Fixed Speed Positive Displacement Blowers Running Speed

18815030 · Jun 5, 2025

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 · Jun 5, 2025

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 · Jun 5, 2025

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.

18815030 · Jun 7, 2025

Thanks all, your responses have confirmed what I thought. The only way to do it is mechanically with a gear or pulley reduction or a VFD.

I thought there may be something I was missing. Thankyou for the responses.

LittleInch · Jun 7, 2025

Well given that what you are controlling is presumably flow, you can always send some of the gas back round to the inlet. Wastes a bit of energy but means you can use a folded speed mirror and still have a variable flow rate.

dvd · Jun 10, 2025

LittleInch said:
folded speed mirror

I have never heard of this term - would appreciate some discussion.

LittleInch · Jun 10, 2025

Sorry, doing it on my phone I suspect.

Bloody auto correct. What did I mean?

Ah yes "Fixed speed motor...."

Cooling Junkie · Friday at 7:46 PM

Without a VFD you’re mostly looking at mechanical options like pulleys or gearboxes, though as you said those can be tough to fine tune. Another approach is to design around the motor’s fixed speed by matching airflow and thermal performance to what the motor naturally delivers.

At YS TECH we’ve worked on setups like this by building custom cooling solutions that operate right in the motor’s sweet spot. Happy to share more if that’s helpful.

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Fixed Speed Positive Displacement Blowers Running Speed

18815030

Mechanical

LittleInch

Petroleum

Gr8blu

Electrical

18815030

Mechanical

LittleInch

Petroleum

dvd

Mechanical

LittleInch

Petroleum

Cooling Junkie

Mechanical

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