I understand that single-phase power is obtained by tapping two lines of a three-phase system. I also understand that the neutral line on a single-phase system is connected to the center tap of the transformer. Therefore, I take it that:
The voltage waveform between the two hot lines is a sine curve. Between one hot line and neutral the waveform looks like the graph of |sin x|+sin x or -|sin x|+sin x, depending on which hot line is used.
I also understand that in a residential circuit, the two hot lines from the transformer enter the box and attach seperately to two buses inside the box. The neutral line from the center tap connects to a neutral bus. 120 V circuit breakers attach to one bus and connect to the hot line of a circuit run. The returning neutral line from that run connects to the neutral bus.
240 V circuit breakers span the two buses. There is no neutral line.
With that in mind, I don’t understand:
Why a 120 V single-phase motor doesn’t rapidly speed up, slow down, stop, and repeat, following the output of |sin x|+sin x?
Why said motor doesn’t run in a different direction when plugged into a circuit that is on a different bus?
Why a 240 V single-phase motor doesn’t rapidly switch directions - i.e. why is it able to run in a single direction if the current flow reverses 60 times each second?
The stationary field windings in a single phase motor establish a magnetic field that starts at zero, say, builds up to a maximum in one direction aligined with the mechanical pole pieces. It then falls back to zero and bulds up to a maximum in the other direction and then returns to zero. That cycle is repeated. If the rotor is turning in some direction there is a magnetic field in it that always produces a torque on it that continues the turning in that direction.
The problem is to get it started turning. There are a couple of ways to do this. The best way is to put on an auxiliary field winding and connect it to the power through a series capacitor. This establishes a phase shift relative to the main field winding resulting in a magnetic field that rotates in the desired direction. When this field is applied to the windings in the rotor a rotor field is established that turns it in the direction of rotation of the rotating field. This is a capacitor run motor. There is a variation on this that uses a centrifugal switch to disconnect the auxiliary field-capacitor winding when the rotor reaches some preset speed. This is a capacitor start motor. These motors have high starting torque and are used for heavy duty machinerry.
Then there is the universal motor that uses a commutator to switch the rotor winding polarity. The field action is as described above and the commutator switches the rotor windings so as to result in a magnetic rotor field that always turns it in the desired direction.
Another starting method is to use a shaded pole. This consists of a slot cut into the field core with a shorted turn or two around the slot. This shorted turn sets up a small magnetic field that is at 90[sup]o[/sup] phase with respect to the main field. The result is a rotating field that varies in strength but is always in one direction. This rotating field will start the motor turning but with low starting torque. This method is used for light duty motors like those in electric clocks, bathroom blowers and the like.
When the input voltage reverses polarity both field and rotor inputs reverse polarity together. As explaine above, the motor is actually working on field and rotor magnetic fields that always have the same relationship to each other because when the input polarity is reversed, everything reverses polarity at the same time…
See above. Changing the voltage of 120 to 240 doesn’t affect the relationship of the magnetic fields.
Couple minor points to address to clean up your understanding, which is largely correct but for a couple small points which David Simmons didn’t touch upon.
To be clear, the primary of a residential single-phase transformer is normally fed from ONE phase of the (typically) 7200 V (L-N) 3-phase local distribution network. The other side of the primary is connected to a neutral return, which is often (usually? always?) bonded to Earth ground at the pole. It is not common to find residential power fed from two phases, but it’s not unheard-of, and in these cases the voltages will be 120/208 V instead of 120/240 V.
No, there often is a neutral included in a 240 V circuit. Many 240 V appliances still need a 120 V circuit for things like control circuitry. These will have a four-terminal outlet–the fourth terminal is the safety ground, which will always be present, even if a neutral is not.
All the breakers I have seen on 240 V. residential circuits consist of two ordinary breakers ganged together. There is one from each hot line to neutral, with the handles tied together with a metal clip or a rod through holes in them. If either breaker trips they both trip which opens both hot lines of the circuit.
Right, but the circuit breaker positions are arranged so that every other one is from the opposite hot line. The rails are interleaved sort of like this:
Neutral line1 line2
| | |
| | |
--X-----------------------------------X Breaker connection to line 2 & neut.
| | |
| | |
--X------------------X----------------- Breaker connection to line 1 & neut.
| | |
| | |
There is a connector on the bottom of the breaker that you can put in either of two slots in the breaker depending upon which line you want the breaker connected to.
It’s probably both, actually. Mine is a typical Square-D box, which is mostly what I’ve dealt with. I can’t recall having encountered one like yours, but I may have and just don’t remember it. That’s why you can’t use a Square-D breaker in a Federal Pacific box.
