Shunt resistors are very common in motor control due to their low cost and simplicity. The voltage drop across the shunt resistor is measured using a current sense amp and an ADC. This measured voltage drop is then used to calculate the current flow. The main disadvantage of shunt resistors is the power dissipation across the resistor. At high current levels, the power loss can be significant and there is a limit to how much current can flow through a resi,=stor before damaging it. For high current applications, in-line transformers or LEM sensors are an excellent choice, although they are more expensive.