# B AC generators and Transformers

1. Dec 31, 2016

### Biker

So Assume that I have an AC generator and I am giving it enough power to keep spinning with the same rate. So I have limited power usage. The generator creates maximum voltage V and because power is limited that leaves me with only 1 specific current. Now if I connect this generator to a transformer. The voltage on the secondary coil will be Vs and it depends on the ratio between the number of turns Which means there is only 1 specific resistance should be placed in the secondary coil circuit to satisfy the power usage. What if I place a different resistance? What will happen to this system?

2. Dec 31, 2016

### Staff: Mentor

If the resistance is higher, the amperage and power are lower and the generator just has an easier time.

If the resistance is lower, the load demands more power than the generator can provide and it fails. The exact mode of failure depends on the specifics of the generator and its prime mover, but it may shut down or it may just slow down and lower the voltage until the power drops back into its range.

3. Dec 31, 2016

### Biker

About high resistance, What about energy conservation? where does the energy go?

The low resistance part, I think in the case of giving it the same power it will probably just fail because it wont be able to slow down if I am giving it energy. Which means a bit of fire I guess.

4. Dec 31, 2016

### cnh1995

The generator will speed up or slow down depending on whether it is underloaded or overloaded.

Let's assume the system without the transformer as the transformer only transfers the power. It won't make any difference.

The input to the generator is mechanical torque. Say your input mechanical torque is 10Nm and your generator is running at N rpm and it generates a voltage of 100V with a load resistance of 100 ohm. Now, the current is 1A and electrical power is 100V*1A=100W. Now, if you added one more 100 ohm resistance in parallel with the load, the equivalent load resistance is 50 ohm, meaning the load is doubled. If your mechanical input torque is constant at 10Nm, the generator will slow down and voltage developed by the generator will drop to 50V. Note that the load current will still be 1A. So the power supplied by the generator will drop to 50W.

5. Dec 31, 2016

### Biker

So basically, If the load doesn't consume all of the energy supplied, the energy will speed up the generator. If it takes more than it should,then it will take energy more than supplied from the generator which means it will slow down until it reaches equilibrium again.

Really nice.

6. Dec 31, 2016

### cnh1995

Yes. In steady state, power generated= power consumed (+losses). Generators speed up or slow down as per the loading. Power plants have automatic systems which regulate the speeds of their generators by adjusting the mechanical torque.

7. Dec 31, 2016

### Staff: Mentor

Energy is conserved. Check the equations; higher resistance = lower power and low resistance = higher power.
V=IR and P=VI so if "R" goes down, "I" goes up and "P" goes up.
If you are giving the generator the same input rotational power and trying to hold the rpm constant, it will have to slow down because there won't be enough torque to keep it spinning at the same rate. Again: check your equations: P= tw [torque times angular speed]

The input and output power must match in a steady-state situation. If they don't, *something* will change to make them become equal. Like an angular acceleration (deceleration).

Last edited: Dec 31, 2016
8. Dec 31, 2016

### Staff: Mentor

Right: unless there is a controller that can regulate the rotational power (actually just torque, at constant RPM) provided by the prime mover.