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3-Phase Power Design. WHY?

01/24/2018 2:38 PM

Hello all,

At our shop we have a benchtop size piece of mixing equipment that uses a 3 phase motor (Machine 1). The equipment's power entry is 220V single phase, but inside the machine (from what I've been told) there is a Variable frequency drive that converts this single phase power into 3-phase power for the motor.

We have another very similar machine (Machine 2) that performs the same function and also has a 3-phase motor. The difference is that Machine 2 requires a 3-phase supply directly from the outlet. Machine 2 is a much higher quality, German engineered, and overall better machine compared to the Chinese built Machine 1.

It seems to me like the much nicer option is to have a VFD inside of the of the machine to convert from single phase into 3-phase power, than to need an outlet for 3-phase power which requires an electrician to install and is costly to the consumer. So my question is:

Why does Machine 2 require a 3 phase outlet connection instead of converting the power internally like Machine 1 does? Can anyone tell me the benefits to this design? It seems like there is a big disadvantage for the consumer to have to hire an electrician to use this type of equipment, and therefore would be limiting to sales.

Thanks

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#1

Re: 3-phase power design. WHY?

01/24/2018 3:06 PM

Why does Machine 2 require a 3 phase outlet connection instead of converting the power internally like Machine 1 does? Because that's the way it's built. Presumably your organisation knew what it was buying. Most industrial installations have 3-phase available, so it wouldn't be an issue. Maybe the German firm could have supplied a 1-phase machine if they'd been asked.

If Machine 1 has a standard motor, this would be 220V delta, 380V star (not 380V delta, 660V star), connected in delta, because only 220V is available from the inverter with 220V 1-phase supply.

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#2

Re: 3-phase power design. WHY?

01/24/2018 3:34 PM

Cost vs benefit. A variable speed drive is expensive but has additional control and operating features compared to a motor alone, however a custom three phase motor is more expensive than a standard three phase motor that may not have the required performance for the application.

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#3

Re: 3-phase power design. WHY?

01/24/2018 3:46 PM

There could be any number of reasons....and it might be a phase converter or a static converter rather than a vfd...might have got a good price, might be used for lighter or intermittent duty...

https://www.wolfautomation.com/blog/phase-converters-vs-vfds/

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#4

Re: 3-Phase Power Design. WHY?

01/24/2018 4:05 PM

Simplicity.

A machine that has a three-phase motor needs nothing more than a starter to operate. (Assuming you don't need variable speeds, etc.) Therefore it will be in theory, much more reliable over the long run. You pay higher up-front cost for the three-phase power, but your long-term savings may be greater. Properly sized and maintained motors will run for decades. Most industrial facilities routinely have three-phase power.

A machine with a VFD is more complicated (adds cost) and more prone to failure. When using single-phase power to drive a three-phase motor, the VFD will need substantial capacitance (and/or inductance, but capacitance is more common) to provide power through the zero-crossings of the input line power. You also have to figure in the life of the inverter. They are getting better, but it remains to be seen if they last 'decades'.

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#5
In reply to #4

Re: 3-Phase Power Design. WHY?

01/24/2018 4:31 PM

Not to mention that I think 3Φ power is even available in households in Germany, if I recall what Andy Germany stated about that.

3Φ power is a better option for price, simplicity of operation, and matches typical industrial and domestic use requirements in Germany. These can be made more powerful (IMHO) than 1Φ motors, so they are better in instances where starting torque is higher, and run torques are not trivial.

In the U.S., you have to have 3Φ supply coming from service transformer (Y or Δ), and carrying through the master control cabinet (MCC) to the motor, and the MCC will typically have the contactor for the motor, with control leads routed through a switch.

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#6
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Re: 3-Phase Power Design. WHY?

01/24/2018 6:08 PM

"but it remains to be seen if they last 'decades'."

You haven't seen my plant; we have VFDs that have been around since the abacus was invented!

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Re: 3-Phase Power Design. WHY?

01/24/2018 11:31 PM

Ha! I still have my first digital watch....

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#8

Re: 3-Phase Power Design. WHY?

01/25/2018 2:31 AM

You are maybe asking the wrong question.

The question better is why a 3phase connection for your company makes more sense.

My answer is that if you load all 3phases equally the power company will have less complains about you and potentially bill you less.

Just attempt to run your whole business with all high powered motors on one single phase and wait until the power company comes knocking on your door.

All else has been said in terms of selection and VFD drive and so on.

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#9

Re: 3-Phase Power Design. WHY?

01/25/2018 9:27 AM

That is a question for the manufacturer, like why do they paint it gray instead of blue. There can be no reasonable answers on this form unless there are members that are in responsible positions at the maker of the machine. -- JHF

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#10
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Re: 3-Phase Power Design. WHY?

01/25/2018 3:21 PM

Quite, as I was trying to say in #1. No doubt a 3-phase machine is better if 3-phase is available, but if not OP has to decide whether it's worth paying to install it, or stick with the 1-phase option, presumably cheaper in mixer cost, apart from cost of installing 3-phase.

On 3-phase the inverter might be internal or separate. Same for 1-phase in general, but it seems like in this particular case it's internal.

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