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Selecting servo geared motors

Selecting servo geared motors

The choice of high performance geared drives is a far from trivial task in demanding automation applications, so we asked the experts at Lenze for some guidance.


Whilst servo motors offer high dynamic response for the control of industrial machinery, their rated output speeds of typically 3000-5000 r/min are too high for most requirements. Gearboxes are needed to reduce the output speed, and this leads to a choice of products - synchronous or asynchronous motors and different designs of gearbox. Lenze offers the machine designer a full range of motors and gearboxes so that the optimum combination can be selected.

Lenze servo motors are available in both synchronous and asynchronous designs. Both allow precise control over a speed range down to standstill, have high peak torques in a range 4x to 9x nominal, and can be supplied with options of brakes, resolvers, incremental and absolute encoders

The choice between them depends on the detail of the application.  Asynchronous motors have higher inertias (which can be useful in some cases), can achieve a wider speed range through field weakening and tend to be lower in cost. However they are less efficient than synchronous motors, physically larger and require a blower if continually delivering torque at zero speed.

Synchronous servo motors use rare earth magnets to generate the magnetic field leading to high efficiency. This in turn means smaller size and no blower requirement at standstill, albeit at a cost premium.  They have very low inertias and give excellent smooth running torques.  Lenze offer asynchronous motors in a range 0.8 to 54kW and synchronous motors 0.25 to 16kW.

Servo motors are commonly combined with planetary gearboxes because they are compact and have low inertia, matching well the motor characteristics. Lenze offers a choice of planetary gearboxes with either shaft or face output. Both are available in four sizes for rated torques 30 to 390Nm in one or two stages for ratios from 3 to 100. The model MPR gearboxes with male output shafts are the most common, offering backlash levels of 2 to 4 arcmins or as little as 1 arcmin on request. The face mounted gearboxes model MPG are axially shorter and torsionally stiffer. They are often used in robotic applications.

Lenze says that it also offers further gearbox options with lower cost. For example, there are medium duty planetary gearboxes with slightly higher values for backlash. Stepping away from planetary designs, helical and bevel gearboxes are available with the same servo motors. Backlash is again higher but this is not a problem for drives that do not reverse. These gearboxes offer lower costs and the option of higher torques up to 6000Nm.

Lenze servo geared motors are all standard catalogued products, which the company says are built to order on reasonable lead times. Further details and technical advice are readily available from the company.
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