Research on the Failure Mechanism and Solution Strategy for Friction Braking of Electric Hoists

The electromagnetic braking motor, which combines cylindrical motors with electromagnetic brakes, has gradually been widely used in the lifting mechanism of electric hoists. This configuration of electric hoists usually has a large difference in lifting speed between fast and slow, and the highest speed can reach the lowest speed Liu Bei. During the process of rapid sudden braking and sudden switching between fast and slow speeds, it is easy to generate a large amount of braking slip, posing a huge challenge to friction braking. In response to the issue of excessive brake slip of a wire rope electric hoist, the mechanism causing the excessive brake slip was analysed, and a solution was provided. This paper analyses the methods of shortening braking time and reducing braking slip from mechanical, electrical, and control aspects, with a focus on the impact of rectifier and brake response delay on braking deceleration time and braking slip amount.


Overview
The electromagnetic brake motor, which is a combination of Y-series cylindrical motor and electromagnetic brake, is a standard configuration for high-end electric hoists.With continuous innovation in motor pole numbers and speeds, such as the application of 2-pole/12-pole dual-speed motors, the electromagnetic brake often cannot perfectly match the motor.This leads to issues in the electric hoist, such as inadequate braking slip level during high-low speed transitions, sudden braking, etc., causing hook slippage [1], and the friction plate of the brake having a much lower lifespan than expected.

Background of the Problem
During a dynamic load test on a newly manufactured wire rope electric hoist with a rated lifting capacity of 20t at 1.1 times the rated load, the electromagnetic brake failed to perform the required deceleration and stop function when the electric hoist transitioned from low speed to high speed, from high speed to low speed, during sudden stops, and sudden power loss during operation.The braking distance greatly exceeded the standard requirement, resulting in hook slippage.

Cause Analysis
In response to the above failure, since it is a new prototype, the technical personnel initially determined, based on experience, that it was an electrical control problem and focused on the control of the electromagnetic brake [2][3][4].Referring to the motor manual and electromagnetic brake manual, on-site inspections were conducted on the brake and electrical control system, and it was found that the terminal jumper between terminals 5 and 6 of the electromagnetic brake rectifier module was not removed, as shown in figure 1. Terminals 5 and 6 of the rectifier modules were not connected according to the drawing to terminals 153 and 151 of the electrical control box.

Analysis of Electrical Reasons:
The terminals 5 and 6 of the rectifier module were not connected to the electrical control box, which means that the DC side switch of the electromagnetic brake cannot be controlled by the electrical control, resulting in delayed braking and an increase in initial braking speed.
The AC power supply of the brake is directly connected to the motor terminal block, and it is controlled only by the AC side switch, without being controlled by the DC side switch.This means that the electromagnetic brake and the motor are powered on and off simultaneously, regardless of the state of the electric hoist (high speed, low speed, lifting, lowering, stopping).
Without the control of the DC side switch for fast braking, the following situations may occur: During the transition from low speed to high speed, the electromagnetic brake suddenly engages.If the brake opens earlier than the motor is powered on, it causes a delay in the motor receiving power.This is equivalent to lifting a heavy load while the brake is open and the motor is not running, resulting in zero torque from the motor and causing the motor to rotate in the opposite direction under the load, leading to hook slippage.
In the absence of fast braking with the DC side switch, when the motor suddenly loses power, it continues to rotate due to the effect of gravity and inertia.The electromagnetic brake remains powered on, in the open state, and cannot brake immediately, resulting in hook slippage.
During hoisting, both the electromagnetic brake and the motor are powered on simultaneously.However, the motor has not yet generated enough upward torque while the brake is already open.This leads to the motor being pulled in the opposite direction by the load, causing hook slippage.

Solution to the Problem
Improve the electrical schematic diagram according to figure 2 and figure 3, ensuring that the AC contactor KM13 has a power-on delay set to 1.7-2 seconds, and the AC contactor KM15 has a poweroff delay set to 0.5-0.7 seconds.
Wire the system according to the electrical schematic diagram shown in figure 2 and figure 3, connecting terminals 5 and 6 of the electromagnetic brake to terminals 153 and 151 in the electrical control box.

Analysis of Brake Time
The total brake time of the electromagnetic brake is the result of combined mechanical and electrical influences.The total brake time is generally related to factors such as control mode, response time of the rectifier module, closing time of the brake, and magnitude of the brake torque [5].
The total brake time is shown in equation ( 1): where:   -Total brake time, the time from when the power to the brake driving device is cut off until the brake shaft speed reaches zero, measured in seconds (s);  1 -Response time, the time from when the power to the brake driving device is cut off until the brake starts to engage, measured in seconds (s);  2 -Closing time of the brake, the time from when the brake starts to engage until the brake pads make contact with the brake disc and establish braking torque, measured in seconds (s).
The rebound velocity of the brake spring is calculated using equation ( 2): where:  -Rebound velocity of the brake spring, measured in meters per second (m/s); ,  -Random variable related to spring parameters and position;  -Modulus of elasticity, measured in Megapascals (MPa).
The deceleration time of the brake is defined by equation ( 3): where:  -Moment of inertia of the rotating system and load, measured in kilogram square meters (kg.m^2);  -Initial rotational speed of the brake disc, measured in revolutions per minute (r/min);   -Dynamic braking torque, measured in Newton meters (Nm);   -Resistance torque of the rotating system, measured in Newton meters (Nm).Based on the above equations: (1) The response time of the brake depends on the control mode of the brake and the response time of the electronic control system and the rectifier module.
(2) The closing time of the brake depends on the parameters and release stroke of the brake spring, including compression and release stroke.
(3) The deceleration time of the brake mainly depends on the brake torque.

