Protecting motor and driven load: let's torque sense

摘要

Motor power is the only purely electronic measurement of motor load which is linear with, and proportional to, the mechanical load on the motor. An alternative is torque measurement; however this requires a speed input from an electro-mechanical sensor and can be significantly more expensive. Motor current is essentially flat line from idle load to 50-60 percent of full motor load and fluctuates with site voltage. Above 50-60 percent of full motor load, depending on the motor efficiency, motor current increases at an almost exponential rate with an increase in mechanical load. It is nigh on impossible to establish a reliable and repeatable trip point on an exponential curve. Take this example for a motor rated 10A full load current: Using the power equation: 1.73 × I × V × Cos φ (I=current; V= voltage).