Min Fan Speed on VFD - Before Damaging 1

[remp]

What is the minimum fan speed I can run at using a Variable Freq. Inverter before damaging the motor. Dont you need a minimum air flow across the motor to keep it cool? I have both axial and DWDI centrif's. Motor sizes approximatly 10-30 kW. The motors ar in the air stream.
Thanks

 

[ClydeMule]

Since the power consumption drops, the heat output drops as well, but not to zero. For a fan application it is usually 10 or 20 Hz. Check the mfg. specs to be sure.

Auxiliary fans are required when you have a constant horsepower, where you can be running 10Hz but nameplate HP. Bascially you get a doohickey that has a fan that clamps onto the motor and runs full tilt all the time. This doesn't really apply to pumps and fans.

Common in the industry are TEAO motors. THe small belly band type. We have had good luck with the more industrial type ODP motors which have built in fans. Maybe more reliable, but don't have the data. But gobs of people other than us have been using TEAO hvac duty motors with VFD's for ages. Just make sure the motor is inverter duty rated.

Check out the tech info section on the Leeson catalog (

http://www.leeson.com)

Good luck,

Clyde

 

[quark]

Agree with ClydeMule. Just check that your motor is inverter duty rated. This ensures you that even at low speed the air flow across it is sufficient to take care of the heat generated.

Generally, I didn't see any application where you require to slow down the motor below 20 Hz.

Keeping motor in the air flow path is an added advantage in motor point of view. But strictly a no no condition when this is used for a pharmaceutical facility.

Regards,

 

[lilliput1]

The type of drive between motor & fan probably determine this. I did a job with a 75 hp motor driving a cooling tower fan through a gear reducer without oil pump. The minimum speed was limited to 650 rpm with the maximum 1750 rpm. The cooling tower manufacturer (Marley) required this minimum limit.

 

[zakh]

Apart from the need to know the limit on motor speed to ensure cooling of the motor windings, one also need to ensure that at lower speeds the fan operating point will not be in the surge region. With the fan curves, the operating point at 100% volume, and the static pressure at 0% volume (which is actually the setpoint), one can draw the system curve on the fan curve and know the limit of the speed before the operating point gets into the surge region. Additionally, we can also determine the minimum speed required for the fans based on these specific conditions.

As Clyde rightly pointed out, the generation of heat from the motor windings at low speeds is very low for the type of application you mentioned, ie variable fluid flow. I have tested the temperature rise of an VSD connected pump in an ambient condition of about 95F, and the results showed a decrease in winding temperature as the speed decreases (down to about 40% of full speed). Perhaps at lower speed, the temperature will start to rise again (due to magnetizing and harmonic currents), but i doubt that it will rise to a point where it will exceed the temp. rating of the insulation. From an energy conservation point of view, the savings on the motor power is minimal at lower speeds due to the cube law.