220 vs 110v

chris_goris

Chris
Senior User
Added question pertaining to the subject: If changing a 120v/15A saw to 220v, what would be the appropriate sized 220 breaker to protect the saw motor?
In theory, a 120v/15a motor now wired for 220v would use half the amperage nominally. But at start up, these induction motors require an inrush of 1.5 to 2x the rated amperage, so a 220V 15amp breaker should be sufficient. Not sure if you can get one that low, I think they start at 20 amps.
 

bob vaughan

Bob Vaughan
Senior User
A circuit breaker in the electrical panel won't really be a good motor overload protection. That device needs to be at the machine and the debate on those devices can be bewildering and worthy of a separate thread.
 

tvrgeek

tvrgeek
User
The motor will have a thermal breaker in it that protects the motor. The panel breaker is to protect the wiring and structure. Size the breaker for the size of the wiring and put in the correct outlet for that rating. 12 gauge wire can usually be fitted with 20A breakers. Check your local code.
 

bob vaughan

Bob Vaughan
Senior User
The motor will have a thermal breaker in it that protects the motor.
That's not always true and in fact it is the exception rather than the rule. That little red Klixon button found on the occasional motor implies that it will protect the motor but I've seen burned out motors where that overload didn't cycle. That red button overload is found on most of the Sears Roebuck radial arm saws.
Like I said, this subject should be a separate thread rather than comparing the advantages and disadvantages of 120 volts and 240 volts.
 

Willemjm

Willem
Corporate Member
Why do these questions always go so far off track?. The OP's question was about tool performance when wired for either 110 or 220. It is common to find that motors in the 3/4 to 1 1/2 horsepower range are dual voltage. The term dual voltage means that the motor has two sets of 120 volt windings for the purpose of being configured for either service -- parallel for connection to a 120 volt service or series for connection to a 240 volt service. Internally, as far as the motor is concerned, there is no difference since the current through each winding is the same. Externally, as viewed from the electrical service panel, there is the difference in the required minimum branch circuit breaker sizing . The only advantage of operating from 240 volts would be the convenience of taking advantage of an existing 240 volt outlet if 120 volts isn't conveniently located close to where you plan to locate the machine. That's not much of an advantage, but I wanted to emphasize that as far as the motor is concerned, there is no advantage of wiring the motor for one voltage over the other.
Why are motors above 2hp generally 240V motors?
Why above 5hp generally motors are 3 phase?
What about back EMF in the stator windings?
What about I squared R losses especially during motor start?
What about switching and arcing when interrupting a low current circuit versus a high current circuit?
Why is it much easier to stall a 3hp 120V router, compared to 3hp 240V shaper?
 
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JimD

Jim
Senior User
Mark, breakers are sized to protect wiring from overheating and starting a fire. Our tools should have their own overload protection. But if we ignore that, you would want a 7.5A breaker.
 

JimD

Jim
Senior User
Why are motors above 2hp generally 240V motors?

The current gets too high for common conductors to work for large motors at 120V. The running current is not so much the issue, it is the starting current. The best 2hp induction motors have an inrush of almost 30A at startup. Fortunately it doesn't last long enough to overheat the conductor but if we went up to a 3hp motor, you can see where we are well above what a 20A circuit can safely handle. The conversion of hp to kw is almost .75kW/hp. So a 2hp motor would draw about 12.5A. So the running load of a 3hp might just squeak under 20A - but we haven't figured in effiency yet - but when we consider startup, it is clearly too much. You'd also need outlets and plugs and flexible conductors capable of the higher current. The biggest commonly available 120V outlet is 20A.

Why above 5hp generally motors are 3 phase?

I am certainly no expert in specifying large electrical motors. I checked only two sources to verify your assertion that motors above 5hp are generally 3 phase. McMaster Carr agrees with that statement, Amazon does not. I would guess there is a difference in audience that may help explain that. In a home, we are typically limited to single phase power and would thus want a motor that is single phase. In a plant, 3 phase power could be more readily available and would save wiring in the plant. But that is just a guess. Large customers sometimes get billed for power factor - kind of like electric usage by phase - and that would make them want to use 3 phase motors.

What about back EMF in the stator windings?

I don't understand the reference. All motors, even DC motors, have back EMF. I don't think it's a difference between single and 3 phase motors. Or a difference between 120V and 230V motors.

What about I squared R losses especially during motor start?

Higher current means 120V motors will have higher resistive (I squared R) losses. I wouldn't count on seeing any difference in your electric bill if you change.

What about switching and arcing when interrupting a low current circuit versus a high current circuit?

The ability to arc is created by the voltage, not the current. Higher voltage switches require more careful design and tend to be more expensive because of this.

Why is it much easier to stall a 3hp 120V router, compared to 3hp 240V shaper?

The router is not really a 3hp motor. Universal motors tend to get a rating based upon their stall current and running voltage which never occur simultaneously. I just bought a PC 7518 for my router table. It is rated 3.25 hp by the manufacturer. But it also is rated to pull 15A. If it was 100% efficient, 15A at 120V would only be 1.35hp. Universal motors are not typically really high effiency so I would guess it's closer to a 1hp router. That is still a big one. But even a 1.35 hp router would be less powerful than a 3hp shaper.
 

chris_goris

Chris
Senior User
Why are motors above 2hp generally 240V motors?
Why above 5hp generally motors are 3 phase?
What about back EMF in the stator windings?
What about I squared R losses especially during motor start?
What about switching and arcing when interrupting a low current circuit versus a high current circuit?
Why is it much easier to stall a 3hp 120V router, compared to 3hp 240V shaper?
My point was, these things tend to suddenly get derailed by tangents, the original posters questions dont get answered. He never asked about 3 phase or anything other than a voltage switchable motor and suddenly, the discussion went to motor overload protection and breakers...
 

Skymaster

Jack
Senior User
Why are motors above 2hp generally 240V motors?
Why above 5hp generally motors are 3 phase?
What about back EMF in the stator windings?
What about I squared R losses especially during motor start?
What about switching and arcing when interrupting a low current circuit versus a high current circuit?
Why is it much easier to stall a 3hp 120V router, compared to 3hp 240V shaper?
because the router is a universal high rpm low tourque motor while a shaper is a high torque tefc industrial motor :D :D :cool: :cool: ;);)
 

Willemjm

Willem
Corporate Member
My point was, these things tend to suddenly get derailed by tangents, the original posters questions dont get answered. He never asked about 3 phase or anything other than a voltage switchable motor and suddenly, the discussion went to motor overload protection and breakers...
I thought post #3 was pretty accurate and answered the OP pretty good?
 

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