# How to size water heater element for VFD DC brake



## bloomingtonmike (Oct 18, 2013)

How do you size a water heater element to use as a VFD DC brake? I have read 500watt and 50 ohm resistors from engine block heaters can be used but water heater elements are 1400 watt 220v and measured 8 ohm on my meter.


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## stupoty (Oct 18, 2013)

Might be easier with an electric bar heater element as you could cut the element untill you got the correct resistance.  Although they mau actualy be too low.  I'm sort of presuming the water heater element is enclosed in somthing.

I have been thinking of using electric lamps , has any one tried that?

Stuart


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## Rbeckett (Oct 18, 2013)

Bloomington,
The idea of a water heater element sounds pretty good, and you could plum it into your flood coolant system to help keep it warm and active if you using a bio cleaner like Ozzy clean.  Some of the brew sites I go to often use systems like this to cook wort while making beer.  Could you please explain exactly what you are trying to do so that other members might also consider whether they  would like to add one to their machines also.   Also the coils are considered Ultra low energy density so there is less likely to burn through and expose live voltage to the coolant.  Thanks!!!

Bob


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## Tony Wells (Oct 18, 2013)

Braking resistors aren't really my thing, but wh elements are inductive, meaning the DC resistance isn't what determines their actual current draw. 8 ohms is pretty low and could draw considerable current, depending on the voltage. I am making the assumption that the braking current would be DC, but I do not know that for a fact. IF you can use a standard 50 ohm resistor, you should be fine as long as you stack up enough elements to get to that resistance (or close......6X8=48). With dissipation capability of 1400 watts each, they probably would not even get warm in that scenario. I'd probably put them in a water bath anyway, if I was using the lathe hard. It would take quite a bit to heat a 5 gallon bucket of water that way.


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## bloomingtonmike (Oct 18, 2013)

Tony Wells said:


> Braking resistors aren't really my thing, but wh elements are inductive, meaning the DC resistance isn't what determines their actual current draw. 8 ohms is pretty low and could draw considerable current, depending on the voltage. I am making the assumption that the braking current would be DC, but I do not know that for a fact. IF you can use a standard 50 ohm resistor, you should be fine as long as you stack up enough elements to get to that resistance (or close......6X8=48). With dissipation capability of 1400 watts each, they probably would not even get warm in that scenario. I'd probably put them in a water bath anyway, if I was using the lathe hard. It would take quite a bit to heat a 5 gallon bucket of water that way.



I have a Hitachi WJ200 2.2kw VFD. I am using it with a large Millport 1630 CNC Mill. The spindle motor is a large 3hp 60hz monster. The drive actually stops the motor fairly fast at least in 7 seconds at 3000rpm, but it over voltage error tripped trying to do it in 2 seconds. You can add DC braking modules to this series of VFD to improve the decel time. I have read of folks who do not want to spend $200 on the hitachi setup that have used engine block heaters and water heater elements. The water heater elements being cheaper than the engine heater blocks (unless I have to buy a few water gheater elements). I thought if I used one I would just suspend the element in the air by mounting one of the wrenches for them - then slip the element in the wrench and the nut would suspend the element. Had not thought of doing that in the coolant but that is a good idea too. 

Really thinking I will just return it though and buy a 300watt engine heater block - they are even magnetized to mount it somewhere.

Here is one:

http://www.fleetfarm.com/detail/Kat...000000047506?gclid=CMTkt6fjoLoCFQhgMgodRVUA2g

Autozone has them for order and possibly for store pickup as well.

Goes without saying - you do not plug these into 120V LOL and your VFD has to have the capability of accepting a DC Brake resistor directly (not all can - some require a braking module before the resistor).


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## DMS (Oct 18, 2013)

I have the same VFD (well, a smaller version), and have thought of doing the same thing. I would expect that element to have a resistance closer to 30 ohms based on the specs, but resistance increases with temp, so maybe it is lower while "cold". 

From the reading I have done, the main thing you need to do is ensure that the current flowing into your braking resistor doesn't exceed the capability of the braking transistor in the VFD. the WJ200 ships with a shorting bar in the "braking resistor" position, and monitors the voltage across there, and presumably the current too. If it gets too high, it faults out, so it should be pretty safe to expreiment. When in doubt, going with a higher resistance is going to be "safer", but will result in a slower stopping time.

One option would be to just wind your own resistor using NiChrome wire. This stuff should be about 1 ohm per linear foot

http://www.mcmaster.com/#8880k77/=ozqamf

Another thing to keep in mind is that the voltage feeding into the braking resistor is going to be high (2-400V), so make sure your connections are well insulated electrically as well as thermally.


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