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I have a photovoltaic power system and an inverter with an overall power rating of 1500 Watts. I want to connect a dishwasher which consumes 2200 Watts when heating the water.

Question: What is the best way to reduce this maximum power rating of the dishwasher? Is it possible to somehow (how?) use a voltage divider or a simple resistor in series with the heating coil?

I assume the only drawback of this idea would be that the washing takes longer, as the water needs more time to heat up.

PS: I have asked this question (in a more general manner) again and got some nice answers and no downvotes.

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    \$\begingroup\$ Why the downvote? \$\endgroup\$ – erik Sep 12 '14 at 11:25
  • \$\begingroup\$ Ok, I will ask the question again without the words dishwasher and so on. \$\endgroup\$ – erik Sep 12 '14 at 21:10
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You don't. Get a dishwasher that can run from 1.5 kW, get a larger solar power system, or wash your dishes by hand.

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  • \$\begingroup\$ +1 Washing dishes by hand seems like the most efficient solution in a power constrained situation. \$\endgroup\$ – PeterJ Sep 12 '14 at 14:17
  • \$\begingroup\$ The solar power system is big enough. But the main 3 kW inverter is broken and my temporary borrowed inverter is to weak for our dishwasher. I cannot imagine that there is no way to reduce the current flowing through a heating element. \$\endgroup\$ – erik Sep 12 '14 at 21:08
  • \$\begingroup\$ -1 as this is no answer to my question. And as you can see in the answers to my other question there were some easy solutions to this problem. \$\endgroup\$ – erik Sep 20 '14 at 8:12
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You would have bigger problems than you might think.

  1. A resistor that could make a difference would have to be laid out for high current and high power, else it would burn out in short order.
  2. Heating elements don't like being operated below their specified voltage (which is what a series resistor amounts to.)

I have lived in house where the line voltage was 20% too low (90VAC instead of 110VAC) and have seen electric ovens and dishwashers burn out the heating elements, as well as motors (water pump for the well.) The low voltage also caused the electric meter to burn out one memorable night in the middle of a Louisiana summer, after which the utility company finally fixed the low voltage problem.

The only way I can see to do it right would be to replace the heating element in the dishwasher - or get a new dishwasher that uses less power.

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  • \$\begingroup\$ While I don't have a problem with most of what you wrote, I don't see how lower voltage will negatively affect a heating element. Higher voltage can greatly reduce the life. \$\endgroup\$ – Spehro Pefhany Sep 12 '14 at 12:42
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    \$\begingroup\$ Most folks don't believe it happens. Having seen it, and cussed over getting the heating elements replaced, I can say this: When the voltage is too low, the elements don't heat as fast as they should. Since the resistance of the element depends in part on the temperatur, they end up drawing a higher current for a longer time. This usually causes the connectors to burn out. The current isn't as high as the normal inrush with the proper voltage, but it lasts much longer. That's what I could puzzle together after seeing it happen. I may be wrong on the details but I do know that it happens \$\endgroup\$ – JRE Sep 12 '14 at 12:52
  • \$\begingroup\$ Doesn't make a lot of sense to me- in another life I headed a company that distributed industrial heaters (among other things) including lots of calrods with the faston terminals. Running them at less than rated RMS voltage was the norm. But I'll certainly file it as an anomaly based on experience.. \$\endgroup\$ – Spehro Pefhany Sep 12 '14 at 14:25
  • \$\begingroup\$ As you can read in the answer to my other question, I’ve found a solution to this. And I will report, if it really will destroy my heating coil (in the two or three months period I will use this setup) – which I doubt! \$\endgroup\$ – erik Sep 20 '14 at 8:14
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There's no easy way in general to reduce the power of a heating element that would be guaranteed not to negatively affect your inverter.

AFIUI, heating elements in a consumer dishwasher are only used to dry the dishes, so you could disconnect the heating element and use a towel. Edit: As Olin points out, the element has a dual use in heating the water during the wash cycle so you'd get inferior cleaning if you disconnect the heater.

They are calrod swaged technology, so another possibility would be to have a custom element made, but as the watt density of calrods is fairly fixed it might not be possible to get one of the same dimensions, and in any case the drying cycle would be fixed and insufficient for a low power heater, so the dishes are going to be wet either way.

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    \$\begingroup\$ Some dishwashers actively heat the incoming water for part of the cycle. This can possibly be avoided by feeding it sufficiently hot water in the first place. However, the hot water temperature in a dishwasher is usually significantly higher than normal "hot" water faucet temperature in a house. \$\endgroup\$ – Olin Lathrop Sep 12 '14 at 13:14
  • \$\begingroup\$ @OlinLathrop Thanks, Olin, that makes sense- the same element is used for both because it's immersed in the reservoir at the bottom. When the water drains it heats the air. Still might be worth a try disconnecting it- however the dishes will not be as sterilized as with the heater operational. \$\endgroup\$ – Spehro Pefhany Sep 12 '14 at 14:22
  • \$\begingroup\$ -1 as there are some easy ways, as seen in the answers to my other, more general question. \$\endgroup\$ – erik Sep 20 '14 at 8:05
  • \$\begingroup\$ Let us know how your inverter deals with a dimmer load at that current level - I reckon even chance of it failing. \$\endgroup\$ – Spehro Pefhany Sep 20 '14 at 15:47

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