Methods to Shorten Brake Time
From the analysis in 4.1, it is evident that the factors affecting the brake time of the electromagnetic brake mainly involve electrical, control, and mechanical aspects [6].Therefore, efforts to shorten the brake time should focus on these three aspects.

Methods to Shorten Response Time.
(1) Directly powering the electromagnetic brake and motor from a DC power source, avoiding the rectification time from AC to DC and preventing residual magnetism from the motor causing a delay in brake engagement.
(2) Converting AC power to DC power in the electronic control box for the electromagnetic brake's power supply.
(3) Powering the brake from an AC power source and converting it to DC power using a rectifier module.Generally, the brake is connected to an AC switch and powered on and off simultaneously with the motor.Control is achieved through motor switches.To achieve fast braking, a DC switch is commonly used.The DC switch is controlled directly by the electronic control, independently of the motor, allowing for sequential control between them.The closing time of the DC switch is approximately 6-10 times faster than that of the AC switch.Rectifier modules are generally divided into half-wave rectifiers, full-wave rectifiers, and overexcitation rectifiers.If faster response time is required, the overexcitation rectifier can be chosen.Compared to a standard half-wave rectifier, the release time of the overexcitation rectifier is 50% shorter, and the wear resistance and lifespan of the brake are doubled.Compared to a standard full-wave rectifier, the brake time is shortened by approximately 30%, resulting in less heat generation and a 25% reduction in energy consumption.

Methods to Shorten Closing Time.
The closing time of the brake is related to the size of the gap between the brake and the brake spring's force.A larger gap increases the working length of the spring, reducing the braking force and prolonging the closing time.A smaller gap decreases the working length of the spring, increases the braking force, shortens the closing time, and improves the brake effect.However, if the gap is too small, it may cause friction between the brake components after engagement, leading to overheating, excessive wear of the friction pad, thermal decay of the friction coefficient [7], reduced braking force, and prolonged deceleration time.Based on experience, a gap of around 0.5mm is generally suitable.

5.3.1.
Increase Brake Torque.According to equation (3), under unchanged conditions, increasing the brake torque can shorten the brake deceleration time.
where:   --Rated brake torque, measured in Newton meters (Nm), and the dynamic braking torque is generally around 90% of the rated brake torque; m --Number of springs; N --Number of friction parts;  --Friction coefficient; R --Effective friction radius of the brake disc, measured in meters (m); P --Compression force of the spring, measured in Newtons (N).
To increase the brake torque and reduce the brake time, methods such as increasing the number of springs, increasing the spring force, increasing the friction coefficient [8], increasing the number of friction pairs, and increasing the effective friction radius of the friction disc can be employed.

Reduce Brake Drag.
If the brake is closed later than the motor's operation when starting, it will cause brake drag, resulting in thinner friction discs, decreased brake torque, and prolonged brake deceleration time.
Methods to reduce brake drag include: (1) Shortening the response time of the rectifier module and increasing the opening speed of the brake to avoid the brake starting later than the motor's operation, causing the motor to drag the brake.Choose rectifier modules with fast response speeds, such as overexcitation rectifiers.
(2) Increasing the electromagnetic attraction force of the brake to enhance the opening speed.Increase the wire diameter of the brake's electromagnetic coil and reduce the working air gap of the brake.
(3) Optimize the control mode to achieve fast braking.Use separate switches to control the brake, and employ a DC-side switch to independently control the opening and closing of the brake.
(4) Brake timing control, carefully controlling the start and stop times of the brake and motor.Use time relays or other control logic to control the sequence and time difference between the brake and motor's start-up.
(5) Increase the brake torque to reduce the brake deceleration time and, consequently, reduce brake drag time.

Brake Sliding Distance Study
The brake sliding distance of the brake can be mainly divided into the acceleration sliding distance S1 during the time from when the brake is de-energized to when the brake is applied (response time t1 + closing time t2), and the sliding distance S2 during the brake deceleration stage.

Brake Sliding Time
Assuming the time from when the brake is de-energized to when the brake is applied is Δt, according to equation (3), the brake deceleration time during this period is:  32 = .(+)9.55(  +  ) (6) The delay Δt in the brake application caused by the delay in the brake's response will significantly increase the brake deceleration time.The increase in deceleration time, can be calculated as:  3 =  32 −  3 =  9.55(  +  ) (7) Dividing equation (7) by equation (3) yields: For example, if the high speed of the dual-speed motor in this study is 2855rpm, the low speed is 430rpm, and the gear ratio of the hoist reducer is 216.6, taking as 9.8kg/m2, equation (8) gives 0.74 and 4.9.This means that the brake deceleration time increases by approximately 0.74Δt times and 4.9Δt times.If the delay time is 1s, the brake deceleration time can increase by about 5 times.