Electric Flight in Australia

 

Using meters in electric flight

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How can I use meters to understand more about my electric model by Phil Connolly

The performance of an electric motor depends on two main factors – the voltage applied to the motor (largely a function of the number of cells) and the current or amps consumed (a function of the gearbox ratio and propeller size)

Without knowing the voltage and current you will be ‘flying in the dark’, especially if you choose to experiment in the search for performance improvements. The performance subject is addressed in another ‘answer’ in this Website, but here we will give guidance on how to use meters to see the light!! Some of the many uses of meters are:
o To learn whether the motor is operating below, at or above its optimum current – this in turn allows you to decide on alternative propeller sizes
o To uncover unexpected or unacceptable voltage losses which will reduce the power from the motor, or , worse, cause actual damage
o To check the state of your battery pack – for example, if the model no longer climbs as well as previously, is it the battery pack , the switcher or the motor which needs attention (or replacement)
o To check the likely motor run time from the battery pack and whether the current is above recommended limits for that pack

It is possible to buy a ready made high current ammeter or even a special wattmeter for reading power directly. There is also a device known as a clamp meter which allows measurements of currents without breaking the electrical circuit (see elsewhere). However, such units are somewhat expensive and an alternative is given below

Many hobby and electrical outlets sell good, but economical digital multimeters at prices around $20 and purchasing one of these is well worthwhile for the performance improvements which can eventuate and, possibly, even savings in motor, switcher or battery damage

We suggest you will need a multimeter with at least the following ranges for direct current (DC):
o 2 volts for measuring individual cells
o 20 volts for measuring pack voltages
o 200 millivolts for chasing losses
o 10 amps for small motors and for calibrating your own high current meter – yes, it is actually very simple to convert your new multimeter to measure much higher currents with sufficient accuracy

Building your own high current ammeter shunt


Our ‘shunt’ will simply be a length of wire sufficiently thick to carry the maximum motor current which we plan to use. The voltage loss along this wire will be used to indicate the current and will be measured using the 200 millivolt range of your multimeter. If you are totally new to things electrical, then ask a friend with a basic knowledge to give you a hand. Better still, find such a friend who also wants a high current ammeter and is prepared to buy or loan you their own multimeter while you make up two such high current shunts.

You will need:
a spare plug/socket of the type you plan to use to connect your battery pack and the speed control unit
a car headlamp bulb of at least 60 watts
up to a metre of wire with a cross section of 1 – 2 sq mm. – about 14 - 16 swg wire
a soldering iron or crocodile clips and a couple of dressmaking pins
a pair of lightweight leads with a plug on one end to fit your multimeter

What to do:
1. Set the multimeter on its 10 amp scale and make sure that the leads are plugged into the correct multimeter sockets (if all else fails, read the instructions which came with the multimeter!)
2. Connect a car battery or other 12 volt source in series with the wire, the bulb and the multimeter. That is to say, connect the length of wire to, say the red positive terminal of the car battery, and the other end to the bulb.
3. Then connect the other side of the bulb filament to one lead of the multimeter (on most multimeters it doesn’t matter which way around you connect them, but you may get a negative reading – this is fine).
4. Connect the other lead of the multimeter to the black negative car battery terminal, having first placed the headlamp bulb in a safe spot where it will not burn or touch anything
5. The headlamp bulb will now light up and the meter should read about 5.0 amps. This will depend on the wattage of the bulb (and whether you have selected the main or low beam filaments).
6. Make a note of the reading, eg 5.2 amps.
7. Now disconnect the meter from the circuit, replacing it with a second piece of similar wire so that the bulb once again lights up. Plug the meter leads into the sockets for measuring voltages and set the meter to the 200 millivolt scale.
8. Push the two dressmaking pins through the insulation of the wire which runs to the positive of the car battery, such that they make contact with the copper inside. Try placing the pins about 15 cms apart to start with and measure the voltage with the multimeter. Suppose the reading is 3.6, you will need to move the pins further apart and re-measure until you get the same answer which you had in step 6 above.
9. If the reading is too high, then move the pins closer together to give that same figure, eg 5.2. Congratulations – you have now calibrated a shunt which will allow you to use your 200 millivolt multimeter range for measuring currents up to 199 amps.
10. For the technically minded, what you have done is to create a length of wire with a one milliohm resistance. All that remains is to ’tidy it up’ for convenient use later
11. If you have the use of a second multimeter, then leave one in the circuit at step 7 above, but proceed with steps 8 – 9 with the second meter. You will be able to read both meters simultaneously for a slightly more accurate result
12. Cut off your calibrated shunt just beyond each of the two pins and connect male and female plugs and sockets to each end. If you use gold bullet connectors, only the single shunt wire will be needed; if you use Deans ULT, Astro zero loss or other twin connectors, then you will also need a second wire to connect the other side of the plug. In the latter case, it is imperative that you connect the +ve of the male plug to the +ve of the female socket and similarly for the two negatives. Crossing +ve and –ve wires will almost certainly damage the speed controller
13. Your spare lightweight leads with the multimeter style plug on one end can be soldered to the shunt at the exact, final, position of the dressmaking pins. This will overcome any problems with pins pulling out of the shunt

