Kibbles & Bytes Blog

I am heartened by the overwhelming response to the executive order on immigration. Unless you are a native American, you are descended from immigrants. I know I am as my grand parents immigrated to the USA to escape persecution. And do not forget that son of a Syrian immigrant by the name of Steve Jobs! It is wonderful to see the peaceful non-violent demonstrations and the statements by Apple and other businesses including Budweiser who is airing an immigration-themed Super Bowl commercial.

Small Dog Electronics recognizes the intrinsic value of our immigrants and the diversity that makes our country strong and abhors the misguided executive order.

Thank you for reading this issue of Kibbles & Bytes!

Your Kibbles & Bytes Team,

_Don, Emily & Hadley_

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I received an interesting question from a reader after my last article and I thought that I’d share it and explain in more detail here.

The question was about the recommendation that an inverter (or indeed, any power source) have a large amount of excess capacity if being used to run electric motors. For example, if you had an electric motor that required 100 watts, you might need a power source that could supply 300 watts, or more. This oversizing of power sources when it comes to electric motors is almost always true, and the reason why touches on many things I’ve discussed in articles before.

All electric motors (AC-induction or DC-brushed/brushless) share at least one thing: coils. These coils are what get energized by the electricity, thereby creating a magnetic field, thereby interacting with either a squirrel cage, or permanent magnets, and resulting in motion. If you really break it down and unwind the hundreds of feet of wire coil, the actual electric motor circuit becomes counterintuitively simple. It’s quite literally just a really long wire loop that we are passing power through. In the simplest form, there are no components in the circuit at all. No resistors, no capacitors, nothing. It’s just power applied to a wire loop. This is often called a short circuit. When power is applied to this uncoiled wire, there is no resistance (other than the wire itself), so the current can just flow at maximum unimpeded. Assuming an unlimited power source, the wire itself will eventually become a resistor (through resistive heating) once the current becomes sufficiently high, thereby preventing any kind of runaway situation. Nevertheless, the amount of current that initially flows through that wire loop (the first few milliseconds) can be immense. All inductors including the electromagnets in electric motors, are subject to this phenomenon.

But we know that electric motors 1) don’t consume infinite amounts of electricity and 2) don’t get super hot from resistive heating. This is because electric motors are also subject to something called **back-EMF** or **counter-EMF**. The EMF stands for ElectroMotive Force. Once an electric motor is spinning, a secondary electromotive force is created that is opposite in direction to the primary electromotive force (the one created by us applying power to the coils). This counter electromotive force interacts with the coils creating the primary electromotive force thereby putting downward pressure on the amount of current flowing through them. This is what ultimately prevents the runaway situation described earlier. However, this counter electromotive force is only present once the motor is up to speed, and its magnitude is directly proportional to motor speed. So when the motor is at a dead stop and power is applied, at that instant, there is NO counter electromotive force, and the motor/coils experience what is called “inrush current”. All inductive (and often capacitive) devices are subject to this inrush current and many are engineered with mechanisms to limit the amount of inrush current so that nothing is damaged. But they only limit it, they don’t typically eliminate it. Therefore, when you power up an electric motor, it may initially (for anywhere up to a few seconds) require more power (sometimes much more) than it would normally consume. A similar phenomenon occurs (in a more controlled fashion) if a load is placed on the motor causing it to slow down. Less rotor speed = less counter-EMF = more current flowing through the coils = more power being used.

An unloaded electric motor, of any size, only consumes just enough power to overcome the counter electromotive force and the friction of the bearings. You’ve actually probably observed effects of counter electromotive force. If you turn on a large electric motor, such as a drill press, circular saw, etc near an incandescent light, you may notice the light dim very briefly when you turn the device on. This is because the motor is not turning yet, and the amount of power it is consuming is very high, thereby causing a brief voltage drop for lights on the same circuit. Devices with large capacitors such as amplifiers can also experience large inrush current. My friends all used to laugh at my old Pioneer SA-8500 (a mere 60-watt per channel amplifier) because when you turned it on, the lights in the house dimmed briefly. Again, this is because the large capacitors in the amplifier need to fill up, and they do so rapidly.

This is why if you’re using a finite power source, such as an inverter on a battery, you need to oversize it relative to the motor you’ll be powering. You need to be able to supply that extra power initially to get the motor started. This isn’t to say that household power from the power company is infinite, but it’s a lot harder to drain all of the power from a regional nuclear plant or hydro station.