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Residential, lite comm., Industrial
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i cant ''see" anything wrong
however you know it wasnt designed that way
time to get out the ohmeter and find out whether the rest of the bank has good continuity
if one section is burning out, then it is over loaded
this means the rest of the bank is not sharing the load as it should

probably best to replace that section of bank and not just the individual strips

on the other hand: if there are contactors to add or remove portions of the bank depending on the situation
you will also need to check on them for continuity, and coil supply

you or someone will have to go over the system and find the problem,
we are not there and cannot put a meter on it for you
 

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Chief Flunky
Field Service Engineer
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3,569 Posts
Wound rotor or brake system.

Common mistake number 1: cleaning those “rusty” resistors. Stainless steel oxidizes to chrome oxide which is a black colored “rust”. This “rust” protects the metal from the heat. Other than blowing off dust, do not CLEAN resistors.

Common mistake number 2: a section of resistor is damaged so jumpers are added to bypass it. This causes a couple unintended consequences. In a wound rotor motor reducing resistance cuts the motor torque. So now it draws a lot more current to compensate. Also as mentioned current imbalances become a big issue. In a motor we have to derate. A 1% imbalance is acceptable. At 5% we derate by 25%! The principle is the same with resistors.

Common mistake number 3: adjusting running resistor taps to increase torque by decreasing resistance. In wound rotor motors it’s the opposite.

Lots more checking you need to do. When I do PMs on these, I test all the timing relays, aux contacts, contactor coils, contactor contact pressure, contact tips, resistor overall resistance on each phase. Even something as simple as a worn contact tip can cause a major problem.
 

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Wound rotor or brake system.

Common mistake number 1: cleaning those “rusty” resistors. Stainless steel oxidizes to chrome oxide which is a black colored “rust”. This “rust” protects the metal from the heat. Other than blowing off dust, do not CLEAN resistors.

Common mistake number 2: a section of resistor is damaged so jumpers are added to bypass it. This causes a couple unintended consequences. In a wound rotor motor reducing resistance cuts the motor torque. So now it draws a lot more current to compensate. Also as mentioned current imbalances become a big issue. In a motor we have to derate. A 1% imbalance is acceptable. At 5% we derate by 25%! The principle is the same with resistors.

Common mistake number 3: adjusting running resistor taps to increase torque by decreasing resistance. In wound rotor motors it’s the opposite.

Lots more checking you need to do. When I do PMs on these, I test all the timing relays, aux contacts, contactor coils, contactor contact pressure, contact tips, resistor overall resistance on each phase. Even something as simple as a worn contact tip can cause a major problem.
Please elaborate a little on why these resistor banks needed. All new to me so just a little curious.
 

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Chief Flunky
Field Service Engineer
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3,569 Posts
Please elaborate a little on why these resistor banks needed. All new to me so just a little curious.
Depends on what they are for

Almost forgot. Steel flows at around 800-1000 degrees. Stainless service temperature is up to 1500-1800 so it might be normal.

In a wound rotor crane such as a log crane the motors are wound rotor motors. You have a standard AC motor with slip rings. As you add resistance to the slip rings, the torque-speed curve shifts down. Think of it like a manual transmission with 1st through usually 5th gears. They generate some heat but it’s not terrible. They get hot but it’s normal. “Stepping” contactors short out parts of the resistor to increase speed just like shifting a transmission. You can recognize them instantly because of the odd jumpers. You will see say the top of A and B jumpered, and the bottom of B and C poles. A lot of older cranes use this.

The heat is normally minor. The resistance is close to the same as the rotor itself…a few ohms. But you might have a few hundred amps. The voltage is the other trade off. A motor is essentially a rotary transformer and the rotor and stator voltages are almost independent. Just check the name plate.

If you need parts or tech support Post Glover is the place to go. Not cheap but good quality. Many other “brands” including our shop just private labels them.

You CAN also put resistors in series with a motor to control speed and torque. It works but performance is horrendous. I haven’t seen that done in cranes. It’s usually a cheapo fan thing.

Another use is “plugging” a motor. If you are say going forward and suddenly decide to reverse, there is still residual magnetism in the motor. If you do it fast enough you will get very high currents (higher than 10xFLA). There is usually a timer to prevent this (about 0.1-0.2 seconds is enough) and some controllers even insert a resistor to dump the energy faster.

This was a lead in to another type. So with DC and AC drive systems, often you have braking resistors. In a modern AC VFD for instance when the crane brakes, the motor becomes a generator. The VFD will convert the incoming power back onto the power line as much as it can. If it can’t keep up it fires the resistor, dumping the extra energy as heat instead of trying to recover it. The same thing can be done with a DC drive as well. The resistor in this case is optional…the “brakes” just get mushy. This is on a modern or modernized crane.

Performance wise, wound rotor motors are awesome once you wrap your head around them. You can get up to almost 300% of name plate torque out (more in a doubly fed arrangement but that’s not used in cranes) AND all the way to stall (0 speed). It is the same capability as a VFD with an encoder but using 1800s technology. Wound rotor motors were one of Teslas original 3 patents for AC motors, practically the forgotten one. The big difference though is it is almost pure torque control as opposed to speed control. In crane operation acceleration/deceleration control is really what we want and that is what wound rotors excel at doing.

You might occasionally see resistors for closed transition wye delta stuff or something like that but they will be small.
 
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