Engel fridge current draw

Submitted: Wednesday, Mar 10, 2004 at 11:46
ThreadID: 11136 Views:6146 Replies:3 FollowUps:6
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Hi all,
I have ben checking my old 15 litre yellow case Engel fridge and have found that the current draw is rather higher than I expected.
When first switched on at 30C room temp, 11.8 Volts at the fridge, the current is about 5.3 amps. Later when the inside was at fridge temp, not freezing, the current was over 6 amps.
Once the fridge was cool, with 4 litres of water in bottles inside, the on time was about 45 sec and the off time over 10 minutes at 20C ambient so average current is low.
The specification plate says 4.5 Amp at 12 volt
Has anyone else measured these sort of figures?
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Reply By: joc45 - Wednesday, Mar 10, 2004 at 12:19

Wednesday, Mar 10, 2004 at 12:19
My first Engel was a '70's version, and it drew over 6A. But the draw current of the fridge will depend on a lot of factors, such as ambient temp, input volts, temp draw-down, etc. The figure quoted is nominal.
However, what the meter reads may not be a true reading. Most digital and analogue meters measure Average current and are calibrated to read out the RMS value. This calibration only applies if measuring either pure DC or a sine wave. When you put a pulsed wave through the meter (as the Engel does), it may read a different value. The only way to get a proper reading is to get hold of a "True-RMS" meter, which actually calculates the true value. The more expensive digital multimeters may have this feature, and it will be stated on the front of the meter.
AnswerID: 49767

Follow Up By: Fishin' Dave - Wednesday, Mar 10, 2004 at 15:26

Wednesday, Mar 10, 2004 at 15:26
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 >
FollowupID: 311586

Follow Up By: joc45 - Thursday, Mar 11, 2004 at 00:09

Thursday, Mar 11, 2004 at 00:09
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.
FollowupID: 311684

Follow Up By: V8troopie - Thursday, Mar 11, 2004 at 01:24

Thursday, Mar 11, 2004 at 01:24
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.
FollowupID: 311691

Follow Up By: joc45 - Thursday, Mar 11, 2004 at 17:26

Thursday, Mar 11, 2004 at 17:26
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..
FollowupID: 311762

Reply By: Fishin' Dave - Thursday, Mar 11, 2004 at 06:14

Thursday, Mar 11, 2004 at 06:14
I didn't relaise they had an inverter in them - I assumed they used the 12DC to drive a small compressor.

What is your opinion of the Peltier effect fridges as compared to a compressor fridge for efficience and current draw?

I recently bought a small generator which advertised that it is a modified sine wave generator. This confuses me - surely it is a modified square wave inverter.
My understanding is that it's easy to create a square wave but to make a sine wave you have to 'modify' a square wave.

Thanks for the info.

Dave >
AnswerID: 49892

Follow Up By: joc45 - Thursday, Mar 11, 2004 at 16:39

Thursday, Mar 11, 2004 at 16:39
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.

FollowupID: 311749

Reply By: Fishin' Dave - Friday, Mar 12, 2004 at 08:07

Friday, Mar 12, 2004 at 08:07
Thanks Gerry - very informative.

I could never understand the term 'modified sine wave' when refering to a generator output - to create the output they modify a square wave.
I suppose the output looks like a sine wave that has been 'modified' and that's why they call it a modified sine wave, but it would more sense to me if it was labeled as a modified square wave.

Cheers mate.

Dave >
AnswerID: 50047

Follow Up By: joc45 - Friday, Mar 12, 2004 at 22:59

Friday, Mar 12, 2004 at 22:59
Yeh, either way, it's modified. A lousy sine wave or a lousy square wave. Reckon "modified square wave" is closer to the mark.
FollowupID: 311951

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