My first semester of physics was a disaster. I really didn’t get it. The basic forces of motion made sense, but I always had difficulty solving the problems. I was interpreting all sorts of other issues that weren’t really solving the problem in question.
A major lesson that I came away with was imparted during one of my attempts to repeat the course with a passing grade. (It took me 3 tries before I passed) “Always pay attention to the units of measure when you are trying to analyze a problem”. In the problem solving arena, trying to figure out the relationships is sometimes a little easier when you just look at the units of measure.
In energy its really important to understand the units of measure. Electrical power is measured as Watts. Watts can be used as a measure of electric light, electric heat or any form of energy which can be directly derived from electricity. As we seek to conserve power it is important to consider how efficient it is to convert electricity to another form of power.
The often ignored unit of measure is time. It is especially important in energy measurements because the ratings of equipment can vary dramatically between starting conditions and running conditions. Most motors and heaters have a significant inrush current when they are first switched on and then settle over time. The ratings need to be examined more closely to get this information.
So a 100 Watt light bulb left on for 10 hours is one kilowatt hour. The cost of a kilowatt hour varies around the US, but here in Texas its around 11 cents. A computer with a high resolution video card can use 400 Watts. In this case we are exploring the “dollars per kilowatt hour” for various appliances. A large refrigerator can use 500 Watts, which is substantially better than older units with ratings over 1000 Watts. And a central air conditioner rated at 5 tons can run as high as 17,585 watts or $2.00 an hour when running at full load.
When comparing electric motors, regardless of type, Wattage and duty cycle are consistent units of measure for the power that can be produced by the motors. In this arena what is really being measured is the amount of work that can be performed within the thermal limitations of the motor and drive. This is really important to keep in mind because at the end of the day it’s all about the amount of work required and the cost of the solution. The units of measure might be $/kW or $/inch pound of torque.
When comparing motor and drive hardware from various suppliers, make sure the units of measure are the same (which they usually aren’t) and the thermodynamic basis for the ratings is similar. The exact size of the cooling plate, what material its made of and how long the motor is run for thermal testing is all part of the rating system.
Some years ago I was confronted with a torque comparison between a 4″ servo from one vendor that claimed to put out the same torque as a 5″ servo from another vendor. After considerable effort, an associate came up with the real answer. The smaller servo was rated at a thermal limit 50 degrees higher than the larger one. Although ratings are generally not so disparate in today’s market, there is still plenty of variation that has to be considered. So make sure you know the units of measure.