The design of a high accuracy rig for the testing of Pratt & Whitney PT6 jet engine starter generators has at its heart a variable speed AC drive from Control Techniques.

Euravia of Kelbrook, near Colne in Lancashire, has a state-of-the-art fully computerised test facility for gas turbine aero engines and Auxiliary Power Units (APUs), including the testing of the PT6, one of the most popular and reliable turboprop aircraft engines in use today. 

One of the range of tests concerns the performance of the compact, yet powerful engine starter generators.  Its function is to spin up the compressor section of the engine until it reaches the required rpm and the engine is self-sustaining.  At this point it becomes a generator to maintain the aircraft’s electrical system – fans, pumps, lights and other electrical equipment.  APU’s provide the same function whilst the aircraft is on the tarmac.

Euravia called in online automation distributor Direct-Industrial.com to look at the requirements for a test rig to undertake a series of tests to check on refurbished starter-generator performance.  Direct-Industrial.com and Euravia pooled technical resources, with Direct-Industrial.com designing the structural aluminium frame and the electrical and electronic system.  Euravia designed and manufactured the precision triple mounting heads and the extremely high-speed mechanical bearing system for the final drive shafts.

Direct-Industrial.com, an official distributor of AC and DC drives for Control Techniques and a long time user, chose a 15kW Unidrive SP (solutions platform) AC drive for the central task of testing the generator function.  The control of the rig is via a HMI with a bespoke test programme written by Direct-Industrial.com incorporated into a PLC.  It was found that the Unidrive SP and the Pixys proved to be the ideal combination, communication being via the standard ModBus built into the drive.  The drive responds to constant acceleration and deceleration commands in high torque and speed situations very rapidly and precisely with no overshoot and absolute accuracy.

The rig comprises three sections for three series of tests.  On the left, the section tests the ability of the starter to run up to starter speed against defined loads.  The central section, driven by the Unidrive SP AC drive, carries out a series of tests at a series of defined speeds; 6,700rpm, 7,800rpm, 12,000rpm and 14,000rpm; with parameters fed back to the PLC. The final test involves the checking of starting torque, with 800 amps being applied to the starter which has a locked shaft – in one test, for instance, looking for a torque level of 20.3Nm (15ft lb).

Readings of shaft torque, temperature, voltage, current, impedance and radial vibration are all recorded during the test sequences and fed back to the PLC, where test results are formulated and automatically compared against preset parameters.
The Unidrive SP AC variable speed drive range spans 0.75kW right up to 1.9MW.  Unidrive SP is the world’s most advanced ‘solutions platform’ AC drive, configurable into five operating modes – open and closed loop, vector, servo and regenerating modes - connectivity to most industry standard networks and accepting most position feedback protocols.