Three-phase motors 101

      Three-phase power and converters; explained in layman's terms. June 19, 2003

Question
Can anyone give me a layman's explanation for three phase motors? How about converters for when three phase is not available?

Forum Responses
(Cabinetmaking Forum)
A three phase motor has three hot leads. This is to say that the peaks in the alternating current are closer together so that the motor has less time between each peak, therefore runs smoother and more efficiently. A three phase motor will generally last longer than a single phase. I have in the past used both types of phase converters - static and rotary. The static converters are a series of capacitors which make up the third leg when connected to a 220 circuit. These are not 100% dependable. I kept the electrician's phone number handy. A rotary converter is, as I understand it, a generator which generates the third phase for the motor. These in my experience are reliable and if large enough, can operate more than one motor. I am neither an electrician nor an engineer but have had experience with both types of phase converters.



From contributor M:
I have a basic idea. Three phase power consists of three legs of power. Two lines have approximately 110/120 volts (220/240 combined). The third line consists of a voltage that is approximately 190. The high voltage line replaces the capacitors found on single phase motors (the reason three phase equipment costs less). The high voltage line jolts the motor into starting, a job that the capacitors perform on single phase motors. Once three phase motors start, they run on single phase. Capacitors store a high voltage charge while the motor is running for the next startup (hence the reason a capacitor that has not been discharged can severely injure or kill you). Because there are no capacitors on three phase motors, they are unable to start on single phase. Static phase converters are boxes that contain a bank of capacitors used to store the high voltage needed to start. Rotophases are simply generators that have 220 v input and 190 v output. Three phase motors are also cheaper to operate. As a rule, once a motor passes 7.5 hp it is difficult to find single phase motors. Now that I've tried to explain this, an electrical engineer will probably make a fool out of me.


From contributor W:
Okay, engineer's turn:

AC Power 101
120 Volt single-phase AC power is just that - you have one 'hot' leg and one neutral. Power is transmitted on the hot leg in a sinusoidal wave alternating (AC = alternating current) between positive and negative voltage 60 times / second (60 Hertz). The neutral provides the return path to complete the circuit back to the power generating point. (Don't do it, but you can disconnect the neutral lead from the plug, connect it to any good ground, and the device will work.)

Now, 220 Volt 'single phase' really isn't single phase - it's two phase. You have two 'hot' legs and a neutral leg. The two hot legs each have the same sinusoidal wave form, but 180 degrees out of phase with each other. When one leg is at the peak of the wave, the other is at the bottom. If you measure between the two hot legs, you will have 220 VAC; between the hot and the neutral you will have 110 volts.

Three-phase power has three legs carrying power, each with the same sinusoidal wave form, but 120 degrees out of phase with each other. When one leg is at the peak of the wave, another is 2/3 the way towards the bottom on its way down, and the third is 1/3 the way *up* from the bottom on its way up. In a generated 230 or 460 VAC 3 phase system, you should always measure about the same 230 or 460 volts between any two legs, (unless your supply power factor is way off).

All of this applies here in the US. Three-phase in Canada, and much of the rest of the world, operates the same but at lower voltages (190 / 380) and at 50 Hz instead of 60.

That's power 101 in a nutshell.



Contributor W is correct. Contributor M, the third leg you mention that has 190 volts is probably because the circuit you checked is wired "y" - it gives a high reading to ground in one circuit "square route of 3 times 110v" 1.73 x 110 =190 v. Don't ask me to explain why - I just know it because I built my own 15 hp phase converter with a Y wound motor. Contributor W, I hope I don't have Y confused with delta. Correct me if I'm wrong.


From contributor M:
The power company mentioned to me that their system is delta common. Does that relate to your Y/delta comment?


From contributor W:
Actually, in all likelihood, the 190 V measurement is the result of a delta-wired 3-phase supply, or if you are using a phase converter, one leg is low due to not quite enough capacitance for the load the converter is seeing.

Okay, time for me to take a crack at Motors 101.

