Amethyst transformer design and transformer manufacturer

Amethyst Designs Atlas

Amethyst Designs Ltd
47/48 Viking Way, Bar Hill
Cambridge, CB23 8EL
Tel: +44 (0)1954 - 789696
Fax: +44 (0)1954 - 789662
transformer design Amethyst-designs © 2017 Manufacturer & design of transformers and wound components

Transformer Terminology

Secondary V A
This is the first and major influence on the physical size of a transformer.
V A = total (Secondary AC full-load voltage) x (Secondary AC current). No phase or power factor is involved.

Magnetising current
This is very often limited by the customer’s requirements. In order to keep it low core losses must be reduced. This can be achieved by reducing the flux density, increasing the physical size or using higher quality core material.

The British definition is the difference between full-load and no-load voltages, as a ratio of the no-load voltage.

% reg = (VNL-VFL) / VNL * 100

Temperature rise
This is dependant on losses. Core loss (flux density, core material and size) and copper losses (D C winding resistance). A further consideration is the ambient temperature the transformer will be operating in. For instance, if the maximum temperature the transformer will stand is 105º and the ambient is 35º C it only allows 70º C rise. A point to remember is if the transformer is to be situated in a small enclosure its rate of cooling will be severely limited. However certain encapsulation materials have a better co-efficient thermal conductivity than air which can assist in the cooling.

Operating Frequency
50Hz is the standard frequency for transformers in the UK (60Hz in USA) but there are cases for 400Hz or even higher. Frequency has two effects, both opposite in result. The higher the frequency, the worse the losses but the higher the frequency, the lower the flux density (all other things being equal) and so an optimum point is reached at between 400Hz – 800Hz where the size can be reduced (on steel cored transformers).

By changing the core material, the frequency can be raised still further and the size reduced further thus entering the realms of switched mode transformers. However at very high frequencies losses in the copper windings, known as ‘skin-effect’, begin to take effect (the current tends to travel on the outer part of the wire). The layout of windings, wire gauge and number of strands of wire become important.

Phase Shift
This is the amount by which the secondary wave form leads (or is “offset” from) the primary wave form. The phase shift angle is usually expressed as either degrees, radians or seconds at zero crossover point. It should not be confused with the phase difference between secondary voltage waveform and secondary current waveform. The cosine of this angle is known as the power factor and is the reason for the difference in watts to VA.

Stray Field
With toroidal wound transformers “low” stray field is often requested to prevent hum and pick-up in amplifiers. Although careful design and manufacture can help, it is usual to run them at a low flux density which may require a larger core.

Where the voltage potential between two adjacent layers (volts per layer) is particularly high, it is often necessary to reinforce the wire enamel with layers of an insulation material. This is usually the case on larger transformers where there are intrinsically few turns which results in higher volts per layer. Interleaving is also common in transformers which involve particularly high voltages.

Short Circuit Current
This is the current drawn by the primary when the secondary is shortened. It is dependant on the square of the turns ratio, the applied primary voltage and the primary and secondary winding resistances.

Tapped Primaries versus Series / Parallel
When a primary winding is tapped for different input voltages (as opposed to two windings for series / parallel connections), the portion of the winding for the lower voltage must carry more current, and therefore use thicker wire. (If the VA is to remain constant, A must increase if V is decreased). It therefore means the primary will occupy more space than it would otherwise and so the frame size must be increased

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Amethyst transformers are manufactured to international standards -  Medical BS EN 60601, Technical Equipment BS EN 60950, Safety Isolating BS EN 61558 (60742) we have full trace ability of production using UL-approved materials.