Resistors

[John Szegda]

Question,

Last week we had gusting winds. My controller was operating as designed, but the dump load resisters were hot and stinking up my garage. I have a 2KW Exmork Turbine and I am using two 2000w 90RJ resisters in parallel. I am not familiar with Ohm capacities and whether adding additional resisters of the same type would solve the problem.

John

[Rob Beckers]

John, no, you can't just add two more resistors. The overall resistance has to stay the same; adding two resistors in parallel will halve the overall resistance, adding them in series will double what you have now.

Can you measure the resistance value of what you have with a multi-meter? The "90RJ" probably means 90 Ohm, it would be a good idea to be sure though. Assuming 90 Ohm, that makes 45 Ohm for two in parallel. To generate 4kW that Exmork must be producing 420 Volt DC!

Alas, I can't think of an easy way to just add a few resistors and keep what you have. IMO it's easier to replace the resistors with (multiple) others of the right value to collectively make up the same resistance value. Another idea could be to aim one or two small fans at those resistors, maybe let them come on based on voltage, or just when the dump load is switched on. Increasing airflow keeps the resistors much cooler.

-RoB-

[John Szegda]

Rob,

I tested the resistors and got 96 Ohms each. I wired them in series, hopefully that will cure the problem. If it does not work i'll try the fans. I bought two Turbines and they came with one 90 Ohm resistor each. I tought two would give the Inverter better protection.

John

[Rob Beckers]

John, just realize that by putting them in series they now put only 1/4 of the load on the turbine (ie. your dump load is pressing the 'brake pedal' at only 1/4 the force of what it was doing before). The dump load is there to keep the turbine RPM under control, if the load is insufficient, your turbine will run away.

-RoB-

[John Szegda]

Rob,

I'm confused and misunderstood what you were saying. So I would assume by alleviating a resistor and only using one as designed, the system should work fine.

John

[Rob Beckers]

John, resistors are like water hoses, each hose presents a resistance to the water flowing through: Put two of them in parallel and twice as much water (energy) will flow through in the same amount of time. Put two in series, and only half as much water (energy) will get through for the same pressure (voltage).

Case 1: You had two resistors in parallel: Their total resistance will be 1/2 the resistance of each, or 96/2 = 48 Ohm.

Case 2: You had two resistors in series: Their total resistance will be 2 times that of each, or 2 * 96 = 192 Ohm.

Case 3: Now you have just a single resistor, so that's 96 Ohm.

For each of these cases, and assuming the dump load comes on at 420 Volt, here is how much power is dumped:

Case 1: Power = 420 * 420 / 48 = 3,675 Watt

Case 2: Power = 420 * 420 / 192 = 919 Watt

Case 3: Power = 420 * 420 / 96 = 1,838 Watt

"Power" here is the load that the turbine sees, or in other words, how hard the "brakes" are put on it. Too little and it'll run away, blow the inverter, and ultimately self-destruct, too much and your alternator burns up.

-RoB-