Stacy E. Apelt - Bladesmith
ilmarinen - MODERATOR
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Knifemaker / Craftsman / Service Provider
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I get a lot of requests about 3 phase and VFDs. There is almost always a thread running about a motor or VFD. I though it might help if we all knew something about the subject. For the purpose of this article, I will call 120 volts as the standard power supply line. There may be some technical errors in my statements, so any power guru guys feel free to post corrections.
What is single phase and three phase power?
In North America, power is generated at the power plant with three sets of windings on the generator. These are 120° out of phase with each other. This is what 3 phase power means. This is sent to the power grid at high voltage, and then reduced to a usable power of 480 or 240 volts as it gets to the users home/business. Along the way, the power is left as three phase until it gets to the transformer station in your local area. There it is changed to single phase. I will explain that in simple terms:
The three phase power at the last transformer is 240 volts per leg volts. Each leg of the three wires is referenced to a neutral leg ( ground). If you read from neutral/ground to any leg you get 240 volts. This is a single generator phase ... called single phase. This is sent to a transformer near your house that is center-tapped in reference to ground, and the voltage is taken as two legs of 120 volts each. This delivers two 120 volt lines that are 180° opposed. If either is read to neutral/ground they will read 120 volts. IF read across the two legs, they read 240 volts. There is no need for the neutral line when drawing the 240 volts. Most all homes have this single phase two line power system. You get three wires, two power legs and a neutral.
Businesses need more and higher power, so the power is sent to them as three phase and at 480 volts per leg. With a transformer, this is usually dropped to 240 volts, but kept at three phase. Any leg read to neutral/ground reads 240 volts. That is dropped by a second transformer to 120 volts. If you read across any two legs you get 208 volts. ( because they are only 120° opposed, not 180°) this is determined by √3 X 120 volts). This is called the "wye" system, and is the North American standard. You will get four wires - three phases and a neutral.
Getting three phase power requires that there are the four needed lines going down the power poles or underground lines in your neighborhood. Unless you live in an industrial zone or business zone, there will only be the three residential wires running to your home.
How do motors use this power?
A motor has windings on its armature. These are designed to be powered by the voltage and "push" that armature in reference to a magnetic field the motor also creates. The number of cycles per second and the number of winding groups ( called poles) determines how fast the motor turns. In basic, the motors we use are normally turning at at 1750 RPM or 3500RPM. They are usually either two pole or four pole.
If the windings are 180° opposed (N-S on a 2 pole and N-S-N-S on a 4 pole), then it can only use single phase power. If the windings are done on wye, they are 120° opposed, and can only run on three phase power. Any attempt to run a motor on the wrong power type will be fighting the magnetic field and will burn up the motor.
Can a single phase motor run off three phase power - Yes and NO. A 120 volt motor can run on one the legs ... so it is still running on single phase. A 240 volt motor can run on two legs ... but it will only get 208 volts. If connected wrong to three phase power, a motor could easily burn up.
Can a three phase motor run directly on single phase power - NO, it won't work at all.
How do I know if a motor is single or three phase?
On every motor plate and power label, there is normally a place marked "phase" or "Ph" it will have a 1 or 3 in it if it is a box, or there will be a 1 or 3 followed by the phase sign - it will look like this - 3╪ ( My computer ascii code isn't printing it right. It is a zero with a slash down it). On some motors, it is at the top right corner of the plate. The simplest way to know is that if you have to look for it, it is almost surely a single phase motor.
How can I control the speed of a motor?
Most larger motors for shop tools can not be controlled by the cheap "Speed controllers" . These devices deliver 60 cycle power, but clip the wave to deliver less of it. They will not run a normal motor winding. They will only control smaller shaded pole, universal type, and DC motors. Controllers that vary the frequency of the power will not work, because the motor will overheat and burn up.
There is a device called a variac. That means variable-AC. It is a variable transformer ( like a rheostat) that drops the AC voltage as it is turned. Some people think one of these will work for VS - WRONG. Just like the speed controllers, they will only work on universal motors and DC motors ( with a rectifier installed). Use of a variac on a standard 2/4 pole motor will burn it up.
Obviously, you can mechanically control the speed of any motor by using step up and step down pulley combinations.
On three phase motors, you can vary the frequency, because things aren't run at 180º opposites. The device to do this is called a Variable Frequency Device - VFD for short.
It takes in either single phase power and electronically changes it to three phase, then changes the frequency electronically ... or takes in three phase power and just changes the frequency. Frequency is read in Hertz (Hz), which means cycles per second. You need to look at the VFD plate to see what power voltage and what power phase the incoming and outgoing power will be. The three phase to three phase units are cheaper, but won't work for most of us. You ant single phase in - three phase out. and either 120 volts in or 240 volts in. The 240 volts in are far preferred. Larger motors will not run on a 120 volt powered VFD.
Note: Some VFD units may state that the output is 0-400 Hz. Motors do not run safely above 120 HZ, so set the programming for that limit.
By varying the frequency, a three phase motor can run from about 20 Hz to 120Hz with no issue. Motors that are "Inverter Rated" are more robust, and have heavier windings. They are made to run from 0Hz to over 120Hz at optimum efficiency. If you see one of that type motor, it is a good choice. Regular three phase motors will be fine on our shop tools, too.
There is another way to get three phase power in the shop - a phase converter!
There are two types. A static converter takes in the single phase power and uses electronic timing circuits to change it to three phase. These are OK for smaller power needs. A VDF can be connected to the converter and a motor run at variable speed.
A rotary converter is a single phase motor turning a three phase motor. The three phase motor has the single phase power connected to its windings so it provides a timing change on the output power to match the wye windings. Any three phase motor connected to this system will run synchronously with the three phase motor in the converter. Actually, the more three phase motors running on the system, the smoother it runs. This is a good system for a shop full of three phase devices.
