We see our charger in many fields across the country and one thing we notice is that the wattmeter is still taped up or has wire nuts and is not set up for use. While you may not think of a wattmeter as an everyday gadget you would be surprised how we end up using ours almost every day. Especially when you are the person helping your buddies out at the field. Below are some questions that your Xtrema's wattmeter can answer almost every day you fly.
Note: If you need a basic understanding of volts amps and watts please read our short Basics of Electrics (without Math) first.
http://www.tmenet.com/pdf/Basics%20of%20Electrics.pdfWhich pack did I just use? Ever fly and take out your battery, start talking to your friends and then notice you laid it in your battery box along with all your other fully charged batteries? The wattmeter is actually a voltmeter and ammeter that multiplies the two together to give you watts. A voltmeter with a battery connector is what you need in a situation like this. Quickly test and determine which packs are low and you know the ones you used.
Why are my batteries or speed controller getting hot to the touch! Batteries and speed controllers get hot for basically 3 reasons. You may not have proper ventilation, You are drawing too many AMPS, or the internal resistance has gone up. How can you tell which one it is? I thought you would never ask. Lets say you have a 1,000mah battery with a discharge capacity of 20C continuous or a speed controller rated 20 amps. That means it should be able to deliver 20 AMPs solidly and not get too warm with adequate airflow.
OK so now we hook up the wattmeter on a freshly charged battery and we measure at full power running wide open.
If you measure under 20 amps and your voltage is staying up nicely, then most likely you don't have adequate ventilation or some transistors have opened up in your speed controller causing the internal resistance of the controller to go up.
If you're under 20 AMPS but your running battery voltage is only a little over 3 volts per cell then this is a significant voltage drop that tells you the internal resistance of your batteries is very high which signals the end of their life for running high powered motors. ( they are probably still good for low current applications around the house )
If you are drawing OVER 20 amps then either your prop is too big in pitch or diameter or your motor may have some shorted windings.
Why does my controller keep cutting off?This question is similar to solve as the one above. Your speed controller should shut off when your cells drop to below 3 volts per cell. For example a 3 cell pack should cut off when the pack voltage goes below 9 volts. Lets assume a 3s - 1,000 mah 20C battery like the example above.
If your current is under 20 Amps but your voltage is going below 9 volts then your battery is shot! It can no longer maintain a voltage below the max specified discharge rate. If however your battery is producing way over 20 AMPS and the voltage starts out high but quickly drops to below 9 volts then you are destroying your battery with an excessive load. If this is the same prop you have used for years most likely your motor has a short. If not reduce the size of the prop to what the manufacturer recommends for the number of cells you are running.
But what if your voltage is nice and high and your amps are nice and low and your motor still wants to quit? This usually indicates a stalled servo pulling excessive current from speed controllers that have built in BEC, ( battery eliminator circuit) to power the receiver and servo, is overheating and cutting off. Check to see if you have a binding servo or one that feels hot. Sometimes a bad gear in the servo can make a servo start drawing ridiculous current. Especially a digital servo gone bad.
Which is the best prop for my plane?This question is always answered by flying the plane. But when you make a change to the prop you are making a change to the power that you are forcing the motor to produce. Electric motors are willing servants. Just because they are rated at say 50 watts doesn't mean they can't produce 75 watts! (for a while!) Put a big prop on a glow engine and it may not even start. Do the same thing to an electric motor and it will fry its brains trying to turn that big prop.
So while your are playing around with prop sizes do your motor a favor and check to see that the combination does not exceed the watt rating and current rating of the motor. As you exceed the current rating (even if you are within the watts), you are forcing the motor into an area of inefficiency and this inefficiency will cause the motor to run hot.
So what do you do if you find two props that fly your plane perfectly but one draws 20% less current than the other? Well that is a perfect display of how a wattmeter can help you spot the more efficient propeller. If you go with the prop that draws 20% less current you may get about 20% more flight time and all your components will run cooler and more efficiently which means a LONG life.
Is my new airplane configuration going to damage my batteries or speed controller?This is the number one reason to use a wattmeter. At least this one time just before the maiden flight. Why? You have spent good money and time building your model and you have gone with all the recommendations or maybe only one slight modification and it would be a shame that a faulty component or poor substitution will create a surprise landing or possibly damage your battery, speed controller or motor.
So what do you check for? Here is a check list...
Is the current at WOT below battery C rating times its mah?
Is the current at WOT below max rating of ESC?
Is the current at WOT below max rating of motor?
Is a fully charged battery staying well above cell count times 3.0 volts?
Is the power enough for the type of flying you expect to do?
- Powered Glider / Scale - 50 watts/pound
- - Slow Flyer - 70-90 watts/pound
- Warbird / Sport Aerobatic - 90-110 Watts/pound
- Aerobatic - High Speed 110-130 Watts/pound
- loaded 3D / Racer - 130-150 watts/pound
- Jet / Unlimited 3D - 150-200 watts/pound
Your actual results may be a little above or below the recommendations above but you should not be off very far from the expectation that you have for performance.
