What is the purpose of a burden resistor in a circuit? My WAG is the term burden is a synonym for load so it’s intended to be a dummy load for some reason. I was replacing a DC drive and the manual says the resistors are sized according to the FLA of the motor but doesn’t say why. At first I thought they were pull-down resistors because of how they’re connected but they’re clearly labeled as ‘burden’ on the PCB. I don’t recall coming across this term before.
Could it be a load for the motor when the motor is spinning faster than the setpoint? For example, when you suddenly lower the setpoint speed?
I wish I could remember the size of them, there are two on the board. They are power resistors, probably 20 k-ohm or so but not big enough to handle the full motor current. The motor is 15hp so fairly large amperage.
Just a guess here: sometimes you have a circuit that, under certain circumstances, will have all loads shut off. In that case, you have a short circuit connecting the hot and ground ends of the power source. That would be bad. So a “burden” resistor is added to the circuit to guarantee that the total resistance never goes below some safe amount, thus avoiding sparks, fires, explosions, and other sub-optimal events.
Like I said, though, this is just a guess from a mechanical engineer…TRM
Not a bad guess at all, especially on a DC drive where there’s always a lot of inductive kickback when the field collapses.
I was about to ask what TRM stood for :smack: …HKF
20KΩ is too large to dissipate significant power (even at 120v, 20k will only dissipate .72W).
The only use of the term “burden resistor” I’ve heard of is the resistor used to generate a voltage from a current transformer. 20K might be a reasonable value in that case.
Ditto, but 20k seems rather high for a CT application. Burden resistors on the CTs we use here range from about .04 to .1 Ohms; these are monitoring circuits drawing up to ~830 amps with CT ratios ranging from 250:5 up to 800:5, depending on the maximum circuit current rating.
That’s what it is in this case too, thanks. There are two CTs monitoring the armature and shunt currents and the troubleshooting chart lists the burden resistors as probable sources for false overcurrent trips. I had never heard the term before (that wasn’t the problem with this board I just found the term intriguing). I’m sure you and Q.E.D. are correct about 20k being too big, I just swapped the resistors from the old board to the new one I didn’t pay close attention to the values since there were other issues to deal with.
Could they have been 20 milliohms? Were their leads doubled at each end (Kelvin wiring)?
The resistor sizes are 13.8Ω and 45.9Ω. I just took a guess initially since I couldn’t open up the drive while the line was in production. I found a better manual and one is for armature feedback and the other is for the overcurrent trip. They’re both monitoring the armature current.
Not doubled no. What’s Kelvin wiring? There’s another new term for me. Are they doubled so the induced field from the current in each leg will cancel out or something?
Kelvin (AKA 4-wire) connections are used to measure very small resistances accurately or to tightly control small circuit resistances by compensating for wiring resistance variances. It uses two wires for each lead of the resistor under test such that the wiring resistances cancel each other out leaving only the test resistance.
And, yeah, those values are more reasonable for low- to medium-current CT applications.
No, Kelvin (or “4-wire”) resistors are designed for precision applications. In addition to the normal two connections a standard resistor has, there are also two “sense” connections right at the point where the wires attach to the resistor body. This allows accurate measurements, since one isn’t measuring the resistance in the wires. You often see this with high-current shunts, where the voltage drop across the leads can be quite significant.
Thanks, I’ve seen it done but hadn’t heard the term for it before.
Glad you like it. Kelvin was a fascinating guy, who insisted the Earth was no more than maybe a million years or so old, who invented a zillion things, who lies buried now next to Isaac Newton. Seems there are a few fields where his name is attached to one of the basic ideas.
More precisely, the point in precision resistance measurement is to use separate leads to measure the voltage across the resistant element and to drive a current through it. Which leads do which job doesn’t usually matter, and they are often colored or marked the same way. For example with the highest precision platinum thermometers, there will be two black wires connected to one end of the element and two red ones connected to the other. In the case of high current shunt resistors, where the leads have to be fat and expensive just to carry the current without overheating, they will only make one set that way, and let the others be thin, but it’s just to save the extra expense where it is not needed. High precision resistors can be bought with 4 leads, and even when the currents are tiny, this can be worthwhile if (as is usually the case) the leads are copper, which has a resistivity about proportional to the absolute temperature. High precision resistors are made of special alloys with very small thermal coefficients of resistance (Manganin is an older example, and Vishay’s Z Foil is a pretty new one). With the copper leads included, the overall performance has a significant TCR, and its magnitude depends on where along the length of the lead you connect to it. 4 lead resistors fix that problem completely.
In some of the nicest places, they measure resistance with DC, first one direction and then the other, and average the two. This cancels out whatever thermoelectric effects there are. (Whether to say it is DC that is driven one way and then the other way, or call that AC, is debateable; but the instruments may use a DC power supply and an H bridge or even reed relays to connect one way, take a measurement, and then connect the other way, and take a second. Most folks call that DC.)