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Three Phase Motor takes too much Current

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  • J Tiers
    replied
    Good that you got that figured out!

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  • nickel-city-fab
    replied
    Originally posted by The Metal Butcher View Post
    The case of the excessive pixies has been cracked by Detective Tiers and others!

    Tl;dr, other than being a noisy POC, low efficiency motor, it's fine, and it was just me, and undersized wire thinking it was not.

    Thanks all!
    When in doubt, over-engineer the >bleeep< out of it I used 10/3 wire on my big Black & Decker grinder....
    (my old boss was noted for his over-engineering.... it actually made sense after a few yrs)
    Glad you got it sorted!
    Last edited by nickel-city-fab; 03-18-2021, 11:35 PM.

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  • The Metal Butcher
    replied
    The case of the excessive pixies has been cracked by Detective Tiers and others!

    You are correct. My aggressive use of the clutch was slowing the motor more than I thought. I attempted to bog it down to get some new current readings, a feat more more difficult than I thought. I couldn't read the current at the lathe (safety) while operating it, so I measured current at the RPC, but isolating the lathe alone this time. It was hard enough to get the clamp on one wire, so I only tested one of the real legs.
    1. Idle current was 10.5 amps. Not running the spindle. I had measured 15 before at the lathe. I attribute the drop to 3 things: Digital meter that is probably more accurate, upgrade to 12 gauge wire from 14, measured at the RPC side instead of at the lathe so voltage hasn't had a chance to drop yet.
    2. Spindle running: 11.3
    3. Light cut: 11.3
    4. Medium cut: 15.5
    5. Heavy cut: 17.6
    So, eh, that's within reason. I'm sure the generated leg was an amp or two lower than the reals. My machining calculator I made says that the last cut was a 5.06HP cut assuming the machine is 85% efficient. The details, should anyone want to calculate it themselves: Cast iron brake rotor, ~9" OD (eyeball, forgot to measure), facing cut, 0.1" stepover, 0.01" radial feed, 238 RPM. K factor used is 1.56.

    Tl;dr, other than being a noisy POC, low efficiency motor, it's fine, and it was just me, and undersized wire thinking it was not.

    Thanks all!

    Leave a comment:


  • farmermachinist
    replied
    I have had similar experiences with running 5hp motors on a 7.5hp rpc. The 5hp Baldor 2 pole motor on the table saw runs great with good phase to phase balance. The Reliance 5hp 6 pole motor on the Sidney lathe has low voltage (200v) on one phase and vibrates somewhat ( I attribute that to the electrical imbalance ). 3hp (both older GE Triclad) motors on a mill and a radial drill press run well balanced and reverse instantly. I have found that most commercial rpc manfacturers recommend a minimum 10hp converter to run a 5hp load. I am looking for a 10hp motor to build a new rpc.

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  • PStechPaul
    replied
    Originally posted by The Metal Butcher View Post
    RPC is fine.

    High current is on two of the legs and they are equal. The third is low as the voltage is low, so it's nothing to be suprised at there.
    How are you measuring voltage? It should be phase-to-phase, or A-B, B-C, and C-A. Assuming the manufactured leg is B, and that current is low, and voltage is low on B-C, that means that the manufactured leg B is collapsing toward the C leg, and its phase with respect to A-C will be far from the 60o that it should be. This is where a proper three phase power analysis meter is useful, being able to read all three phase currents, voltages, power, and phase angle.

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  • J Tiers
    replied
    Originally posted by The Metal Butcher View Post
    Interesting. I did not know that the current could be spiked over nameplate. Well I got some free brake rotors, I will turn them to dust and see if I can't stall it out. Results... possibly later today. Stay tuned.
    The way that works is:

    The motor resistance is far less than what would draw nameplate current. The motor, when running, develops a reverse voltage ("back EMF") which opposes the input voltage. This happens because the stator current induces current in the rotor, and the rotor current produces a magnetic field. Spinning that magnetic field generates the back EMF.

