Portable test stands
SPANGLER – the Plus of Security.
PM drives are motors whose rotor is equipped with permanent magnets. Due to their type of construction they promise users a higher degree of efficiency than the conventional asynchronous motors. However, the small engines do also have a negative side. Permanent magnet motors cannot even be operated directly on the power supply on a trial basis but only with frequency converters. SPANGLER Automation has developed a portable test stand for the company HUBER SE in order to be able to test machines in the production hall as well as on construction sites at different speeds. In these machines there are installed both asynchronous and permanent magnet motors.
Personal security is above all
So that the test stand can be connected to different motors and machines, the supply line and the motors were designed pluggable. Although this increases the flexibility of the plant, it required special demands on the personal security. This means it is not allowed to connect electrical equipment with a frequency converter with no further ado to a power socket which is monitored with a fault-current circuit breaker (RCD) of the type A. These RCDs of the type A are only designed for detecting errors with alternating voltage. In case of a direct-current error, this RCD could both not detect the error and thus injure people or rather the RCD could be damaged forever what could lead to personal injuries later.
Limitation DC residual current 6mA
The RCDs of the type A are still used in many production sites. SPANGLER has found a solution to guarantee personal security for pluggable equipment of 30mA and, in addition, to limit the DC residual current proportion to less than 6mA. With a special fault-current circuit breaker type B MI for mobile units it is allowed to connect the plant downstream of a type A. The tripping level of 6mADC prevents a dangerous pre-magnetising of the upstream fault-current circuit breaker of the type A as a result of a smooth DC residual current.
Proof of efficacy in the test laboratory
By installing a low-leakage filter in front of the frequency converter the limit values for emitted interferences for variable-speed drives (150kHz – 30MHz) class C1 could be adhered to and proven. Moreover, by installing the filter another demand (leakage current <3,5mA) for mobile plants has been fulfilled. The efficacy of all these measures was proven by means of an EMC testing in the test laboratory at the company EPA GmbH.