Variable speed drive booster pump sets became very popular in the past decade.
It cannot be denied that it works very well, but it has some surprising drawbacks:
- VSD pump sets are not nearly as efficient as the sales people claims. The reason is that the inverter already has an electrical loss of about 8% - a very significant amount given the mechanical efficiency of pumps these days. To make up 8% of losses is already a tall order.
- It uses multi-stage pumps which are not mechanically as efficient as larger single stage pumps. The advantage though is a sharp rising pressure give which helps with accurate pressure control.
- VSD electronics are robust, but can only take so much hammering. Constant load shedding increased the failure rate dramatically.
To select a constant speed pump correctly for a purpose, takes a bit of patience and skill - which many designers of today don't have any more.
Interestingly, pumps with small impellers are not mechanically very efficient - usually as low as 45%, whereas large pumps can reach efficiencies of 85% or higher.
For larger flows then, a correctly selected fixed speed pump will outperform a VSD pump set all day and it is significantly cheaper. The biggest advantage is that fixed speed pumps can still be bought over the counter at the local co-op - vital for users in small towns and rural areas.
For this purpose, I have developed a 4-stage booster pump controller that can be programmed in multiple ways.
- It can have 4 of the same pumps which can be switched in stages, in equal runtime fashion with the last pump as backup if so required.
- All 4 pumps can be completely different so that stage 1 can be selected for best efficiency at low flow and so forth until stage 4 which can be a highly efficient large pump for peak demands periods of the day.
- There is dryrun protection on all pumps.
- The controller also manage the storage tank supply level.
This is controller is highly recommended for all engineers who are capable of good pump selections and who are concerned about maintenance of sets in the long run.