Varaible speed motor for small spray booth - DC or 3ph?

richard newman · May 26, 2021

Picked up a nice little 4' x 4' Devilbiss spray booth, has the usual tube axial fan with 1/2hp 110v motor. Will be used for spraying lacquer on banjos and small objects, low production, occasional use. Would like to have a variable speed motor, so after spraying can have the fan run slowly to vent fumes as the lacquer dries.

Wondering if I should use a dc motor instead of 3ph w/ vfd, as dc seems happier at much lower speeds. I already have some dc controllers, as well as a vfd. Are dc motors with brushes a fire hazard even tho motor is isolated from booth air flow?

Also thought of piggy-backing a smaller motor to the drive train, with small pulley for just venting fumes. Motor would go for a high speed ride during spraying, is that a crazy idea?

DDoug · May 26, 2021

richard newman said:
Picked up a nice little 4' x 4' Devilbiss spray booth, has the usual tube axial fan with 1/2hp 110v motor. Will be used for spraying lacquer on banjos and small objects, low production, occasional use. Would like to have a variable speed motor, so after spraying can have the fan run slowly to vent fumes as the lacquer dries.

Wondering if I should use a dc motor instead of 3ph w/ vfd, as dc seems happier at much lower speeds. I already have some dc controllers, as well as a vfd. Are dc motors with brushes a fire hazard even tho motor is isolated from booth air flow?

Also thought of piggy-backing a smaller motor to the drive train, with small pulley for just venting fumes. Motor would go for a high speed ride during spraying, is that a crazy idea?

Use the existing motor, damper to slow down flow.

Just a Sparky · May 27, 2021

Paint booths are covered under hazardous locations within the code. Depending on the location of the motor, it may be situated within a Class 1, Div 2 or Class 1, Div 1 hazardous location. Your installation may require an explosionproof motor and wiring method. Rigid metal conduit with at least five threads engaged, explosionproof flex and 'Chico' seal-offs are typical.

Bill D · May 28, 2021

Consider one belt drive fan with two drive pulleys driven form two separate motors. run only one motor at a time for two speeds.
Bill D

Joe Gwinn · May 28, 2021

richard newman said:
Picked up a nice little 4' x 4' Devilbiss spray booth, ...

Wondering if I should use a dc motor instead of 3ph w/ vfd, as dc seems happier at much lower speeds. I already have some dc controllers, as well as a vfd. Are dc motors with brushes a fire hazard even tho motor is isolated from booth air flow?

Do not use a brushed motor, unless it is rated as explosion-proof, even if this is not for a business.

If this is for use in a business, than there are multiple probably code and/or OSHA regulations to be followed, as Sparky mentioned. I have a friend who was manufacturing electric guitars some years back, and the spray booth was a very big deal, both to avoid explosions, and to avoid inhalation of paint mist/dust - would this disease be called "rainbow lung"? With glitter?

Anyway, it was booth plus full mask.

richard newman · May 28, 2021

To put this in perspective, I'm "semi-retired", working alone, only spraying 3 or 4 banjos a year, with a tiny Sata minijet gun. Pretty far under the radar, but I want to stay safe as I can.

Joe - point taken about brushed motors, I suspected they would be a bad choice.

Bill D - I like the idea of two motors on the fan.

The auxiliary motor for low fan speed would have a much higher(?) drive ratio, which would mean that at higher fan speed it would be turning a lot faster than nameplate rpm. Assuming a 1750 motor, how fast can it spin without frying bearings, or causing some sort of electrical problem?

Gordon Heaton · May 28, 2021

There are a lot of 'what if' ers around here, but I wouldn't balk at using a brushed motor if its located outside the booth and not in the exit stream.

Dieseldoc · May 28, 2021

We have used an attic power ventilator, pulling through a 14”x14” a/c filter and a track light dimmer switch for years with no issues in a Cerakote booth.

Terry Keeley · May 28, 2021

I just went through this and went with a three phase 3/4 hp motor and cheap ABB drive.

Works great.

Bill D · May 28, 2021

You could use the fan to blow clean air into the booth with the exhaust exiting through a duct with no elctrical stuff in the airstream out.
Bill D

Gordon Heaton · May 29, 2021

Bill D said:
You could use the fan to blow clean air into the booth with the exhaust exiting through a duct with no elctrical stuff in the airstream out.
Bill D

This typically results in somewhat forceful air currents in spots, not conducive to painting. Pulling through filters with good surface area tends to slow and even out the flow.

Just a Sparky · May 29, 2021

Also FWIW every spray booth I've ever seen has been fitted with either a wet fire sprinkler system or a stored pressure fire suppression system.

DDoug · May 29, 2021

Just a Sparky said:
Also worth noting every spray booth I've ever seen has been fitted with either a wet fire sprinkler system or a stored pressure fire suppression system.

Yup...because even belts spark...
Simply install booth as designed, damper down output (or input if easier to enable)
for less flow.

Scruffy887 · May 29, 2021

digger doug said:
Yup...because even belts spark...
Simply install booth as designed, damper down output (or input if easier to enable)
for less flow.

This^^^. The damper will limit the air flow and less electricity will be used as well. Simple. Get a fire suppression system of some sort. And never scrape out lacquer overspray with steel tools of any type.

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Varaible speed motor for small spray booth - DC or 3ph?

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