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بواسطة Serdar Tavaslıoğlu

Q1. What Should Be The Ratio Between The Motor-Machine Lifting Capacity And The Rated Load in Friction-Driven Elevators?

In friction-driven elevators, the rated load of the elevator and the cabin dimensions corresponding to this load are specified in clause 5.4.2.1.1 of the standard.

“5.4.2.1.1 General

The usable cabin area must be limited to prevent the cabin from being overloaded by people.”

To achieve this, the relationship between the rated load and the maximum usable cabin area is provided in Table 6.

The standard prevents more people than the rated load from entering the cabin by limiting the cabin area. The number of passengers corresponding to the rated load defines the maximum number of people who can be forced into the cabin. Nevertheless, in the event this load is exceeded, the elevator’s load and motion control is managed via overload contacts.

“5.12.1.2 Control of cabin load

5.12.1.2.1 The elevator must be equipped with a device that prevents the cabin from moving normally — including automatic leveling — in the event of cabin overload. In hydraulic elevators, this device must not prevent automatic leveling.

5.12.1.2.2 If the rated load is exceeded by more than 10%, provided that the rated load is 75 kg, an overload must be detected.”

As specified in Section 5.12.1.2.2, if the rated load is exceeded, the additional load that the motor or machine must handle is 10% of the rated load, with a minimum of 75 kg. Once this load is exceeded, the overload contacts are activated. If an overload is detected during movement, the elevator must proceed to the nearest floor and stop there. In the event of an overload, the following actions occur.

“5.12.1.2.3 In the event of an overload:

a) Users must be informed via an audible and visual signal inside the car, b) Power-operated doors must open fully, c) Manually operated doors must remain unlocked, d) The preliminary measures specified in Section 5.12.1.4 must be deactivated.”

The additional load to be applied to the elevator machine is a 10% overload, provided that it is at least 75 kg for a very short period of time. This is a situation that can be resolved with a current load that all electric motors can handle for a short time. The elevator machine’s specified rated load must not be lower than the elevator’s rated load. This check is sufficient. The machine’s declaration of conformity is paramount.

If the elevator machine’s specified capacity is equal to or greater than the elevator’s rated load, the machine’s or motor’s drive capability must be deemed sufficient. If the drive machine cannot lift the elevator’s rated load despite the specified rated load, this should be addressed as a matter of product PGD activities rather than a control or elevator design issue. The problem should be evaluated as a non-conformity of the declaration of conformity. The standard requires not only load control but also speed control.

“5.9.2.4 Speed

When the power supply is at its rated frequency and the motor voltage is equal to the equipment’s rated voltage (6), the car’s speed — excluding all acceleration and deceleration periods — must not exceed the rated speed by more than 5% during half-load operation, upward and downward movement and steady-state travel.

The following speed tolerances also apply: a) Leveling (Section 5.12.1.4 c)), b) Automatic (repeat) leveling (Section 5.12.1.4 d)); c) Inspection operation (Section 5.12.1.5.2.1 e) and Section 5.12.1.5.2.1 f)); d) Emergency electrical intervention (Article 5.12.1.6.1 f)).”

If the rated load and rated speed of the elevator drive machine are compatible with the elevator’s rated load and speed, this condition is already met. A requirement such as lifting a 125% load on the machine-motor during inspections is not specified in either Annex 1 or the machine-motor inspection required by the standard. In the elevator inspection, there are three instances where a 125% load is involved in the inspections and tests to be conducted before putting the elevator into service (TS EN 81-20 Section 6.3).

“6.3 Inspections and tests conducted before the elevator is put into service

6.3.1 Braking system (Clause 5.9.2.2)

The test must verify the following:

a) The electromechanical safety gear must be able to stop a cabin loaded with 25% more than the rated load while moving downward at the rated speed.

In this case, the deceleration of the car must not exceed the decelerations that cause the safety device to activate or the car to come to rest against the buffers.

6.3.3 Check of drive capability (Clause 5.5.3)

The check of drive capability must be performed with several stops involving very hard braking (stopping with a stop, author’s note) appropriate for the elevator installation. In each test, the car must be brought to a complete stop. This test must be conducted as follows:

a) At the uppermost level of upward travel with an empty car,

b) At the lowermost level of downward travel with a car loaded to 125% of the rated load.

