October 23

RC Brushless Outrunner Motors Testing – Wattmeter

The Essential Digital Power Meter

Digital Wattmeter
Digital Wattmeter

A “Wattmeter” is an invaluable, and inexpensive piece of measuring equipment. It is the best way of ensuring that your rc brushless outrunner motors and associated electric power train is correctly matched.  All of the components within an electric power train (Motor, ESC & Lipo) are designed to operate within specific parameters, and we need to ensure that our system is going to produce the power we are looking for without exceeding the upper limits of these parameters.

In order to test your setup before you commit the model to flight you need to have some method of checking the various power consumption components. This is exactly what a Digital Power Meter or Wattmeter does.

Using Your Wattmeter

Here’s how it works. You will have seen in the main website under “Installing Electric Motors” how the motor, battery and ESC all connect up.

The Wattmeter is temporarily inserted between the battery and the speed controllers’ main cables and will provide you with a readout of exactly how much Current your power train is consuming, how many Watts of power the system is providing, and also how many Volts the battery is maintaining when under full load.

watts up meterIf you look at the image on the right you will see that the meter is reading a Voltage of 11.93V. This probably tells us that a 3cell lipo battery is supplying volts to the circuit.

Next we see that the current being consumed is at 2.87 Amps. So if we multiply 2.87 x 11.93 we get a value of Watts = 34.2391 Watts. Our meter only has three digits to display the watts so it gives us 34.2W. this is more than adequate for our purpose.

You can ignore the Ah figure at this stage but for the inquisitive amongst you, this is the value of current being consumed each hour and is determined by the length of time this current has been drawn from the power system.

Let us  quickly look at the circuit that is used to take these measurements.Wattmeter Diagram

This simplified diagram shows the supply from our Battery and the load which comprises the ESC and Motor. The current (A) in Amps is measured in the positive line. The Voltage (V) is measured across the positive and negative lines. The product of these two values appears as W (Watts). All of this takes place inside the Wattmeter and is shown digitally in the display.

A Practical Exercise

Normally the rig is secured safely on the bench or in the plane, and the throttle is advanced (ensure you stand behind the prop / fan and that if the worst happens and the prop flies off, it will not damage anything).

The following diagram shows the correct position of the Wattmeter in relation to the other components.

Wattmeter circuit

Let the motor run for around 30 seconds or so, and make a note of the figures mentioned above as shown on the wattmeter display.

Current (Amps) =

Volts (V) =

Watts (Volts x Amps) =

The figures are likely to fluctuate a little during the test, but you will get the trend. One of the main things this will show for you is the power (Watts) that the set up is producing. You will see that I have given you the components that need to be multiplied together to give the Watts

Watts are derived from multiplying the Volts of the battery (under load) by the Amps consumed.

W =V x I (I is the symbol for current, or amps).

The more Volts you have the higher the Watts or the more Amps you have the higher the Watts.

Selecting A Propeller Using Your Wattmeter

Assuming that the battery you select is not likely to be changed for a higher voltage version, then changing the propeller is the biggest single factor in affecting the current that the motor will consume.

Let’s say your motor is designed for a 6 X 4 prop, on the chosen battery, but you want to try and get a little more thrust, or climbing performance from your setup. With the meter in circuit, you could now fit a slightly larger diameter propeller, and monitor the current being drawn.

Now you can watch the display and see the Amps that the motor is pulling and the Watts being produced, and therefore ensure you do not “over prop” the motor to the point where the maximum figures allowable are exceeded.

These maximum figures are also applicable to your battery, and your speed controller, and indeed exceeding any or all the limits of these items could prove very expensive, or even dangerous.

Saving Money

The modest cost of a suitable digital power meter or wattmeter will be recouped the very first time you use it and discover that your current consumption and resultant power figures are beyond the maximums allowed for your power train. Without it you could be in for a great deal of heat and smoke and a major replacement expense for your rc outrunner motors circuit components.

You can obtain a Wattmeter on-line by clicking this link:-

RC Wattmeter (USA) or RC Wattmeter (UK)

This is probably one of the most important posts I will offer, especially if you intend to fly electric planes. I hope you have found it useful and will share it with others. Please don’t forget to visit my website: www.rookiercflyer.com for everything you need to know about starting up with RC model planes.

Be safe and Enjoy your flying.

Colin

 



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Posted October 23, 2015 by Colin Bedson in category "Electric Flight

2 COMMENTS :

  1. By coughlanmaureen on

    I found your post on the wattmeter very informative. You explained the technical workings of the wattmeter very well so I understood what it does after reading your post .

    I was very surprised that it cost so little I thought it would be very expensive well worth the money in my opinion.

    Reply
    1. By Colin on

      Thank you for your kind comment. I am glad that I managed to help you understand its purpose. For what it costs it is a very affordable accessory.

      Colin

      Reply

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