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Thread: Coping with start up current on large single phase motors?

  1. #1
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    Coping with start up current on large single phase motors?

    Hi guys. Over here in in Ireland/UK we rarely see 5hp single phase motors used due to the high start up current, and machines like large bandsaws are not offered with single phase motors.

    At our 220V single phase the plated current is roughly 16A, but they apparently can draw x5 or up to 80A on start up. Which is a problem, since most incoming service provider fuses are only 60A, with (not always) the possibility of an upgrade to 80A.

    At least some of the few items with large e.g. 4kW 'single phase' motors like Felder saws actually seem to use 3 phase motors driven by VFDs (inverters) to control the start up current.

    Yet in the US many of the big bandsaws etc seem to be offered in 5HP/3.75kW sizes.

    I guess i'm wondering (i have a big 3 phase bandsaw to run from single phase, and am exploring rotary converters, dual voltage motors with a VFD and conversion to single phase) how you guys get around this issue? Maybe you have a 100A supply or something???

    Does anybody know if the single phase motors supplied on these machines stock single phase types, or do they use Felder's VFD/dual voltage trick or capacitors enable them to start against load?

    Do stock single phase motors present any problems when starting a big bandsaw with cast iron bandwheels, or with the slightly less robust performance of single phase motors?

    Thank you,

    ian

  2. #2
    If all wiring had to be rated for start up current, most homeowners couldn't run a 1 1/2 hp saw. That's where 'slow blow' fuses and breakers come into play. They allow for the momentary surge at start up.

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    I would also think 5x current draw is a bit high... 2-3x is more likely in all but the most odd of cases.

    Have you considered running a VFD, as you mentioned? Hitachi makes a really robust line, and interestingly enough I was just looking at prices... several models support up to 5HP motors and only run about US$350-$500 (depending upon model). You can set it up for soft starts, etc. which would drastically reduce the surge current rating, and you get variability of speed via keypad.
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    Quote Originally Posted by Dan Hintz View Post
    I would also think 5x current draw is a bit high... 2-3x is more likely in all but the most odd of cases.

    Have you considered running a VFD, as you mentioned? Hitachi makes a really robust line, and interestingly enough I was just looking at prices... several models support up to 5HP motors and only run about US$350-$500 (depending upon model). You can set it up for soft starts, etc. which would drastically reduce the surge current rating, and you get variability of speed via keypad.
    5x isn't unrealistic. Inrush currents are pretty intense.

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    Ian

    Without going nuts with theory. The inrush peak on start-up is a short duration spike, measured in milliseconds. The breakers have a "time delay", if you will, that will not trip the breaker unless the inrush current stays high for too long, or the voltage stays too low, which would cause the current to remain high.
    If you were to plot the current on initial startup, versus voltage, you would see that as voltage increases, current decreases. Remember though that this is all happening in milliseconds, or cycles. It's very fast, and highly doubtful that you would ever be able to see it on any "meter" a homeowner could be expected to own.

    Wiring, and insulation resistance, are rated for continuous ratings, and peaks, at temperature, and have an "envelope" to maintain conductor integrity.

    Rick Christopherson, a member here on the board, has ( or used to have ) a really nice website for understanding electrical theory as applicable to the shop. It's in pretty straight forward terms. If he still has it up, it's a good read.
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    Quote Originally Posted by Matt Armstrong View Post
    5x isn't unrealistic. Inrush currents are pretty intense.

    I agree 5X is not that unusual. As others have mentioned inrush currents last a very short time on typical motors, I have rarely has an induction motor trip even a "standard" breaker even when the motor current rating is right at the breaker amperage.

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    Well, yet again I'll point to the fact that I was a solid 'C' student when it came to my power classes... didn't like it then, don't like it now. I'm just trying to imagine the large inductance of such a sizeable motor allowing 5x current in, even for a short time...
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    Hi Ian,

    To use the Grizzly G0568 (5 HP, Single Phase) as an example if we were to buy that saw we would have to feed it with a 40 Amp Circuit and Wire Size #8 per the Grizzly specifications. The standard voltage for a single phase service here in America is 115/230.

    If I'm not mistaken European Voltages are higher which would lend itself to lower amperages.

    In your case of having a three phase motor and single phase service I think your options are two get the circuit to the band saw converted to three phase or change the motor to a single phase. I'm not sure which way would be the most cost effective.

    PHM

  9. #9
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    Hi guys, thanks for that. Don't worry Dan, my 5X was just from a casual comment by a local electrics guy so your number is as good as mine.

    The slow 'pop' of the mains fuse or a local breaker sounds promising Mike. I can fit a slow 'C' type breaker, but i wasn't too sure if it would cope with the saw given the heavy flywheels. Presumably the incoming fuse is 'slow' too.

    An inverter would be my first choice as well Dan - it's both cheaper, and as you says controls the start current. Unfortunately the saw manufacturer (Agazzani) so far as i can establish fitted a 3 phase 400V delta only motor to the saw - not the dual voltage star & delta type than can run at 220V/3phase - which i gather are usually plated 220/400V. The inverters sold over here don't step up voltage, reportedly something to do with feeding harmonics/interference back into the system.

    I've a query in on this with Agazzani, but haven't had an answer yet. I don't know how to tell from an inspection of the terminal box if this is definitively the case (photo attached), but so far as i know dual voltage types usually have copper dog bones on the terminals to facilitate the change over.

    The only other suitable option i'm aware of that can deliver the required 400V/3phase is a rotary converter, but so far i've not been able to dig one up at an affordable cost. I wasn't sure about start up current either, although i've been told they limit it to about X3. New is expensive at about $900 over here, but if going that road i should really buy a much larger one that would enable me to go 3phase in future with more equipment.

