Think you can measure the voltage and current right at the motor? Think again.
Motor Input Power
When measuring the outrunner motor constants, you might think that you can just hook up a couple of multimeters directly to the motor to measure the voltage and the current. After all, the amount of power being used by the motor is what we are after. Why not measure it directly at the motor?
Well, you cannot. It is very frustrating to me because I like to keep things simple. But there are complications that just get in the way. Let me explain why. It is a little hard to understand, but it is interesting to see how our motors really work.
Three Phase Voltage
Brushless outrunner motors are really three-phase alternating current motors. I’m talking about how they behave electrically. We are all familiar with alternating current as it comes out of a wall socket. This is called single-phase power. Our motors are very similar, except that they use three wires instead of two. The oscillating voltage in each wire is out of phase with each of the other two wires. The voltage is never zero in all three wires at the same time. It is actually a very efficient way to transmit electrical power, which is why it is used for long distance power lines.
If you watch the voltage in an alternating current circuit, it will resemble a sine wave. If it is shaped exactly like a sine wave, then any ac-capable multimeter can be used to measure the voltage. In real life and in our application it probably will not look much like a sine wave. It can be discontinuous, squared-off, or whatever. A “true RMS” multimeter can compensate for this choppiness and still show an accurate reading. So before you even get started, you need a fancy multimeter that uses RMS to measure alternating voltages.
An RMS multimeter could be used to measure the voltage going into one of our motors. You would still need to multiply out the value to account for all three phases, since the multimeter would only be measuring one phase. But it can be done.
Three Phase Current
The real problem is measuring the current. You see, when the voltage goes into the electric motor, the windings develop an inductance. This is when a changing current flowing through an electric wire creates a voltage. Bottom line is that this inductance delays the current surge with respect to the voltage surge.
What this means is that if you try to measure the current at one of the phases of the motor, you will only see a portion of the full amount. This fraction is what is called the power factor. If you knew what the power factor was, then you could use it to compute the actual electrical current value.
Problem is, the power factor changes with the motor and even the speed of the motor. I have seen books that say just to assume that the power factor is 0.8, but in practice that value could be very wrong. No, the power factor cannot be easily measured.
I have seen electrical circuit designs that combine the power flowing through all three phases of a motor into a steady direct current circuit. Then it is trivial to use a dc multimeter to take readings. But I think that building and using a circuit like this is really more trouble than it is worth for most of us.
What to do? I just measure the voltage and the current at the input to the speed control. These are easy to measure direct current values. The downside is that you are now taking measurements of both the motor and the speed control. Modern speed controls are very efficient, so I expect their losses to be very small.
If you are paranoid about it, take measurements of your motor using different speed controls. Then use the values from what appears to be the most efficient one. This is a convenience/accuracy trade-off, and to be honest, I normally do not bother doing it. I just make sure I use a high quality modern speed control before I take my measurements.
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