Measuring and Using Motor Currents

This should normally be carried out by connecting your shunt between the battery pack and speed controller. In the case of brushed motors, you can also connect the shunt between the controller and motor, but do not attempt this second technique with brushless motors

Taking all normal precautions and ensuring that the extra wires are safely out of the way of the propeller, turn the motor on with the radio control and watch the reading on the 200 mV multimeter scale. With a freshly charged Nicad pack, you would expect the current to drop several percent over the first few seconds and then to reach a steadier value. The number you read will be the current in amps if you have carried out the calibration successfully.

Divide the capacity of your battery pack (in milliamp-hours) by this current and multiply by 3.6 to give an approximate motor run time in seconds. Eg with 1300 SCR and 20 amps expect 1300 / 20 * 3.6 = 234 secs. At high currents, most cells will deliver less than their rated capacity. On the other hand, as the model picks up speed and the battery voltage falls, the current will reduce, tending to return the motor run time back to your original estimate.

Many manufacturers quote a maximum current and a current for maximum efficiency for their motors. Let us suppose these figures are 30 amps and 15 amps respectively. If you measure your current at less than 15 amps, it would be well worth considering a larger propeller size for improvements to both motor and propeller efficiency. Needless to say, any maximum current quoted by a manufacturer should be considered. Sometimes, these are given for continuous and sometimes for short periods such as 10 seconds. They also vary according to the degree of cooling available and the ambient temperature

You should also compare the rating for the speed controller with your measured motor current. Most controllers come with a continuous and a short term rating. Your measurements will help you decide how long a motor run to use and whether you should use a different propeller size.

Measuring and understanding battery voltages

Now that you know the motor current, you can get a good idea as to whether your battery pack is performing well. Measure the battery voltage by choosing the 20 volt scale on your multimeter and connecting one lead to each side of the battery pack – if possible, directly onto the battery cells or onto a dressmaking pin pushed through the wires as close as possible to the cells. The voltage you read with a fully charged pack will depend on the number of cells, the current and the internal resistance of each cell

This last figure is a little elusive and variable, but for a 1300SCR pack, you could use 8 milliohms, for a 1700CP cell, use 6 milliohms and for 2400RC cells 4 milliohms. If you have a 10 cell 1700CP pack, then the total resistance is about 6*10 = 60 milliohms. Multiply this by the current, say 20 amps and you get 60 * 20 = 1200 millivolts or 1.2 volts lost. Now you should expect about 12.2 volts from a 10 cell Nicad or NiMh pack, less the 1.2 volts lost giving 11 volts. If you have much less than this figure, then you have cause for further investigations. Perhaps one cell has failed, or the battery was not fully charged?

If, however, you draw a much higher current of, say, 50 amps, then similar arithmetic shows that you should only expect 9.2 volts from the 10 cell 1300SCR pack – quite a large difference!

Use the 2 volt scale on your multimeter and those two dressmaking pins as probes to measure the voltage of each cell in turn, preferably whilst the battery is delivering a current. Your 12 volt car headlamp bulb is a suitable load for a battery pack of up to 10 Nicad or NiMh cells, but don’t leave it connected for too long a time, or you will flatten the battery pack completely, possibly causing damage

Chasing lost voltages is covered in the ‘How do I improve the performance section’, but, hopefully by now you will understand many of the advantages of knowing what your powerplant is really doing and know what to expect!