I don’t think this is true. This is what a (half-wave) rectified sine waveform looks like, but not the hot-neutral voltages that I’ve looked at. (Q.E.D. or David Simmons, is this right?) The voltage between one end (“hot”) tap and the center tap of the transformer is half the voltage between the two ends; it’s a sine wave with half the amplitude. Equivalently, the voltages between the ends and the neutral are 180° out of phase. (But note Q.E.D.'s point: if the wiring is using two of the three phases, the two voltages will only be 120° out of phase.)
Of course, line voltages only bear a passing resemblance to sinusoids anyway.
Right. The equations given are for half-wave rectification. The voltage anywhere on the line are just Asin(2πft), where A is the peak amplitude (about 170 for a 120 V. system.)
I haven’t looked at line voltages for years but I don’t remember them as being all that bad.
Ah. So does that mean that an AC motor really does repeatedly speed up and slow down with respect to t in your equation, but it happens often and fast enough it’s not noticable?
There is doubtless a really little variation in motor speed because the magnetic fields pulsate in strength. However, the inertia of the rotor and the load effectively filter them so that the resisdual variations are small.
No necessarily; in a circuit that uses half-wave rectification, the current will only flow in one direction.
So to be uber-precise, the voltage is always a 120 VAC[sub]rms[/sub], 60 Hz sine wave (or very close to one), which means its polarity reverses twice every second. The current can be just about anything… it might be zero, it may or may not reverse direction, and it may or may not look like a sine wave.
I am now starting a campaign to call it AV instead of AC, since the voltage is always alternating, while the current may or may not be.
It may be a matter of how one inteprets Cleophus’ statement of tapping 2 phases of a three-phase system that leads to the confusion as he didn’t mention whether the single-phase power was derived by tapping a distribution step-down transformer to 2 phases of the primary or whether he meant 2 phases of a three-phase service.
For the primary side, either a phase and a neutral or 2 phases are fine; either will give you single-phase service. As to which is more common or ‘typical’ depends entirely on where you are. Common system voltages are 4160, 12,470, 13,200, 13,800, and 24,490. A typical Los Angeles Department of Water and Power system is 4160, a typical Mississippi Power system is 12,470 and a typical Seirra Pacific Power system is 24,490. System voltages are expressed as phase to phase; the actual system voltage of the example you gave is 12,470, a minor point in this discussion, but important when identifying systems. Regardless, your assumption that the more common connection is to use 1 phase and the neutral is correct. Much of this has to do with simple economics; a transformer that uses the neutral as one of the primary connections only has to have one bushing rated at primary voltage and one fused cutout for the primary connection. While this may only save a hundred dollars or so in the cost of the transformer installation, given the thousands of transformers a utility sets in a year, the overall savings can be substantial.
A 120/240 single-phase service can also be derived from a three-phase service. The secondary voltages are not dependent on how the primary is connected. The secondary voltage is determined by how the secondary is connected. A WYE connected secondary (using 120/240 windings) will result in a 120/208 3Ø service; 120 volts phase to neutral and 208 volts phase to phase. When setting up a 120/208 service, the secondary windings in each transformer are connected in parallel. A Delta connected secondary with a grounded center tap will give you a 120/240 3Ø service with a ‘wild leg’; all phase to phase voltages will be 240, 2 phase to neutral voltages will be 120, the other phase to neutral voltage will be a nominal 208. In older residential neighborhoods the Delta connected secondaries are still common. Older air conditioners were very inefficient and to get units capable of cooling larger homes often required three-phase compressors. Utilities would hang an open-Delta bank (commonly referred to as a ‘light and power’ bank) to provide the 3Ø service necessary for those homes that needed it. Those homes that were strictly single-phase would be connected to the two 120 volt legs and the neutral. New residential services of this type are exceedingly rare as air conditioner efficiencies have improved greatly but can still be seen in multi-family dwellings with central cooling plants.
And yes, with the exception of very specific transformers (buck or boost), transformers are connected to the neutral and the ground.
Finally, although i can’t speak to all of them, breakers are not connected to the neutral. The case of the breaker may be connected to ground for safety reasons, but the breaker is not meant to disonnect the neutral. With the exception of very specific circuits, the NEC does not allow the neutral (or a ground) to be fused or behind a breaker. The 240 circuit itself will carry a neutral that ties to the neutral bus in the panel for reasons mentioned by Q.E.D. and i believe that the NEC requires a neutral in any residential 240 volt circuit whether it is used or not.
Right. The connection to the neutral bus is mechanical only, Its sole purpose is to furnish a secure mechanical connection of the breaker to the box. The only electrical connection is to the hot bus.
