Agree.
Simplistic answer: Motors attempt to be constant-power loads, so they use less current, and thus run cooler, when the applied voltage is nearer the upper end of its rated voltage range.
Disagree. Simplistic and wrong answer. Above name plate flux increases as well and you are overfluxing the motor resulting in an increase in current that overcomes the integral fan cooling. Efficiency decreases. The only real gain from overvoltage is higher starting torque. Everything else is a negative.
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UL and NEMA recommend +10/-15% worst case. CBEMA allows temporary sags/surges but generally recommends +/-5%. That’s what you need to shoot for both loaded and unloaded. Standard taps are at 2.5% and 5% which is in keeping with CBEMA limits. IEEE has tighter limits (Red Book +/-10%).
On long lines voltage variation is always a problem. I didn’t say drop because it’s really a problem both loaded and unloaded because the bus at the starter has a high impedance. What we call a soft bus. The extra impedance of the cable means when large loads are off (right after power up) voltage is often really high but when loaded it might even be too low. So you try to split the difference and keep it “between the ditches”. The obvious solution is a bigger transformer but that would be a better option if the 208 bus is soft. Think of going from a coffee cup to a 24 ounce “big gulp” cup. Sure you have a bigger drink but it’s still a coffee straw! You need a bigger straw on long lines. increased pressure (voltage) helps but some easy calculations would have told you this won’t work. Finally there is another option with motors. 10-15% of the motor power draw at full load is reactive power. That power is fixed so if you don’t run 100% loaded it just loads up the line with amps and causes voltage drop but you get nothing for it. So either put in the right size motor (smaller), add power factor correction capacitors, or both. On slow speed motors (6+ poles) not under full load current can decrease by 50% very easily. But if it’s a two pole fan running close to FLA don’t waste time on this. Again...depends on the application.
Next time do a little math of voltage drop and make sure you are going after the best solution. Wire is cheap. There is nothing wrong with running 4/0 cable with only 100 A load. For just one motor I would not have bothered with the transformer other than the fact that I despise 208 and 240 V power.
This is called voltage drop but I prefer to call it voltage variation because usually we are interested in both the high and low voltage condition. It gets worse with regenerative loads where the load becomes a source where true voltage variation takes place. Voltage drop implies all we are concerned with is how low it gets but that’s without considering startup conditions.
Let’s look at some cables aiming for roughly 100 A ampacity.
#2, $4.87/foot.
THHN-PVC Tray Cable. Cut to length - sold by the Foot.
www.wireandcableyourway.com
4/0, $10/foot
THHN-PVC Tray Cable. Cut to length - sold by the Foot.
www.wireandcableyourway.com
So an increase of about $5000 for 1000 feet but labor and all other costs are roughly the same, and that web site is much more expensive than your local electrical distributor. You would spend way more than that just on the motor and starter or labor building the raceway. By the time you added disconnects, fuses, and the transformer with installation, the extra large cable will look pretty cheap.
Utilities practice transformers all the time but their idea of distribution voltages is say 4160 or 13,500 V compared to 208 or 480
V. At those voltages your voltage drop issue vanishes for 75 kVA of load. The utility is dealing with miles of cable where transformers are a small part of the overall cost, never mind I-squared-R losses. I had a situation a few years ago where I looked at going from 22.9 kV to 35 kV over about 10 miles with several megawatts of power to decrease voltage variation. Even at 69 kV the cost justification just wasn’t there. So even though 22.9 kV is sort of an oddball nonstandard voltage raising it to another distribution voltage was pointless and going to transmission voltages (115 or 230 kV) blew the cost up. It ended up making more sense to move the utility’s substation (so essentially working at 230 kV) or running several conductors per phase. If a huge mine can’t make it work over miles of cable, the economics won’t work at smaller sizes either. There are other good reasons to do it, just not voltage drop. It helps reduce equipment and cable costs for higher power distribution. Above about 1000 kVA there are big advantages to 4160.
I can think of exactly one time changing voltages made tons of sense. This was a copper mine in Arizona. The time was the 1990s. Medium voltage VFDs were both very expensive and spectacularly unreliable but low voltage VFDs had just become really good due to the IGBT. They needed a 1600 HP VFD. It was cheaper and more reliable to step down from 4160 to 480, go through three parallel 500 HP VFDs, then step up to 4160 for the motor. The other option is a mechanical variable speed drive (scoop drive) but again reliability is not good on those.
