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3 phase converters
3 phase converters are used to power 3 phase equipment when only single phase utility power is available. Typical applications are small workshops, farms, garages, etc.
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In some countries three phase power is readily available but in Australia, Canada, New Zealand, Republic of Ireland, the USA and the UK it is limited to large industrial premises. In these countries if a domestic or light industrial consumer asks for 3-phase to be installed, the answer is likely to include a 'please go away' price. If one has three phase equipment, inevitable if it is above 3 kW power rating, then the only economic option is to buy a single to three phase converter, commonly abbreviated to 3 phase converter or just phase converter. The 3 technological approaches adopted by manufacturers are inverters, 'static' converters or rotary converters.
Inverters convert a direct current (DC) input into an alternating current (AC) output. A logical extension is to use the same technology to produce 3 phases from a single phase. The result is a relatively small converter that is economic to produce in low rated powers. Unfortunately, the economics are unfavourable for the higher power ratings where phase converters are most needed. Also inverter efficiency is poor, resulting in excessive power loss and the generation of unwanted heat.
Static converters use a transformer to produce 2 phases from one, and at the required voltage, plus a capacitor to produce a third phase when a load motor is connected. Such a converter is cheap to manufacture but has four serious disadvantages:
a) the load must be a motor and it must be known in advance if it is star or delta connected
b) if the load has more than one motor then the highest power motor must always be switched on first
c) the load motor will only run at 70% of normal torque
d) there is a minimum power limit as well as a maximum so the capacitor value must be manually switched to match the load.
Rotary converters add an unloaded motor-generator to the static converter, eliminating problems a) to d). The motor-generator can be considered as analogous to the flywheel of an internal combustion engine; it smooths the flow of power and maintains the equal time relationship between the three phases. Efficiency is much higher than for an inverter solution, reaching 95% for most of the power range. The usual options adopted by static and rotary converter manufacturers are an autotransformer and a delta connected motor-generator but star-connected motors are used for the lower power ratings. There is usually a ‘boost' control to produce 500-600 % power for a few seconds to cope with hard starts or other momentary overloads. Rotary converters sometimes dispense with the transformer and incorporate its function within a more expensive motor-generator. This is economic for a manufacturer that makes motors in-house but not transformers. Conversely a transformer manufacturer that has to buy in motors finds the economics reversed.
The Booster range of converters from Boostek LLC in the USA, Isomatic Phase converters in the UK, K2C Industrial Automation Inc. in Canada, Phasetech Ltd. in New Zealand and Power Change Pty Ltd. in Australia add some major features to the basic rotary converter. All Boosters feature a solid state fully automatic boost facility and use no electromechanical contactors. This leads to a theoretical mean time to failure of 50000 hours and allows a 3 year free parts replacement warranty. The Booster E series offers +/- 10% variation between phase to phase voltages, whereas the Booster D series offers +/- 5% variation, plus a measure of power factor correction, thus keeping down electricity costs.
For more information on Boosters browse:
http://www.isomatic.co.uk/3phConverters.htm
http://www.powerchange.com.au
http://www.phasetech.co.nz
http://www.boostek.com
http://www.k2ciautomation
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