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Thread: VSD power tests

  1. #106
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    Quote Originally Posted by cba_melbourne View Post
    . . . . . Be aware, that (almost) every VFD is capable of automatically feeding the motor with a 50 to 100% larger power than rated, for a short time of about 60 seconds. This means that you should let stabilize your test setup for maybe 1.5 to 2 minutes, before taking a reading.
    I can't see how this matters as I am measuring the phase current directly so whether it is over loading or otherwise seems immaterial. The max I can let the setup running with near motor stall load without smoking the paint on the brake drum is about 30 seconds. I will strip the paint off the drum and then see what happens. I reckon within a minute or so of load the brake will be glowing red. I'm not sure I want to be this for many tests especially at low speeds

    Several times during the testing the input current to the VFD briefly reached 10A but I should state that I'm doing these test with a 2.2kW VFD so its should not overloading in terms of current. The HP curves in the graph of post #94 are run right up to the stall point of the motor but I do not include the stall point HP as by then the motor has slowed well below its no load RPM.

    Anyone care to predict what might happen if I use a smaller or larger capacity VFD on the same motor?

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  3. #107
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    Quote Originally Posted by cba_melbourne View Post
    Something is not right with that plot or with the test setup, or with the VFD settings, or with the VFD itself. The motor power at 60Hz should be about 10% higher than at 50Hz, certainly not lower. I believe that even an older motor should remain more or less constant power from 50Hz up to about 70 or 80Hz, before slowly dropping down again and reach zero shaft power at about 120Hz.
    I don't think so.. what is under test is a V/Hz drive, the voltage and frequency increase up to nameplate frequency , and then voltage remains constant (at a maximum) so bearing in mind that the torque and hp readings are taken at the point where current reaches nameplate current, I can't see how hp could ever be higher at 60hz than it is at 50hz.

    Rated frequency is just a line in the sand, it can be anything within reason, it's just the point at which maximum voltage is applied to the motor. You could reprogram the VFD to make the base frequency 100Hz if desired. The logic doesn't change. You won't get more power above base frequency. You just get less power everywhere else

    That said, I'm surprised the 60hz data is so low, I would have expected about the same hp as 50hz.

    Here's the standard hp vs torque graph.
    speed torque hp.jpg

    Ray

  4. #108
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    Quote Originally Posted by BobL View Post
    Anyone care to predict what might happen if I use a smaller or larger capacity VFD on the same motor?
    I'd expect the same results if a larger vfd was used. But if a smaller vfd was used, you might hit current limiting and the vfd trips on over current. If the vfd didn't trip, you should see the same results as the larger vfd. ( that's my guess, anyway )

    Ray

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    Quote Originally Posted by RayG View Post
    I don't think so.. what is under test is a V/Hz drive, the voltage and frequency increase up to nameplate frequency , and then voltage remains constant (at a maximum) so bearing in mind that the torque and hp readings are taken at the point where current reaches nameplate current, I can't see how hp could ever be higher at 60hz than it is at 50hz.

    Rated frequency is just a line in the sand, it can be anything within reason, it's just the point at which maximum voltage is applied to the motor. You could reprogram the VFD to make the base frequency 100Hz if desired. The logic doesn't change. You won't get more power above base frequency. You just get less power everywhere else

    That said, I'm surprised the 60hz data is so low, I would have expected about the same hp as 50hz.

    Here's the standard hp vs torque graph.
    speed torque hp.jpg

    Ray
    Hi Ray,

    I think the makers of these small HP motors have been standardizing for the world markets. They like to use the same stators and rotors for 50Hz and 60Hz countries. This means that the efficiency of the motor must be optimised for 60Hz. Say a motor is rated at 1HP at 50 Hz, with a given voltage and a given current. If the same motor is driven at 60Hz with the same voltage and the same current, and the current results in the same torque, it is producing 20% more power (power = rpm*torque). The fan works harder though, and losses are greater at higher frequencies. But I would guess the motor is still delivering at the shaft some 10% more power.

    Larger motors are still designed for a specific base frequency

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    Quote Originally Posted by cba_melbourne View Post
    Say a motor is rated at 1HP at 50 Hz, with a given voltage and a given current. If the same motor is driven at 60Hz with the same voltage and the same current, and the current results in the same torque, it is producing 20% more power (power = rpm*torque).
    No, you don't get more power, what happens is that the torque drops as the rpm (Hz) rises, ( after you get to maximum voltage that is.. ) and as you correctly said..

    power = rpm*torque

    And strangely enough the drop in torque is *exactly* the amount required to balance the increased rpm so that the power stays constant..

    Well except for the fact that the field is weakening (causing slip to increase) and hysteresis losses go higher and stator laminations get hotter. So the power starts to fall away at some point. Depending on construction details, rotor slip increases to where it starts poling and things go south pretty quickly as torque falls in a heap.

    Ray

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    Interesting discussion here guys.

