Fluid Couplings and VVVFs

Your title refers to VVVF, then you mention a soft starter. They are two different things. I also seriously doubt you have a 330,000 volt motor. You probably mean 330kW, about 450HP.

A solid state soft starter varies voltage only. It will start your motor softly, but only within the limits of the motor's thermal capacity. In other words, if your load is such that it takes a long time and a lot of energy to start, a solid state starter may not work, or it may cause a voltage drop. Not as severe as DOL, but sometimes it is still too much. If you have plenty of power, it may still be the better choice, solid state soft starters are typically 1/5 the cost of a VVVF drive.

A VVVF Drive (better known as a VFD) can extend the acceleration time of the motor for as long as possible without adding undue thermal stress to the motor. It's drawback is that even at full speed, there are still losses in the VFD that represent a cost of operation to be considered. Typically this cost would not be as high as that of a fluid coupling however.

It sounds as though this is a long heavy, conveyor that you have to start under quite a high load.

In this case, the use of a soft start can have some associated considerations:

1. High torque to 'breakaway' will require quite a high voltage and current and then you want to ramp up over a period of time to get your soft start. This presents a high thermal load to the soft start unit which will need sizing to cope with this.

2. Such a prolonged, high current start is also arduous for the motor as well, so you may suffer starting overheat but must provide good motor thermal protection.

3. The above conditions may severely restrict the number of starts/hour for both units.

4. This also presents a very high electrical load during this starting phase giving potential problems on your power supply

Soft starts are not so easy on this type of application, I have run long belt conveyors in quarries on VFD, even these need sizing adequately relative to high prolonged breakaway torque, particularly when its been raining which adds load to the material already on the conveyor. Then, you can also stop and start much more often if needed under certain circumstances (e.g. production is full or halted from upstream)

If you need accurate information, I would recommend talking directly to a specialist e.g. www.fairford.co.uk who have many years manufacturing, development and application experience of soft start units, before you decide which method to use to replace your coupling.

Good luck, hope this helps!