480 volt motor quandry
480 volt motor quandry
(OP)
We are build our own custom 240 vac 3 phase inductive motor in our shop from scratch. This is a 2 hp small motor that is only about 3 1/2" diameter by 8" long. Last week we built a 480 volt version of the same exact motor, and it has drawn very amps.
We have checked and re-checked our windings and connections and are confident that it is correct for 480v. About the only thing I can think of is that we didn't put phase seperators in the windings.
Should be drawing about 1.5 amps and it was at about 8.
Any thoughts or input will be much appreciated.
We have checked and re-checked our windings and connections and are confident that it is correct for 480v. About the only thing I can think of is that we didn't put phase seperators in the windings.
Should be drawing about 1.5 amps and it was at about 8.
Any thoughts or input will be much appreciated.





RE: 480 volt motor quandry
Also pictures of your motor are welcome.
Zlatkodo
RE: 480 volt motor quandry
RE: 480 volt motor quandry
This is a two pole motor. The core has twelve slots. There are six coils. For the 240v motor we are using 50 turns of double strands of 22awg spike resistant, inverter duty wire. (these motors are run on a VFD at various rpms). For the 480v motor we made our coil windings 100 turns with one strand of the same wire. The 240v motor has a full load rating of 6.4 amps and draws about 2.6 amps no load. The 480v motor was up around 8 amp running on a vfd. When we hooked directly to our 60 hz 480 volt shop power, the motor fried within a few seconds.
The stator stack length is about 2.150" The I.D. of the stator is 1.612" The air gap is .012- .015" The bare slot area is .135 sq. inch.
The squirrel cage rotor has fourteen rows of double bars,
Let me know if you need more info. THANKS!
RE: 480 volt motor quandry
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(2B)+(2B)' ?
RE: 480 volt motor quandry
The volts per turn stays same so flux density roughly the same as the first motor.
With twice the turns, the magnetizing reactance should be nearly 4 times as much as the first motor, which would give roughly half the no-load current (~magnetizing current) with 480vac applied as the first motor with 240vac applied.
If 240vac worked fine, it's hard to see where the problem is on the 480vac, unless there was a miswiring or a failure.
I haven't done any detailed calcs on whether the original 240vac design was reasonable. Maybe someone else can chime in.
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(2B)+(2B)' ?
RE: 480 volt motor quandry
RE: 480 volt motor quandry
RE: 480 volt motor quandry
"If 240vac worked fine, it's hard to see where the problem is on the 480vac, unless there was a miswiring or a failure."
I agree and this is what has us baffled. We tested two 480 motors and both performed the same. As mentioned, we fried already. The remaining one was run at 208v and worked fine. Only drew about .6 amp. Tomorrow we will try to dig out the connections and do a high pot test between phases at 1000vdc to see if thats where the failure is. As I stated earlier, we didn't install phase seperators/insulation on these prototypes but the inverter duyt wire and our vacume impregnation of the resin before baking should have been sufficient.
Also, note that we ran this 480v motor on a VFD, varying the Hz from 20-120 and it consistently drew about 8 amps before kicking out the overloads. All things point to a wiring error but have not found anything yet...
RE: 480 volt motor quandry
Bill
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"Why not the best?"
Jimmy Carter
RE: 480 volt motor quandry
Need : tooth width, back-iron ( the smallest measured value), span, wye or delta.
Whether the internal connection and wye (or delta) are the same in both cases?
Zlatkodo
RE: 480 volt motor quandry
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If we learn from our mistakes I'm getting a great education!
RE: 480 volt motor quandry
Try first connecting the motor with out a VFD, and test with no load at 60 Hz.Is the no load amps OK?
RE: 480 volt motor quandry
If a winding is reversed, the applied voltage angles will be 120 Deg. The back EMF will be 60 deg, 60 deg and 240 deg. Bound to get hot.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 480 volt motor quandry
Zlatkodo
RE: 480 volt motor quandry
Since the exact same motor performed well at 208vac but not at 480vac (19 Feb 12 18:38), that would seem to rule out internal wiring error of the phase-polarity-swap type. (Although I'm not suggesting to rule out anything.)
Focusing on that single behavior reported 19 Feb 12 18:38, we'd be inclined to think the core is pushed far into saturation at high voltage, but not at the lower voltage. But if wired as stated, the original 220vac motor should have had the same problem.
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(2B)+(2B)' ?
RE: 480 volt motor quandry
RE: 480 volt motor quandry
Now that we see it, it seems obvious:
Our motor is a high speed motor designed for a top Hz rating of 250. When we run it at 480v and 250 Hz. the amp draw is fine -about 1.6 (what we expected).
When we run it at 120 Hz, as we did, when we got 8 amps, or 60Hz when we fried it, all makes sense now.
We were able to run our 240v motor at the 120Hz needed for our application without any trouble and so assumed wrongly that we could also run the 480v motor at the same Hz. Big mistake...
So, we switched to a 240v motor and are running it on the 480v VFD with parameters to make it have the needed voltage (about 110) at the 120Hz. Sounds unorthodox but it works fine and allows the use of the 480v line voltage without a transformer.
Thanks again for all help and input...
RE: 480 volt motor quandry
I will add one comment, due to your motor application involves a VFD, your motor windings needs a proper design to support the VFD action, so I will suggest you the use of proper materials like Inverter Duty or Spike duty magnet wire and extra insulation( insulating each of 12 coil) and aplying two varnish cycles or resin to the winding.
This actions will contribute to extend the winding life.
Regards
Carlos
RE: 480 volt motor quandry
Thanks for the feedback.
Unfortunately, there is no data for the width of the teeth and the back-iron.
I assume that these values are 4.5 mm and 9 mm. With these inputs I have made a test of flux densities for the case of 480 V, 250 Hz, 100 turns/coil. From the results it can be concluded that it is advisable slightly to increase the width of the teeth and thus reduce the flux density in the tooth. At the same time slot depth can be slightly increased to get the same slot area. In this way we can further reduce the heat in the teeth. Also, I would not choose 14 bars in the rotor. Such combinations could cause noise and cusp. Yes, the cusp can be avoided with skewed rotor bars , but however, increased noise may occur.
RE: 480 volt motor quandry
RE: 480 volt motor quandry
Using the VFD to produce 480V @ 60Hz = 8V/Hz which is 4 times the ratio the motor was designed for and using the VFD to produce 480V @ 120Hz = 4V/Hz which is 2 times the ratio the motor is designed for. Both are caused by an incorrect VFD setup.
There is no reason you can't run the motor at 120Hz. The VFD can surely supply the correct 230V @ 120Hz.