In order to deal with the topic of efficiency in the field of lifting machines (gearless and gearboxes), it is first necessary to make clear some points regarding the efficiency of electric motors. A first very important point concerns the connection between speed and efficiency: with the same power and engine technology, the faster the engine rotates, the higher its efficiency will be. So a classic 4-pole asynchronous motor (typical of gearboxes) rotating at 1500 rpm (revolutions per minute) will generally have a higher efficiency than a 6-pole asynchronous motor rotating at 1000 rpm.
This obviously also applies to a permanent-magnet synchronous motor (typical of gearless motors) which, when rotating at 160 rpm, will certainly have a higher efficiency than one of equal power rotating at 80 rpm.
The second important point concerns the comparison of efficiency between asynchronous and permanente magnet synchronous motors. The latter, for the same power and rpm, always has a higher efficiency than the asynchronous one and has no rotor losses. The third and final point concerns the total efficiency of the gearbox; in this case, in addition to the efficiency of the motor, we have to consider also that of the gear (worm crown) which, depending on the ratio, can vary on average from 65% for slow ratios to 85% for fast ones. In general, the faster the ratio, the higher the efficiency. A gearbox that has a gearbox with 70% efficiency coupled with a motor that has 80% efficiency will have as a final value: 70 x 80/100 = 56%. In a gearbox, the electric motor always rotates at high speeds (1000-1500 rpm) and thus the motor efficiency remains in a fairly high range (80-92%). In the light of these considerations, we can state that: in the worst cases the gearbox efficiency is above 52% while in the best cases it is around 78%; the maximum efficiencies of the gearbox are certainly lower than the maximums of the gearless; the minimum efficiencies of the gearbox are certainly higher than the minimum efficiencies of the gearless. With the same system (load, speed and balancing), the output power of the lifting machine does not vary according to the type of machine, the roping, the diameter of the traction sheave, the reduction ratio, etc., so all the posible machines that we will use will certainly give the same output power (this is strictly valid only if we disregard the differences of shaft frictions in the various system configurations). What, on the other hand, can change significantly depending on the chosen machine is the electrical input power. With the same output power, the higher the efficiency of the machine, the lower the ”consumed” input electrical power will be. In this regard, it is good to clarify an aspect that many people confuse: the nameplate power of all rotating electrical machines always refers to the mechanical power that can be delivered to the shaft and never to the electrical input power. The misunderstanding arises from the fact that power is expressed in kW and many associate it with electrical power, mistakenly thinking that it is the power absorbed from the mains. Absolutely not! The power of the motor only depends on the features of the lift (capacity, speed, balance, shaft friction) and the efficiency of the gearbox in the case of the gearbox. Let’s take a typical example of a gearless system: 480 kg load at 1m/s balanced at 50% (more content is available in the digital version of this journal, ed.(. Assuming a shaft efficiency of 80% due to friction, the power required to lift the full load will be 3kW: (480[kg] x 9.81[m/s^2] x 50/100 x 1[m/s] /0.8). The 3kW does not represent the electrical power absorbed from the mains, but the mechanical power at the shaft. The electrical power absorbed from the mains will depend on the efficiency of the lifting machine:
- If it is 90%, the power absorbed from the grid will be 3.3 kW;
- If it is 50%, the electrical power absorbed from the mains will be 6 kW.
As stated above, if the goal is to maximise efficiency, it is necessary for the motor to rotate at the highest possible speed. How can this be achieved? In gearless, there are essentially 2 parameters that can be managed, the diameter of the traction sheave and the roping:
- The smaller the diameter of the traction sheave, the aster the motor rotation;
- The higher the roping factor (e.g. 2:1, 3:1, 4:1, 6:1, 8:1, etc.), the faster the motor rotation.
Obviously, both options clash with the practical aspects of installation and cannot be chosen at will: the diameter of the sheave is connected to the diameter of the rope, which in turn is connected to the capacity of the installation and the maximum number of ropes that can be used. In practice, there is a lower limit to the diameter of the pulley. The roping factor, on the other hand, complicates the system and makes it more expensive (longer ropes, more pulleys, more complicated assembly) and so the best compromise is sought, which in most cases results in 2:1.
A gearless in a 2:1 system with a 240 mm pulley, therefore, will certainly have a higher efficiency than a gearless in stalled in a system of the same capacity and speed made in direct roping and a 520 mm traction pulley. Another fundamental parameter that needs to be known when setting the layout of a lift system is the nominal torque of the electric motor, i.e. its ‘force’. In the field of lift applications, approximately 85%-95% of the motor size depends on this very parameter and NOT, as many believe, on the power, which is the product of torque times rotational speed (P=C x W). This is why, by playing on torque and rpm, you can have very small motors (low torque and high rpm) that have the same power as giant motors (high torque and low rpm). With the same power to be delivered (which in the case of the lift, we repeat, depends on load, speed and balancing), one can therefore choose whether to deliver it with more torque and less rpm or vice versa. Since the size of the motor, and indirectly also the cost, depends mainly on the torque, it would make more sense trying to supply this power by increasing the rpm in order to have as little torque as possible.
The levers we have to make this, are again the diameter of the traction sheave and the roping. In fact, the smaller the traction sheave and the greater the roping, the lower the torque. Therefore, a reduction in pulley diameter and an increase in roping has both the advantage of increased efficiency (higher rpm) and a reduction in size (lower torque), weight and consequently also price.
A comparison Table 1 between a gearbox and a gearless is shown below, considering a lift with a load capacity of 450 kg and using different roping configurations and pulley diameters as described in the columns (lifting systems 1,2,3,4,5). Scrolling down the rows, the comparison is made at different lift speeds (0.68m/s, 1m/s, 1.5m/s).
Some considerations based on the data in the Table 1.
When the gearless has a low rotational speed because it is in direct roping and with large pulleys, as in the case of system 2), its efficiency is lower than that of the gearbox.
Scrolling the table horizontally, it can be seen that at the same lift speed, using smaller pulleys and 2:1 ropings:
- the torque decreases;
- the gearless becomes lighter (and therefore has a smaller size and lower cost);
- total efficiency increases despite the fact that the shaft efficiency is lower (due to the diverting pulleys).
Scrolling the table vertically, it can be seen that for the same system, as the speed increases, the machine efficiency increases with both gearbox and gearless traction, but the one with gearless traction increases more.
In gearless systems, the power of the brakes (Pbrake) is not negligible. In gearless, in fact, the brake acts directly on the shaft of the traction sheave and therefore must
have a significant braking torque, which requires not always a negligible amount of power that generates a reduction the overall efficiency of the machine.
On the winch the brake is on the fast shaft of the gear box so the braking torque required is considerably lower and the brake requires less power.
In conclusion, we can say that the gearless traction machine is not a priori more efficient than a gearbox. It becomes so if you set up the system in such a way as to increase the rotation speed.
With regard to MRL systems, where space is limited, the use of small pulleys and indirect ropings leads to a smaller and therefore easier to install machine.
When designing the lift system, there are many aspects to be considered when choosing the right layout, and it is also right to be aware of those that allow real energy savings and optimisation of space and resources.
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Source: ANACAM Magazine
