Using a wattmeter

Anyone researching electric flight for the first time can’t help but notice the advice to buy a wattmeter which appears in so many magazine articles and forum posts aimed at beginners. Let’s look at what they do. 

  Quite simply a wattmeter is an electrical measuring instrument that is connected within a DC electrical circuit and measures and displays the current flowing in the circuit and the voltage present. It also multiplies the voltage and current together to present the watts consumed by the circuit. Watts are a measure of electrical power and 746 of them are equivalent to one horsepower.

  A wattmeter has two connections, one for the source which is usually the flight battery and one for the load, which is usually the ESC (because this is effectively the rest of the circuit). Here we hit our first snag. 

When you buy a wattmeter it will almost certainly be supplied with bare wires so you will need to supply your own connectors and solder them onto the wires. Since there is little or no standardisation of connectors within electric flight then we can’t really blame the wattmeter manufacturers for not supplying their products ready to use. A wattmeter supplied with Deans type connectors would be of little use to a modeller who used XT60s after all. Most flyers, however, do tend to standardise on a particular type of connector for their batteries and ESCs so it makes sense to fit the equivalent connectors to your wattmeter for ease of use. 

  You may well find that you occasionally use different connectors for other projects so it’s a good idea to make a conversion lead for both ends of the wattmeter that has your standard connector for connection to the wattmeter on one end and the different connector on the other. This will allow you to easily connect up the wattmeter to your circuit and measure what’s going on. 

  When soldering connectors to the wattmeter or making conversion leads do be especially careful to respect the polarity of the connections. (i.e. the “positive” and “negative”). Get these the wrong way round and at best your wattmeter won’t display any data, at worst you might irreparably damage it.

Our wattmeter now has the appropriate connectors fitted so lets use it and do some measuring. Before we do however a quick word of warning – spinning propellers hurt if they hit you. When using a wattmeter by definition we will be running the motor up to and including full power. This means that the propeller will be spinning at several thousand rpm and will simply disappear. You will also be concentrating on the figures on the wattmeter and this makes it all too easy to put a hand or a finger into the propeller, so do be careful and also make sure that there are no trailing wires that can catch in the spinning prop. 

  It is also essential that the model is adequately restrained during the motor run and you should monitor the wattmeter from behind the propeller. 

  So, let us begin. As mentioned earlier the wattmeter has a connection for the load and for the source. Connect the wattmeter to the load first (usually the ESC) and then the freshly charged battery (source). 

  At this point the ESC should beep to say it is now armed. Your model is now ‘live’ and ready for testing. The wattmeter will show the battery voltage and quite possibly a very small current drain (0.1A or so). This is the current being drawn by the ESC and the BEC (assuming you are using one) which is effectively the current being drawn by the electronics in the model. 

  Now assuming this is the first run of the power train in a new model then you should have to hand, or at least knowledge of, the maximum current that both the ESC and the motor will tollerate. Slowly advance the throttle and the motor will start to spin and the figures on the wattmeter will change; the voltage will reduce slightly and the current will start to rise and the power this represents, in watts, will also start to increase. Continue to open the throttle and keep monitoring the wattmeter. You will notice that the voltage will tend to stabilise whilst the current and watts figure continue to rise as the RPM increases. 

  If at any point whilst opening the throttle you notice that the maximum current of either the motor or the ESC is being exceeded then immediately close the throttle and abandon the test (this is why you should know what the max current figures for your motor/ESC are). 

  Something is wrong with your set up and if you continue then either the ESC or the motor will burn out; in all probability the reason for the overload is that the propeller you are using is too large for the motor/battery combination and you should switch to a smaller propeller before re-testing. 

  Assuming the current is within the capabilities of your power train then continue to steadily open the throttle until full power is reached. Allow the system to stabilise for 20 or 30 seconds and make a note of the numbers displayed on your wattmeter, specifically the Voltage, Current and Power (in Watts). 

  If you are fairly new to electric flight then I suggest that you try varying the throttle and observing how the current drawn changes with the speed at which the propeller is turning. You will note that at low throttle settings the current is extremely low…a few amps at most. Open the throttle further and you will see the current rise sharply. The current will rise at a faster rate as the throttle is steadily opened. Once you have finished experimenting you should close the throttle and disconnect the wattmeter from the circuit. The test is at an end. If this is your first electric set up you might like to swap the propeller for a different size. Repeat the above and again observe how the current changes with motor speed. Be aware that the current can rise alarmingly quickly with larger propellers so keep an eye on the current to avoid overloading your motor and/or ESC.

* Is the maximum current less than the maximum allowable current for both the motor and the ESC? I’m a big fan of running components well within their rating and would always like to see currents not exceeding 90% of the motor or ESC maximum limit.

* How does the ‘under load’ voltage compare with the rest voltage of the battery? Assuming you are using LiPos then the under load voltage should never be less than 3V per cell and ideally not much under 3.5V. If it is then your battery is simply not capable of supplying the current you are demanding of it either because it is past it’s best or you are asking more current than the battery is capable of supplying (i.e. you have exceeded it’s C rating). There is a judgement to be made here however; if you are using short bursts of very high current (say in a hotliner during the climb phase) you may find this dip in voltage acceptable and choose to live with it in the interests of your high performance aircraft. Pushing your batteries beyond their limit will inevitably shorten their life however.

* Is the power produced what you expected and/or wanted to fly the aircraft? If so then congratulations! You can fly and enjoy your model secure in the knowledge that all the powertrain components are working within their limits. If not then it’s time to try another prop and repeat the test.