Characteristic curves of a hydraulic turbine are the curves, with the help of which the exact behavior and performance of the turbine under different working conditions, can be known. These curves are plotted from the results of the tests performed on the turbine under different working conditions.
The important parameters which are varied during a test on a turbine are :
1. Speed (W)
3. Discharge (Q)
4. Power (P)
5. Overall efficiency (o)
Out of the above six parameters, three parameters namely speed (N), Head (H) and discharge (Q) are independent parameters.
Out of the three independent parameters (N, H, Q), one of the parameter is kept constant
(say H) and the variation of the other four parameters with respect to any one of the remaining two independent variables (say N and Q) are plotted and various curves are obtained. These curves are called characteristic curves.
The followings are the important characteristic curves of a turbine :
• Main Characteristic Curves or Constant Head Curve.
• Operating Characteristic Curves or Constant Speed Curve.
• Muschel Curves or Constant Efficiency Curve.
• Main characteristic Curves or Constant Head Curves.
Main characteristic curves are obtained by maintaining a constant head and a constant gate opening (G.O.) on the turbine. The speed of the turbine is varied by changing load on the turbine. For each value of the speed, the corresponding values of the power (P) and discharge (Q) are obtained. Then the overall efficiency (o) for each value of the speed is calculated. From these readings the values of unit speed (Nu) unit power (Pu) and unit discharge (Ou) are determined. Taking Nu as abscissa, the values of Qu, Pu, P and o are plotted. By changing the gate opening, the values of Qu Pu and o and Nu are determined and taking Ns as abscissa, the values of Qu, Pu and o are plotted. Fig. below shows the main characteristic curves for a Pelton wheel and Fig. below shows the main characteristic curves for reaction (Francis and Kaplan) turbines.