VFD brake chopper and resistor wire sizing

[Swan]

Looking to get some help on determining wire size for a vfd brake chopper unit and resistor Bank

The company I work for ordered a large drive as a replacement for one that got damaged but the drive they ordered needed a brake chopper unit where the previous one only used the resistor Bank with number 12awg wires feeding it.. would I use the original wire sizes because the motor did not change ??...its is a 600 volt 3 phase 100 horsepower 97amp but the new drive they ordered is double the size therefore 600 volt 200 amp and a brake chopper unit to be paired with it....long story short ,because of the drive being double the size will I need to accommodate that in calculating the new wire size for the chopper unit and resistor feed?? The Manal calls for fuse protection and a way bigger wire size assuming we are driving a 200hp motor...
Thanks.
Resistor bank value is 10.4ohms 8746watt

 

[paulengr]

Looking to get some help on determining wire size for a vfd brake chopper unit and resistor Bank

The company I work for ordered a large drive as a replacement for one that got damaged but the drive they ordered needed a brake chopper unit where the previous one only used the resistor Bank with number 12awg wires feeding it.. would I use the original wire sizes because the motor did not change ??...its is a 600 volt 3 phase 100 horsepower 97amp but the new drive they ordered is double the size therefore 600 volt 200 amp and a brake chopper unit to be paired with it....long story short ,because of the drive being double the size will I need to accommodate that in calculating the new wire size for the chopper unit and resistor feed?? The Manal calls for fuse protection and a way bigger wire size assuming we are driving a 200hp motor...
Thanks.
Resistor bank value is 10.4ohms 8746watt

The drive will work. So the drive itself just has a 7th transistor connected to the resistor. When the DC bus voltage gets high, it turns on the 7th transistor. It is common to adjust the brake resistor size based on how much braking you need. The only limitation is that your brake resistor amp rating can’t be too large or the brake transistor can’t take it. The fuse is there to protect the transistor. Smaller won’t hurt anything.

 

[petersonra]

Looking to get some help on determining wire size for a vfd brake chopper unit and resistor Bank

The company I work for ordered a large drive as a replacement for one that got damaged but the drive they ordered needed a brake chopper unit where the previous one only used the resistor Bank with number 12awg wires feeding it.. would I use the original wire sizes because the motor did not change ??...its is a 600 volt 3 phase 100 horsepower 97amp but the new drive they ordered is double the size therefore 600 volt 200 amp and a brake chopper unit to be paired with it....long story short ,because of the drive being double the size will I need to accommodate that in calculating the new wire size for the chopper unit and resistor feed?? The Manal calls for fuse protection and a way bigger wire size assuming we are driving a 200hp motor...
Thanks.
Resistor bank value is 10.4ohms 8746watt

8746 Watts @ 10.4 Ohms is about 30 Amps.

 

[Swan]

The drive will work. So the drive itself just has a 7th transistor connected to the resistor. When the DC bus voltage gets high, it turns on the 7th transistor. It is common to adjust the brake resistor size based on how much braking you need. The only limitation is that your brake resistor amp rating can’t be too large or the brake transistor can’t take it. The fuse is there to protect the transistor. Smaller won’t hurt anything.

Okay that make sense but I'm trying to figure out the wire size... original setup had 12awg running to resistor right from the vfd without a chopper. Now they bought a chopper because new vfd requires it. According to the manual they want 250kcmil...that's huge.

 

[topgone]

As mentioned above, your braking amps depend on the amount of braking power your VFD is required to provide. That 250 MCM thing is possibly the conductor size required for the motor phase conductors, not the braking resistor lines.
#12 can withstand a continuous current of 25 amps. But since the braking duty will only last for a few seconds, you can choose a brake conductor with a lower current carrying capacity conductor. Most VFD braking resistors are sized to 20% duty with full motor torque. Heavy-duty braking resistors are rated at 50% at 150% motor torque. That is why your brake resistor and lines are only required rated to handle that amount of brake current. At a brake duty of 50%, your brake resistor conductors are required to carry about 35 amps! Your old #12 wire when serving a 50% duty can carry 25A/sqrt(0.5) = = 36 amps!