What kind of switch do I need?

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Old 04-20-12, 07:49 PM
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What kind of switch do I need?

Hello all, I am in the process of building a whole garage air filtration system for woodworking.

I am interested in using a scavenged 240v AC blower motor as the heart of the system, but wanted to know what kind of switch I should use. I am assuming that the blower motor may likely be a multi speed unit, perhaps three speed.

Do I need a rotary switch with 2 pole and 3 ways or ? I don't seem to find much doing a search this way, at least nothing in the correct $$ area. Is there a different way of controlling the motor and speeds? I would prefer to have both feeds of 120v switched off when the switch is off for safety reasons, although I don't believe NEC requires it, IIRC.

Thanks in advance!
 
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Old 04-20-12, 08:55 PM
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Old 04-21-12, 06:12 AM
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Depending on the amps of the motor you could use a standard 15 or 20 amp two pole toggle switch.

Note: you do not have two 120v feeds, you have one 240v feed.
 
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Old 04-21-12, 01:29 PM
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A individual motor- circuit requires the following-- (1), an over-current device ( fuse or breaker) to protect the conductors supplying the motor, the rating of such devices based on the full-load current of the motor -- (2) motor conductors with an ampacity rating based on the full-load current rating of the motor--- (3) , a dis-connect switch , the rating of such usually based on the HP rating of the motor--- the location of the dis-connect switch relative to the motor location is subect to Code requirements-- ( 4), a controller-- (5) , overload protection devices , the rating of such devices based on the full-load current of the motor.
 
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Old 04-21-12, 06:22 PM
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Thanks SeaOn, the 3A254 looks like a promising unit. I am still concerned about having 120v on one side of the motor though...

1. Are there any other switches you know of that would solve this issue?

Tolyn, thanks for correcting me. I always think of the 240 coming in on residential as two phases of 120v, 180 degrees out of sync. I know its a misnomer because its only single phase, but referred to split-phase, sorry ;-)

It's going to be a smaller fractional hp motor, 240v under 8a.

PATTBAA - this will be wired to a 240v 20a breaker since I have leftover 12g thhn. I also have some 14-3 so I could find a 15a breaker, I will have to check and see what breakers I have on hand. Either way, the circuit will be protected.

2. Does the disconnect switch rule apply to small motors under 8a?
3. Are you speaking of local building code or NEC?
 
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Old 04-21-12, 06:37 PM
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Thanks SeaOn, the 3A254 looks like a promising unit. I am still concerned about having 120v on one side of the motor though..
You wouldn't. You would have 240v on both sides of your motor. No 120 volts to a 240 volt motor.
 
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Old 04-21-12, 06:55 PM
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Taking a hot lead from my breaker and going to the common of the motor and the other hot lead from the breaker going to the switch to either speed winding - thats 240v on either leg of the circuit when complete, this I get. But as referenced to ground, there is 120v on either side. Am I not understanding this correctly?
In my mind I see a floating leg of the circuit after the switch breaks continuity with the speed windings. If the motor should short to ground, yes the breaker could trip, but still there is a possibility for harm to occur should the breaker not trip in time.
 
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Old 04-21-12, 08:12 PM
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But as referenced to ground, there is 120v on either side. Am I not understanding this correctly?
Yes, 120v to ground. A two pole switch is safer.
 
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Old 04-21-12, 08:57 PM
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Am I then looking for a dp3t type switch or dp4t (off plus 3 other positions)? Or is it called something else?

Thanks in advance.
 
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Old 04-22-12, 12:05 PM
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A dis-connect is required for all motors; for a motor 2 HP or less , a two-pole toggle-switch with a ampere rating that equals the full-load current of the motor X 2 can be used, presuming this is a 240 volt , single-phase motor

If this motor is a single-phase , 240 volt motor with two speed-windings ( "Hi" / "Low" ) = three motor-connection leads , then the controller can be a single-pole , double-throw toggle -switch. The controller need not open all motor-leads.

Please understand that the ampere-rating of the fuse or circuit-breaker protecting the conductors that extend from the fuse / breaker panel to the motor is not neccesarily equal to the ampere-rating of the conductors so protected.

Any motor is required to have "Over-load" protection , and it's possible the OL protection for your motor may be "internal" , i.e. , connected inside the motor between the motor line-connection terminals and the motor windings. It would behoove you to determine this.
 
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Old 04-22-12, 05:55 PM
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"Any motor is required to have "Over-load" protection , and it's possible the OL protection for your motor may be "internal" , i.e. , connected inside the motor between the motor line-connection terminals and the motor windings. It would behoove you to determine this."

Indeed it would otherwise I might let out the magic smoke :-)

Although not required, I still would prefer to have the common motor lead as well as L2 open when switched.

Disconnect requirement is 2x full load current of the motor - nameplate rating then?

OL protection.... I doubt the motor in question has it built in since its a furnance blower motor and typically there are a lot of other electronics to monitor and control the device.
What type of OL do you suggest? Where should I start looking for something like that. Would a mini breaker of some sort be in order? I intend to mount this into a j box or a 4 square on the wall near the sub that will feed this circuit.

Thanks for your time and input!


"Please understand that the ampere-rating of the fuse or circuit-breaker protecting the conductors that extend from the fuse / breaker panel to the motor is not neccesarily equal to the ampere-rating of the conductors so protected."

Can you please clarify this statement: The premise of circuit breakers as I understand their function are to prevent over current in the conductors...
 

Last edited by chopnhack; 04-22-12 at 08:28 PM.
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Old 04-24-12, 01:16 PM
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When voltage is initially applied to the windings of a motor "at rest" , there will be a "starting" current value that could be 3 X the full-load current. If the full-load , or "running" current value of motor was 15 amps, the value of the current when starting the motor could be 45 amps , which would "trip" a circuit-breaker with a current-rating equal to the full-load current , i.e. , 15 amps.

AS to the question of the motor being designed with internal over-load protection ,I advise that you determine the make & model of the motor after which you can possibly find the needed detailed motor information "On-line" , or consult a motor repair shop that possibly could provide the needed information.
 
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