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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

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).

Questions:

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

ABB ACS550 drives

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.

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  • \$\begingroup\$ What does the VFD say the current is?. If it's zero then the motor won't get hot and the outputs are off. However if it reads anything else it probably the holding current (in AC drives it could be the magnetizing currents). The exact way it is handled is probably VFD dependent. And possibly determined by the mode the drive is operating in. \$\endgroup\$ – Spoon Jan 7 '16 at 13:29
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VFDs used for HVAC applications are often special models designed for that industry. You should look carefully at your manual and other documentation to determine what features are provided in your VFD. Rather than braking, it may be equipped with a feature that only allows the load to decelerate as fast as it will decelerate without regenerating more energy than is absorbed by losses in the motor and VFD. It may essentially "sleep" when the speed command is zero. It may have a feature to find the motor speed and synchronize with it if it is turned on when the motor is already turning due to air flow produced by the other drive. Various features can be enabled or disabled using drive set-up parameters. It is essential to have full documentation for the VFD.

Edit re added information:

I believe the drive is configured as for sensorless vector control. When zero speed is commanded it will energize the motor with magnetizing current, but try to drive the motor to zero speed if the shaft is turning. It will not provide DC braking unless that is configured, and I suspect that it is not configured. The drive has a braking chopper built in, but apparently no braking resistors. There will be no dynamic braking unless optional external braking resistors are connected. When deceleration is called for, the drive will decelerate at the slower of the rate called for by the deceleration ramp adjustment or the rate that prevents regeneration in excess of losses.

This drive has a group of preset adjustments that are loaded by selecting a "Macro." The PFC macro is intended for use with fan and pump loads. It sets the control method to scalar frequency control rather than vector speed control. I am pretty sure that would cause the drive to completely de-energize the motor when zero speed is called for. Since centrifugal fans and pumps produce zero or near zero flow at some speed above zero, I believe this drive has a configuration setting that would de-energize the motor if any speed below some set minimum speed is called for. I don't think this is configured by the PFC macro.

I have written technical literature under contract for ABB USA, but I have not done that for quite a while, so I am not completely familiar with this drive.

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  • \$\begingroup\$ Thanks, Charles. I was wondering when you'd show up. ;^) I've updated my question. \$\endgroup\$ – Transistor Jan 7 '16 at 17:18
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All VFDs that I have worked with or designed maintain closed-loop velocity control. That is, if the commanded speed is 0 Hz, then the torque command will be such to force the velocity to 0 Hz. Therefore, the current regulation loop is still active and the switches keep PWM'ing. Since the bEMF is effectively zero, the control system will mostly command the zero space vector anyway.

However, most better VFDs have features that reduce the switch utilization in such circumstances. For example, the switching frequency might be cut in half or a quarter to prolong the life of the gate drivers in case there is little to no current flowing through the motor windings.

The braking resistor engages if the drive absorbs energy. The resistor should be designed for continuous operation up to certain power but such operation should not be prolonged because the drive was designed to drive motors and absorb the motor inertia energy only.

Here I'd suggest to contact the drive manufacturer directly. They might be willing to share some info on how much power and for how long the drive can keep absorbing energy.

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A1: It depends on VFD and type of operation. In vector control with feedback, setting the 0Hz will output magnetizing current to the motor (only d part - excitation) if there is no load applied. The sensorless vector control does not work under certain frequency margin. A2: If you don't deal with servo loop, then this is unwanted mode of operation.

By the way, if you want to control fans, there is a special V/f curve that is used for. As the torque of a fan vs. fan speed is rising quadratic, the ideal V/f curve is also quadratic, this reduces heating of the motor and matches the motor and fan torque. You would normaly use a RUN command from PLC + analog input set speed, and you can set the minimum frequency (which indeed is needed to prevent motor overheating). Therefore for your application, no 0Hz is ever needed.

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