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Thread: Any benefit to run DC on 110 vs 220?

  1. #16
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    Quote Originally Posted by Mike Henderson View Post
    Just a correction to Eric's comment about an advantage to running a motor at a higher voltage giving a reduced current and heating effect.

    The way dual voltage motors are wired is that they have two sets of coils. When the motor is wired for low voltage (120 volts) the coils are in parallel. When the motor is wired for high voltage, the coils are in series.

    When the coils are in parallel (low voltage), the current is double the current when in series (high voltage) but only half of the current goes through each coil and the voltage drop across each coil is 120 volts.

    When the coils are in series (high voltage) the motor only requires half the current as when wired for low voltage and that current goes through both coils (in series). The voltage drop across each coil is 120 volts, half the high voltage of 240 volts.

    In either case, the voltage and current in the coils are identical and the heating effects are the same. There's no free lunch.

    Mike
    When starting, in either case, you won't be getting 120V across the coils. The locked rotor current can be 5-10x the full load current which can produce a significant voltage drop even if your wiring is up to code. The voltage drop will be less in the series (240V) configuration. Running off of 240 will generally bring the motor up to speed faster. Whether that makes a difference is another matter.

  2. #17
    Quote Originally Posted by Bill Dufour View Post
    120 does have one advantage. It can be switched with a single pole switch on the only hot leg. A 240 circuit needs a two pole switch, to be safe, one pole for each of the two hot legs. So a 240 volt switch is harder to find, costs more, and requires a trivial increase in wiring time at the switch box.
    Note that any proper motor contactor can be used for 120 or 240 and the control voltage can be 24, 120 or 240 depending on the coil. The coil can be replaced with a different voltage one if needed.

    Bill D
    The NEC permits a 240V single phase load to be switched only with one leg, and a 3 phase load switched with only 2 poles, as long as it is not the disconnect, and it is a very common practice in packaged air conditioning units, my point is that it is not a preferred practice, it is not wrong to do it that way.

  3. #18
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    Quote Originally Posted by Mike Henderson View Post
    Just a correction to Eric's comment about an advantage to running a motor at a higher voltage giving a reduced current and heating effect.

    The way dual voltage motors are wired is that they have two sets of coils. When the motor is wired for low voltage (120 volts) the coils are in parallel. When the motor is wired for high voltage, the coils are in series.

    When the coils are in parallel (low voltage), the current is double the current when in series (high voltage) but only half of the current goes through each coil and the voltage drop across each coil is 120 volts.

    When the coils are in series (high voltage) the motor only requires half the current as when wired for low voltage and that current goes through both coils (in series). The voltage drop across each coil is 120 volts, half the high voltage of 240 volts.

    In either case, the voltage and current in the coils are identical and the heating effects are the same. There's no free lunch.

    Mike
    Mike I agree that if you only look at steady state and assumed the voltage at the motor was exactly the same as ideal (120/240). But the first thing is the line losses leading up to the motor will be more in the lower voltage case and the V/f variances will cause differences. Additionally, my use of all motors in my shop are much more in a transient than steady state constant load case which cause even more differences.

    I completely agree that it is completely functional if you only have the lower voltage. But there are a number of reasons why you get improvements using the higher voltages.

  4. #19
    Quote Originally Posted by Eric Arnsdorff View Post
    Mike I agree that if you only look at steady state and assumed the voltage at the motor was exactly the same as ideal (120/240). But the first thing is the line losses leading up to the motor will be more in the lower voltage case and the V/f variances will cause differences. Additionally, my use of all motors in my shop are much more in a transient than steady state constant load case which cause even more differences.

    I completely agree that it is completely functional if you only have the lower voltage. But there are a number of reasons why you get improvements using the higher voltages.
    The differences you mention are in the circuit feeding the motor. If the circuits are properly sized so that the voltage drop during startup or running is the same percentage in either the 120V or 240V case, the motor will perform exactly the same and have the same heat characteristics.

    The problem is that the circuit feeding most motors running on 120V does not meet that requirement so you get better performance on 240V.

    Theoretically, they're the same. In real world situations, 240V operation often works better because of the circuit.

    In general, I prefer to run any motor over about 1.5HP on 240V.

    Mike
    Go into the world and do well. But more importantly, go into the world and do good.

  5. #20
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    Quote Originally Posted by Bill Dufour View Post
    120 does have one advantage. It can be switched with a single pole switch on the only hot leg. A 240 circuit needs a two pole switch, to be safe, one pole for each of the two hot legs. So a 240 volt switch is harder to find, costs more, and requires a trivial increase in wiring time at the switch box.
    Note that any proper motor contactor can be used for 120 or 240 and the control voltage can be 24, 120 or 240 depending on the coil. The coil can be replaced with a different voltage one if needed.

    Bill D
    Partially correct.

    Starters and switches for 120 volts need to have twice the current carrying capacity at 120 volts as they do at 240 volts.

    This increases the starter costs in many cases since you have to go up a size for 120 volts for the same motor power….Regards, Rod

  6. #21
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    Amp draw is higher on 110v and you will need a larger wire gauge and breaker. Power consumption is identical though.

  7. #22
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    Most tools come with a switch so why would you need a separate one?

  8. #23
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    Another consideration is what else if anything is on that 120 volt circuit. A typical 120 volt circuit is could be pulling 15 amps just running a 1.5 h.p. motor under load. Add in a few lights or a TV upstairs or who knows and lights dim/flicker or breakers trip. 240 volt circuits are less likely to see additional loads. I imagine this would mostly be a consideration in basement shops.
    Last edited by Curt Harms; 06-29-2022 at 7:42 AM.

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