    So, that back EMF is dependent on RPM. Slower means less of it. When running with light load, the back EMF comes up to nearly mains voltage, with the difference being just enough to allow the idle current to flow through the motor resistance.

    If you apply a sudden load, the motor slows down, and the back EMF drops. That allows more current to flow, which produces more torque. The added torque allows the rotor to speed up, and the voltage will balance at a new current that provides just enough torque to spin the new load.

    If the "load" is just inertial, requiring to speed up added mass that has little added friction etc, then the motor will draw more current and speed up until the speed returns to near the previous speed, and will hold at whatever speed allows just enough added current/torque to run the new load. But while it is accelerating the inertial load, it will draw quite a bit more current as it needs a lot of torque to speed up the added mass.

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  • The Metal Butcher
    replied
    RPC is fine.

    High current is on two of the legs and they are equal. The third is low as the voltage is low, so it's nothing to be suprised at there.

    Leave a comment:


  • PStechPaul
    replied
    You might try altering the phase connections ( A-B, B-C, C-A ), to see if the high current still occurs on the same motor lead. In that case, the motor is OK and the RPC is at fault. You might try fiddling the compensation capacitors at normal load.

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  • Rich Carlstedt
    replied
    Some motors are Pigs !
    Especially if they are 2 speed motors.
    Like all things---tradeoffs are made during manufacturing or in the design department
    That motor was made in Korea...just because a GE stamp is on there, does not mean it's A OK
    Rich

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  • The Metal Butcher
    replied
    Interesting. I did not know that the current could be spiked over nameplate. Well I got some free brake rotors, I will turn them to dust and see if I can't stall it out. Results... possibly later today. Stay tuned.

    Leave a comment:


  • Ringo
    replied
    Originally posted by The Metal Butcher View Post
    Hi All,

    Dad and I were playing with a new digital amp clamp multimeter the other day, and we discovered that the 5hp motor in the Sidney lathe pulls way too much juice. At idle, the combined current on L1 or L2 of the phase converter and lathe was 17 amps. The phase converter idles at about 5. I've measured the Sidney once before at 15 amps by itself with an old analog meter, so it's not the meter. It pulls around 55+ starting. I didn't have have time or material to waste to put it under a hard load, but I did yank the clutch a few times pretty fast in high gear to try to bog it slightly. And even though I never heard it bog much of any, we could spike the combined current draw into the 40s and even 50s once.

    In comparison, the Lagun's 3.5/7 HP motor draws between a combined 28-35 starting, and 9-12 idling.

    So what does that mean? Internal short?

    I guess I should keep my eyes peeled for quality motor, especially an old Babbitt bearing one...
    Since I been adjusting caps on my own RPC, and researching the data from others, L1 will read lower than T1.
    Assuming L1, L2 is your single phase side, and T1, T2, T3 is your 3phase side, then the total amps on 3ph side reads higher than 1ph side. Don't ask me why, it just shows up that way on a amp clamp.
    There is certain amount of VooDoo, Black Majic, and false readings in a RPC. One research item said to balance your caps to equalize your 3ph amp legs idling the (5hp) load motor. I did that and later turned on a small grinder, the amps on L1 went down with the grinder coming on. That right there tells me there is voodoo & false readings in a RPC.
    I wouldn't get too wrapped around axle on this.

    Leave a comment:


  • rdfeil
    replied
    I am with Jerry on this. Spiking the motor with a sudden load IE: dumping the clutch, WILL cause a high spike, which will return to a lower level quickly. Another thing that add unknowns to this mix is the phase converter. Phase converters want constant loads for constant performance. By spiking the load you are causing current, voltage and phase changes which ALL interact. As long as the motor current is 15 amps or less when you are in a constant condition (Not starting or bumping the clutch etc...), you are good!! Motors can take a lot of abuse as long as they do not get excessively hot or the overload conditions do not instantly melt wires or arc through the insulation. Do not worry about the differences between the two motors.

