Tuesday, August 9, 2011

Speed and Load Torque of PLC

At the starting, the induction motor performance supplied from a standard 380 V, measured at 50-Hz network. After that, the control system experimental was run between full load (1, 0 N m) and no load in the two different modes:
1. Induction motor supplied by the inverter and with the control of PLC;
2. Induction motor supplied by the inverter.

The range of speed and of load torque matches to the PLC hardware and software design. The torque versus speed characteristics were considered in the range 500–1500 r/min. The results illustrate that configuration works with diverging speed-varying the characteristics supplied load torque for different speed set points. Configuration that works with constant-speed-varying load torque characteristics in the speed range 0–1400 r/min and 0–100% loads. However, in the loads higher than 70% and speeds range higher than 1400 r/min, the system works with varying-speed and varying-load and the stable speed was not probable to be kept. This reality illustrates that PI control for stable speed as realized by the PLC software is effective at the synchronous speeds lower than 93%. The efficiency is displayed normalized, using as 1 p.u. or base value the induction motor efficiency supplied from the network of standard.

The normalized efficiency is meaning that the acquired efficiency with control of PLC is higher than the induction motor efficiency worked from the standard 380-V, 50-Hz network without the PLC control and without the inverter at operation with loads higher than 70%. The PLC-controlled system efficiency is enlarged up to 10–12% compared to the motor operation of standard. If we abandon magnetizing current, an estimated value for the efficiency is where is the slip and are the rotor and stator winding resistances, in that order from a theoretical point of view. The PLC controlled system operates with very low slip values. In all load and torque speed conditions, therefore the upper values of efficiency can be acceptable and specially at high frequencies and speeds. The flux of magnetic enlarges and, as a result, there is an raise in magnetizing current resulting in augmented losses at lower frequencies.

The stator frequency versus stator voltage attributes of the inverter with PLC control for the same speeds range and torques. The relationship between stator frequency and stator voltage is constant for each one of the speed–torque characteristics. Nonetheless, which communicates to the motor flux, raises with the reduce of frequency from 8, 3 at 50 Hz up to 11, 25 at 12 Hz. Therefore, where the accessible torque reduces from 100% at 50 Hz up to 60% at 20 Hz, when both frequency and voltage reduce, there is a raise in magnetic flux with a maximum available torque reduce.

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