    I'd like to be contributing more data but I still many distractions. Refurbishing my 38 year old table saw (the one I have been using as a metal cutting saw) returning it to wood cutting for my son as I now have a plasma cutter. I also started to service my big chainsaw mill and found that one of the custom bar bolts has lost some of its thread so I have to turn up a new one and one of the welds on a cam lock has cracked. Have also been playing a bit with my new soldering reworking station and now I need to wire up a HD 15A extension cord for my new plasma cutter.

    Perhaps too many projects on the go at one time?

  8. #112
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    Quote Originally Posted by BobL View Post
    Perhaps too many projects on the go at one time?
    NEVER!!!! Thats only 4 or 5. Even if i take my shoes off i'd fast run out of digits if i counted mine!

    As Ray said, the big question is what is the difference between a motor running on star or delta.

    Cheers,
    Ew
    1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.

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    Quote Originally Posted by BobL View Post
    I
    Perhaps too many projects on the go at one time?
    I've seen a lot of ridiculous statements made on the net, but that one could win prizes. No such thing as too many projects running simultaneously.

    PDW

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    Quote Originally Posted by RayG View Post
    No, you don't get more power, what happens is that the torque drops as the rpm (Hz) rises, ( after you get to maximum voltage that is.. ) and as you correctly said..

    power = rpm*torque

    And strangely enough the drop in torque is *exactly* the amount required to balance the increased rpm so that the power stays constant..

    Well except for the fact that the field is weakening (causing slip to increase) and hysteresis losses go higher and stator laminations get hotter. So the power starts to fall away at some point. Depending on construction details, rotor slip increases to where it starts poling and things go south pretty quickly as torque falls in a heap.

    Ray
    Ray,

    I have a 3/4HP motor here, made in the EC, it is about 12 years old. I believe it belongs to the sort of "standardized for world market" breed that I mentioned. Its nameplate reads as follows:

    Th.Cl. F IP55
    50Hz: delta/star 230/400V 0.55kW 0.75HP
    60Hz: delta/star 265/460V 0.63kW 0.84HP
    50Hz: delta/star 218-242 / 380-420V 3.1A / 1.8A
    60Hz: delta/star 254-277 / 440-480V 3.1A / 1.8A
    1380/1700min-1 cos phi 0.67 / 0.67

  11. #115
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    Quote Originally Posted by Ueee View Post
    NEVER!!!! Thats only 4 or 5. Even if i take my shoes off i'd fast run out of digits if i counted mine!

    As Ray said, the big question is what is the difference between a motor running on star or delta.

    Cheers,
    Ew
    OK this will be relatively easy as the ASEA motor I have been doing the tests on can be quickly reconfigured to Y.

  12. #116
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    Quote Originally Posted by cba_melbourne View Post
    Ray,

    I have a 3/4HP motor here, made in the EC, it is about 12 years old. I believe it belongs to the sort of "standardized for world market" breed that I mentioned. Its nameplate reads as follows:

    Th.Cl. F IP55
    50Hz: delta/star 230/400V 0.55kW 0.75HP
    60Hz: delta/star 265/460V 0.63kW 0.84HP
    50Hz: delta/star 218-242 / 380-420V 3.1A / 1.8A
    60Hz: delta/star 254-277 / 440-480V 3.1A / 1.8A
    1380/1700min-1 cos phi 0.67 / 0.67
    Good example, that illustrates my point... the difference in power is purely due to the higher supply voltage, nothing to do with frequency.

    Ray

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    Quote Originally Posted by PDW View Post
    I always thought I'd have to go from a 3HP motor to a 7.5HP VFD driven motor if I ever got rid of the Kopp variator and still need a 2:1 intermediate shaft somewhere. That may even be optimistic.
    As you have real 3 phase couldnt a two speed motor replace the 2:1(or maybe both would be the go?*)

    Quote Originally Posted by BobL View Post
    Perhaps too many projects on the go at one time?
    Certainly not.............. and it gets one out of my shed

    Quote Originally Posted by cba_melbourne View Post
    50Hz: delta/star 230/400V 0.55kW 0.75HP
    60Hz: delta/star 265/460V 0.63kW 0.84HP
    But isnt the V increase is where the extra power is coming from???


    Stuart

    *Though not with a 2:1 gear ratio

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    Quote Originally Posted by Stustoys View Post

    But isnt the V increase is where the extra power is coming from???
    Maybe this helps:
    Motors: Changing between a 50 and 60Hz supply. - Electric motors, generators & controls engineering FAQ - Eng-Tips

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    Quote Originally Posted by RayG View Post
    Good example, that illustrates my point... the difference in power is purely due to the higher supply voltage, nothing to do with frequency.

    Ray
    Yes, it is the changed V/Hz that makes the higher voltage possible at 60Hz in the first place. If you tried to apply the higher voltage at 50Hz you would saturate it and it simply gets hot instead of delivering more power. The VFD supplied from 240V AC has 240 * 1.41 = 338V DC available at its DC bus and should be able to provide the motor with the higher voltage that will lead to a higher HP output at 60Hz .

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    I'll let you think about that some more before I reply, maybe you will see your mistake

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