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To increase the hydraulic horsepower and the upward speed af mud stream, rotor is manufactured into hollow rotor with nozzles. Thus, motor total flow rate is equal to the sum of the flow rate through seal cavity and rotor. Each type of motor has its own recommended maximum and minimum flow rate value.
If the flow rate is too high, the rotor may run over speed, and the stator and rotor may be more severely damaged. If it is too low, motor will stop operating. Therefore, the flow rate through seal cavity shall be ensured or even higher than the recommended maintenance value while selecting nozzle size, only in this way, can motor be kept in normal operation. (see Fig)
When mud density, nozzle size and total flow rate is a constant value, the flow rate through nozzle and seal cavity always changes with the load. when the bit-leaves the bottom hole, the load is next to zero. Meanwhile, the flow rate through nozzle on the rotor becomes minimum, but the flow rate through the seal cavity becomes maximum. While the bit dropping down, motor-pressure loss will continuously increase, and the flow rate through rotor nozzle will become higher. But the flow rate through the seal cavity will become lower.
