Technical Papers

Integrated Load Indicator in Frequency Converters and Servo Amplifiers

The drive specialist SIEB & MEYER has equipped all frequency converters and servo amplifiers of series SD2x with the function load indicator as standard. This function reliably detects load changes of the connected motor. To achieve this only the torque-forming part of the motor current is measured which ensures an extremely accurate functionality. If a preset motor load is exceeded, the load indicator generates a message. Then, the operator can see whether a tool is, for example, broken or worn out. Additional expensive measuring sensors, such as acoustic emission sensors, may not be required.

"The load indicator is not only integrated as standard in our devices of the SD2x series", says Rolf Gerhardt, Head of Sales Drive Technology at SIEB & MEYER AG. "The user particularly benefits from the fact that the function is based on high-precision current measurement. This is quite a rarity on the market." The reason: Standard frequency converters are not optimized for high-speed tasks. Therefore, the manufacturers use mainly simple techniques for the current measurement. This, however, affects the dynamics and speed of functions like the load indicator because these functions are based on the derived signals. In addition, standard converters often measure and evaluate only the total motor current. This is misleading because the motor current consists of both inductive and torque-forming components. Only the torque-forming current is relevant for load detection, though. Since high-inductive currents occur particularly in the small-load and partial-load ranges, small sudden load variations cannot be detected in the total signal.


High Precision, Speed and Dynamics

With the load indicator of the frequency converters and servo amplifiers of series SD2x things are different. This advanced function evaluates only the torque-forming part of the motor current and can reliably detect even the smallest variations. In addition, special software functions are capable of filtering also slow variations in the current. These are, for example, thermal deviations that affect bearing and stator losses. "We are specialists for high-speed applications that require quick and accurate measurements. That's why we have developed a load indicator that works exceptionally dynamic and above all highly precise," Rolf Gerhardt points out. "By means of oscillograms users can visualize the high sensitivity of the function, which is very impressive." The load indicator even detects a drill stroke that is executed with a small drill of 0.2 mm – that means the load change is only 1 % compared to the rated torque of the driving drilling spindle. The motor current, by contrast, shows no change at all in the oscillogram and can therefore not serve as basis for an adequate evaluation.


Detection of Tool Breakage and Tool Wear

What conclusions can be drawn from the load changes of the motor that the load indicator has detected with such accuracy? Users can, for example, assess the actual status of the respective machine: If an expected current change does not occur during a machining process, tool breakage is likely. The operator can stop the machine immediately and prevent further damage to the machine. Tool wear can also be detected thanks to the load indicator, since worn-out tools need more power. If a preset load threshold is reached, a message is generated and the corresponding tool can be replaced quickly. This is how a continuous machining quality can be guaranteed.


Versatile Application Scenarios

The load indicator integrated in the frequency converters and servo amplifiers of series SD2x can also detect the contact between tool and work piece (gap eliminator). This process occurs frequently, for example, during grinding and requires immediate downscaling of the machining feed rate. This is usually covered by expensive acoustic emission sensors. In many cases the load indicator can take over this function.

Last but not least, feed rates can be flexibly adjusted by means of the load indicator. This is particularly helpful for processing work pieces with different material densities (e.g. wood).


By means of different working modes and parameter settings the load indicator by SIEB & MEYER can be adapted to the various possible applications. Operators benefit from the easy-to-use oscilloscope function which can also be installed on higher-ranking controls. For evaluation during the control sequences the function supports different signals via the I/O terminals and via serial data connections. "When developing the load indicator we looked to it that the function is particularly easy and flexible in use," says Rolf Gerhardt. "This way our customers benefit as much as possible from the function." In conclusion, the load indicator clearly adds value for the user – and SIEB & MEYER has another unique selling point in this industry sector.


Special Information:
More safety during the operation of machine tools

Tool breakage does not pose the only possible danger to processing machines and machine tools. Tools that are still rotating or rotating at too high speeds are also a risk to the operator or the service technician. The operator is on the safe side with devices of the series SD2 by SIEB & MEYER, as these devices feature safety functions for motors without speed sensors. The sensorless safety functions SFM (Safe Frequency Monitor) and SLOF (Safe Limited Output Frequency) of the SD2 drives help the manufacturers of processing machines and machine tools to comply with the increasing demands regarding machine efficiency and safety of people.


Load Indicator by SIEB & MEYER
Load Indicator by SIEB & MEYER: Drilling Hits

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