Making accurate measurements of the current drain for marine motors is the subject of this tech tip. A lot of hobbyist are baffled by the results they get when they attempt this kind of measurement. In the paragraphs that follow I will try to explain why such measurments often fail to give accurate results.
Before we start, I would like to say a few words about test equipment. The basic piece of equipment most hobbyist own is the digital multimeter or DMM. This is a meter which can measure volts, amps and resistance (or ohms) and is a necessity if you are doing R/C modeling. Those of you who have been in the hobby for a long time may have the older analog-type meters.
Most DMMs have the ability to measure currents up to 10 amps. The Fluke model 70-2 (priced at about $100 US) is an example of a good quality DMM with a 10 amp scale. Usually the meter has a separate connector for this current range. If your motor is small and your battery voltage is high (12 volts for example), you may get a accurate reading by a direct measurement as shown in the diagram below. The motor should loaded as it will be under the final operating conditions (I will explain more about loading later).
If we try using this setup with a larger motor and/or a lower voltage we may have problems. The most common problem is that the motor will not start. To understand why this happens, consider the way a DMM measures current, which is to pass current through a known resistor inside the meter and then measure the voltage across this resistor. The problem with most DMMs is that the value of this resistor may be quite large, around 1/2 ohm. Now consider a typical hobby motor like a Mabuchi 380 connected, direct drive, to a shaft and prop (to use an example from my own personal experience). This motor normally draws around 4 to 5 amps when running at 7.2 volts. However, when the motor is first starting to turn, it needs a brief burst of current about 3 or 4 times the normal operating current to get started; thus we require a burst of current in the range of 12 to 20 amps, just for a fraction of a second. But with the 1/2 ohmm resistance of the meter in series with the motor, the maximum current we can pull is ( using Ohm's law, I=V/R ) 7.2/.5 = 14.4 amps. if the motor needs 20 amps, it will not start. If you are using a larger motor like a Mabuchi 540 or 550 you will need even more current to get started.
Let's say, for a moment, that we can get the motor started. We look at the DMM and it is reading 4 amps. With 4 amps going through a .5 ohm resistor we will have a voltage drop of ( Ohm's law again V = I*R ) 4 * .5 = 2.0 volts across the DMM resistor and so our 380 motor is only getting 7.2 - 2.0 = 5.2 volts instead of 7.2 volts. So now we have a measurement, but it's lower than the real current because the voltage is too low. But, some will argue, when I install the speed control there will be some voltage drop across the control. True enough, but the resistance of a control like the SC-5 (at full throttle) is much lower than the DMM, around .1 ohms, and so the voltage getting to the motor will be higher.
One obvious solution is to use a better meter. A good indication that a DMM has a very low internal resistance is if it has a 20 amp maximum current scale rather than a 10 amp scale. After my problems with measuring the current on the 380 motor, I bought a much better meter, a Metex M-3650. I did some tests on this meter and found the internal resistance on the 20 amp scale was about .05 ohms, about the same as a SC-8 control. I don't recommend going out and buying a Metex because I heard the company (in Korea) went out of business. B&K makes a DMM called model 388A with a 20 amp scale for about $150 (US).
If you are using large, high performance motors such as the Graupner Speed 700s, a DMM just will not cut it. What we need in an industrial-strength solution. This problem is not new to the electronics industry, so there are a couple of ways to get around it. The simplest way to do it is to buy a Hall-effect current probe. This probe plugs into your DMM and you clip it over one of the motor leads. It measures current by indirectly, by measuring the strength of the magnetic field produced by the current in the wire, no voltage drop at all. The problem with the Hall-effect probe is that it costs a lot more than most of us want to spend (about $300 US).
Alternativly, for about $30 US, you can buy a Bach-Simpson high-current shunt. These shunts, about 3" long, are 2 large blocks of metal (with screw terminals for connecting wires) connected by a smaller block of metal, all mounted on a bakelite base. When you see one you will understand why they don't build them into DMMs. The model 6709, a 50 amp shunt is the easiest to use, because these are 50 millivolt shuts and every amp going through the shunt produces 1 millivolt. So we just connect a DMM to the shunt as shown below and measure the millivolts. If the DMM says 12 millivolts, then the current is 12 amps, no math required.
Now a few words about loading. For most motors, the current draw will vary widely depending on the loading of the motor. This effect is greatest in direct-drive situations, if you are using gear or belt reduction, the gears (or belt) tend to isolate the motor and reduce the effects of loading. To get an accurate measurement the motor should be connected to the shaft and prop. Also, the prop should be spinning in water not air. Furthermore, the deeper in the water the prop goes, the more current the motor draws. What I usually do is this: set up my test equipment next to the sink or bathtub, depending on the size of the boat, and push the stern deep into the water while running the test. This way I get a "worst case" current. If you change to a larger prop you can assume the current draw will increase so you may want to do the test again.