All motors are driven with ac current signals which are synchronized to the desired motor rotation. A key aspect to correct operation is the constant and precise monitoring of the currents going into the coils. Getting this wrong will damage the motor! In the picture above, the currents are monitored by measuring the voltages across low impedance, < 100mΏ resistors. But, voltages can be as high as 630V rms with respect to each other. So the current monitoring has to be high-voltage tolerant with respect to each phase and with respect to the controlling circuitry. These are regulated by safety standards which require up to 3.75kV reinforced isolation voltages. Isolation requirements are region dependent but 3.75kV is one of the toughest. Current monitoring options include current shunts with isolators, current transformers, and isolation amplifiers. The isolated A/D converter method with current shunt resistors has the fewest analog to digital conversions. It has the fewest support components and lowest cost. However, there is a limit to the current ranges of the shunts as high power can cause heat. These are useful for 80% of motor control applications. Current transformers are good for high power, large-current motors. But, these are relatively large and relatively expensive solutions. Isolation amplifiers can increase flexibility.