Let's start with a simple DC motor. An armature or rotor is made to spin by magnetism - N attracted to S inside a magnetic field. The field can be created either by permanent magnets or an electromagnet. The armature is simply an electromagnet whose poles are switched every time they get close to where they 'want' to be, forcing the armature to keep rotating towards its attraction.

Once that is understood, a single-phase AC motor is a little easier to explain. If you were to build the armature with permanent magnets, and alternate the magnetism of the field (AC, or Alternating Current, does this for us), the armature would rotate to try and satisfy the magnetic attraction between the poles of the armature and the windings of the field.

The problem is: which way should the motor turn if the motor is where it wants to be when the power is turned on ? In single phase motors, this is solved by either using brushes to flip-flop the polarity of an electromagnetic armature (like in a DC motor) or by using a capacitor to create a shifted sine wave that can be used to 'pull' the armature in the direction we want it to go. Once it is turning, a speed switch inside the motor cuts out the capacitor.

In reality, permanent magnets are rarely used in AC motor armatures. Instead, short loops of wire wound into the armature "get magnetic" (are *induced*) from the electrical power flowing in the field around them.

Finally, three-phase motors. With all of the above behind us, this gets pretty simple. Remember the discussion about three-phase power being three lines of Alternating Current power, each 120 degrees (1/3 of a cycle) apart in their timing? If you give each of these lines its own winding in the motors field, then they will each take turns producing North and South magnetic fields. 1 North, 2 N, 3 N, 1 N, etc.

The sequence or direction in which these step determines the direction in which the armature will be pulled to rotate! No brushes, no capacitors! Just the simplest, most efficient AC motor.

If you want to change the direction that the motor rotates, swap two leads so the order becomes 1N, 3N, 2N, 1N etc... The rotation heads the other way.

There are a lot of other factors that go into the design and operation of AC and DC motors to determine the speed of the motor, its efficiency, ratings, etc. but I hope that this helps you understand the basic differences between basic motor types.

If I haven't confused everybody, should I try and do one more for phase converters?



From the original questioner:
Thanks very much for all your info. I've learned that a static converter would cost me approximately $300 but would only run the tool at 2/3 of its available horsepower. A rotary converter would give me full juice, but cost approximately $1200. Does this seem accurate?


The ratio of those prices seems about right. Static converters are about a quarter of what a rotary costs, so your numbers are spot on. But the prices vary somewhat from one manufacturer to another. For complex matters such as this, I believe that service and support are worth the extra cost of dealing with a reputable company, who uses quality parts and necessarily must charge a little more for the seemingly same product.

These are some considerations you'll need to make before you decide on a converter purchase:

What do you need to run now and what are your short and long term requirements? Actually choosing the proper converter can be very confusing. Different machines with the same HP ratings often require different HP rated converters. I'd recommend that you get a converter that is rated higher than your current and short term needs, based on your heaviest draw machine. You'll also have to factor in whether or not you will be running more than one 3Ph machine at a time.

I recently purchased a rotary converter for an edgebander, and a hinge boring machine requiring 3Ph power. I intend to purchase a slider and double line drill in the future so I bought a converter that will handle up to a 7.5 HP saw motor, which will be the largest (read: heaviest use) motor I plan to run. I will be able to run any 2 machines at a time on the converter I purchased, with the exception of having to run the slider all alone.

The edgebander was the most complicated machine to match a converter because it runs on single phase (while the glue heats up), so the wiring had to be done in such a way as to bypass the third leg while heating the glue. Once the glue is heated to temperature, the bander can (only) then be run, which requires 3Ph power. After my own internet research and conferring with the tech support of the Cehisa edgebander, along with the tech support of the selected converter company, I settled on a rotary converter rated 7.5/15 HP. As I understand it, this converter will start a 3Ph motor that requires up to double (hence the 15HP) amperage draw (at initial startup) and will comfortably run a 7.5 HP motor indefinitely. My edgebander rep referred me to Allied Electric Motor Service, Inc. Don Holcomb of AEMS, Inc is very knowledgeable and was helpful in answering all my questions and willingly took the time to help during every step of the way, until I got the converter up and running. I did hire an electrician to do the wiring, which I strongly recommend unless you completely understand what you are doing.



