Conversion of Air Compressor Motor Controller from Wye-Delta to VFD

Irrelevant.

<...the effect on lubrication or mechanical wear and tear...> will be insignificant.

<...converting the motor controller of an air compressor...from wye-delta to vfd...> Why on earth would one want to do that?

You should consider other aspects like Cost,necessity for your decision,smooth running,torque/speed characteristic,maintenance cost,trouble shooting etc etc

Can't see why you would want to do this.

The motor is at its most efficient when running flat out. It has good control via a pressure switch so nothing to be gained like when running a fan or pump on continuous duty.

Issues with starting could be addressed with a soft start but you already have a cheap and manageable system installed with your star/delta starter.

Yes Sir I agree but I think this is good only if compressed air demand is at the same requirement all the time but what if the demand is low? For example, I have a 100 HP compressor and it is capable of delivering 446 CFM of compressed air, what if, the requirement is only 80%? By slowing down the speed of the motor, 357 CFM can be achieved at lesser power usage.

Not really (or not by much). If you run at 80% speed, you will run 125% as long, for essentially the same energy. However, you could save somewhat on fewer start/stop cycles.

Why don't you have a storage tank with pressure switch to automatically ON/OFF the compressor to maintain a maximum/minimum pressure?.

When vfds were first applied to compressors there were a host of mechanical failures due to insufficient lubrication. The oil pumps were diven at compressor speed and when the vfd slowed the machine down there was not enough oil going round to lubricate the bearings etc.

However buying a new compressor with a vfd is a great way of saving energy if the machine is part loaded for any length of time. When a fixed speed machine hits control pressure the unloading system in essence makes the pumping action less and less efficient and the motor continues to draw a large proportion of its normal load current. With a vfd the motor slows down and draws less power whilst maintaining the outlet pressure. Manufactureres are quoting longer lifespans for vfd machines as the average shaft speed is reduced.

Maybe that's the reason why other manufacturers only allowed down to 60% of rated speed of the motor inorder to satisfy the minimum requirement of lubrication, and/or also to address the cooling requirement of the motor???

I assumed this was a compressor with a an accumulator tank. Does it have a tank?

If the compressor was a reciprocating machine then it would have a tank. If it was a screw compressor it may or may not have a tank but all the tank does is damp down the load/unload frequency.

Unless the compressor provides base load only and never unloads there is always a period of time when it is running but not actually pumping any air into the downstream pipework. If you were stood at the side of the compressor you would hear the change in the noise as the load changes. It is more noticible on a recip machine than on a screw but it is still audible.

thank you sir Gasman!

If it doesn't have a tank then VSD is a good way forward. The pump must be wired to a seperate supply i.e. not from the VSD.

check the motor cooling. A big problem with VSD motors is that the are cooled from the fixed fan on the end of the shaft. Turning it more slowly massively reduces the cooling. You are usually ok down to about 30% or you must consider a way to cool the motor.

thank you sir Silverfox!

the mail went before I intended. You need to consider cooling the compressor motor. If you want to use the full range of a VSD you should consider a small fan, powered from the normal supply with the oil pump.

All in all adding a tank to the system might simplify things a lot.