I don't remember reading how well a VFD works on this setup, so perhaps Professor Tony or another will chime in on that.
What is single phase and three phase power?
In North America, power is generated at the power plant with three sets of windings on the generator. These are 120° out of phase with each other. This is what 3 phase power means. This is sent to the power grid at high voltage, and then reduced to a usable power of 480 or 240 volts as it gets to the users home/business. Along the way, the power is left as three phase until it gets to the transformer station in your local area. There it is changed to single phase. I will explain that in simple terms:
The three phase power at the last transformer is 240 volts per leg volts. Each leg of the three wires is referenced to a neutral leg ( ground). If you read from neutral/ground to any leg you get 240 volts. This is a single generator phase ... called single phase. This is sent to a transformer near your house that is center-tapped in reference to ground, and the voltage is taken as two legs of 120 volts each. This delivers two 120 volt lines that are 180° opposed. If either is read to neutral/ground they will read 120 volts. IF read across the two legs, they read 240 volts. There is no need for the neutral line when drawing the 240 volts. Most all homes have this single phase two line power system. You get three wires, two power legs and a neutral.
Businesses need more and higher power, so the power is sent to them as three phase and at 480 volts per leg. With a transformer, this is usually dropped to 240 volts, but kept at three phase. Any leg read to neutral/ground reads 240 volts. That is dropped by a second transformer to 120 volts. If you read across any two legs you get 208 volts. ( because they are only 120° opposed, not 180°) this is determined by √3 X 120 volts). This is called the "wye" system, and is the North American standard. You will get four wires - three phases and a neutral.
Getting three phase power requires that there are the four needed lines going down the power poles or underground lines in your neighborhood. Unless you live in an industrial zone or business zone, there will only be the three residential wires running to your home.
How do motors use this power?
A motor has windings on its armature. These are designed to be powered by the voltage and "push" that armature in reference to a magnetic field the motor also creates. The number of cycles per second and the number of winding groups ( called poles) determines how fast the motor turns. In basic, the motors we use are normally turning at at 1750 RPM or 3500RPM. They are usually either two pole or four pole.
If the windings are 180° opposed (N-S on a 2 pole and N-S-N-S on a 4 pole), then it can only use single phase power. If the windings are done on wye, they are 120° opposed, and can only run on three phase power. Any attempt to run a motor on the wrong power type will be fighting the magnetic field and will burn up the motor.
Can a single phase motor run off three phase power - Yes and NO. A 120 volt motor can run on one the legs ... so it is still running on single phase. A 240 volt motor can run on two legs ... but it will only get 208 volts. If connected wrong to three phase power, a motor could easily burn up.
Can a three phase motor run directly on single phase power - NO, it won't work at all.
How do I know if a motor is single or three phase?
On every motor plate and power label, there is normally a place marked "phase" or "Ph" it will have a 1 or 3 in it if it is a box, or there will be a 1 or 3 followed by the phase sign - it will look like this - 3╪ ( My computer ascii code isn't printing it right. It is a zero with a slash down it). On some motors, it is at the top right corner of the plate. The simplest way to know is that if you have to look for it, it is almost surely a single phase motor.
How can I control the speed of a motor?
Most larger motors for shop tools can not be controlled by the cheap "Speed controllers" . These devices deliver 60 cycle power, but clip the wave to deliver less of it. They will not run a normal motor winding. They will only control smaller shaded pole, universal type, and DC motors. Controllers that vary the frequency of the power will not work, because the motor will overheat and burn up.
There is a device called a variac. That means variable-AC. It is a variable transformer ( like a rheostat) that drops the AC voltage as it is turned. Some people think one of these will work for VS - WRONG. Just like the speed controllers, they will only work on universal motors and DC motors ( with a rectifier installed). Use of a variac on a standard 2/4 pole motor will burn it up.
Obviously, you can mechanically control the speed of any motor by using step up and step down pulley combinations.
On three phase motors, you can vary the frequency, because things aren't run at 180º opposites. The device to do this is called a Variable Frequency Device - VFD for short.
It takes in either single phase power and electronically changes it to three phase, then changes the frequency electronically ... or takes in three phase power and just changes the frequency. Frequency is read in Hertz (Hz), which means cycles per second. You need to look at the VFD plate to see what power voltage and what power phase the incoming and outgoing power will be. The three phase to three phase units are cheaper, but won't work for most of us. You ant single phase in - three phase out. and either 120 volts in or 240 volts in. The 240 volts in are far preferred. Larger motors will not run on a 120 volt powered VFD.
Note: Some VFD units may state that the output is 0-400 Hz. Motors do not run safely above 120 HZ, so set the programming for that limit.
By varying the frequency, a three phase motor can run from about 20 Hz to 120Hz with no issue. Motors that are "Inverter Rated" are more robust, and have heavier windings. They are made to run from 0Hz to over 120Hz at optimum efficiency. If you see one of that type motor, it is a good choice. Regular three phase motors will be fine on our shop tools, too.
There is another way to get three phase power in the shop - a phase converter!
There are two types. A static converter takes in the single phase power and uses electronic timing circuits to change it to three phase. These are OK for smaller power needs. A VDF can be connected to the converter and a motor run at variable speed.
A rotary converter is a single phase motor turning a three phase motor. The three phase motor has the single phase power connected to its windings so it provides a timing change on the output power to match the wye windings. Any three phase motor connected to this system will run synchronously with the three phase motor in the converter. Actually, the more three phase motors running on the system, the smoother it runs. This is a good system for a shop full of three phase devices.
I don't remember reading how well a VFD works on this setup, so perhaps Professor Tony or another will chime in on that.
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