Engel fridge current draw

Hi Gerry

Sound like you know a lot about it - would you be able to explain that:

"When you put a pulsed wave through the meter (as the Engel does), it may read a different value."

I understand your comments on the ambient temp, temp draw-down and supply voltage affecting the current draw but how and why does the fridge put a "pulsed wave" on the supply voltage.

Thanks mate.

Dave >

Simple; the Engel uses a 50Hz 12v to 20v square wave inverter to drive the swing motor. This involves a transistor switching circuit, which by design switches the 12v alternately in pulse form. No filtering is used on the primary side of the inverter, hence a ripple in current will be present on the 12v DC line.
As an example, some users of 2-ways will testify to a "hum" on their transmitted audio when the Engel is running, caused by this very pulsed DC current.

If you measure the current with an old fashioned 'pointer' meter then the pulse frequency is too fast for the pointer to respond and you get a fair average current reading.
The swing motors compression is very sensitive to input voltage, you'll find they work a lot more efficient while the battery gets charged from the car's alternator than when running on battery alone. The sound of the fridge 'hum' changes noticeable on my old Engel 15l fridge when the battery gets charged.
For that very reason I would never buy another Engel, they are too tough on batteries. Positive displacement (rotary or piston) compressors are the way to go for car fridges.
Klaus

Klaus,
it's a bit more than just the needle being too slow to follow the pulse frequency; both the old analogue (pointer) meters and the cheaper digital meters measure a mathematical Average of the waveform, which, in the case of a sine wave, is only slightly different in value to RMS value (RMS=Root Mean Square, and is effectively the value used to calculate true power in a circuit). This average reading is corrected in the factory calibration of the meter offset to the RMS value, and the meter then reads true rms for sine waves only. When a waveform other than a sine wave (eg, a square wave) is measured, the meter no longer reads true rms.
True-RMS meters sample the waveform and do a mathematical transfer to come up with the real value.
I'd suggest that the fridge mfrs use the RMS value of current in their specs.
Re the change of the fridge "hum", as supply the voltage increases, the piston may have a slightly longer throw, changing the sound. It may also bring the fridge up to the optimum gas pressure in the system, improving efficiency. (the throw of the piston in the swing motor is not constant, like a rotary compressor, and can vary depending on the gas pressure and the input voltage)
Re the efficiency of Engels (I own a Danfoss-compressor fridge) they are still pretty good, esp the latest models. If the insulation was better, I'd probably buy one. Horses for courses..
Gerry

The swing motor in the Engel is just a piston driven back and forth by a "voice coil" and permanent magnet assembly, with coil springs centering the assembly. The piston mass and spring tension are set up to resonate at 50Hz. The coil requires about 20v AC to drive it. The earlier Engels used a simple square wave inverter to give the 20v AC, but I'm unfamiliar with the latest dual voltage cicruit. The one transformer did both mains stepdown to 20v and 12v up to 20v.
One advantage with the Engel is that it does not require high currents to get the compressor up over compression from a cold start, as the piston starts off with a small oscillation, getting bigger with time, till gas pressure is up in the system. A disadvantage is that if the gas is lost, the compressor can destroy itself (expensive).
Re Peltier-effect devices, they only achieve a max of about 30degC temp differential, and with rather poor efficiency, compared with compressor fridges. So if it's 40deg outside, then it's 10deg inside the fridge. The efficiency is generally limited by the hot side of the peltier junction moving heat back to the cold side (unavoidable), and the more heat which can be removed externally (usually by the fan), the better the efficiency. Peltier junctions can be cascaded to improve the temp differential (and are in special applications), but the efficiency is very low by this stage, making it pretty unsuitable for general purpose fridges. Stick to your compressor fridge.
Re the generator, if it is advertised as modified square wave, then it most likely has a DC output, like a car alternator, which feeds an inverter. The reason for doing it this way is to reduce the size of the alternator, since a true 50Hz alternator would require much more iron in it, and would have to run at a fairly accurate multiple of 1500rpm. The modified Sq wave alternator would allow the use of a light small-capacity motor to run at fairly high revs.
A "modified sine wave' is a 50Hz square wave with steps in it (wish I could draw a pic), making it less of an absolute square wave, but not a true sine wave, and is cheap and simple to achieve. Sine wave inverters (modern ones, anyway) use a series of high frequency pulses, all the same amplitude, but with varying width, to give an average output forming a sine wave after filtering. Much more circuitry rquired to do this. Both modern modified sq wave and sine wave inverters use a high frequency (20-100KHz) invertor as the primary source, significantly reducing the core size of the step-up transformer required, compared with old designs which ran at purely 50Hz.

Gerry

Yeh, either way, it's modified. A lousy sine wave or a lousy square wave. Reckon "modified square wave" is closer to the mark.
Gerry