6.3.4 Car safety device (Clause 5.6.2)...

b) Gradually engaging safety device: In rope-driven elevators, the car must be loaded with 125% of the rated load and must perform the travel movement at the rated speed or a lower speed.

This 125% loading in all three cases does not pertain to conditions related to the machine’s drive power. In the safety gear system test described in Section 6.3.1, the holding capacity of the electromechanical safety gear, as explained in Section 5.9.2.2.2, is tested. In the control of drive capability (Section 6.3.3), the friction drive capability of the sheave and rope, as described in Section 5.5.3 (Rope Drive), is tested. Section 6.3.4 pertains to the cabin safety device test. None of these inspections relate to the drive power of the machine or motor. A machine or motor with sufficient capacity to lift the elevator’s rated load is adequate. As mentioned above, the drive machine must also be capable of handling an additional 10% load in the event of an expected overload. It is not required to handle an extra 125% capacity; such a condition does not exist. The drive unit will never operate under such a load.

Table 18 lists the tests that must be performed on the motor or machine. This is defined in Section 5.9.2; while the relevant section covers everything related to the drive system, the 125% lifting capacity is not mentioned at all.

Q2. Should The Drive Unit Be Able to Lift The Elevator Upward After The Safety Device Is Activated?

The motor drive power is defined as an additional 10% of the rated load in the elevator. In fact, all electric motors have the lifting capacity to handle this additional load for a very short period. Torque values in motors depend on various factors, including current; it is clear that motors can withstand much higher currents for a short time when the wire cross-section and current density are correctly calculated. Just as the windings of asynchronous motors can withstand starting currents five-six times higher than the rated current for a short time, they can also withstand excessive loads and currents for brief periods while in motion. Overcurrent control is already a monitored factor in the elevator. If the current reaches a level the motor cannot handle, necessary safety measures are taken to prevent the motor from continuing to draw excessive current.

However, depending on the type of safety device used, the load the machine attempts to lift may far exceed the rated value due to the compression caused by the braking effect on the rails. Even in the gentlest step-by-step safety device, the compression resulting from the kinetic and static energy generated during free fall exceeds the force the drive unit can handle. In safety device tests conducted during routine inspections, however, since there is no free fall, the speed is at the elevator’s rated speed, and the test is performed using the load (P+Q-G=Q/2) instead of the (P+Q) load with counterweight balancing, the compression may be at a much lower level. This compression force varies depending on the type of rail, the type of safety device compression mechanisms, material hardness values and shapes. While the compression force, which is relatively smaller compared to free fall, may or may not fall within the machine’s lifting capacity, this is not a requirement specified by the standard. Depending on the state of the force generated after braking, the drive machine may not be able to lift the jammed cabin upward. The drive machine is expected to lift the unloaded cabin upward at 110% of its rated capacity. Even if you assume the (k1) coefficient to be as high as 2 under optimal conditions and disregard bending forces, you are effectively multiplying the (P+Q) load by two. A negative finding should not be recorded simply because the drive unit cannot lift the cabin upward after braking. Sometimes such jams occur that the braking system must be removed from the chassis to move the cabin. Additionally, the criteria in the inspection form published by the Ministry are clear and are binding criteria, not merely recommendations. Criteria not listed in the inspection form should not be cited as deficiencies.

Interventions based on personal requests made with “little knowledge but many ideas” are harming the industry. Custom manufacturing cannot be done based on everyone’s knowledge and preferences. Please do not attempt to “fix” the industry based on your first impulse; verifying your knowledge is a simpler approach. If you are correct, let’s do it together, but if you are applying incorrect practices, you will cause significant harm to our own industry. Demanding a 125% lifting capacity means requiring not one but two upper machine groups to be installed in every elevator. In a period where international competition is so fierce, every unnecessary additional financial burden reduces our sector’s competitive strength. If the rules are the same for everyone, unfair competition does not arise.

Response to Objections Regarding Answer to Question 2

Let’s take an elevator with a rated load of 800 kg as an example. Since counterweight balancing is applied, the load the drive motor must lift is 400 kg (P+Q-G=Q/2). This applies whether a full car is going up or an empty car is going down.

