When Will Power Engineering Finally Get Some Respect?
Electrical engineers generally get respect from the public, but it seems to me that some EE “types” get more than others. Designers of low-power systems get kudos from reviewers, “Can you believe what they did, it runs for weeks on a tiny battery!” The same goes for engineers who do most or all of their work sitting at a keyboard, “See that kid, she’s a coder, so she’s set for life!”
While I do have a bias towards hardware (aka circuitry) engineers, I recognize that a project usually needs a wide set of technical skills. So, OK, I’ll give those micropower and keyboard engineers their due recognition.
Still, there seems to be an as of yet largely unseen class of EEs which gets little attention or acknowledgement for the challenges they face: those who work in the higher power regimens above several hundred volts and amps, and tens of kilowatts. Some may argue that’s because these applications are at a distance from the public, but that’s not the case at all. These higher power applications are not limited to those which are relatively removed from the consumers, such as industrial settings or even 25 kilovolt (kV) catenary lines for electrified trains.
Consider the electric vehicle (EV) with which many consumers have had direct or at least indirect exposure. EV battery packs range in energy capacity from about 25 kilowatt-hours (kWhrs) to over 70 kWhrs, and these packs can deliver 300 volts to 400 volts at about 1000 amperes (A) (those traction motor sets can provide up to 300 horsepower (HP) or more). Any of those numbers – energy capacity, voltage, current – means that the EV power pack, conversion, management, and distribution are pretty serious issues when it comes to design, test, and maintenance.
The difference between these design environments is not just a matter of raw numbers or numerical scaling. Instead, it takes an entirely different mindset and approach to doing anything and everything in the high-power world. In low-power design, it’s no big deal to try a temporary hack such as moving and spot soldering a wire or running a quick impromptu test to check out an idea. But when you are dealing with those higher power levels, every action has to be planned, simulated, evaluated, assessed, and double-checked before anything is done. There’s a significant amount of densely stored energy to be managed here.
There’s also the issue of testing. Every aspect of determining what the system is doing and the impact of any changes needs a carefully constructed test plan and arrangement. There’s no quick clipping of some digital voltmeter (DVM) leads to the points of interest. Even a routine requirement such as measuring current via an inline shunt requires careful consideration of components, interface circuit, galvanic isolation in many cases, and even the implementation of the physical connections.
Consider this scenario: you’re planning to use a shunt resistor to measure the current in a high current conductor. While that’s a well-known technique, you’re looking at hundreds of amps in an EV, so you need to keep the value of the shunt resistor as small as possible to reduce both IR-induced voltage drop, as well as I2R thermal dissipation of the sense resistor.
Luckily, there are standard shunts available with extremely low resistance values. For example, the Vishay Dale WSBS8518 family has standard ratings of 100, 500 and 1000 µΩ (that’s just 0.1, 0.5 and 1.0 mΩ) (Figure 1). The shunt, a deceptively ordinary looking metal “strap” measuring about 85 mm long × 18 mm wide, is made of solid metal nickel-chrome alloy with a temperature coefficient of resistance (TCR) as low as ±10 ppm/°C.
Figure 1: This microohm (µΩ) range shunt resistor may look simple compared to other electronics components, but it’s a carefully engineered and manufactured piece of solid metal nickel-chrome alloy with extremely low temperature coefficient and Kelvin contacts. (Image source: Vishay/Dale)
But how do you physically connect this resistor to the load lines? After all, even a few milliohms (mΩ) of contact resistance will dissipate power and drop voltage, so the assembly of the shunt connections is yet another design issue. Also, you still have to connect the voltage sensing leads; fortunately, this particular shunt has integral Kelvin contacts to make that task somewhat easier; many shunts do not.
It’s not all “power engineers” who get little respect; I think it is mostly electrical power ones who face that issue. With all the interest in the 50th anniversary of the Apollo moon landing, it was beyond amazing to see the liftoff thrust developed by the five F-1 rocket engines which powered the first stage of the Saturn liftoff vehicle (Figure 2).
Figure 2: There’s less visible power and very visible power; the Saturn V launch vehicle with its five F-1 engines is definitely in the latter category. (Image source: NASA)
The numbers tell it all yet are hard to actually grasp: the Saturn V first stage carried 203,400 gallons (770,000 liters) of kerosene fuel and 318,000 gallons (1.2 million liters) of liquid oxygen. Each F-1 fuel pump was driven by a 55,000 horsepower turbine to deliver about 15,000 US gallons (just under 60,000 l) of kerosene per minute while the oxidizer pump delivered 25,000 US gal (94,000 l) of liquid oxygen per minute; each turbopump also had to withstand input gas at 1,500°F (820°C) to liquid oxygen at −300°F (−18 °C). At liftoff, the five engines produced 7.5 million pounds of thrust.
Just think of the fixturing needed to hold those F-1 engines in place at the test stand, or the holding clamps which kept the Saturn on the pad after ignition while the rocket motors came up to full power. Not only did they have to hold back the millions of pounds of thrust, but they also had to release smoothly and consistently despite the exhaust environment (and how do you test that?).
I think the very visible power of a rocket, whether the launch is successful or not, gives rocket engineers the respect they deserve. However, since electrical energy is less “visible,” electrical power engineers don’t get that respect. That huge tail of rocket exhaust makes it all so real, while electrons in a battery pack are silent in normal operation and thus seem like “no big deal.”
Will EEs who deal with these higher power levels get more respect in the future? I don’t know, of course. But it would be nice because mass-market applications such as EVs, solar power, and a smarter grid will need that kilowatt and megawatt expertise.
1 – Roger E. Bilstein, “Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles” (free downloadable 168 MB file available here; free chapter-by-chapter download here)
2 – Charles Murray & Catherine Bly Cox, “Apollo: The Race to the Moon”
3 – Wikipedia, “Rocketdyne F-1”