Kaplan afternoon problem # 37

[Gnana]

A 3 ph, 4 pole synchronous motor and a 3 ph 4 pole slip ring induction motor are mechanically coupled. Their stators are also connected to a balanced 3 ph 60 hz supply. A 3 ph resistive load is connected across the slip rings. The frequency of the current in the resistive load is?

a)30Hz b)60Hz c)90hz, d) 120hz

The answer given is d) 120HZ but the explanation doesn't quite make sense.

Can anyone help out?

 

[DK PE]

A 3 ph, 4 pole synchronous motor and a 3 ph 4 pole slip ring induction motor are mechanically coupled. Their stators are also connected to a balanced 3 ph 60 hz supply. A 3 ph resistive load is connected across the slip rings. The frequency of the current in the resistive load is?a)30Hz b)60Hz c)90hz, d) 120hz

The answer given is d) 120HZ but the explanation doesn't quite make sense.

Can anyone help out?

If you have the reference Theodore Wildi: Electric machines drive and power systems I think it is covered as a frequency converter or frequency changer. One way to think about it is when a motor is started, the frequency in the rotor is the 60 Hz and if you had a scope on the terminals of a wound rotor machine you would see 60Hz for a quick instant. In normal operation, the flux is rotating around at (in this case) 1800rpm and the rotor at say 1745 rpm and the rotor frequency is the slip * 60Hz or a Hz or two.

Now in this case, the rotor is driven IN THE OPPOSITE DIRECTION from the rotating flux and since the rotor is spinning 1800 rpm (driven by synch. motor, say clockwise) and the connections at stator of the wound rotor machine are connected so as to make flux travel CCW, the induced frequency in the rotor of the wound rotor machine is 120Hz.

Now what happens if the driving synch motor is changed out to a 6 pole machine vs. 4 pole?

 

[Gnana]

A 3 ph, 4 pole synchronous motor and a 3 ph 4 pole slip ring induction motor are mechanically coupled. Their stators are also connected to a balanced 3 ph 60 hz supply. A 3 ph resistive load is connected across the slip rings. The frequency of the current in the resistive load is?a)30Hz b)60Hz c)90hz, d) 120hz

The answer given is d) 120HZ but the explanation doesn't quite make sense.

Can anyone help out?

If you have the reference Theodore Wildi: Electric machines drive and power systems I think it is covered as a frequency converter or frequency changer. One way to think about it is when a motor is started, the frequency in the rotor is the 60 Hz and if you had a scope on the terminals of a wound rotor machine you would see 60Hz for a quick instant. In normal operation, the flux is rotating around at (in this case) 1800rpm and the rotor at say 1745 rpm and the rotor frequency is the slip * 60Hz or a Hz or two.

Now in this case, the rotor is driven IN THE OPPOSITE DIRECTION from the rotating flux and since the rotor is spinning 1800 rpm (driven by synch. motor, say clockwise) and the connections at stator of the wound rotor machine are connected so as to make flux travel CCW, the induced frequency in the rotor of the wound rotor machine is 120Hz.

Now what happens if the driving synch motor is changed out to a 6 pole machine vs. 4 pole?

Boy oh boy am i lost.... How did u conclude the rotor is driven in the opposite direction? And since i did not grasp the concept yet, i don't know what will happen if it's a 6 pole machine.

 

[DK PE]

Boy oh boy am i lost.... How did u conclude the rotor is driven in the opposite direction? And since i did not grasp the concept yet, i don't know what will happen if it's a 6 pole machine.

It doesn't HAVE to be connected that way but if the synch. motor is driving it the same direction as the stator flux on WRotor motor, the frequency generated is 0 Hz which is not one of the answers given and also not very practical. Highly recommend borrowing a copy of Wildi text as he covers a couple examples.