If you load an 800 kg elevator with an additional 1000 kg (a 125% overload) and expect the machine to pull it, the load the drive motor will bear in an elevator designed with counterweight balancing for 800 kg will be 1000 - 400 = 600 kg. This means the machine is not loaded at 125%, but at 150%, which implies the motor current values will exceed the limit values by a significant margin. It is unlikely that the inverter or thermal settings would allow this; if the settings are not properly configured, motor burnout would not be surprising. It is clear that insisting on this approach makes no sense. A basic understanding of motor principles is sufficient to recognize that this application is incorrect. Moreover, this practice can lead to hazardous situations. The standard requires that the rope be held in place without slipping from the sheave while the elevator is stationary at 125% load, but the $\mu$ friction coefficient is assumed to be 0.1 when the elevator is stationary; this value decreases as a function of speed once the elevator begins moving. This can lead to rope slippage in the elevator when it is at a standstill with a 125% load. This loading may occur while the elevator is stationary, but if it does, the elevator is prevented from moving by overload contacts. Therefore, requiring such a condition is incorrect.

The inability of the machine group to lift the car may also arise after the safety device activates. Depending on the type of rail, the surface width of the clamping device, and the force of the springs used, different clamping forces may develop on the rails. The standard requires the stopping acceleration to be between 0.2 and 1 gn. If the safety device ensures this, it meets the required conditions. The standard does not specify a specific value for the compression force on the rails. If this were a necessary requirement, the standard would certainly have specified it, but it is impossible to determine such a value. Colleagues who have conducted braking tests on different rails have already observed how results vary significantly from rail to rail, how sliding distances can differ and, consequently, how compression forces differ in each test.

For example, in a braking test where an elevator is decelerated at 1 m/s, with an average deceleration of 0.6 gn (equivalent to 0.085 m) and an average friction coefficient of 0.2, the force exerted by the motor to pull the car upward after braking is very close to half the rated load (Q/2). This corresponds to the elevator’s rated load. However, if the test is conducted under slightly unfavorable conditions — such as poor rail lubrication, surface conditions of the clamping mechanism or minor dust accumulation on the springs — the deceleration value may approach 0.8 gn, and the friction value may approach 0.3 (equivalent to approximately 0.06 m at a speed of 1 m/s). Moreover, since the braking spring force — designed to overcome the force generated by the (P+Q) load during free fall — exceeds the force generated by the (Q/2) load at the rated speed, the resulting high stopping acceleration and shorter stopping distance are to be expected. For this reason, the force on the rails may exceed expectations. In this case, the load the machine must lift after braking exceeds the Q/2 value, which means the machine’s motor is subjected to a force two to three times its pulling force. (See “Sliding safety gear in Elevators and Design Problems,” by Fatih C. BABALIK and Kadir ÇAVDAR) If, after the safety mechanism engages, the machine-motor cannot rescue the car despite the car being unloaded, there is no point in forcing the motor or overloading it by adjusting current control values; the intended operation is incorrect. This will result in the motor burning out or damage to the cabin frame. Unnecessary and incorrect practices must be avoided. This is a matter of mathematics: if the pulling force is less than the holding force, the result is damage to the components in between. What needs to be measured here is the deceleration. It is incorrect to look at the pulling force without measuring the deceleration.

سؤال جديد

Q3. The Standard Requires Lifting The Cab to Release The Safety Device. Should The Engine Be Able to Lift The Cab After Braking?

The provision regarding the release of the brake is defined in clause 5.6.2.1.4.1 of the standard.

"5.6.2.1.4.1 The release and automatic reset of a safety device on the counterweight or balancing weight shall be possible only by raising the cabin, counterweight, or balancing weight."

What this clause intends to specify is that no additional mechanism is required for the release of the mechanical safety gear; rather, the mechanical safety gear must release automatically and reset itself directly in response to the movement of the cabin. How the cabin is raised is specified in a subclause.

"5.6.2.1.4.2 Under all load conditions up to the rated load, the safety device must be released in the following ways: a) through the means specified for emergency operations (5.9.2.3 or 5.9.3.9) or b) by applying procedures available on-site (7.2.2)."

a) The clauses referenced in this paragraph pertain to emergency operation procedures for friction-driven and hydraulic elevators. Emergency operation is a condition established to move a car loaded with the rated load. In the event of a malfunction, if the car comes to rest against the buffers or the safety brake disconnects the circuit, the requirement for rescue via emergency electrical intervention in accordance with 5.12.1.6 applies under clause 5.9.2.3.3.