    Also, remember the rated current on the nameplate is "Full Load" current only. Starting current and accelerating current can and will be MUCH higher.
    Last edited by rdfeil; 03-18-2021, 04:40 AM.

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  • J Tiers
    replied
    Originally posted by The Metal Butcher View Post

    30 amps on the MFG leg when starting. Only a 7.5 hp converter. I will try to get some current values under load next time I run it. But I don't want to test it to death. I can replace it faster than that. I just wanted to hear the electrical gurus opinions on here for why this thing draws so damn much power.
    Originally posted by The Metal Butcher View Post

    ............................................But it shouldn't pull 50 amps under load when it's rated for 15. That's BS.
    Originally posted by The Metal Butcher View Post

    Right. It does start at over 50 amps. Not a huge issue, though it is considerably higher than the aforementioned 7HP.

    Yes. I can make it draw 40-50 amps just by putting it under a load (engaging the clutch).
    Hold on there.....!

    Are you saying it pulls 50A under load "continuously"?

    Per your earlier posts, you said you could "spike it" into 40 or 50 A by suddenly applying a load. I would regard that as perfectly normal...... so long as it comes back down once equilibrium is again established. That's only 3x FLA, and perfectly normal for a sudden load change. The motor may draw double that when starting, but unless the meter has a "peak hold", you will not see the peak, you will see a lower number.

    As for drawing "power", we have no idea how much power it is drawing. All we have is current readings. We do NOT know what the voltage was when that current is drawn.

    Normally, an unloaded motor will draw 40 to 50% of the FLA at idle, although some may draw less, as little as 25%. But the phase angle relative to voltage is so large that the net power is low. The "power factor" (Cosine phase angle) is as low as 0.1.

    What is your local voltage? Low voltage will generally cause a larger current, and that can even occur at idle.

    There are a lot of variables here. You have a motor that is running from an RPC, where the current on one phase is known to be low, and the voltage is almost sure to be low and/or at a somewhat incorrect phase angle.

    That motor would likely draw its expected current when supplied from a good 3 phase source. An RPC is an "OK" source. They work well, but are not the same as a well balanced powerco 3 phase. The motor is not operating under the normal conditions, and so will not draw the normal currents. The unbalanced voltages which are inevitable will likely lead to excess circulating currents in the motor, and those have to come from somewhere.

    If you really suspect a problem, then pull the motor and let a motor shop check it. If there is anything funky about it, they should find it.

    Leave a comment:


  • The Metal Butcher
    replied
    Originally posted by rdfeil View Post
    The Metal Butcher , Sorry I am not following exactly what you are saying. I thought you said the motor would draw up to 50+ amps on startup. If that is what you said and mean, it could easily be very normal. It is not uncommon for a motor to draw 5 to 10 times the rated full load current when starting. Many things effect this including, load, voltage, voltage balance and phase relationship along with others....
    Right. It does start at over 50 amps. Not a huge issue, though it is considerably higher than the aforementioned 7HP.

    Originally posted by rdfeil View Post
    Now if you are saying the motor draws 50 amps when running steady, now you have a problem!! High start currents are 100% normal for motors and different motors will have different current draws. It all depends on the engineering that went into the motor design....
    Yes. I can make it draw 40-50 amps just by putting it under a load (engaging the clutch).

    Leave a comment:


  • rdfeil
    replied
    The Metal Butcher , Sorry I am not following exactly what you are saying. I thought you said the motor would draw up to 50+ amps on startup. If that is what you said and mean, it could easily be very normal. It is not uncommon for a motor to draw 5 to 10 times the rated full load current when starting. Many things effect this including, load, voltage, voltage balance and phase relationship along with others....
    Now if you are saying the motor draws 50 amps when running steady, now you have a problem!! High start currents are 100% normal for motors and different motors will have different current draws. It all depends on the engineering that went into the motor design....

    Leave a comment:

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