Hi everybody,

I need some help with soft-starters. I have a 3-phase 15kW motor which is used to drive a large blower fan and I'm planning to use a soft-starter to start the motor to limit the starting torque. The motor nominal speed is 1460 rpm and this speed is reduced to about 500 rpm by a gearbox coupled to the fan. The motor rotates the fan in the clockwise direction.

The problem is that the fan may rotate in any direction when the motor is not running due to the wind blowing into its blades. When there is very strong winds the fan may rotate at speeds close to 500 rpm or even more. Its not a problem as long as the wind blows the fan in the clockwise direction. It may be a problem when the wind blows the fan in counter clockwise direction.If when this happens the soft-starter starts the motor, will the regenerative braking action harm my soft-starter?

Furthermore, the start ramp will be used to brake the motor rather than starting it, thus the motor will be started as if it was operated direct online.How can i solve this problem?

 

My guess is that you need more sophistication than a typical soft-starter. You will need to have a flying start feature, where the drive will sync with the existing motor frequency. Many VFD's offer a flying start capability. You will also probably need a braking resistor or regenerative braking unit to absorb the negative torque during a regenerative start. I am not aware of any soft-starters with these features. If the fan runs at line speed most of the time, it might also make sense to use a bypass contactor to bypass the VFD once the fan is at speed. This will increase the efficiency of the system.

You should really contact a local vendor who sells and understands soft starters and VFD's and discuss your application with them. Their advice and support may be very helpful before you spend any money.

Bill Sturm

 

Responding to JulesCeasar's 29-Aug-09 (04:54) comments regarding fan "windmilling?"

Have you searched Control.com's archives for "anti-windmilling" topics?

Regards, Phil Corso

 

Lets get simple with this guys. Replace one of your bearings, does not matter which, with a sprag clutch.

"KISS"

 

Was even thinking of putting in a mechanical brake on motor or shaft, or am i missing something? Brake can be operated directly from soft starter