Synchronous Motors versus Induction Motor

The only advantage of an induction motor is less maintenance.

The synchronous motor will not change its speed with different loads, while the Induction motor will. This will cause slower response to load changes, and thus more pressure fluctuations.

You may find that newer synchronous motors are more efficient. You will have to do some calculations to see if it is worth the cost.

How close are the old ones to the end of their expected life expectancy? Has the maintenance cost started to escalate? Are parts starting to be hard to find?

There may also be some serious ramifications in Power Factor penalties if the Synchronous motors are removed. In many cases, Synchronous motors are used on loads that are somewhat continuous because they can be run in a leading power factor and compensate for lagging power factors from other smaller motor loads that are running throughout the day. Replacing them with Asynchronous motors will remove that capability and even if you correct the pf on the bg motors with capacitors, you may find yourself having to go through and do that with all of the others as well.

It should not be undertaken lightly.

It may be worthwhile to to consider replacing them with AC sq.cage induction motors from the maintenance point of view.The DC system and slip rings wound rotor all these and associated maintenance will be avoided.Further induction motors are rugged and trouble free.Only thing to be taken care is the starting current and Poor factor.Pf can be improved by providing capacitors across each motor.Starting current if system cannot withstand then suitable means can be adopted.If system can withstand the starting then dol starting will make it more simple.Considering that driven equipment is a pump acceleration time and hence duration of starting is likely to be low.