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    So why does every new freight loco use AC traction, with approx twice the starting torque for a similar size/weight/HP machine?

    Likewise, look at where there are new series universal or DC motors in heavy industry. There just aren't. Even places like ball mills are moving to variable frequency AC.

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    Quote Originally Posted by SomeoneSomewhere View Post
    So why does every new freight loco use AC traction, with approx twice the starting torque for a similar size/weight/HP machine?
    You can actually research all those options if you care to do.

    Most of it is even published in English, though not so much of it is being done where English is actually spoken, day to day.

    US rail for freight is still mostly Diesel-powered, doesn't even have a lot of electrified routes, so it isn't on my radar.

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    Quote Originally Posted by SomeoneSomewhere View Post
    So why does every new freight loco use AC traction, with approx twice the starting torque for a similar size/weight/HP machine?

    Likewise, look at where there are new series universal or DC motors in heavy industry. There just aren't. Even places like ball mills are moving to variable frequency AC.
    more robust motor, might have solid copper bars in the armature = twice the amount of copper in the rotor as does a dc motor, so higher power density by default, lower losses, no energy robbing commutator or brushes to arc over.

    As for your grinder: most induction motors can produce about 2-2.5 times the rated torque at about 2-3 times the slip (1700rpm range for a 1750 rpm motor).. pull up torque might be as low as 1.25-1.5 times full load torque (500 rpm ish). the vfd will need to deliver about 2x the FLA to deliver 2x nameplate current, plus a bit more.

    if your grinder only needs 1.5 times the motor's rated torque at standard operating conditions to start up then your standard 2x oversized vfd (due to the single phase rectifier limit) will probably work for you. if it doesn't then the machine is overloaded. i suspect this could be the case given that grinding wheels probably have velocity dependent friction...

    running the vfd off of a RPC will help take the stress off the diodes but without a current probe and an oscope you don't really know if its doing any good. good news is the bandwidth required for this task is so low that you can buy a 50$ battery powered oscope off ebay and you can buy a current transformer on ebay for a few bucks and a 1-100 ohm resistor to load the ct and hook it up to your scope.

    also there is the matter of the generated leg typically being too high under no load and causing the drive to fault due to overvoltage.


    anyhow if your machine is not overloaded then your vfd does not need to deliver more than the nameplate FLA of the motor in order to start the machine.

  4. #124
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    Quote Originally Posted by SomeoneSomewhere View Post
    So why does every new freight loco use AC traction, with approx twice the starting torque for a similar size/weight/HP machine?

    Likewise, look at where there are new series universal or DC motors in heavy industry. There just aren't. Even places like ball mills are moving to variable frequency AC.
    I have been out of touch with the railroads since GE closed the shop that sent me work, but the last I heard the railroads were buying GE Dash 8 DC locomotives because they had so much trouble with the semiconductors in the AC ones. I never had anything to do with the AC locos but I gather that the advantage is startup control, bringing the torque up gradually. The Dash 8s generate AC, which is rectified and fed to traction motors, one for each axle. They can control the generated voltage and the traction motor fields. The problem is that if the wheels a DC motor is driving slip a little, the motor speeds up and turns it into serious slip. The AC traction motors act as synchronous motors and when they have wheel slip, only speed up a small amount, making it easier to re-establish traction. They also can reverse by changing the pulses rather than needing a big reversing switch with big contacts.

    In any case, it is moot here. Everybody is arguing about something that really only depends on how much energy comes through the user's power lines. Everything else is a dependent variable.

    DC would have one possible advantage here. Make a DC supply for the running voltage and add to it another higher voltage one connected through blocking diodes to pull the no load voltage up much higher, charging a capacitor large enough to give the motor a starting kick to break it loose, then falling to the normal voltage.

    Bill

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    Quote Originally Posted by 9100 View Post
    .....
    In any case, it is moot here. Everybody is arguing about something that really only depends on how much energy comes through the user's power lines. Everything else is a dependent variable.

    .......

    Bill
    His real issue is the single phase line, that forces the use of some kind of converter.

    I had thought he also said he had somewhat limited supply, but did not find it in a quick look through the thread.

    Anyway... Given that there is "some converter" and there is three phase to the motor.... And that the motor has a big starting load, mixed between inertia and startup friction....

    The choice is between a "slam it onto the line and let it do whatever", which would have been what was done before, at the 3 phase equipped shop, and "some" form of device to limit the draw to something that the converter can handle.

    Most regular soft start devices are designed to limit current from the source, trading that off with limited current /voltage to the motor.

    Resistive and inductive soft starts limit current, and therefore torque. They may keep the RPC happy, but may not do much for the machine. And, they typically have a time limit, which may or may not be compatible with the start time of the machine given that current limit (which we do not know). All of them do that by limiting the voltage.

    Transformer types also limit the voltage, bit do not put any limit on current other than whatever the limited voltage provides. They DO limit line draw, in proportion to the reduction in voltage (turns ratio), because lower voltage at the same impedance draws less current. They do not change the frequency, so they have a current limit due to motor inductance. They can do a good job of limiting line current, while NOT limiting motor current quite as much as the resistor etc type, but they also have a limited duty cycle in start time, and number of starts per hour, etc.

    The advantage of a VFD is as stated before. Because it changes both voltage and frequency, it can provide more current at start. The overload time adds to the capability, and any de-rating that may be required also adds current capability. You get this because you are transforming "power" and not voltage or current. It can also be the "phase converter".

    The good news s that it can be pretty well guaranteed to work. The bad news being that it uses nothing that is now present in the shop except the machine itself, and costs significant money

    No smoke and mirrors, no "free lunch", everything I have suggested is pretty much all basic physics, Thermite's protests notwithstanding.