“5.9.2.3 Emergency operation

5.9.2.3.3 If the manual force required to move the cabin loaded with the rated load upward exceeds 400 N, or if the mechanical devices specified in 5.9.2.3.1(a) are not provided, an emergency electrical intervention device compliant with 5.12.1.6 must be provided.”

In this clause as well, the load refers to the car loaded with the rated load. If the load exceeds the rated load, the emergency electric intervention device may not function. For this reason, paragraph b) has been added to clause 5.6.1.4.2 regarding the release of the safety gear, and a reference has been made to clause 7.2.2.

“7.2.2 Normal Use

The operating instructions must include the necessary information specified in EN 13015:2001+A1:2008 regarding the normal use of the elevator and rescue operations, and in particular the following points: i) Rescue operations: specifically regarding safety gear release, devices to protect the ascending car from excessive speed, devices to prevent unintended car movement, the pipe rupture valve, and safety devices, including the identification of any special tools, detailed instructions must be provided.”

As stated in paragraph i), detailed information regarding the release of the safety gear and, if applicable, the identification of special tools is required. The release of the safety gear is not defined as a direct function of the machine-motor. In this case, if a safety gear that sinks into the rail is used during braking — due to the surface shape of the track, the stiffness of the spring calculation, the large braking angle or the use of an unsuitable rail (which should not actually be the case but is being used);

1. The safety gear must be mounted on the upper part of the cab frame to allow for the rescue operation and even the removal of the mechanical safety gear from the frame to be performed easily, but the tensile and shear calculations for all studs and bolts must be performed,
2. As specified in Section 7.2.2(i), preparations must be made for a special mechanism capable of lifting the cabin upward (e.g., a lifting mechanism attached to the upper consoles to raise the cabin).

The procedure for testing the cabin safety device and the rescue operation is specified in Section 6.3.4.

“6.3.4 Cabin Safety Device (5.6.2)

The purpose of the test conducted before the elevator enters service is to verify that the complete installation—comprising the cabin, cabin interior, safety device, guide rails, and their secure attachment to the structure—is properly installed, correctly adjusted, and structurally sound... To facilitate the disengagement of the safety device, it is recommended that the test be conducted in a manner that allows the car to empty, opposite the door.”

As clearly stated in the clause, this test is a simulation of the safety device’s operation. In reality, we assume that the safety device will activate due to the drive sheave losing its drive capability, causing the ropes to slip off the sheave; the sheave becoming loose due to the sheave and shaft connection breaking; or wear, breakage or rope failure at the rope connection points, resulting in the incident occurring at an acceleration close to free-fall acceleration. It is clear that following such an incident, the drive unit’s connection to the car will no longer exist, so lifting the car upward is not an option.

The subparagraph of the article requires that the car be unloaded to facilitate the rescue operation. For those who argue that the definition of “unloading” is not entirely clear, a quote from the English standard can also be provided.

“In order to facilitate disengagement of the safety gear, it is recommended that the test be carried out opposite a door in order to be able to unload the car.” Load = noun; load, verb; to load Unload = noun; unloaded, emptied, verb; to unload, to remove the load

The article clearly states that the car must be unloaded to facilitate car movement after the test. Attempting to lift a cabin that is braked, jammed on the rails, and loaded to 125% using engine power cannot be a correct approach, as I explained in my previous calculations. Simply saying “it didn’t work” and issuing a red label is not behavior consistent with the standard or control criteria.

I would like to reiterate that the control criteria were established after extensive work by all industry stakeholders and have been compared with the standard. These are binding decisions, not mere recommendations. No additional rules or requirements should be created outside of them. If you believe there are gaps, report them to the Ministry for sector-wide discussion; however, creating personal additional conditions — especially in this sector — will harm everyone unless they are formally incorporated into the inspection criteria. Designing elevators based solely on individual judgment serves no purpose other than to put our own sector in a difficult position and reduce our competitive strength. We must be more cautious in these matters and avoid creating additional conditions based on our own preferences.