Drive solution for PA fan drives.

The problem

PA (primary air) fans are essential in thermal power plants and anywhere boilers are used. These fans carry very large inertia, and that inertia has to be accelerated by squirrel-cage motors of single- or double-cage design. The result is a long, punishing run-up to full speed:

• A 1250 KW drive takes about 19–23 seconds to reach full speed.
• A 360 KW drive takes about 50–55 seconds.
• A 190 KW drive takes about 90–95 seconds.

Through most of that run-up the starting-current kick of around 600% sustains for 80–85% of the period — staying in a 600% to 500% band — before decaying to normal in the last 20–15% of the time. Held for so long, that current puts severe stress on the motor.

What it does to the motor

The heat generated in the motor during acceleration is proportional to I² × R × time, where I is the motor current and R is the motor resistance. Because the heat rises with the square of current and a large starting current sustains for a long time, the heat generated is very high. In practice this shows up as:

• Large heating of the motor during starting.
• Cracking of cage bars.
• High and localized stresses on the insulation.
• Stresses on the switchgear, shortening contact life.

Together these cause premature failures — motors burning out and insulation breaking down — while switchgear contact life is adversely affected.

Worse at low voltage

When the supply voltage is low, or sags under the heavy in-rush of starting current, the motor and fan take far longer to accelerate, because motor torque falls with the square of voltage. At 80% voltage the torque drops to 0.8² = 0.64 of normal, so the acceleration time stretches to roughly 1.56 times. With reduced voltage every problem above becomes more severe and the risk of motor failure multiplies.

The Fluidomat solution

Fluidomat provides an effective, field-proven answer by installing Fluidomat SMD delayfil fluid couplings between the motor and the fan. The delayfil design decouples the motor start from the fan inertia, with clear advantages:

• The motor starts on no load; torque builds up gradually on the coupling output as the motor speeds up.
• The motor reaches 85–90% speed — corresponding to its pull-out torque — within just 2–3 seconds.
• That pull-out torque is then put to work accelerating the fan.
• Because starting current depends on motor speed, the current kick decays from 600% to 250–400% within 2–3 seconds, then holds while the fan accelerates to 90–92% speed, before decaying to 100% (normal) over the final stretch.
• That fast decay — 600–700% down to 250–400% in 2–3 seconds, against 16 to 45 seconds with a direct coupling — is a major relief to the motor.
• Total fan-and-motor acceleration time reduces to 50–60% of the original.
• Heat generated in the motor during acceleration is reduced by 2.5× at normal (100%) voltage and by about 7× at 80% voltage.

With current decaying quickly and the fan accelerating faster, heating of the motor and stresses on it and the connected switchgear are greatly reduced. The drive motor on your PA fans stays safe — and you avoid the losses of expensive motor repairs and downtime.

Proven in the field

Fluidomat SMD couplings are already proven on PA fan drives across many installations and supplies, including MPEB Sarni, MSEB Paras, MSEB Bhusawal, SAIL Rourkela, HZL Udaipur, HZL Chanderia, HZL Vizag, GHCL Veraval, Birla Copper Dahej and Indian Oil.