Six And Twelve Pulse Rectification Components in Lift Transformers
by E.S. Barker – Chief Engineer
For many years now, Express Transformers and Controls has supplied lift transformers and chokes for use in the elevator industry. We produce single phase control transformers and three-phase power transformers which provide the drive voltages for many of the passenger and goods lifts in the UK. We also supply single and three phase chokes and reactors which are used in various circuits to prevent electrical noise being passed back into the power lines as a result of the rectification systems used to create DC voltages for use in lifts.
We are currently specified as the sole UK-based supplier of three phase rectifier transformers and linear DC chokes and ripple filters by the American company Magnetek as an essential partner in their efforts to market their range of thyristor DC drives in Europe. This company has its products in 9 out of 10 of the worlds tallest buildings.
Six-Pulse Drive Systems
One requirement of a DC lift motor is for a smooth supply of high power DC voltage. This is often obtained by feeding the secondary of a three-phase power transformer into a full-wave rectification circuit comprising six controlled thyristors. These thyristors are part of the DC Drive unit, and along with a further set of six thyristors they form the main switching network in a reversible drive system. The transformer is necessary to improve the power factor of the system by matching closely the DC motor armature voltage requirements . It also assists in the operation of the drive by providing the necessary reactance to assist rectification, and to reduce the transmission of switching harmonics into the supply system.
The six pulse DC current feeding the motor armature is smoothed by the inclusion of a DC choke and 300Hz ripple filter in series with the armature to ensure silent operation of the motor. The continual stop/start duty of the lift creates large overload currents in the choke which must be designed to maintain its inductance without falling below 85% of its design value at the current peaks. We have built a specialised tester to plot the linearity of the chokes that we provide for lift duty and are consequently able to guarantee their performance under these arduous conditions.
Twelve-Pulse Drive Systems
An alternative approach to the provision of a smooth DC supply, often employed as a measure in the reduction of harmonic distortion, is the twelve pulse rectification system whereby two controlled six pulse systems are connected together. In the elevator industry, in which the DC motor voltages are not too high, it is usual to shift the phasing of one set of thyristors by 30 degrees and connect the outputs of the two rectifier bridges in parallel. The fact that the instantaneous outputs of the two bridges are not equal requires the inclusion, in the circuit, of a centre tapped inter-bridge reactor to allow for correct operation. The reactors that we manufacture for this application are designed to avoid saturation at the extremes of DC bridge current unbalance as well as having a guaranteed inductance when operating at the high ripple frequency of 600 Hz.
The twelve pulse drive system provides a smoother DC output than the six pulse rectifier circuit thus reducing the cost of the linear choke and filter unit. The cost savings of this must however be balanced against the added cost of the inter-bridge reactor and other additional circuit components.
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