Your numbers are over-simplified. You never did say what you are planning to run or your long term plans. You can get a static for a lot less and you can spend a lot more on a rotary or you could make a rotary using a static and a regular motor as a slave and possibly do it for a few hundred dollars. Then there are inverters if you need variable speed for something like a drill press and shaper. Full power but only one machine at a time. More bells and whistles than static and rotary. Contact your local motor repair shop for more info. This is something they probably do a lot of if they are anything like my local motor shop. A static is probably less than 2/3 power in reality. A static only starts the motor, then its job is over.


From contributor J:
Here is a very simple and crude way to make your very own rotary phase converter for $53.00.

First buy a used three phase motor which typically runs about $10 per hp. Second, go to the hardware store and buy a lawnmower string (seriously!).

Run your single phase power to the motor (2hots and ground). This needs to be a switch or a plug so you can turn the motor on in an instant. Splice the power leads from the machine you want to run into these wires. Connect the third leg from the machine you want to run to the third leg in the motor.

To start your phase converter, wrap the lawnmower string around the shaft of the motor, and pull on it just like you would to start a lawnmower. While the shaft is still turning fast, either plug the motor in or flip the switch.

The motor will run on single phase power. Your lawnmower string took the place of the capacitors that commercial phase converters have wired to the third leg. Somehow, once the motor is running, it generates the power in the third leg.

I know this may sound strange, but it is not very different from your $1400 phase converter. They are nothing more than a typical 3ph motor, a switch, and a capacitor bank wired into the third leg.

I used this setup to run a 7.5hp inverted pin router until I had a shop with three phase. I learned this trick from a guy who had a machine shop in his house. He used this to run his CNC mills and lathes.



From contributor W:
Contributor J, you are 100% correct - your setup *will* run a 3-phase motor. However, there are a couple problems with creating 3 phase this way:

1) Without proper capacitors, the three legs will be *very* unbalanced, both in terms of voltage and current draw, resulting in an uneven load on the machine's motor windings, less than full HP availability, and possibly premature failure of the machine's motor.

2) Without a proper isolating means (disconnect or plugs), circuit protection fusing, and motor running over current protection, don't let your insurance guy or the local inspectors see this arrangement!

I don't recommend that anyone who is not comfortable with electrical codes try to build and install their own converter.



Contributor J is correct in that you can build a 3 phase converter for low cost. The shop I worked in used one for 15 years. What he forgot was that if you wind your rope around the 3 phase motor backwards. your machines will run backwards.


I recently bought a used 3 phase RAS which came with what I think is an inverter. It is a Square D Altivar 16. I was told a variable speed dial was added for the setup to work. It does. However, it seems at a somewhat reduced HP even at full. Also it is necessary to follow a strict on/off procedure. I must wait 10 seconds after turning on the inverter before powering up the saw. If I don't wait long enough, it hums and fails to start. I must also power down in reverse order after each use. I am told that I cannot leave the inverter on, even for a short time, or I would burn it up.


I have a three phase rotor motor because my snowboard factory is located in rural VT mountains with no 3 phase. I have a 20hp rotary motor to run my CNC and other 3 phase machines. You have to be careful with the motor you choose because some motors don't provide consistent power and when there are dips in the current the equipment can shut down. With a CNC it will stop in the middle of the program and can cause scrap. I had to upgrade the service from 100 amp service to 200 amp to run the rotor motor. These motors are used regularly in the Midwest on farms for irrigation. Mine cost about $2500 and came with motor and control board. Buy one that has at least double the hp of the 3 phase machine you are trying to run. The larger the motor, the more consistent the current will be.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Engineer "W" has made a very good description of the 3 phase setup, but has the facts wrong on Canada. We are at 60 hertz, and the normal 3 phase levels are either 208/220 or 575 volt, though there is some 440 volt.



Comment from contributor B:
Canada uses the same voltages as the U.S. It is only Europe that uses 50Hz. Canada uses 60Hz.


