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Old 08-09-14, 06:10 PM   #31
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Under 10% THD on the LINE so that high power, high harmonic devices that cause power quality issues and distribution inefficiencies do not get spec'd out.

You're comparing electric car propulsion which must literally operate from zero to operating speed over a widely varying loads. Use of VSD is very well accepted in traction CONTROL or accelerating something that is very very very heavy, like refinery machinery.

Why can't synchronous motor be made to work as 2 pole/4 pole configuration?


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Old 08-09-14, 09:45 PM   #32
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The number of poles a permanent magnet rotor has is determined by its physical construction. A 2 pole rotor would have one north pole and one south pole. A 4 pole rotor would have 2 north poles and 2 south poles.
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Old 08-10-14, 02:22 AM   #33
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Do you have credible references, not just your words that show a combination of <10% line side THD VSD + motor combination that shows actual performance differences compared to comparable top of the line line frequency motor when 3450 and 1725 are all that's needed?

The power savings from switching from 1.1kW to 0.2kW by switching to two-speed is huge.

Further reduction on low speed from 0.2kW to say... 0.15kW would not justify the higher purchase cost and increased risk factor from reduced resiliency can not be warranted. We shall keep the use of electronic crap to the absolute minimum unless it is unavoidable.
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Old 08-10-14, 09:19 AM   #34
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If those two speeds are really all that's needed, a true two speed motor would probably be a good choice. A VFD will give you many more speeds in between that can be used for more savings in some applications.

Also, if there's already a single speed pump installed, adding a surplus VFD is cheaper than replacing a perfectly good motor. If it's due for replacement anyways, it would probably make more sense to just replace it.
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Old 08-11-14, 03:59 PM   #35
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Quote:
Also, if there's already a single speed pump installed, adding a surplus VFD is cheaper than replacing a perfectly good motor. If it's due for replacement anyways, it would probably make more sense to just replace it.
The efficiency gain, the cost of addition do not downscale well. The added fragility is an acceptable compromise when the added control adds productivity in industrial facilities.

In residential settings, embedded systems and power electronics are part of the complexication disease that just makes devices more susceptible to failures while only producing negligible savings that is more than offset by increase in e-Waste that is generated. Not only that, semiconductor processes use tons of toxic chemicals and creates water and air pollution while some are stuck in la la land about the advertising tune of state side reduction in "carbon foot print" while having all of pollutant sh!tting done in China. It's truly questionable if the net eco benefit of using VFDs, especially China sourced ones where environmental regulations are lax and letting pollutions pour out.

Quote:
three phase motors are more efficient than single phase, which can easily offset the efficiency loss from a VFD.
I'd like to see proof of that. We're talking about single phase backyard fractional to 1 hp pool pump motor, not double digit hp industrial machines.

Efficiency of drives... OUCH. This is just the loss in drive. Poor efficiency is even more likely with using some surplus VFDs. Add the motor's own loss, addition to motor loss due to harmonics on the motor side, then power quality pollution in the neighborhood with the use of harmonic spewing device that is relatively high proportion of total demand on the transformer.


Now the 50-60% of efficiency of two-speed motor on low don't look so bad when you look at what you'll lose in a VFD that small. It will be even worse if you're using a oversize surplus unit, like 2-3 hp one driving a 0.75 to 1 hp motor.

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Old 08-11-14, 09:05 PM   #36
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I have done a lot of work on power electronics and it's pretty easy to get way more than 48% efficiency at 12.5% load. In fact, it's not uncommon to find super cheap automotive type inverters that get 70-80% efficiency at around 10% load, and those have a huge efficiency penalty from the DC/DC boost stage.

The efficiency penalty from oversizing a VFD (within reason) is very small. In some cases, it would actually be more efficient due to lower losses in the output transistors.

It's pretty clear you're the type who prefers tried and true technology. Maybe you do, but what about those who want to try new stuff? (Not that VFDs are new technology...)

What we really need is an open source inverter that is scalable for a wide range of residential applications, with a focus on converting existing single speed equipment to variable speed. The open source nature would make it easy to service.
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Old 08-11-14, 09:51 PM   #37
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Are you trying to discredit such a credible source? They're comparing whatever off the shelf motor drives they reviewed in 2005.

Adjustable speed motor drives are generally VVVF that raises and lowers voltage proportionately. Another viable option is a second piggy back synchronous motor driving through a one-way clutch and low tension toothed belt.

So, we're talking about providing a considerable magnetizing current and the loss associated with providing this reactive power and additional deductions must be made for motor efficiency losses if driven at a less then ideal waveform.

You're comparing it with a fixed voltage, fixed frequency China sourced stuff providing power to resistors. If the VSD is to standby, the standby power at off is counted as usage.

So, you're saying a surplus VFD driving a 1-hp 3450 RPM 60Hz motor at 1725 RPM at 1/8 hp will surpass the line-to-shaft efficiency compared to the best available 1725/3450 dual wound motor 1hp motor on both speeds while staying within 10-15% current THD on line side, and provide the same level of fault tolerance? (accidental short, motor lock up due to a sock getting sucked in, etc)

It's a freaking pool pump. You don't need a full range of ramp up, down, and speed control.

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Old 08-11-14, 10:34 PM   #38
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If there's already a single speed pump (or whatever) with plenty of years of service left, what do you propose to add multi speed capability?
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Old 08-11-14, 11:21 PM   #39
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$200-300 for 2 spd core and coil isn't bad considering it comes with a new warranty and don't have the failure susceptibility of power electronics.
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Old 08-11-14, 11:37 PM   #40
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Which costs more than a surplus VFD. A VFD will also give you a nearly infinite number of speeds to use. Just another option if you rather not replace a good motor.

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