Abstract
This paper analyzes the theoretical basis of measuring the elevator balance coefficient using the no-load power method and discusses its applicability for worm gear drive elevators. It is pointed out that the applicability depends on whether the transmission efficiencies during no-load upward and downward travel are the same. The paper also demonstrates the applicability of the no-load power method for permanent magnet synchronous gearless and worm gear drive elevators. Through comparisons with traditional current-load curve method, it is found that the no-load power method has smaller relative deviations and is more convenient and efficient, reducing time and labor costs for elevator inspection and testing.
1. Theoretical Basis of the No-load Power Method
Physics defines power as the rate of doing work per unit time, i.e., power is the product of force and velocity. For an elevator in no-load condition, the motor power can be expressed as:
Where:
- W is the counterweight mass (kg);
- P is the mass of the no-load car and components supported by the car (kg);
- v is the car speed (m/s);
- g_n is the gravitational acceleration, taken as 9.81 m/s²;
- η is the transmission efficiency of the elevator;
- q is the balance coefficient of the elevator in no-load condition;
- Q is the rated load of the elevator (kg).
When the elevator is traveling no-load upward, the motor is in a regenerative braking state, and when traveling no-load downward, the motor is in a motoring state. The motor powers N_x and N_s at the same level position of the car and counterweight during no-load downward and upward travel, respectively, can be derived as:
Where:
- v_x is the car speed when the car and counterweight are at the same level during no-load downward travel;
- η_x is the transmission efficiency during no-load downward travel;
- v_s is the car speed when the car and counterweight are at the same level during no-load upward travel;
- η_s is the transmission efficiency during no-load upward travel.
By summing N_x and N_s and simplifying, the balance coefficient q in no-load condition can be derived:
The applicability condition of this method is that the transmission efficiencies during no-load upward and downward travel are the same.
2. Applicability of the No-load Power Method for Permanent Magnet Synchronous Gearless and Worm Gear Drive Elevators
Table 1. Comparison of Transmission Efficiencies
| Drive Type | Main Components | Transmission Efficiency |
|---|---|---|
| Permanent Magnet Synchronous Gearless | Permanent Magnet Synchronous Motor, Traction Wheel, Braking System | ~95% |
| Worm Gear | Worm, Worm Gear, Bearings, Oil Stirring | ~70% |
2.1 Permanent Magnet Synchronous Gearless Drive Elevators
Due to the absence of intermediate transmission mechanisms, the transmission efficiency remains the same regardless of the direction of rotation, making the no-load power method applicable.
2.2 Worm Gear Drive Elevators
The efficiency of worm gear transmission is mainly affected by the friction coefficient between the worm and worm gear teeth. For no-load worm gear drive elevators, the moment acting on the traction wheel remains constant (except during acceleration and deceleration), and the force direction on the worm remains unchanged. This results in the same friction coefficient and hence the same transmission efficiency during no-load upward and downward travel, making the no-load power method applicable.
3. Case Study on the Application of the No-load Power Method in Worm Gear Drive Elevators
Table 2. Parameters of the Elevators
| Elevator ID | Manufacturing Date | Type | Drive Type | Rated Load | Rated Speed | Floors/Stops |
|---|---|---|---|---|---|---|
| A | July 2002 | Traction-driven Passenger Elevator | Worm Gear | 1000 kg | 1.75 m/s | 22/22 |
| B | July 2002 | |||||
| C | July 2002 | |||||
| D | July 2002 |
Table 3. Balance Coefficients Measured by Current-load Curve Method
| Elevator ID | Balance Coefficient (%) | Load Rate (%) | Actual Load (kg) | Upward Current (A) | Downward Current (A) |
|---|---|---|---|---|---|
| A | 44.2 | 30 | 300 | 13.2 | 15.8 |
| 40 | 400 | 13.4 | 14.2 | ||
| … | … | … | … | ||
| B | 43.6 | 30 | 300 | 9.4 | 13.9 |
| … | … | … | … | ||
| C | 43.1 | 30 | 300 | 13.0 | 15.7 |
| … | … | … | … | ||
| D | 42.9 | 30 | 300 | 9.5 | 14.5 |
4 Significance of Power Method in Inspection and Testing of Old Elevators
Since there is no mandatory retirement age for elevators in use in China, some elevators that have exceeded their designed service life are still in operation. Generally, elevators that have been in use for more than 15 years are classified as old elevators, and these old elevators pose certain safety hazards. For example, there may be excessive wear on the suspension steel wire ropes of the car, excessive wear on the曳引轮grooves, insufficient braking torque of the brake, and excessive or insufficient balance coefficients. Moreover, many accessories of these old elevators are no longer produced and sold, which leads to difficulties in later maintenance and results in these old elevators operating with “deficiencies.” This poses a serious threat to passengers’ personal and property safety. Therefore, the inspection and testing of old elevators should be more cautious, especially the inspection and testing of the balance coefficient, which is the top priority. During the inspection and testing of old elevators, once it is found that the wear of the曳引轮grooves may affect the曳引ability, the balance coefficient of the elevator should be checked first to see if it meets the requirements, and then the曳引ability verification test should be performed.
Currently, some elevator manufacturers have directly integrated the power method functional module into the control cabinet of newly installed elevators, which can monitor the balance coefficient of the elevator in real-time and provide a more intuitive and rapid understanding of the elevator’s operating status. However, most old elevators are still mainly driven by worm gears, and the balance coefficient can only be obtained through on-site testing. The traditional current method requires repeatedly placing weights in the elevator car, which takes a long time. In smooth situations, it takes about 1 hour to test one elevator; whereas the power method only takes about 10 minutes, greatly reducing time costs. Besides time costs, the power method requires less labor, and often only one person is needed to complete the balance coefficient testing of the elevator.
5 Conclusion
The power method, as a new method for detecting elevator balance coefficients, has gradually been applied in the elevator industry nationwide. The State Administration for Market Regulation is also vigorously advocating and promoting innovation in special equipment inspection and testing technology. As practitioners in special equipment inspection and testing, we should keep up with the times, respond to the call, and make our own contributions to the development of special equipment inspection and testing.