3 Phase 480 Volt 2 Pole Motor
From the previous conversation (High and low speed motor) I understand that a 3 phase 480 volt 2 pole motor has 6 windings creating 2 magnetic poles at 180 degree apart. Is the rotor just an iron core between the two poles or is there something more to the rotor?
Last edited by kda3310; 02-21-11 at 04:21 PM.
check my last post in the other thread - I described how the rotor is made there.
It should be some material with a high relative permeability, see table in the link below.
Originally Posted by kda3310
The fundamental point you need to understand about AC motors that are not of the universal type is:
1. The stator coils produce a rotating magnetic field in physical space from a 3 phase source. In the simplest form of a 3 phase motor it has 6 coils and 6 poles, with 2 each for each phase. When you analyze the stator portion of the motor when excited with 3 phase power you will find that a magnetic vector of approximately constant magnitude is produced from the stator coils that makes one physical revolution in space per cycle. If you double the number poles and coils and appropriately wire them, then the vector will rotate at 1/2 the speed of the 6 pole version or once each two cycles.
If you insert a shaft in the middle of the stator poles, and on this shaft mount a permanent magnet with a N and S pole so positioned to forcefully interact with the rotating magnetic vector, then you have a synchronous motor that in the 6 pole 60 Hz version rotates at exactly 3600 RPM. In the 12 pole version it is 1800 RPM.
Instead of a permanent magnet on the rotor you can mount a coil of wire connected to 2 slip rings and a DC supply. This produces the equivalent of a permanent magnet on the rotor.
2. If instead of making the rotor a permanent magnet a shorted coil is mounted on the rotor, then a current is induced in the coil from the changing magnetic field. This induced current produces a magnetic field that interacts with the rotating magnetic vector and causes rotation of the rotor. The rotor will never run at synchronous speed because slip is needed to induce a current in the rotor coil.
3. Three phase motors are inherently self stating.
Go to the library and find a book on the theory of alternating-current machines. A book I can recommend is "Alternating-Current Machinery", by Bailey and Gault, 1951, McGraw-Hill. This may not be easy to find.
True single phase motors are a different animal. There is no rotating magnetic vector in space. There is simply a vector at one spatial angle that oscillates from a + magnetic value to a - value and back to a + value in one AC cycle in a 2 pole motor. Put a permanent magnet rotor in this motor and it will probably just vibrate or hum. Give the rotor a little kick in either direction you choose and the rotor will rotate in that direction at synchronous speed.
This is like a child on a swing and each cycle of the swing at the correct time point you add some energy to the swing.
Does this help provide you with some part of a basic understanding?
If it is is the common cage induction motor, the rotor will have Cu (or maybe Al) conductors in slots in the rotor and connected together at either end to make it look like a squirrel cage. Or hamster cage....or whatever analogy conjures up the right image.
Originally Posted by kda3310
I like how you described how these motors work. I would like to add to the single phase motor information.
Originally Posted by gar
In a single phase motor there is no magnetic rotation. There is a few different methods used to make rotation. With squirrel cage motors they need to create a phase shift in an auxiliary winding. By phase shift I mean current in the main winding is at its peak at a different point in time then the current in the aux winding. You then have some magnetic rotation in the stator fields to make the rotor start turning. capacitors in the aux circuit is one of the most common ways to do this. Some are only needed to get the rotor turning and then shut off, others remain in the circuit while running or have different value of capacitor switched into circuit after motor is started. Keeping a capacitor in the circuit while running will help develop more torque. Keeping a capacitor in the circuit while running essentially makes the motor run as a two phase motor, with a single phase source as the input.
The shading coil on a shaded pole motor serves the same purpose as the aux winding, but it has its current induced by the field of the stator.
While all responses have been of quality, I'd highlight Gar's point in that you do need to grab a book on machine theory or take a course at a local college. These descriptions, as well produced as they may be, will allow only a fleeting overview. To gain a deep understanding you will need to dedicate some time to the material to allow the information to sink in. As far as reference material I'd recommend Electric Machinery by Fitzgerald, Kingsley, & Umans., McGraw-Hill, 2002.
When trying to explain how things work I try to get back to kitchen sink fundamentals. In others words the simple basics that you may be able grasp intuitively.
When I had courses in DC and AC machinery my teacher was Dr. J. G. Tarboux. His approach was to try to instill fundamental concepts. For the most part he did not teach using absolute constants, but rather on how changing one parameter affected something else.
Consider a shunt wound DC motor. What fundamentally determines the speed of this motor? Basically there is a balance between applied voltage to the armature and the counter EMF produced by the armature rotating in the magnetic field of the stator poles. How are speed (velocity) and magnetic field intensity related? v = K * df/dt where v is the voltage, K some constant, and df/dt the rate of change of magnetic flux. So what happens when you lose field excitation? You have all the information needed to determine what will happen.
Thank you for all the time y’all have given me. I now have a better understanding of how motors work. I have hooked up many motors without knowing how they function. The bulk of my knowledge has been field experience with the exception of a few courses form Mike Holt. I am very thankful for the help. Going back to school is not feasible at this time so I will check out the books that were referenced.
I had not much field experience at all when I finished school. Let me emphasis a point you may already know: having a good understanding of theory is a very useful skill but equally so is field experience. The fact that you are interested in understanding the concept behind the workmanship that you have experience with will put you ahead of those that simply follow manuals and instructions in the field just the same as persons who think pure theory is enough to get things done.