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1st July 2019, 09:22 AM #31Member
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Sorry correction my last post should have read " Richardson" 1/2 HP 415 volt 3Ø drill press not "Robertson".
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1st July 2019, 09:56 AM #32.
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The math is correct but given that all that can be done is to compare the relative change in resistance, for both a ∆ and Y connected motor the relative change in R between the unshorted and shorted state is 0.75 so it is not possible to tell which it is. I have tried doing this in practice with many motors and could never see a difference in the relative change in resistance.
Y connection
If the resistance of any one coil is R
unshorted connection between any 2 coils has a resistance of 2R.
Shorting any two coils resistance is now 1.5R
Relative change in resistance is from 2R down to 1.5R or 1.5/2 = 0.75
∆ connection
unshorted connection between any 2 coils has a resistance of 2/3R.
Shorting any two coils resistance is now 1/2 R
Relative change in resistance is from 2/3 down to1/2R , (1/2)/(2/3) = 0.75
you will most likely find these days that the stator winding is encapsulated in resin and not readily modified whether it's delta or star, that said even some of the old star wound motors could have there center tap separated and wired into a six tail delta configuration but this is unlikely with a mass produced European machine.
The larger the motor the easier it usually is so motor rewinder will find it relatively easy to convert such a large motor. The hardest motors to work on are the smaller ones. The smallest one I have worked on was a 1/30th HP where the wires were like hair and very easily broken.
Within my own workshop I have a vintage "Robertson" 1/2 HP 415 volt 3Ø drill press, that I'm considering upgrading with an Cheep $300 eBay 3Ø to 3Ø VSD/VFD so as to avoid changing the speed belts all the time. Should this cheap 3Ø to 3Ø controller ever fail it can be simply switched out of circuit and I will still have a functioning machine.
With respect to your N4400 band saw should the single phase to 3Ø controller fail you will be with out your critical machinery until the controller is replaced, therefore I would recommend that a quality derated VSD/VFD be installed, in commercial applications we currently consider the brands SEW Eurodrive, Danfoss and Siemens safe bets and match the quality of the Hammer Saw.
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2nd July 2019, 04:44 PM #33Member
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BobL is quite right and I stand corrected the results from a resistance test would be relative for both star or delta and of no help in determining the wiring configuration of the motor, best bet is to open up the motor and have a look to see if it's star wound. I have not had the experience of converting a 3 wire motor from star to delta winding, I have only worked on 6, 9 & 12 wire motor before where the configuration is made in the terminal box of the motor, in the old days when we used Star/Delta motor starters.
BobL is also correct about the cost of some of the VSD/VFD's I had a quick look on eBay and was surprised at how cheep some electronic drive have become. I was in the opinion that reasonable electronic drive was in the $600 to $700 range but I have seen some on eBay in the $100 to $200 price range.
Maybe Felder can help with more information about their motor windings configuration. All the best with converting the motor if you find it star wound.
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2nd July 2019, 06:35 PM #34.
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If all motors of a given power used the same resistance coils (R) then measuring the resistance across any two coils would give 2R for a Y, and 2/3R would be enough to distinguish between ∆ and Y but unfortunately they seem to vary quite a bit. However it can still be used as a rough guide.
The typical single coil resistance for a 3kW motor is about 2.5Ω.
Then a ∆ wired motor should show around 1.7Ω across any two coils while a Y wired motor should show about 5Ω.
This is only an indication because I have measured different motors of the same HP vary in single coil resistances by almost much as this.
If you have a multimeter with a resistance capability (with the motor completely disconnected from the mains) you could try measuring the coil to coil resistances and see what they are.
In your case measure Red to Blue, Blue to White and White to Rd.
This method works better for smaller motors with higher resistances
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2nd July 2019, 06:47 PM #35.
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If all motors of a given power used the same resistance coils (R) then measuring the resistance across any two coils would give 2R for a Y, and 2/3R would be enough to distinguish between ∆ and Y but unfortunately they seem to vary quite a bit. However it can still be used as a rough guide.
For example, the typical single coil resistance for a 3kW motor is about 2.5Ω.
Then a ∆ wired motor should show around 1.7Ω across any two coils while a Y wired motor should show about 5Ω.
This is only an indication because I have measured different motors of the same HP vary in single coil resistances by almost much as this.
If you have a multimeter with a resistance capability (with the motor completely disconnected from the mains) you could try measuring the coil to coil resistances and see what they are.
In your case measure Red to Blue, Blue to White and White to Rd.
It would be interesting to know what these are.
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2nd July 2019, 07:34 PM #36GOLD MEMBER
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I appear to have killed my good multimeter, so I wouldn't bet on the chinese sweatshop special accuracy here, but it does appear consistent: 4.8 ohms across all three.
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2nd July 2019, 07:38 PM #37.
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