As part of a HVAC system we have a supply fan and extract fan running on independent VFD1. With the current configuration we usually don't need to run the return fan as the supply fan pressurises the room adequately to force air out through the return duct and room is at the required pressure. So far, so good.
The control system ramps down the return fan speed to control room pressure and so we're mostly running the return fan at 0 Hz. We are wondering what effect this will have on the drive.
Variable Frequency Drive outline schematic thanks to Stephen Collings.
We know that in normal VFD braking the normal commutation of the output transistors and generation from the motor result in a rise in DC bus voltage. If this exceeds a preset level the drive will bleed off power into a braking resistor (not shown on the schematic).
Q1. What typically happens at 0 Hz? Will the drive hold the three motor phases at a particular DC levels (by PWM2)? Or will the output transistors turn off completely.
Q2. Is this a bad thing? (I have experience of servo motor brake resistor burn-out. The resistor turned out to be more like a toaster element and I repaired the drive by connecting to an external high-wattage heater cartridge of the same resistance.)
I'm going to check the DC bus voltage and motor current readings and will update the question. Is there anything else I should read from the drive?
Update after inspection
I visited the drives and recorded the following data. Both are ABB ACS550 series.
╔═════════╦════════════════╦════════════════╗ ║ ║ Supply ║ Extract ║ ╠═════════╬════════════════╬════════════════╣ ║ Model ║ ACS550-01-059A ║ ACS550-01-023A ║ ║ Rating ║ 30 kW ║ 11 kW ║ ║ DC bus ║ 555 V ║ 564 V ║ ║ Power ║ 12.4 kW ║ 0 kW ║ ║ Current ║ 30 A ║ 11 A <--- ! ║ ║ Torque ║ 44% ║ -4% to +10% ║ ║ Speed ║ 900 RPM ║ 0 RPM ║ ║ Temp ║ 30°C ║ 30°C ║ ╚═════════╩════════════════╩════════════════╝
The only thing that looks odd is the 11 A on the idle return fan. It could be that the voltage and current are 90° out of phase or that the motor voltage is zero.
1 VFD = variable frequency drive for AC induction motor.
2 PWM = pulse width modulation.