Control box for a central vacuum system


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Old 12-08-03, 12:32 AM
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Control box for a central vacuum system

This is an electrical question but I need to provide an intro.

I installed a central vacuum system using “smart inlet valves” that include a low and high voltage connection for auto-start and the electric broom motor power. The vacuum unit I constructed from 4 vacuum motors obtained from Fein dust collection units (shop vacuums), as spare parts. Each motor is rated at 120vac/8 amps. I had a sheet metal shop make up the housing for the motors which are connected in parallel/series as far as air flow (not electrically). This improves the negative pressure (series connection) and airflow (parallel connection). You can draw an analogy between negative pressure as voltage and air flow as current. All works very well but I need to find a way to get my wall-to-wall carpet out of the vacuum hose. Seriously…

I had to make a control box to remotely turn on the vacuum motors using a 24vdc signal and to provide the 120vac to power the electric broom. I used a 120/16vac transformer with a bridge rectifier to get 24vdc to power a relay coil. The 4 sets of relay contacts are rated 10hp inductive. I stuck all this stuff in a small sub-panel enclosure (control box). I ran a12/3 cable with ground using a 2-phase 20amp breaker from my basement sub-panel to the control box. Also going to the control box is a 14/2 cable to provide power to the electric broom via the “smart inlet valves”, and the low voltage turn-on cable. The electric broom uses 2 amps, so I used an appliance fuse holder in the control box and 3 amp fuse on the 14/2, 120vac cable, and a 0.5amp fuse on the step-down transformer primary winding. The low voltage wire insulation is rated at 300volts while the other wires are rated at 600volts. Lastly, I cut out part of the control box to accept a duplex outlet. Each outlet was wired to a different phase, and only energized when the relay closes. The vacuum motors are wired so each pair is plugged into a different outlet via modified (cut female end off) 20amp 4 foot extension cords. When not in use only the transformer is powered, all else is powered when relay contacts close.

Although I have some electronic/electricity expertise, I know little about the NEC. Your comments regarding compliance would be appreciated.
 
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Old 12-08-03, 10:36 AM
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Since the maximum voltage of your system is only 120VAC, the 300V insulation is sufficent. If there are no qualifications on any piece of eqipment in you system that it be powered from a Class 2 low voltage/limited energy circuit then you should be OK. I would caution that if you intend to sell this home, you may have trouble with your homemade system. I have heard lenders not wanting to loan money on a house with non-standard systems.
 
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Old 12-09-03, 12:50 PM
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You lost me when you crossed the bridge rectifier. Were you trying to see how complex of a system you could build? I've wired "homemade" house vacuum systems before, but each one consisted of one vacuum motor and collection can with nothing more than a 30' vacuum hose on the business end. Several "smart inlets" are placed throughout the house - there is a set of contacts that close when the inlet cover is lifted and the hose is "plugged" in. The contacts allow a 24V AC current to energize a relay coil and turn on the vacuum motor. Nothing more than a small transformer and 24V relay with one set of NO contacts are required.
 
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Old 12-09-03, 04:54 PM
texsparky
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Dis I read that you used 14/2 NM on a 20 amp breaker?
 
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Old 12-09-03, 07:26 PM
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Mcjunk – I already had a suitable relay with a DC coil so I added a bridge rectifier to the step-down transformer. Other than that it’s much the same as the systems you’ve built. The reason for using 4 vacuum motors was to achieve the same performance as the very high-end, brand name systems for about $250.00. If you recall, would you mind telling me what brand and model vacuum motors you used?

Texsparky – Although the sub-panel breaker is 20amps, in the control box I used a 3amp fuse to provide protection for the 14/2 cable (brush motor) and a 0.5amp fuse for the transformer primary.

scott e. – Other than the control box, all components are standard, off-the-shelf, vacuum components. Would such things be considered a Class 2 low voltage/limited energy circuit?

Anyhoo, I decided to retire a few months ago so I'll probably kick the bucket in this house. I'll let the kids worry about selling the place. Meanwhile I gotta keep busy or I'll drive my wife nuts

Thanks for replying folks.
 

Last edited by 1Geniere; 12-09-03 at 07:43 PM.
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Old 12-10-03, 05:16 AM
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Actually, I have used a "shop-vac" in such systems. That way, when I need a vacuum in the shop or to clean out a car, I simply unplug it from the receptacle mounted on my control box, pull the hose attachment out of the piping and am ready to go.
 
 

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