Performance of Motor With Soft Starter And Fluid Coupling

If your going to use a soft start why not simplify the system and get rid of the fluid coupling?

It's one less thing to go wrong. I can't see the point in having both.

The equipment is ordered with fluid coupling.

As motor size is big it is having impact on DG sizing. To reduce DG size this option is exploerd.

The equipment supplier is not confident about performance on equipment with fluid coulping and soft starter together.

Also function of both are not identical when compared with respect to starting and running characteristic of driven equipment.

If the soft start has a relatively short ramp up time to control the peak loading on the generator the fluid coupling will pick up as normal.

One system I've worked on was from the 1930's it had a 150HP slip ring motor an a magnetic clutch driving a fan, you could hardly tell it was starting up

It would depend on the start-up torque against speed curve and inertia of the machine-train. It is ideal, if you can take the operating fluid away from the coupling during motor starting, then restore it with motor at normal speed. Have you asked the makers if they have experience with your problem?

I agree with your statement that It would depend be function of start-up torque against speed curve and inertia of the machine-train.

As motor size is small the fluid coupling shall be of simple construction without provision of taking fluid during starting.

The manufacturer of equipment has give no satisfactory reply and have no experience with such arrangement i.e. Soft Starter+Motor + fluid coupling + Agitator.

Any one who have worked with similar arrangement? Pl. reply

It's done all the time. Despite what the fluid coupling people say about it providing soft starting, it's a MECHANICAL soft start, not an electrical one. A Solid State Soft Starter would technically be capable of both functions, but if, as in your case, the FC is integral it's kind of a waste to duplicate the capabilities. All you really need now is a Reduced Voltage Starter of any sort for the generator.

I absolutely HATE Star-Delta starting, I think it's a cheat: neither soft nor reduced voltage and certainly not benign. Don't go that way, you will regret it later. But a better method of reducing the effects of starting current on a DG than a Solid State Soft Starter is actually a Reduced Voltage Autotransformer (RVAT) starter. The transformer action further reduces the starting current drawn from the line by the transformer ratio, which helps the DG, where as in an RVSS the starting current is the same as the line current. It's a subtle difference and generally I don't use RVATs because they don't really provide the same MECHANICAL soft starting benefits as an RVSS. But in this case the FC is taking care of that. I would use an RVAT starter then; simpler and easier to set up in a retrofit. It may be physically much larger though, and that's sometimes an issue. In that case, go with an RVSS.

If you were to install a soft start, given one of the benefits of fluid couplings such as you have here, I would not remove the fluid clutch and replace it with a hard and fast mechanical coupling.

One of the aspects of fluid clutches is that they have the ability to "absorb" based on the shearing action inherent in a such a coupling, sudden shock loads that present to the mechanical system. That is one of the reasons they are used in agitator drives and larger centrifuge based systems.

By removing the fluid coupling, if you were to do so, you remove some of the systems ability to absorb sudden shocks and introduce another failure mode to the system.

Since I posted question on CR4, I have checked net and have come across Soft starter sizing program from ABB PROSOFT3.0 and various product catalog of fluid coupling.

From those inputs It can be concluded that addition of Soft starter in equipment train would further help in smoothening starting stress imposed by motor on equipment train.

During starting of motor as driven equipment is at stand still total energy is desipated in fluid coupling. As mechanical load accelerated this loss reduces and finaly equipment shall come to full speed.

With introduction of soft starter the torque generated by motor would be less when compared to DOL starting, resulting in less power to be absorbed by fluid coupling.

Overall effect on equipment train is expected to be

1. Motor subjected to lower starting stress due to Soft starter
2. Coupling subjected to less severe service.
3. Probably more no. of starting per hours for equipment train.