    Just out of interest. What is the maximum current you guys are normally allowed to draw on a single phase 220v (?) domestic supply?

    ian
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    Last edited by ian maybury; 09-02-2010 at 6:26 PM.

  10. #10
    Quote Originally Posted by ian maybury View Post


    Just out of interest. What is the maximum current you guys are normally allowed to draw on a single phase 220v (?) domestic supply?
    200A is standard.

  11. #11
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    A number of the newer (and bigger) homes are often supplied with 300A (and I've heard of 400A service in the big McMansions). I drooled a bit at seeing a 300A panel while doing a walk-through... that would make a nice sub-panel for a workshop and have practically zero effect on even larger households
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  12. #12
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    Hi, I have two Hammer machines with 4 HP motors rated at 16 Amperes.

    The maximum allowed circuit ampacity for those two machines is 20 Amperes.

    Both of them start and run on a 15 Ampere circuit at my shop.

    As was previously posted, you can purchase fuses with time delay characteristics suitable for motor starting, from your local electrical supplier.

    Regards, Rod.

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    Motor Starting (Long)

    Hi, Ian. I'm mostly a lurker here, but maybe I can shed a little light on the motor issue, since I make my living as a EE.

    The 5X full load value given for starting current is a reasonable guess. It could be even higher, as others have mentioned. When an induction motor is started "across the line" at full voltage, there is a very brief period of "inrush current" - this is due to energizing the large inductance that is the stator winding. This will last a few cycles at most and could be 8 X motor full load amps. After this, while the motor is coming up to speed, it will be drawing "locked rotor current". This is the 5X value mentioned earlier. The terms 'inrush current' and 'starting current' are often used interchangeably, but technically, they are not the same thing.

    The motor will draw close to locked rotor current until it is nearly at full speed, then close to full speed, the current drops off drastically.

    Also, on a weak system, such as a residence, there is often a lot of voltage drop during starting due to the resistance in the wiring used. This tends to reduce the voltage at the motor terminals and this does reduce (not increase) the motor locked rotor current. This is the basic principle behind reduced-voltage starters.

    As far as your fuses and/or breakers: circuit breakers have two tripping elements - a thermal overload element and a magnetic short circuit trip. The thermal element is time delayed based on the amount of current. At 85 A, it might take a couple of hours to trip. At 400 A, it might be a second or two. The short circuit "magnetic" trip is instantaneous. The magnetic trip will probably actuate somewhere between 5X and 10X of the breaker rating, or 400 A to 800 A for an 80 A breaker. Fuses have somewhat similar characteristics.

    So long story short, if your 5 hp motor is drawing 80 A locked rotor current it is probably NOT going to trip an 80 A breaker during starting, since the acceleration time is too short to cause the thermal element to trip.

    But if you have a three-phase service, a three-phase motor is a step up in reliability and performance over a single-phase motor, all other things being equal.

    Hope that helps - sorry for the length.

    Cheers,

    Dave

  14. #14
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    Based on what you have to say (Rod and David) it looks like a single phase motor conversion is possible. Even if it exceeded the nominal 80A of my supply for short periods, it looks like with the right breaker and fuse fitted it should be OK.

    That 24in Grizzly GO568 @ 5hp with its cast iron wheels must be very similar to my 24in Aggazani Paul, so 40A @230V is probably a good bench mark. That's a bit more than 2X the name plate amps i suspect, which is not that high and again suggests that it's normal to presume that circuitry can and is routinely expected to handle short duration currents much higher than their nominal rating - presuming that is there's nothing special about the Grizzly that keeps the current down vs a stock single phase motor set up.

    16, 32 and i think 63A are standard ratings for single phase sockets here, although the 63A would be pretty unusual.

    Given that an inverter is likely to require me to buy a dual voltage motor as well the single phase conversion or rotary converter look like the most cost effective options - probably the latter is best given that it retains the 3 phase motor, and i gather reduces starting current to 2 - 3 X.

    All i need now is for a used one of the right kW to come up for sale. Most bought here are used to power whole shops and tend to be OTO 10 - 15kW - and consequently even second hand cost about $1,500....

    Thank you guys, some great info there.

    ian

    PS I'd hate to be paying for the power consumption in a house with a 200A supply - our electricity costs about 18.5c/unit (kWhr). Probably multiples of what you guys pay - yet another gift from the state/legacy of yet another unreformed state monopoly gone private - having been sold to maximise short term government gain and to hell with the consumer.

  15. #15
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    My 2HP Dewalt RAS (17A @220V on the motor plate, what they used to call a TRUE 2, probably more like a 3HP today) runs fine on a 20A circuit, draws 93A momentary on start up! My FIL (former electrician) measured it with a neat device that looks like a little hand cuff that goes around the wire. Thats nearly half my service for a few milliseconds.

    Ian, I bought a Gentec American Rotary phase convertor, 10HP, to push an SCMI shaper (5 HP), cost around $700 new with slave motor and control box. Add several hundred dollars in wire and incidentals to set it up. The voltage issue will add to the cost I imagine considerably. I was converting 220 single to 220 three phase. I think Gentec American rotary also handles transformers for the voltage step up but I have no idea what the cost is. It might be a better option if three phase is the norm. Three phase is the norm here in most industrial or commercial situations, but is very difficult to get in many residential areas and cost more per Kw.

    I saw a picture in a museum of a large wooden band saw being powered by two apprentices on a sort of tandem bicycle whose out put could be switched via leather belt to a lathe also. So good luck getting that big bandsaw running, and remember things could always be worse! It could have come with pedals!

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