how can I tell the design voltage of an unlabeled 3 phase motor?
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  1. #1
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    Default how can I tell the design voltage of an unlabeled 3 phase motor?

    Just acquired a 16" Fay and Egan table saw from an estate. The saw was in the process of being restored, motor was rewound, but no control transformer, starter has a 110v coil. It was still attached but hanging out. This saw has the arbor integral to the motor, 3 phase. There was an unmarked 3 phase transformer in the room with other machinery, not connected, and no phase converter around. There was a variety of machinery in various stages of completion all around the place, in 440 and 230v. There is no data plate on the saw or motor, and I can't even rely on the heaters installed on the starter because I don't even know the motor hp... Is there a way to tell by resistance or something?

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    ? Be practical machinist maybe.
    Unless it runs, or is really big and valuable, just put a new 3-phase motor on it.
    Anyone with a lathe can couple a new motor to the arbor.

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    Simple hook it up to low voltage [220-240] ,if it runs slow it is set for 440.Won't hurt anything as no real load and if slow you will shut it right down.

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    Well, its a beautiful, fully restored (except for the control) saw with the blade mounted directly on the specially made, long yet small diameter, and just rewound for the rebuild motor so a new one is not an option. I'll give it a try on 240 with a tachometer. But I still wonder if there isn't some way to tell by testing- what about if I spin the motor at its rated speed, would I get the design voltage or some useful reading out of its leads? it has 3 leads only....

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    Quote Originally Posted by tomjelly View Post
    if I spin the motor at its rated speed, would I get the design voltage or some useful reading out of its leads?
    Not for an AC motor, and not for DC motors without permanent magnets.

    If the 220V test won't work out, try taking the motor in to a local motor repair/rebuild shop and ask them to take a look.

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    I'd, first, check if the lids have some resistance between them and none of them is shorted to the motor body (ground). Just to make sure it's not a single phase motor.

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    If the starter has a 110v coil, I would ASSUME it is 230v. How many leads coming out of motor?

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    Definitely not single phase.... 3 power leads and ground, and he had a 3 pole contactor partially wired in there, but I'll be checking resistances prior to applying power anyway.. I also wonder what the RPM of a 16" direct drive table saw is supposed to be.. He had this in a residence, not a commercial building, but I got a 440v only drill press along with it, also restored but in pieces... What would a motor shop actually "do" to tell?

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    If it's a direct drive 16" saw, the motor is 3750 rpm with all probability.

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    Voltage on a standard 3 phase induction motor will not make a significant difference in speed until it is loaded, because the base motor speed is determined by the frequency, which is going to be 60Hz regardless. Voltage will determine how much TORQUE the motor will produce, but if you don't have a point of reference to compare to, it can be difficult to tell if what you are seeing is correct or not.

    If it is designed for 460V and you give it 240V, the motor will only develop about 1/4 of it's rated torque (torque varies at the square of the voltage drop). A little sleuthing seems to indicate that the 16" saws came with 5HP motors. So assuming a 2 pole (3450RPM) motor, it should put out about 7.6 ft.-lbs. of torque. You could push a 4x4 through the saw and if SEEMS too easy to stall it, the motor is likely designed for 460V. Kind of subjective though since you've never seen it work correctly. You don't want to test it by applying 480V to it though, because it it's designed for 230V, you could possibly damage the windings trying to test it.

    What a motor shop with a dynamometer can do is connect it to 240V and if it puts out only 1.9 ft.-lbs., it's designed for 460V, if it puts out the full 7.6, its designed for 230V. That is still however ASSuming it is a 5HP motor...

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    Agree with all of what Jraef states.

    As for..."What would a motor shop actually 'do' to tell?"

    Here's the sequence of inspecting a polyphase motor having no nameplate.

    1. The very first tool a technician in a motor shop would use is a "megger" to
    determine if the apparatus is grounded.
    You can also use a multi-meter, but a megger is actually applying potential to the
    stator winding (typically 500 or 1000 volts). A VOM can't do that.

    2. The next subtle act would be in physically turning the shaft to learn whether
    the shaft of the motor even turns.
    (No further electrical testing takes place if shaft does not rotate.
    At this point it becomes a candidate for dismantling if the item is even worth repair.)

    3. If the motor is not grounded and the output shaft rotates freely, the next step is to
    determine if the motor has a functioning circuit. This can be accomplished using an ohm
    meter, but in an adequately equipped motor repair facility, typically a device called a
    "surge tester" is used. Again, this device applies potential to the motor winding and compares
    each phase of the stator winding to verify a functioning 3 phase circuit.

    To compare the windings using an ohm meter in some cases may require that the meter can
    measure very low resistances. Not all off-the-shelf multi-meters have the ability to measure
    very low resistances. (Resistances below 1 ohm.)

    4. If the above electrical tests all pass, the next step would be to apply power to the motor.
    The lowest output of a test panel is applied first. For a low voltage NEMA frame motor
    the voltage applied first is typically 230 volts... but can be lower if the supply has that option.

    Upon applying power, the reaction of the motor is observed and operating current is measured
    and compared between the three phases of potential.

    If the motor is connected for 230 volts, upon applying power to the device, it will have a sharp
    snap, or jump as it quickly comes up to its no-load speed.

    If the motor has a somewhat "sleepy", sluggish start as it ramps up in speed, the motor is
    suspected of being connected for 460 volts. The current drawn or indicated will also help
    confirm this.

    Note: A competent motor repair facility would not release a rewound motor without a name plate.
    Additional steps required to determine the motor's frame size and horse power output are pursued to gain
    data for affixing a new nameplate.

    Here's a fun fact related to NEMA frame motors seldom mentioned in motor conversations.

    If you take the first two numbers of a motor frame, and divide that by 4, it provides the foot
    to shaft center line dimension.

    John

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    "If you take the first two numbers of a motor frame, and divide that by 4, it provides the foot
    to shaft center line dimension."

    Thats handy to know, thanks! I'm going to take the table off and see if there is some hidden tag or if maybe someone has at least written the voltage on some area I can't see. but I can see most of it and there was nothing tucked in to the wire box

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    Quote Originally Posted by d'Arsonval View Post
    If you take the first two numbers of a motor frame, and divide that by 4, it provides the foot
    to shaft center line dimension.
    In what units?

    And thank you for the informative post.

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    The units are in inches. Hence clarifying the little trick works only with 3 digit NEMA frames and not metric IEC motor frames.
    And of course the math does not workout on smaller two digit NEMA frames such as in a 56, 48, 42 etc.

    Where would someone in industry use this little novel tid-bit of knowledge?

    I'm not sure. Maybe use it during conversation in the lunch room... or on a forum board ; )

    John

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    Quote Originally Posted by d'Arsonval View Post
    Hence clarifying the little trick works only with 3 digit NEMA frames and not metric IEC motor frames.
    In IEC frames the frame size is the shaft height in millimeters.

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    Does the motor have nine wires in its junction box? If so, if 4-5-6 are wired together, it's a 230 volt motor. If not, it's a 460 volt motor.

    No?

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    A competent motor winder could have a look at the connections on the stator winding under the endbell, and tell you how many poles and what voltage the connections are spliced for.

    They can be cut apart, and re-spliced for the other voltage, if that is necessary. When I take motors in for rewinding, I always request that all leads are brought out to the connection box, on dual voltage machines.

    Many times they only bring out the three leads for the intended use voltage, because its faster, easier and less costly.

    SAF Ω

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    3 wires only. As it turns out, I tested it on 230 and it works fine, jumps right up to speed even with a 16" blade on it.

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    So, is it 3500rpm?

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    I didn't put a tach on it, but I think so.


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