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Novel technology controls air-motor speeds
Published:  10 June, 2011

A German air-motor specialist claims to have overcome one of the traditional limitations of this type of motor – controlling their speed. Deprag Schulz says that its technology will allow motors driven by compressed air to hold preset speeds, and could open up new applications for these machines.

Air-motors are small, lightweight and robust. Compared to similarly-rated electric motors, they are about one-third of the size and one-fifth of the mass. They are also oil-free and unaffected by cleaning solvents, making them suitable for use in food and cleanroom applications. But traditionally, it has been difficult to control their speed.

“Air-motors can be operated over a wide range of changing loads,” explains Deprag product manager, Dagmar Dübbelde. “When operated without load, the motor runs at its rated no-load-speed. If the load increases, the speed of the motor decreases. So, an air-motor can operate from 15–100% of its nominal speed, depending on its load.” For applications where the torque varies, the speed varies as well.

This can cause problems. In an agitator or mixer which needs a constant speed for accurate production, for example, if the viscosity of the mixture or its volume change in the mixing container, an undesirable change in speed can occur.

Deprag developed its speed regulator in response to a request from a chemical industry customer which wanted to maintain a constant speed in an agitator that it uses to mix liquid chemicals. The agitator (above) is used with an impeller mixer or blade powered by compressed air.

Deprag’s answer has been to integrate a non-contacting speed sensor (below) between the vane air-motor and the gearing. The sensor sends a digital speed signal to a controller which compares it with a value set on an LCD. If there is a difference, a valve is actuated to regulate the airflow and thus the motor speed.

The nominal speed of the 1.2kW air-motor that drives the agitator is 1,250 rpm, the no-load speed is 2,500 rpm, and the nominal torque is 9.2Nm. Its operating pressure is 6 bar. The “controlled” air-motor maintains the agitator speed, unaffected by any variations in the viscosity or volume of the mixture.

According to Dübbelde, the regulator can be used to optimise either the rate-of-change of the motor speed, or its accuracy. In the case of the agitator, there is no need to vary the speed quickly when fluctuations occur. Rather, an accurate operating speed is needed to ensure quality. The regulator can maintain this constant speed to within ±5 rpm. If any load changes occur, it adjusts the speed to the nominal value within two seconds.

The speed-controller is suitable for use on low-speed vane-motors. It evaluates speeds from 1/32 of a rotation to a full rotation and controls them to match the nominal value. It can also help to maintain speeds as high as 80,000 rpm.

“With the addition of speed regulators, we intend to optimise our product range and further expand our market position,” says Dübbelde. “Problems due to speed fluctuations during a process are now a thing of the past.”

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