Analogy

 

Some time ago, to help those who had trouble understanding electrical terms and how they applied to the real world, a list of some analogies was put together. These analogies were electrical to mechanical. Gear heads got it right away, others not so fast.

The reason for this started when several newly minted EEs and technicians had some troubles or misunderstandings. An example: A newly hired engineer started work in a test equipment lab. A pilot lamp on the front panel of a piece of test equipment was not lighting. He measured 28volts on both terminals of the socket and thought it should light because it had 28volts on it. The trouble turned out to be a failed driver chip with an open collector output that should have grounded one side of the lamp. A new technician was hired straight out of a two year school after getting an AA degree. He was being shown around the test equipment area and the equipment he would be operating, The techs showed him a power test set which contained two large HP power supplies with volt and amp meters. He looked at the power supplies and said, “Oh VU meters”. The other techs said no they are volt and amp meters, to which he replied, “ volts amps same thing”. Many times newly minted engineers and techs know all of the equations and buzz words like Nyquist, Bode, and vectors but have trouble applying their knowledge to the real world.

Some times the following analogies are not a direct fit but are close enough to get the idea across. Years ago I ran these ideas by Bob Pease  and he thought they were close enough. https://www.autodesk.com/products/eagle/blog/whats-all-this-bob-pease-stuff-anyhow/ 

Here are a few analogies:

Volts → RPM
Amps → torque
Watts → horsepower 746 watts= 1 horsepower

An electrical power supply with no load can output a voltage but no current will flow. An engine with no load can be running and rotating its output shaft, but there will be no torque on the shaft. If a load is put on an electrical power supply outputting a voltage, then current will flow out of the power supply and back to the power supply completing a circuit. A running engine with a load on the output shaft will be putting torque on the shaft. The torque will return to the engine through a common mount or support, completing a circuit. In a car or truck with rear wheel drive, the torque from the drive shaft is returned through the rear suspension to the frame and back to the engine through motor mounts. Check the following video to see a torque return path through a truck frame. Watch the left front wheel. https://www.youtube.com/watch?v=Pe42q8ZmjGU
                 

Electrical to mechanical analogies:


Electrical power supply, Engine or motor.

Resistor, I cannot think of any mechanical device which closely
resembles a resistor. A slip clutch sort of resembles a
resistor but has a high breakaway torque before slipping starts.
and transmit’s the same amount of torque independent of RPM.
so it more resembles a constant current source/sink than a
resistor.

Capacitor, A flywheel is a good mechanical representation of a capacitor.
If an attempt is made to rapidly change the speed of a flywheel,
a large amount of torque would be needed to do so. Just
as changing the charge on a capacitor (voltage) rapidly would
require a high current. Both store energy.

Inductor, A torsion bar. A torque input would wind up the bar, When
Torque transferred through the bar lessened, the bar would
unwind.

LC filter, An engine driving a load would produce slight RPM variations
not unlike the ripple on an AC derived power supply. The power
stroke would produce a slight increase in RPM and the scavenge
and compression cycles a slight decrease in RPM. A torsion bar
on the output of an engine driving a fly wheel would smooth out
these variations in RPM.


Zener diode, A centrifugal clutch as used on mini-bikes or go carts with a
locked output sprocket. No load (torque) would be placed on
the driving engine until the RPM reached the clutch
engagement speed at which time the RPM would be held
constant by increasing the torque to a value greater than the
engine could provide.
Another example would be a constant speed propeller on an
aircraft engine. At low RPM the prop would
remain in flat pitch, putting a low load on the engine. As the
throttle is advanced the prop would remain in flat pitch until
the desired operating RPM is reached, at which time more
pitch would be thrown in increasing thrust and loading the
engine to hold it at the desired RPM.


Shunt voltage regulator, A slip clutch following an engine or
motor.(constant torque source) followed by a
locked centrifugal clutch. (RPM limiter).
See “resistor” and “ Zener” above.


Differential amplifier input, A car or truck rear-end with a two gear rotation
reversal gear box in one axle. That is CW in
CCW out. The two axles represent the differential
inputs and pinion shaft the output. If both
axles are rotated at the same speed in the same
Direction, the pinion shaft will not move.
(common mode rejection). If axels are moved
In any direction at different speeds, the pinion
shaft will move. (output due to differential
input). By the way, Bob Pease liked this
analogy and remarked on how good the
common mode rejection ratio would be.


Notes:

When referring to “engines” I mean piston engines. Steam ,gas, Diesel. Not turbine. When referring “motors” I mean electric.

These analogies are just to get ideas across and are not to be taken as absolute equal values except the watts to horsepower comparison. So please be kind in your criticism of this page.

If anyone has any other ideas of electrical to mechanical analogies please let me know. I may be reached at prarie.grass@yahoo.com

 

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