# Blower and vfd's



## Big John (May 23, 2010)

The issue with VFDs is that harmonic current creates additional heat on top of the nameplate RMS load. So in theory this extra heat takes up the thermal capacity the OEM included when figuring the service factor.

That's why the rule of thumb is VFD = SF 1.0

In your case you're running at peak service factor, plus a dozen starts per hour, plus harmonic heating. That sounds like pretty severe duty and you should probably look into if you need to derate the motor HP.

The only thing that might save you is if the ambient environment of water or forced air will provide additional cooling.


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## micromind (Aug 11, 2007)

Big John said:


> The issue with VFDs is that harmonic current creates additional heat on top of the nameplate RMS load. So in theory this extra heat takes up the thermal capacity the OEM included when figuring the service factor.
> 
> That's why the rule of thumb is VFD = SF 1.0
> 
> ...


Agree.

Basically, a motor being supplied by a VFD is already well into the service factor (winding temperature-wise) when the actual current is at FLA.


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## Cow (Jan 16, 2008)

Just curious, why are you using a VFD for a carwash blower? This sounds like an application that may be better suited for a soft start unless you're varying the speed.


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## JRaef (Mar 23, 2009)

Reducing the speed between cars during a long lineup saves energy. You often can't turn the fans all the way off because there is a dryer with heating elements involved and if you cycle that on and off with the blowers, it fails quickly. So the heating element is left on and the fan speed is reduced to just enough to keep them from burning themselves out. It's a common practice in car washes.


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## John Valdes (May 17, 2007)

Agree. Allow the motor to "idle" if you will, between car washes. Let it run all day long and only come up to speed when a car is present.

Edit: You could program zero speed if no cars present for extended period.


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## garfield (Jul 30, 2009)

Cow said:


> Just curious, why are you using a VFD for a carwash blower? This sounds like an application that may be better suited for a soft start unless you're varying the speed.


Utility has a rebate for vfds on centrifugal fans and pumps.


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## garfield (Jul 30, 2009)

JRaef said:


> Reducing the speed between cars during a long lineup saves energy. You often can't turn the fans all the way off because there is a dryer with heating elements involved and if you cycle that on and off with the blowers, it fails quickly. So the heating element is left on and the fan speed is reduced to just enough to keep them from burning themselves out. It's a common practice in car washes.


It's typically 10 cars a day If it's busy it takes 5 minutes per wash. These particular blowers don't have heating elements.


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## garfield (Jul 30, 2009)

John Valdes said:


> Agree. Allow the motor to "idle" if you will, between car washes. Let it run all day long and only come up to speed when a car is present. Edit: You could program zero speed if no cars present for extended period.


I was thinking with a vfd you had unlimited starts because you didn't have inrush from across the line starting.


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## JRaef (Mar 23, 2009)

garfield said:


> I was thinking with a vfd you had unlimited starts because you didn't have inrush from across the line starting.


That's somewhat true, but as noted, just being on the VFD consumes the service factor. And the only way to say "unlimited" starts is if you never exceed FLC on starting. If you are starting so fast as to get to 200% FLC, even briefly, you are heating the motor and limiting the starting. It's still going to be more than ATL, but not really "unlimited".

If you are not varying the speed of the blowers though, the VFD is likely a waste of money. I would use s soft starter and do a 200% current limit.


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## Big John (May 23, 2010)

And my understanding with RV starting is that it often results in longer overcurrent while the motor gets up to speed.

So ultimately the amount of energy consumed and heat produced during each start is similar to DOL starting and you don't really get more starts-per-hour.


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## DriveGuru (Jul 29, 2012)

I would set the current or torque limit to fla if the drive is capable of running there without starting an overload counter. This might increase the accel slightly, but the motor will be happier


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## danhasenauer (Jun 10, 2009)

JRaef said:


> I would use s soft starter and do a 200% current limit.


I did a big 100+ car a day wash tunnel a few years ago. The blowers were made specifically for car wash duty and that was the exact set-up.


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## tates1882 (Sep 3, 2010)

Car washes are my thing, I have done several that are 300'+ long and crank out around 1500-2500 cars per day. 

I wouldn't waste my time or money on a vfd for a blower that starts only 10 times per day. Or a soft start for that matter, unless we are talking a 50+hp @ 208-230vac. With a soft start I would over size it for sure, just in case you have a busy day where you lots of cars.


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