what happens in a seperately excited DC generator when load increased
"Load" is a term for how hard the device has to work to power whatever is attached to it. "High load" means the output power is high, "low load" means the output power is low. You can increase the load by, say, connecting a second device to the output of the generator. Assuming the voltage is constant, the current will be higher (since there are now two deices powered by the generator). That means that the equivalent resistance of both deices in parallel will be lower than that of a single device.

So, increasing the load means decreasing the resistance of the load. This may be counter intuitive, but usually devices are not rated in resistance - they are rated in input current or power, so the "load" follows that. If I have a 1kW load attached to the power supply and now increase it to 2kW, the current will increase (assuming the voltage stays constant).