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24 Cards in this Set
- Front
- Back
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What is the displacement angle between the rotor and stator rotating flux poles when operating at no load in a 3 phase synchronous motor? |
It is near zero and the poles are approximately in alignment. |
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What happens withe the displacement angel in a 3 phase synchronous motor when a load is initially put on it? |
The angel shifts with the rotor lagging behind the stator flux. The rotor is still locked in step at synchronous speed with the stator. |
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What happens when the displacement angel in. 3 phase synchronous motor goes beyond 90 degrees? |
The rotor will begin to slip and will no longer be synchronous. |
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What other names are used for displacement angel in 3 phase synchronous motors? |
Torque angle, power angel or load angle |
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At synchronous speed, what frequencies will the rotor and stator have in a 3 phase synchronous motor? |
The frequencies will be equal. |
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What can be said of the rotor phase angel and torque angel in a 3 phase synchronous motor when it has a load on it. |
They are approximately equal. |
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What opposing force does the rotor create in the stator of a 3 phase synchronous motor? |
The rotor field will induce a voltage in the stator. This voltage will oppose the applied line voltage and reduce the stator current. This induced voltage is called counter EMF, back EMF, or excitation voltage. |
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Equation for counter EMF produced by the rotor in the stator of a 3 phase synchronous motor. |
n is the speed of the rotor ( which is synchronous speed in a 3 phase synchronous motor), Omega is the flux created by the counter EMF flux created by the rotor and k is a constant. |
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How can the rotor caused counter EMF field flux be controlled in a 3 phase synchronous motor? |
By varying the DC current flowing through the rotor. |
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Which is the armature in a 3 phase synchronous motor, the rotor or stator? |
The stator |
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What is the flux produced by the stator in a 3 phase synchronous motor called? |
The armature reaction flux |
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What is the air gap flux in a 3 phase synchronous motor? |
It is the vector sum of the rotor flux and the armature reaction flux. |
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What counter EMF flux does the stator armature reaction flux produce in a 3 phase synchronous motor and where is it produced? |
The counter EMF force produced by the armature reaction flux is called the armature reaction voltage and the stator basically induces this voltage within the stator. |
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What does the equivalent circuit model for a 3 phase synchronous motor look like? |
R sub a is the armature resistance, jX sub l is the leakage reactance, jX sub ar is the armature reaction reactance. E sub f is the counter EMF. I sub f is the field current of the rotor. I sub a is the armature current. The left side of the model is the stator side. |
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What is used to control the field current in the rotor of a 3 phase synchronous motor? |
A reostat |
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What is the equation for applied line voltage V sub T from the equivalent circuit model for a 3 phase synchronous motor? |
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What does the equivalent circuit model phasor diagram look like for a 3 phase synchronous motor? |
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What expression combines these two reactances? |
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What is the effect of increased shaft load on I sub a, torque angle and Power Factor in a 3 phase synchronous motor? |
As the load on the rotor shaft increases, the torque angle ( power angel) increases which increases the armature current (stator). This decreases the current phasor angel for I sub a which in turn increases the power factor ( improves the PF). |
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In a 3 phase synchronous motor, with increases load, does the magnitude of the counter EMF increase? |
No, but the torque angel increases and the magnitude of the stator current does as well, but the angel of the stator current decreases( and remains at 90 degrees from the rotor current angel). The decrease in the rotor current angel means that the PF improved (increased/ improved mean the same). |
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When maintaining a constant mechanical load on a 3 phase synchronous motor and we increase the DC field excitation on the rotor, what happens? |
The stronger field produced by the rotor will pull harder against the stator flux and decrease the torque angel. It will also cause a stronger counter EMF in the stator. |
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With the combination of the decreased torque angel and larger counter EMF when increases the DC field excitation in the rotor in a 3 phase synchronous motor, what results from the phenomenon? |
The combination of the decreased torque angel and larger counter EMF can push the phase angel of the stator current (I sub a) so that it can be lagging, in- phase or even leading. |
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In general terms, in what ways can a 3 phase synchronous motor appear to the power line? |
By adjusting the DC field excitation of the rotor, it can appear as an inductor, pure resistor or capacitor. This means 3 phase synchronous motors can be used for power factor correction. |
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What are the different types of internal losses in a 3 phase synchronous motor? |
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