Apparatus and process for determining the instantaneous and continuous loads on a lifting mechanism

An apparatus for determining a load on a lifting mechanism including a three-phase induction motor for lifting and lowering the load and having three motor phase leads, said apparatus comprising:

• a reversing switch connected to the three motor phase leads for reversing a direction of rotation of the induction motor by reversing two of the motor phase leads;
• a current transformer connected to measure current flow in one of the two reversed motor phase leads;
• a load measuring device connected to said current transformer and the three motor phase leads for measuring a voltage between the one motor phase lead whose current is being detected and one of the other motor phase leads and for determining an instantaneous load on the induction motor based on the measured current, the measured voltage and a reference power output of the induction motor as the lifting mechanism lifts a nominal load, wherein the reference power output is calculated using
• B=(m.multidot.g.multidot.R.sub.TR)/(.eta..multidot.i.multidot.z).multidot.2 .pi..multidot.f/p,
• where
• m is the nominal load;
• g is acceleration due to gravity;
• R.sub.TR is a drum radius of the lifting mechanism;
• f is a power supply frequency to the induction motor;
• i is a transmission ratio between gears of the lifting mechanism;
• z is a shear load on a cable of the lifting mechanism;
• p is a number of pole pairs of the induction motor;
• and
• .eta. is a transmission efficiency of the lifting mechanism.