120volts down to whatever voltage you need and will handle the current for all four motors.  You do have to still keep in mind your total current draw for all motors used cannot exceed the transformer's rating.  Now you need a separate rectifier and capacitor for each motor as you need to do the DC conversion AFTER the SS relays.

Now there is a caveat and that is some AC relays have a certain lower voltage in order to switch.  I seen some as low as 5volts and yet others that require more than 60volts.   I believe the Crydom relays that EFX-TEK sells with the RC-4 has a lower limit of 24volts.   This may pose a problem as most DC motors fall in the 12 volt and under range.  So this means no windshield wiper motors or other 12volt car type motors.

Now you may be getting to think, well, why not use a 120v to 12v transformer for each motor?  Well the problem with that is two fold.  A transformer is seen as an inductor by the SS relay and switching that in and out will carry with it it's own inrush problems.  But even a bigger problem is that now you need FOUR transformers, one for each motor.  Now it no longer becomes cost effective.  You would be better off buying the Crydom DC variants of the relays. 

I am not sure if Crydom does in fact sell a DC variant that will fit the RC-4.  That is something that Jon or someone else may know.

I think that for the most part your best bet is to stick with low powered AC motors.  Fuse everything.   In the case of the RC-4 you would put a fuse on each block as per the relays rating.   If you are using a master transformer for lower voltage, you need a fuse on the primary side of that transformer as well.

To answer another question of yours in regards to measuring peak current draw on a motor under load...well, you can do so, but you need a peak detecting meter so this way you can measure the absolute peak draw.   Another thing you can do is go for an absolute fail safe and measure the current draw on your motor under stall conditions.  That means what current the motor is pulling with full voltage applied and the shaft is prevented from turning.   If your maximum current draw under stall conditions is less than what the Crydom relays are rated for, then you should be good to go.

Now, of course, I am referring to small and medium sized, shaded pole type motors here.  More than likely these motors will pull than less than an amp continuous running.  But the juice these little motors can draw under stall conditions is pretty significant.  So it is something you want to measure.

But by the time you get to large motors such as a 1/4hp (or more) capacitor start motor...you will want to use a motor controller.

Oh!  As a final note...if you are going to mess with 120volt line current, do know what you are doing or get someone to help you.  You are dealing with voltages that can kill at this level, so it does go without saying that you must be careful.  Whenever possible, always turn off the 120volt current when working on the circuit.  TRIPLE check everything before applying power.

Do you have a variac with an AC current draw meter?  If not, get one...you will need it anyway to test the surplus motors you were talking about.

Once you have the variac and are ready to apply power to your circuit, you can bring up the voltage slowly and watch the current draw.  If all of a sudden your current shoots up with only a few volts dialed in on the variac...you have a short circuit.

Variacs can run a bit of change but fear not, there is still a safe way to bring up power to a 120volt circuit.

Read this article:

http://antiqueradio.org/dimbulb.htm

I know this applies to working on old radios, but the concept works for anything you want to test using 120volts AC.  In the case of a short, the bulb sucks up the juice and you don't fry anything.  You still have to fuse things though...so don't skip that step.   At any rate, when you find that you are increasing your bulb size past the point that one of your SS relay's fuses pops...then you are drawing too much current someplace and an investigation is necessary.

Again, when in doubt get help.  Don't assume anything, that is how accidents happen.

Geo

[livinlowe]

Looking at a wire size chart, 20 gauge will handle the max current per channel on a FC-4, but what size do you all recommend? I think 20 gauge would be too small (I've got some from radio shack). 18? 16?

[JonnyMac]

Yeah, 20 is a bit on the smallish side, I'd go with 16-gauge lamp wire.  I also tin the stripped wires to keep frayed ends at bay.