Fixed Speed Positive Displacement Blowers Running Speed

[18815030]

Hi all,

I have fixed speed (no VSD) positive displacement blowers designed to operate at X (e.g. 2000 RPM) speed. Its speed within the datasheets is rated to operate between 1478-2955 RPM.
Our electrical supply is 50 Hz.
In terms of size, differential pressure is 30 kPa and flow rate is 29.53 Nm3/min @ 2955 RPM

How do we achieve X speed within the operating range of 1478-2955 RPM when we can't control the frequency?
I have heard from electrical engineers that we can give the drive/motor a fixed reference, then the motor will run at that reference speed? How do we give the motor a reference speed physically?

From research you can also use with gear or pulley systems to mechanically get the specific speed you want but I imagine these would be difficult to fine tune. Are there alternative methods?
Happy to hear people's thoughts.

 

[LittleInch]

I'm not following this. Your two choices are strong.pole motor at 2955 or a 4 pole motor at 1490.

To vary the speed you need to change frequency or add in a mechanical system such as belt or gear

 

[Gr8blu]

Second LittleInch's response.

The synchronous speed of an AC machine operating directly-on-line (e.g. no variable frequency drive) is directly proportional to the supply frequency. The proportionality is derived from the following equation.

(Synchronous speed in rpm) = 120 * (line frequency in hertz) / (number of poles).

Thus, at 50 Hz a two-pole motor will operate at a nominal 120*50/2 = 3000 rpm. A four-pole design will operate at 1500 rpm. A six-pole design would operate at 1000 rpm. And so on.

To complicate matters a bit, the machine the OP is using is a squirrel cage induction machine - which means it does not operate at synchronous speed, but rather at something less. The difference in speed is due to a parameter called "slip" which can vary from a few tenths of a percent to more than eight percent of the synchronous speed.

Slip will not get the OP to the desired operating point of 2000 rpm if the only power source is a 50 Hz supply. The only two alternatives are to invest in a frequency changer (variable frequency drive or actual electro-mechanical frequency changer) or use other mechanical means (gearbox or belt/pulley) to move the motor and load shafts at different speeds.