Unbalanced Loads

For efficiency and maximum performance 3 phase alternators are the most common alternators on the market, however due to the very nature of the design they are required to work at a balanced load. Unfortunately, it is almost impossible and unrealistic to run 3 perfectly balanced loads. Any variation on any of the loads will cause unbalance in the alternator. An unbalanced load can cause a number of unwanted effects on a alternator which can in-turn affect the load.

A key requirement for the alternator when driving isolated AC loads is to maintain a tight voltage regulation, a clean sinusoidal waveform and high efficiency over a range of load conditions. These conditions must also be met during the certain unbalanced loads that an alternator will experience. Voltage regulation should operate within ±5%, and Total Harmonic Disorder (THD) should be less than 5%. RFL Alternators has analysed the performance of unbalanced loads, in both star and delta configuration. Figure 1 illustrates the equivalent diagrams of these configurations.

 

Figure 1

Figure 1: Equivalent alternator diagrams

 

Conventional Alternators

A conventional alternator has a high source impedance. When operating at a full single phase load the input source impedance (the rotor winding impedance) becomes measurable with that of the phase impedance this will naturally cause a high voltage variation. In turn this could potentially burn out the rotor and stator windings.

In-order to overcome this supplier’s use an Automatic Voltage Regulator (AVR). This adds cost and complexity to the alternator, the AVR is an extra component in the system which will decreases the reliability of the Alternator. Furthermore, as the AVR tries to keep the voltage regulated on the loaded phase, the unloaded phase will have a large and unacceptable voltage spike up to ±20%.

 

RFL Alternators

RLF Alternators specialise in Interior Permanent Magnet Alternators (IPMGs). These alternators outperform other alternators in:

  • Efficiency
  • Size
  • Light weight
  • Cooler operating temperature
  • Absence of brushes, mechanical commutators, slip rings and electronics
  • Highest power to weight ratio

Taking advantage of using Permanent Magnets eliminates the need for a rotor winding and consequently the AVR. Allowing for a cheaper, simpler and more reliable alternator. Furthermore, a load on one phase does not have a negative effect on the other 2 phases.

 

Star Configuration

In general, the standard configuration will almost certainly be a star configuration supplying single phase voltages to separate loads and as stated earlier it will never be possible to run perfectly balanced loads.

RFL has performed considerable testing on their alternators and found that an RFL alternator can run a 16kW alternator on a full single phase load (5.3kW) and maintain a voltage regulation of 4.7%, and the THD increased from 4% to 8%.

Although the THD is slightly higher than preferred, it would be unrealistic that there would ever be only be a full single phase load. If it was the case that the alternator was needed to only run a full single load a Delta configuration would be used.

 

Delta Configuration

If it was desired to use an RFL alternator purely for one phase an RFL alternator can easily be reconfigured to form a delta configuration. The results indicated that the voltage variation was around 4.3%. The biggest advantage using an RFL alternator in a Delta connection is that the measured THD went from 2% at no load to 5% at full load.

 

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