Drum motor technologies explored

29 November 2021

Belt conveyors form the heart of most material handling systems. Drum motors – which are compact drives for belt conveyors – are available in asynchronous and synchronous models. Interroll explains the different technologies and their benefits.

Products being conveyed are becoming ever more complex and more customised all the time. Products requiring transportation can range from small to large, light to heavy, soft to hard, with their properties fluctuating widely depending on the season and the market conditions.

Although electric motors are traditionally used to drive belt conveyors, a trend towards drum motors is emerging. In contrast to classic gear motors, which usually have to be fitted underneath or next to the conveying line and be driven by a power transmission system or several conveyor rollers, the drive for drum motors is built into the roller, eliminating the need for a power transmission mechanism. This makes drum motors well-suited for creating compact and energy-efficient conveying solutions. They are also very easy and cost-effective to fit, and comparatively easy to service and repair. This type of motor also has a hermetically sealed design, which offers benefits in food processing applications where hygiene is of utmost importance and areas requiring thorough, regular cleaning.

Asynchronous or synchronous?

Belt conveyors are often in operation for very long periods of time, if not continuously. Brushless motors are preferred for such applications as they naturally suffer very little wear and have low susceptibility to faults. Here, a basic distinction must be made between synchronous and asynchronous motors. Both technologies generate torque using the electromagnetic interaction between the coils fitted on the stator and the rotor of the motor. In addition in asynchronous motors, electrical voltage is induced in the rotor by the stator's magnetic field. This is not the case with synchronous motors, which have permanent magnets on the rotor.

In industrial applications today, asynchronous motors are by far the most commonly used type of motor. They are robust and comparatively cheap to manufacture. They are efficient and can be operated at constant transport speeds without the need for an additional electronic control system. Asynchronous motors from Interroll are well-suited for use in applications that involve a steady flow of medium-heavy goods in standard speed ranges. They can normally be used to achieve material transport speeds of around 0.1 to 1m/sec.

Yet this is where this drive principle starts to show its limitations. When using synchronous drum motors the range of possible transport speeds is significantly larger, ranging from around 0.01 to just under 3m/sec. For this reason, these drives are also suited to very slow-flowing processes, during which goods have to be manually worked on, for example. Steady flow processes at high speeds – such as those required in distribution centres, for luggage belts or in automated packaging systems – can be implemented, too.

When necessary, asynchronous motors can even be operated at variable speeds (within the limitations of their design, of course) by installing a suitable adjustable-frequency drive upstream. Adjustable-frequency drives can be costly, however, so users looking to use an asynchronous motor with one should consider opting instead for a synchronous motor, which always requires the use of an adjustable-frequency drive due to its design. This solution offers a whole host of additional advantages –including a wider range of speeds.

Synchronous drum motors can make start/stop applications and exact positioning in the material flow possible. Only synchronous drum motors enable very quick and frequent stopping and starting, something that always makes them a good choice for handling intermittent flows of material, such as in automated filling or machining processes.

Synchronous motors equipped with the right sensor technology also boast very precise speed and positioning behaviour, which is required in automated cutting processes. Furthermore, they generate substantially higher torque than asynchronous motors in every speed range, making them capable of transporting heavier goods.

Synchronous motors are far more ‘overloadable’ than asynchronous motors. A synchronous motor can be overloaded multiple times in a short period without any problems, which is especially useful when a lot of power is needed quickly. This means that even heavier goods can be sped up or slowed down quickly and dynamically.

Synchronous motors are also energy efficient. Due to their design, asynchronous motors – particularly those with a lower motor power in – have a considerably lower level of efficiency than synchronous motors. This means that synchronous motors use less electricity and therefore cost less to run in a material handling system. But they offer other benefits, too: As a direct result, synchronous motors remain cooler than asynchronous motors, as less of the electrical energy they are supplied with is turned into heat. This makes them more suitable for applications such as food processing, where temperature can be critical. Furthermore, the lower operating temperatures mean that this type of motor has an extremely long service life.

A compact solution

A further advantage of synchronous motors is that they offer the same power but are smaller in size, due in large part to the significantly smaller permanent magnet rotor. The smaller diameter of synchronous drum motors makes conveying systems equipped with them considerably better suited to conveying both smaller and larger goods. The smaller deflection allows smaller goods to be transferred more efficiently to a downstream conveyor belt.

Switching from asynchronous to synchronous technology has a number of advantages when designing new systems and modernising existing material handling systems, too. Replacing asynchronous motors with comparable synchronous motors offers higher efficiency and flexibility, enabling any future changes in material handling requirements to be accommodated to.

Considerable savings can also be made during day-to-day operation, and not just because of the high level of reliability and low energy costs. Thanks to the wide range of uses to which these motors can be put, users need significantly fewer – and potentially no other – types of motors; as a result, the number of spare parts neded can be reduced.

Anyone looking to build a new conveying solution or upgrade their existing system would be well advised to learn more about the basic capabilities offered by asynchronous and synchronous drum motor technologies before making a purchasing decision.