    A big ass RPC would act more like a 3 phase power line. But the OP does not have one, It would have to be added, at a cost.

    Cutting the motor size would make the existing RPC more "big ass" in comparison. It might allow slamming the machine onto the output of the RPC just as if it were the powerco line. A possibly workable method, IF the smaller motor can get the machine running, which some "objectors" do not seem to think it can. Well, they don't know, I don't know, nobody can be sure about that. But if it were OK, it may be the least cost option.

    As for DC.... it can be good. A series traction motor will provide on the order of 200 to 300% torque, and that will occur at only about 1 3/4 x normal current.

    But controlling DC current efficiently still requires electronics, the motors are not cheap, and the machine already HAS a 3 phase AC motor.

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    7 pages of this! It's a wonderment?

    Stuart

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    Quote Originally Posted by atomarc View Post
    7 pages of this! It's a wonderment?

    Stuart
    Nah. Just part of the standard therapy covered by the combination of multiple competing retirement plans.

    Metal-mangling? Yah can enjoy staying "hands on" practically to your end-of days.

    Lethal-Voltage electricity?

    Anything BUT "hands-on"! Or your days could end "too soon".

    So it goes to key-bore-ed'ing the ISSUE to death!



    Twelve Dee Cee motors under roof here. Only slightly more than I have 3-Phase.

    I like BOTH.

    I *think* I still understand the merits - or lack therof - of EACH tribe?

    They are different.

    For-real.

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    10hp replaced by 5hp. inrush 170-180 now 70-72 but there is a trade off. tested with exh manifold and if no down pressure on manifold and waiting until full speed it works well. if down pressure during startup it won't come up to speed. won't come up with a cylinder head resting on the table so at this point i am restricted to manifolds only however that is them reason i bought it for.

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    Quote Originally Posted by cg285 View Post
    10hp replaced by 5hp. inrush 170-180 now 70-72 but there is a trade off. tested with exh manifold and if no down pressure on manifold and waiting until full speed it works well. if down pressure during startup it won't come up to speed. won't come up with a cylinder head resting on the table so at this point i am restricted to manifolds only however that is them reason i bought it for.
    "Maybe..." you could "mechanically" team TWO x 5 HP and stagger-start them?

    It takes VERY little time - in human terms - for the first one to get past its inrush mode, freeing resources for the second one to join it.

    Seems to me this rig is badly designed, though.

    It "should" have a mechanical means of "soft" start, such as a clutch as can be slipped - even a torque converter in the drivetrain.

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    Quote Originally Posted by thermite View Post
    "Maybe..." you could "mechanically" team TWO x 5 HP and stagger-start them?

    It takes VERY little time - in human terms - for the first one to get past its inrush mode, freeing resources for the second one to join it.

    Seems to me this rig is badly designed, though.

    It "should" have a mechanical means of "soft" start, such as a clutch as can be slipped - even a torque converter in the drivetrain.
    problem (now) is if it needs to come up to speed to kick in another motor that will never happen. coming up to speed is the issue now - and i guess before but before it had the hp just not the current

    i think a clutch that engages when the motor is up to speed may be disastrous with a work piece sitting there.

    plus two hands on the work, one foot on the pedal now another lever to pull :-)

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    Quote Originally Posted by cg285 View Post
    problem (now) is if it needs to come up to speed to kick in another motor that will never happen. coming up to speed is the issue now - and i guess before but before it had the hp just not the current

    i think a clutch that engages when the motor is up to speed may be disastrous with a work piece sitting there.

    plus two hands on the work, one foot on the pedal now another lever to pull :-)
    Not what I had in mind. Yah don't WAIT for it to come up to speed. The much shorter wait is for the worst part of the energy hit on the line to pass by.

    Both motors fully coupled at all times. The "mechanical" I had in mind was belts if shafts-parallel, Lovejoy if shafts-inline.

    At start, you hit a switch.

    - First one either moves the abrasive band (manifolds) or NOT. Good enough.

    - Got a cylinder head on the platen? Flip first switch, immediately flip the SECOND switch. And only about a quarter second later.

    - Common human reaction time is two-tenths of a second.

    - @ 60 Hz, pulses are only a tad over 8 Milli-Seconds apart.

    Breaker on the first motor "may not" have tripped by the time breaker on the second motor has thrown its shoulder to the wheel.

    If I'm right? We each buy our own pizza.

    If I'm wrong? We each skip the next pizza.

    Mind - unless borrowed // temporarily re-purposed, you could end-up with a 5 HP motor surplus.

    Worse things have happened?


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    Quote Originally Posted by thermite View Post
    Not what I had in mind. Yah don't WAIT for it to come up to speed. The much shorter wait is for the worst part of the energy hit on the line to pass by.

    Both motors fully coupled at all times. The "mechanical" I had in mind was belts if shafts-parallel, Lovejoy if shafts-inline.

    At start, you hit a switch.

    - First one either moves the abrasive band (manifolds) or NOT. Good enough.

    - Got a cylinder head on the platen? Flip first switch, immediately flip the SECOND switch. And only about a quarter second later.

    - Common human reaction time is two-tenths of a second.

    - @ 60 Hz, pulses are only a tad over 8 Milli-Seconds apart.

    Breaker on the first motor "may not" have tripped by the time breaker on the second motor has thrown its shoulder to the wheel.

    If I'm right? We each buy our own pizza.

    If I'm wrong? We each skip the next pizza.

    Mind - unless borrowed // temporarily re-purposed, you could end-up with a 5 HP motor surplus.

    Worse things have happened?

    i'm tired already just reading all that :-) plus now i have to hire an employee to flip all those switches while my hands and feet are full


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