Dear sqanesh,

Firstly, you have to follow the local electric code.

A reasonable setting of an overload device would be between 1.05 to 1.1 times the nominal or full load of the electric drive motor.

As a centrifugal fan has to be started in delta (high starting currents) and the starting time can be rather long (high rotational moment of inertia), the higher setting might be necessary.

A lower setting in combination with an overruled overcurrent protection device for the starting period is also possible.

Again local rules have priority.

have a nice day

Teus ■

Dear Mr sganesh,

During the starting period of the fan, the overload protection device is made inactive by a closed contact over the current input of the overload protection.

After the starting period, that contct is opened again and the protection becomes active.

As far as I know a standard procedure when acceleration times are long.

All for now

Teus ■

Originally Posted by Teus Tuinenburg

Dear Mr sganesh,

During the starting period of the fan, the overload protection device is made inactive by a closed contact over the current input of the overload protection.

After the starting period, that contct is opened again and the protection becomes active.

As far as I know a standard procedure when acceleration times are long.

All for now

Teus

Better surely to reduce the motor load during starting. You don't want 6 x full load current (from DOL starting) for an extended period as something is going to get a bit warm! ■

Dear designer,

As stated by Mr sganesh, there are no valves in the centrifugal fan installation. That rules out the possibility of starting under no load conditions.

Thus, starting is subject to the fan laws.

A star/delta starting will fail in the star phase by not reaching the full rpm’s.

A Volt/Hertz regulator is also a possibility.

If the power supply is strong enough, I would prefer a simple and reliable Delta start.

Have a nice day

Teus ■

Just fit a fluid pulley ■

How would a fluid coupling make the fan reaching its full rpm in time?

BR

Teus ■

Originally Posted by Teus Tuinenburg

How would a fluid coupling make the fan reaching its full rpm in time?

BR

Teus

It's the characteristic of fluid pulleys/couplings (at least the ones I use) to allow a motor a "light" start enabling the motor to get up to speed quickly when started DOL so avoiding a high current draw over a long period.

The characteristics are on most manufacturers web sites. ■

A fluid coupling for start up purposes must be soft enough to limit the current of the electric drive motor and later on be stiff enough to minimize the slip and energy losses.

That requires minimum oil during start-up and a gradual increase of oil to maximum during running. In the mean time, the dissipated energy has to be absorbed by the oil and cooled away.

Quite a complex installation.

Visit the Voith Turbo website for a start-up fluid coupling.

DOL with a strong power supply is to be preferred. ■

Maybe we need to get things in perspective as the original post omitted any reference to motor power.

I'm thinking of traction fluid couplings, motors say < 100 kW, used with either low or high inertia loads. (Of course there's always the delay chamber version, or the fancy scoop control version.)

What are you thinking of? ■

Of course, every situation requires its own evaluation.

In general, I prefer a DOL start, if possible.

If a drive is small compared to the power of the system where it belongs to, there should be no problem. All the, already, running motors form also a source of energy (kind of flywheel) which will be drained for starting the last motor.

If the appropriate starting and acceleration calculations show an unacceptable influence on the power supply stability, then one must investigate other means of starting consumers.

That is a matter of matching the drive characteristics with the equipment characteristics.

The drive characteristics can be adapted by the chosen solution and there are a lot of those.

From modern frequency regulators (V/Hz converters), soft starters, Star/Delta switches, fluid couplings, etc. to old-fashioned rotor resistors, Ward Leonard drives and controlling the start up load of the driven machine.

This domain of engineering requires extensive knowledge of power supplies, drives and driven equipment.

This is why rotating equipment engineers do exist.

All for now

Teus ■

Tues,

I note your comments, but can you clarify what motor powers you are thinking about. ■

Dear designer,

The biggest motor, which I installed in my career was 500 kW DOL, driving an Aerzen blower on a 5 MW transformer power supply.

A special case was a 450 kW diesel engine, driving a 360 KVA generator, connected to a 270 kVA squirrel cage motor, driving a low rpm Broomwade piston compressor.

Starting in star/delta did the diesel engine collapse in rpm and the generator voltage dropped to 60%.

To get the piston compressor running, we took the control voltage for the switchgear from another stable source to prevent the switchgear from switching off.

Then we bridged the field excitation voltage regulator of the generator during the starting period, causing full field excitation current.

Also all tripping devices were bridged.

Starting this system began with a sudden noise from the diesel generator when the field current was set to maximum and then after switching on the compressor a diesel rpm drop with full fuel injection and after that a slow build up from the rpm of the diesel as well of the compressor.

The piston compressor was kept at no load by keeping the inlet valves closed.

At the end, the voltage regulator was given free and everything operated properly.

I think we broke all rules of careful and prudent engineering, but it worked.

Actually, my comments relate to all motor powers, because it is just a matter of relative figures.

BR ■

Dear Mr S. Ganesh,

For the proper operation of a 3-phase electric motor the ratio Volt/Herz needs to be constant.

It is obvious that a power supply grid complies with that requirement.

You are right that the various electric codes should be comparable.

Nevertheless, you can check your local code for applicable rules.

If there are no rules, you can start with a setting of f.i 1.05 times the nominal load and check whether the overload device does or does not trip.

In case it trips then set the overload at f.i. 1.07 times.

After cooling for a while, repeat the test.

Continue until you are satisfied.

If you have a feeling that you are doing things of which you are not sure, consult an electrician.

Have a nice day

Teus ■

How large a motor are we talking about? VFDs are good for limiting starting current as well as varying fan speed, if it is a regular thing. But surely the simplest way is to close a damper during fan start-up?

Michael Reid. ■