Fan start up problem -over current

[remp]

Im having problems starting up a fan - 75kW motor, connected to a Variable frequency drive. At present we are testing only and we are leaving the doors of the air handling unit open onintake and discharge side. There is therefore no resistance on the fan so the fan is blowing large amopunts of ai, I guess. Could this be causing the fan motor to trip out on over current. Is high air volume directly preportional to high motor current.???? Sombody says that high fan resistance means high motor current. Which is it please???

Regards

 

[mechanoid]

Yes it will probably overload with the door open----

Although it depends what type of fan you have. I guess its a FC forward curved bladed fan and not BC backward curved

Close the door and ADD resistance. The current draw should fall.

 

[HVAC68]

If it's a Forward curved fan, there could be a problem with high current. Not so with a backward curved fan, since they are the non-overloading type. Look at the fan curves and you will be able to make out what I am trying to say.

HVAC68

 

[quark]

For a fixed flow rate if the resistance is more then power drawn will be more. But when we speak of fixed system then power consumption varies as per fan performance characteristic. The power curve of all the fans increases with increasing flow. When the system resistance increases then the operating point of a fan shifts to the left of the initial operating point, thus reducing flow and so reducing power consumption. When the resistance decreases then the operating point shifts to the right of the initial point, flow increases and power consumption.

For forwarded curved fans, power consumption increases as long as flow increases(with reduction in resistance) till the motor trips on overload. But for backward curved fans, ther is a dip in power curve at the end. i.e though flow increases beyond some point, power consumption will not increase. This is called as a non overloading type fan.

Regards,

 

[friartuck]

Hi BillyQ

I used to work for a large UK fan manufacturer, and this problem was common with FC blades. FC fans actually speed up when resistance is added and slow down when they are left to run in free air. Most peculiar but true.

I always spec BC blades to avoid this occurence.

FC fans have overloading power characteristics which means that the more volume it shifts, the higher the power drawn. So with your door open, the airfow is higher and the power rises dramatically.

What the fan maker should do is give you a fan curve (with a power curve attached) This will show you what is happening.

I also spec motors with a safety margin on the motor and a thyrister safety cut out (this is embedded in the motor windings and measures the core temp of the motor)

Shut the doors and add resistance...and the fan motor will be happy...and so will you.

PS good luck in setting up the inverter, they can be bitches to do. Hope you are using a good manufacturer.

Friar Tuck of Sherwood

 

[HVAC68]

The flip side of using backward curved fans - They are expensive, run at higher speeds and are generally more noisy as compared to FC fans.

If you are sure of the static or resistance, then go for a FC fan from an economical point of view.

Advantage of using a BC fan is (a) they are non-overloading type, (b) more efficient.

HVAC68

 

[chicopee]

very good chance that your CB is underrated for running load amps.

 

[chicopee]

ALSO CHECK THE LOCK ROTOR AMPS RATING OF THE MOTOR-THAT MAY BE TOO HIGH FOR THE CB'S.

 

[boyceg]

billyg,

What overcurrent device is tripping? The CB feeding the variable frequency drive (VFD) or is the VFD shutting down on over current?

I agree with others that low air resistance will cause the motor load to go up with a FC blower.

However, the VFD may not be providing the correct Volts per Hz for the motor. When the drive is running at 50% speed, the motor volts should be about half the rated motor volts. If the voltage is more that about 1.2 times this value, the motor magnetics will saturate and draw excessive current. Since blower power is approximately proportional to the cube of blower speed, the motor power at half speed should be about 1/8th of the motor power at full speed. I would expect the motor current at half speed to be about 1/4th the motor current at full speed. The phase currents should be balanced.

Again, all bets are off until air is being blown through the duct system.

Experience requires the following question to be answered: Is the blower turning in the right direction? Now, who could be that stupid? Well, me, once...
Boyceg

 

[sspeare]

Make sure the fan has proper rotation. I've seen many cases where the fan is turning backwards.

 

[ozmosis]

You might want to also increase the acceleration time of the VFD. The faster the accel the higher the start-up current, the longer the less.