Hello,
I'm sizing a motor control system. While trying to select a correct DBR I received some recommendation from a supplier. The recommended DBR is very different from the minimal requirements coming from my calculations. I'm also confused by the method of calculation specified by Rockwell (PowerFlex Dynamic Braking Resistor Calculator - Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017).I would be very thankful for your comments and advises.
The motor is 150 HP, 1200 RPM, 447TC FRAME, 460/3/60HZ.
- The selected VFD is PowerFlex 753 AC Drive, with Embedded I/O, Air Cooled, AC Input with Precharge, no DC Terminals, Open Type, 302 Amps, 250HP ND, 200HP HD, 480 VAC, 3 PH, Frame 7.
- Total inertia of the load is 84 kg/m2.
- Max.speed 1200 rpm.
- Min.speed 0 rpm.
- Deceleration time - 11 sec.
The specified VFD has a DC bus regulation voltage of 790 V and Minimum brake resistance of 2.4 Ohm.
According to Rockwell method of calculation, the peak braking power for 11 sec. deceleration time is 120.6 kW. Based on this, the Maximum allowable resistance satisfying the DC bus regulation voltage of 790 V is 5.2 Ohm. The required joule rating of the resistance in order to stay this power (120.6 kW) during this time (11 sec.) must be at least 663.2 kW*sec. Once again, I did the calculation according to the Rockwell manual mentioned above.
The supplier proposed me a resistance with the following characteristics:
5.07 Ohm, 15.7 kW, 294 kW*sec. With this proposed resistance, I'm noting several problems regarding this offer:
1. The joule rating, 294 kW*sec, is much lower than required, 663.2 kW*sec, which means that the resistance will not be able to dissipate the heat, it will basically burn out.
2. The proposed resistance will be overloaded during about 9.5 seconds of 11 seconds of the deceleration time. The load will count about up to 768% of the rated power of the resistance descending linearly during the deceleration time. I have a doubt about the ability of the proposed resistance to stay this rate of overloading.
Am I right in my calculations and conclusions?
Finally, I wondered, according to which value should I specify the size of the cables to for the DBR. If I calculate it as a ration of DC bus regulation voltage of 790 V divided by the resistance of 5.07 ohm, it will result to 155.8 A, which means that I have to use 2 AWG single wire cable.
Is it correct calculation?
Any advice or recommendations would be appreciated.
Thanks.
I'm sizing a motor control system. While trying to select a correct DBR I received some recommendation from a supplier. The recommended DBR is very different from the minimal requirements coming from my calculations. I'm also confused by the method of calculation specified by Rockwell (PowerFlex Dynamic Braking Resistor Calculator - Rockwell Automation Publication PFLEX-AT001L-EN-P - September 2017).I would be very thankful for your comments and advises.
The motor is 150 HP, 1200 RPM, 447TC FRAME, 460/3/60HZ.
- The selected VFD is PowerFlex 753 AC Drive, with Embedded I/O, Air Cooled, AC Input with Precharge, no DC Terminals, Open Type, 302 Amps, 250HP ND, 200HP HD, 480 VAC, 3 PH, Frame 7.
- Total inertia of the load is 84 kg/m2.
- Max.speed 1200 rpm.
- Min.speed 0 rpm.
- Deceleration time - 11 sec.
The specified VFD has a DC bus regulation voltage of 790 V and Minimum brake resistance of 2.4 Ohm.
According to Rockwell method of calculation, the peak braking power for 11 sec. deceleration time is 120.6 kW. Based on this, the Maximum allowable resistance satisfying the DC bus regulation voltage of 790 V is 5.2 Ohm. The required joule rating of the resistance in order to stay this power (120.6 kW) during this time (11 sec.) must be at least 663.2 kW*sec. Once again, I did the calculation according to the Rockwell manual mentioned above.
The supplier proposed me a resistance with the following characteristics:
5.07 Ohm, 15.7 kW, 294 kW*sec. With this proposed resistance, I'm noting several problems regarding this offer:
1. The joule rating, 294 kW*sec, is much lower than required, 663.2 kW*sec, which means that the resistance will not be able to dissipate the heat, it will basically burn out.
2. The proposed resistance will be overloaded during about 9.5 seconds of 11 seconds of the deceleration time. The load will count about up to 768% of the rated power of the resistance descending linearly during the deceleration time. I have a doubt about the ability of the proposed resistance to stay this rate of overloading.
Am I right in my calculations and conclusions?
Finally, I wondered, according to which value should I specify the size of the cables to for the DBR. If I calculate it as a ration of DC bus regulation voltage of 790 V divided by the resistance of 5.07 ohm, it will result to 155.8 A, which means that I have to use 2 AWG single wire cable.
Is it correct calculation?
Any advice or recommendations would be appreciated.
Thanks.