Sizing motors - Help

[PennStateIE]

Hi, I'm far from being an EE and I don't try to be one. I recently took a new position where I'm recommending machinery for warehouses and part of the job is recommending motors. I do have some help available to me through vendors, but I want to be able to speak reasonably intelligently with them.

Can anyone give me the basics? What is the difference between single phase and 3 phase motors? Why would you want 115V vs. 120, 240, or 460?

Thanks for all help!

 

[PennStateIE]

One more part to the question....which is better. DC or AC? I have a very crude basic understanding of the difference.

 

[davidbeach]

My guess is that for at least 99.5% of what you need, you will need AC induction squirrel cage motors. Motors 1/2 HP and below will usually be single phase, motors 1 HP and above will usually be 3-phase, different people have different opinions about whether 3/4 HP motors should be single or 3-phase. The single phase motors will almost all be used on 120 VAC systems, so the motor rating will be 110V or 115V. The 3-phase motors will be used on what ever 3-phase voltage is available; could be 208V, 240V, or 480V. Many motors have dual- (multi-) voltage ratings and will list 200/230V - 460V. The 200V motor rating is for use on 208V systems, the 230V motor rating for use on 240V systems, and the 460V motor rating for use on 480V systems.

 

[PennStateIE]

Thanks David...
99.9% of the motors I'm looking at are 1/2 HP to 1 HP. I'm a little confused what is the difference between 110V, 115V and 120 VAC.

Also, what is a phase?

Why would you sepcify 120 over 240 or 460? Is that just dependant on what type of electricy is running into the facility?

 

[davidbeach]

The difference between 110V, 115V, and 120V, is generally just nomenclature. For various historical reasons motor ratings do not match power system ratings, but you can consider these all the same.

A phase, well without having the IEEE dictionary available, lets try this: An AC circuit will be derived from a transformer or generator winding; where there is more than one winding, the windings may be connected either wye or delta. If there are parallel or series windings, consider them combined as single equivalent windings. The number of windings powering the circuit is the number of phases present. At the system voltage, with all windings properly connected, you will measure the system voltage from each phase to all other phases. Common voltages in the US are 208 (from a wye connected set of three windings), 240 (from a delta connected set of three windings), and 480V (from either wye or delta connected set of three windings).

There may also be another conductor, called the neutral, which originates at the center of the wye, or from the mid-point of one winding. Other usable voltages are available from a phase conductor to the neutral, 120V (from any phase to neutral on a 208V wye system, or from two of the phases to neutral on a 240V delta system) or 277V (from any phase to neutral on a 480V wye system).

The one that gets a bit confusing is that when you have two phase conductors, you only have a single phase circuit; the same as when you have one phase conductor and one neutral conductor. Think of it as what is the maximum voltage in the circuit, and how many combinations of two conductors at a time provide that voltage. In any single phase circuit, there is only one combination of two conductors that provides the maximum voltage of the circuit, therefore it is single phase. In a three phase circuit, there will be three combinations that provide the maximum voltage.

120V single phase may also come from a single 240V winding with a mid-point tap. This is the most common household service voltage.

For small equipment to go into a facility, I would always just go with the available voltages. In the US, you would be hard pressed to find anyplace that has AC electric power and does not have 120V available. Beyond that "it depends".

 

[PennStateIE]

Wow. Thank you once more. I feel very dumb when talking about electricity.

Is there a "laymens" description of phases? Is 3 phase more energy efficient that 1 phase?

Thanks again.

 

[itsmoked]

Following up davidbeach's excellent response there is really no difference between 110V,115V,117V,120V. They are interchangeable and amount to the same thing.

Single phase has a supply wire and a return wire.

Three phase has three supply wires.

Single phase motors are larger than an equivalent three phase motor and are not as robust.

Three phase motors inherently spin because of the three phases and single phase motors spin due to clever tricks and an extra start winding hence larger and less robust.

You always want to specify the motor for the available power or you may need extra stuff like transformers.

Higher voltages are usually better because you will have less current draw and be allowed to use smaller wire.

If you have two voltages available use the higher one.

 

[PennStateIE]

Wow. Thanks a lot! One more follow up, if there's no difference b/w 110/115/117/120, then why are there those designations?

 

[itsmoked]

It's kinda funny.. Originally I believe electricity was delivered in the USA first at 110V. So some older areas have that but as time went by the standard voltage was cranked up slowly to 120V Most new stuff I see is 120V. But of course depending on a huge number of things the voltage can always be lower right where a person uses it.

So often if you look at the voltage coming out of a "120V" outlet you may see only 118V or 116V or 121V. It will even change during the day. So as guys in motor shops make their name plates they may tend to make/rate their motors according to what the voltages are like in their personal experience. This is my take on it anyway. Just realize that motors are pretty robust creatures and are designed to run anywhere close to their name plate voltage +/- 5 or so percent. That is why all these would be interchangeable.

 

[PennStateIE]

Thanks! I think I just learned more in 15 minutes then I did in a whole semester of Physics.

 

[jraef]

PennStateIE,
You last question was answered in this thread thread238-131909. There have been several other more comprehensive threads on this issue in here in the past, you can do a "keyword search" from the box at the top of the page for things like 110V, US power rsupply etc. if you really must know more. By the way, there were several older discusiions about the difference in efficiencybetween 1 phase and 3 phase as well.

Bottom line IMHO;
If you have 3 phase available in the facility, use in instead of 1 phase wherever you can on motors. If you don't have 3 phase available in your facility (many warehouses do not), and you only have a handfull of motors, it probably is not worth the cost of paying the utility to give you 3 phase.

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[coconutalley]

To answer which is better AC or DC motor, depends on applications!
AC is better for:
fans, pump, appliances etc. usually higher power application (I'd think >1hp) and where AC power is available.

DC traditional is better for:
precision machinery, portable tools, cassette players, disk drives, printers, toys, lower power <1 hp
DC motors are sometime classified as servo motor. Servo motors really are just motors optimized for servo application (may be more poles added) and are controlled by closed loop servo amplifier controller as opposed to just connecting two leads from a DC supply.

But now the DC brushless motor (usually 3 phase but is not limited to this--I onced worked on a project that has >50 individual phases) is confusing things more.
BLDC motors are really a special case of ac motor but has permanent magnets and are more powerful and efficient as compared to your ac induction type. They definitely need a drive to work. They probably cost a little more. These can go up to MW in sizes.

Advancement in AC motor drives technology has enabled AC motor application into the precision machinery areas. BLDC, servo, and AC motors all need drives or controllers so, this is not helping the confusion.

 

[cswilson]

PennStateIE,

The key issue between 1-phase and 3-phase power is that power from any given single AC phase is delivered intermittently, because as it passes through zero (120 times per second!), there is no power delivered. With 1-phase power, you have to coast through these points. (Think of it like the old flickering fluorescent lights before solid-state controls.)

With 3-phase power, as each phase goes through zero, the other two phases are near maximum, and in fact, the power delivered is essentially constant over the cycle.

The second key issue is that 3-phase power gives you a natural direction of rotation. Swap any two phases into the motor and you reverse the direction of rotation. As itsmoked pointed out above, with 1-phase motors you need extra stuff in the motor to start it and give it a direction of rotation.

 

[PennStateIE]

Thanks for the information. I do appreciate it all. Like I said, I'm a far cry from being an EE....or undrestanding motors.