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I just started studying AC motors and I found myself in some trouble here. Basically the exercise asks me to calculate the number of rotor and stator poles in an AC motor, given the nameplate data.

Data:

  • Nominal speed: nn = 1440 rpm
  • Stator voltage frequency: fn = 50 Hz
  • Line voltage: Un = 380 V
  • Line current: In = 30 A

Basically I found out that:

  • The synchronism speed n1 = 1500 rpm
  • n1 = (60 * fn)/p => p = 2 for stator poles

My question is the following. How do I calculate now the number of rotor poles?

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  • \$\begingroup\$ Isn't it 4 poles for the stator if you get 1500 rpm with 50hz? \$\endgroup\$ – Harry Svensson May 30 '18 at 10:16
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    \$\begingroup\$ @HarrySvensson he's talking about pole pairs. \$\endgroup\$ – Andy aka May 30 '18 at 10:20
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    \$\begingroup\$ he is also talking about rotor poles \$\endgroup\$ – JonRB May 30 '18 at 11:01
  • \$\begingroup\$ Since it is not a synchronous motor (rated speed less than synchronous speed), I am assuming it is an induction motor. The concept of rotor poles does not really apply to an induction motor as far as I know. \$\endgroup\$ – mkeith May 30 '18 at 16:24
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Synchronous speed, n1 = (120 * fn)/p where p = poles

Synchronous speed, n1 = (60 * fn)/p where p = pole pairs

Most text book presentations perform calculations using poles, but some use pole pairs.

In an induction motor, the rotor poles are formed by the action of the stator magnetic field. The number of rotor poles must always match the number of stator poles. For most other types of motors, the rotor is constructed so that the number of rotor poles matches the number of stator poles.

In induction motors, the slip at the rated frequency and load is between about 1.5% and 3% of synchronous speed for "standard" motors. To calculate the number of poles, use the formula to find the synchronous speed that is about 1.5% to 3% higher than the loaded speed. For "high slip" motor designs, the synchronous speed might be as much as 15% above the loaded speed. A wound-rotor motor with external resistance could operate over a wider speed range, but should have a speed rating stated without external resistance. In that case, the loaded speed would probably be 3% to 5% below the synchronous speed.

Single-phase motors generally have higher slip at full load than three-phase motors, but most of them will have 3% to 5% rated slip. Shaded-pole motors may be a little higher.

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An induction motor (such as a squirrel cage rotor type) can be regarded as a rotating shorted turn and as such the (north and south) poles created by induction are irrelevant for speed calculation. There is one pole pair if you want a number.

If it helps, try thinking about the rotor as being a fixed magnet with N and S poles that align with the stator poles as they are driven. This would be a synchronous machine rather than an induction machine but the same principle is involved. In fact a synchronous machine that uses a rotor coil brought out on slip rings behaves identically to a non-synchronous induction motor if the slip rings are shorted out (rather than a DC supply placed on them).

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Someone is playing a joke on you. Synchronous motors don't go 1440 rpm on 50Hz. Induction motors don't have rotor poles.

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