Comment from contributor C:
Contributor B is not entirely correct either; Mexico operates on 50Hz as well.


Comment from contributor H:
50 Hertz machines can run on 60 Hertz frequencies, and vice-versa. It won't hurt them.


Comment from contributor X:
Contributor W said...
"Now, 220 Volt 'single phase' really isn't single phase - it's two phase. You have two 'hot' legs and a neutral leg. The two hot legs each have the same sinusoidal wave form, but 180 degrees out of phase with each other. When one leg is at the peak of the wave, the other is at the bottom. If you measure between the two hot legs, you will have 220 VAC; between the hot and the neutral you will have 110 volts."

While in essence these comments are correct, they are in fact misleading.

The common household 120/240 volt system is not two phase as mentioned in W's comment. It is actually what is referred to as "Split Phase" and consists of two single phase windings (with no phase shift between them) connected so that their polarities are additive. As in (L1 = W1 +) (Neutral = W1 - & W2 +) (L2 = W2 -) so that the voltages in the two windings are additive, hence the 240 volt output.



Comment from contributor Z:
Contributor X - Please note the difference between "split phase" and two phase. 220 VAC is two phase, and 240 VAC can be split phase from the transformer, by wiring in series or parallel. Also note that most motors can be wired for 208 VAC, or 230/460.


Comment from contributor D:
Someone mentioned earlier about inverters. Using invertors is one of the best ways to run a three phase motor either from a three phase or single phase source. They are becoming very inexpensive and are energy efficient (no huge start up current) plus the output frequency can be regulated so you can achieve different speeds. We do many upgrades on overhead cranes with these now and they are becoming the standard in motor control. One drawback is that you should only control one motor at a time unless the motors are identical and always operated together. Also, if using a single phase source you may want to oversize the inverter slightly.


Comment from contributor G:
As an addendum to the poster commenting on single phase 220 volts, in the NEC it is referred to as 240 volts. There is an error. You run two hots to a 240 volt motor and "NO Neutral", because now you are placing 120 volts at the motor, along with the 240. Current for a 240 volt motor leaves line A and returns on line B. This is still single phase and not two phase. That is a system that has been phased out, because it is inconvenient. Two phase was 90 degrees out of phase not 180.

Three phase is an entirely different animal. The three phases are natural; as soon as the motor is started a three phase magnetic field is created and starts to rotate at the synchronous speed of the design of the motor.

Where single phase the motor must be tricked into thinking there is an extra field there to get it to rotate. Induction start, repulsion start, capacitor start, and capacitor run, shaded pole all methods to start a single phase motor.

If a 3 phase motor is running in the wrong direction reverse any two leads of the T leads and change direction. Where as a single phase it either cannot be done or not done easily.

Three phase is the way to go, but if your shop is at home it is mighty difficult to convince the power company you need three phase for one or two machines.

You are better off staying with 240 volts on all machines as far as possible.



Comment from contributor E:
Three phase allows you to more easily reverse by simply swapping two legs on a reverse switch or speed control your motors with a VFD or old school speed selector. Reverse may be handy for sanders, shapers, feeders or drill presses and speed control for band saws, sanders, drill presses, shapers, feeders or even blowers.

Smooth running powerful tools are well worth the investment and properly set up will outlast single phase motors. A VFD will save you on time and consumables like band saw blades, drill bits, cutters and sand belts/discs from running them at the wrong speeds (oak vs. pine, 1/4" vs. 6", etc.). In addition, it will open you up to a whole new class of machines and ways to use them which are exclusive to three phase equipped shops. If using a converter, go with rotary, one size bigger than your biggest motor if you can swing it, but same size will work. Using proper motor starters and protection are very important and can save life and limb. Sparks and burning wires do not mix well with sawdust. You can buy good old iron, get a good used or surplus three phase motor and new bearings for much less than you would spend on a new pot metal tool of equal power or capacity. Running it on a VFD will cost you more up front, and may reduce the life of your motor but well worth it on big jobs to find the right speed setting for your work.



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