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

• US 5,859,373 A
• Filed: 04/17/1997
• Issued: 01/12/1999
• Est. Priority Date: 04/19/1996
• Status: Expired due to Term

• Current Assignee: Demag Cranes & Components GmbH (Konecranes Oyj)
• Original Assignee: Mannesmann AG (Vodafone Group PLC)
• Inventors: Hulsmann, Michael, Munzebrock, Anton, Schroder, Eberhard
• Primary Examiner(s): Noori, Max H.
• Application Number:
  US08/840,859
• Time in Patent Office: 635 Days
• Field of Search: 73/862.381, 73/862.68, 73/862.191, 73/862.193, 73/826
• US Class Current: 73/862.68
• CPC Class Codes: B66C 13/16 Applications of indicating, registering, or weighing devices in crane hooks B66C1/40; in safety gear B66C15/00; weighing-apparatus G01G; remote indicating in general G08
• CPC Class Codes: B66D 1/54 Safety gear
• CPC Class Codes: G07C 3/00 Registering or indicating the condition or the working of machines or other apparatus, other than vehicles
• CPC Class Codes: H02P 29/40 Regulating or controlling the amount of current drawn or delivered by the motor for controlling the mechanical load

Abstract

A process and apparatus for determining a load on a lifting mechanism including a load measuring device and a three-phase induction motor operable for lifting and lowering the load. The effective power consumption of the induction motor is determined by the load measuring device based on a measured current and measured voltage of the induction motor. The measured effective power consumption is then compared with a reference power, which is calculated as the power output of the induction motor driving a nominal load of the lifting mechanism, to calculated an instantaneous load on the lifting mechanism. The inventive apparatus and process advantageously requires no modifications or additions to load carrying parts of a lifting mechanism or carrying gear and provides accurate real time measurement of the load.

Claims

1. A process for determining a load on a lifting mechanism including a load measuring device and a three-phase induction motor for lifting and lowering the load, wherein the induction motor has a stator winding, said process comprising the steps of:

(a) determining an effective power consumption of the induction motor using the load measuring device based on a measured current and a measured voltage of the induction motor and compensating the determined effective power consumption by an offset amount for heat loss in the stator winding of the induction motor; and

(b) calculating an instantaneous load on the lifting mechanism by comparing the determined effective power consumption with a reference power comprising a power consumption of the induction motor driving a nominal load of the lifting mechanism.

2. The process in accordance with claim 1, wherein said step (b) comprises the steps of:

(c) calculating the instantaneous load on the lifting mechanism as a difference between the determined effective power consumption and the reference power; and

(d) displaying the instantaneous load of the lifting mechanism on a display device for monitoring of the load.

3. The process in accordance with claim 1, further comprising the step of switching off the induction motor while lifting the load when the calculated instantaneous load exceeds a predetermined permissible load.

4. The process in accordance with claim 1, further comprising the steps of:

(e) integrating over time the calculated instantaneous load using the load measuring device to determine a continuous load on the lifting mechanism;

(f) storing the determined continuous load in a memory device; and

(g) displaying the determined continuous load on a load display device.

5. The process in accordance with claim 1, wherein said step (b) further comprises the step of calculating the reference power using

7. The process in accordance with claim 1, wherein said step (a) further comprises determining the heat loss in the stator winding of the induction motor based on a measured current in and an electrical resistance of the stator winding.

8. The process in accordance with claim 7, wherein the electrical resistance of the stator winding is a cold resistance of the stator winding.

9. The process in accordance with claim 8, further comprising the steps of:

(i) detecting a temperature of the stator winding of the induction motor; and

(j) measuring the stator winding resistance at the detected temperature; and

(k) calculating the cold resistance of the stator winding using the measured resistance and the detected temperature.

10. The process in accordance with claim 1, wherein said step (a) further comprises adjusting the effective power consumption by a no-load offset loss of the lifting mechanism.

11. The process in accordance with claim 1, wherein the induction motor is connected to a power supply by three motor phase leads, and wherein said step (a) further comprises reversing a direction of rotation of the induction motor by reversing two of the motor phase leads using a reversing switch.

12. The process in accordance with claim 11, wherein said step (a) further comprises measuring current flow in one of the two reversed motor phase leads using a current transformer.

6. A process for determining a load on a lifting mechanism including a load measuring device and a three-phase induction motor for lifting and lowering the load, said process comprising the steps of:

(a) determining an effective power consumption of the induction motor using the load measuring device based on a measured current and a measured voltage of the induction motor, wherein said step of determining an effective power consumption includes the steps of reversing a direction of rotation of the induction motor by reversing two of the motor phase leads using a reversing switch, measuring current flow in one of the two reversed motor phase leads using a current transformer, and determining the measured voltage between the motor phase lead whose current is being detected and one of the two other motor phase leads; and

(b) calculating an instantaneous load on the lifting mechanism by comparing the determined effective power consumption with a reference power comprising a power consumption of the induction motor driving a nominal load of the lifting mechanism.

13. The process in accordance with claim 6, wherein the voltage is measured between the motor phase lead whose current is being detected and each of the two other motor phase leads, and wherein the voltage measurement of the motor phase lead which is not reversed is measured upstream of the reversing switch and the voltage measurement of the reversed motor phase lead is measured downstream of the reversing switch relative to a potential of the motor phase lead which is not reversed.

14. 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

space="preserve" listing-type="equation">B=(m·

g·

R.sub.TR)/(η

·

i·

z)·

2π

·

f/p,

where

m is the nominal load;

g is acceleration due to gravity;

R_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

η

is a transmission efficiency of the lifting mechanism.