Control transformer (machine tool control transformer)

01

Set the rated capacity and the primary and secondary voltages

Taixi lists its BK industrial control transformers at 25, 50, 100, 150, 200, 250, 300, 400, 500, 700, 1000, 1500, 2000, 3000 and 5000 VA, with primaries of 220 V or 380 V and secondaries of 6, 12, 24, 36, 110, 127, 220 or 380 V, or to the user's needs, for 50/60 Hz circuits supplying control power, local lighting and indicator lamps in machine tools. [1]

Carel describes its BK series as suitable for 50–60 Hz circuits up to 500 V, as power supply for machine tool electrical appliances, local lighting and indicator lamps. Leipole's JBK7 table lists 40 VA to 2500 VA units with an AC 380 V or 220 V ±5% input and separate control, signal and lighting windings, for example 110 V control with 6 V or 12 V signal windings; it notes that the capacity of each winding can be distributed to the user's requirements. [2] [3]

Editorial calculation: a 100 VA transformer with a single 24 V secondary can deliver about 4.2 A (100 ÷ 24); split across more than one winding, the VA is shared between them. [1] [3]

Give the supplier the primary voltage and tolerance, each secondary voltage and its VA, total and inrush VA of the coils it will feed, frequency, efficiency or loss limit, insulation class and test voltage, protection (fuses or thermal), mounting dimensions and terminals, ambient temperature, altitude and humidity, and any approvals needed.

02

Check regulation, efficiency, insulation, ambient and mounting

Carel lists for its BK series an input of rated voltage ±10%, output of rated voltage +5% at no load, efficiency of ≥95%, over-current protection, insulation resistance of ≥50 MΩ and no breakdown or flashover under a 2000 V power-frequency voltage for one minute, for ambient temperatures of −15 °C to +40 °C, altitude below 2000 m and relative humidity below 90%. [2]

Leipole's JBK7 table lists efficiency of 80% for 40–160 VA units, 85% for 250–800 VA and 90% for 1000–2500 VA. Taixi gives ambient limits of −5 °C to 40 °C at altitudes up to 2000 m, with relative humidity not above 50% at 40 °C, and asks for special instructions when the transformer is used outside that range. [3] [1]

Mounting is by footprint and hole pattern: Taixi lists BK-100 at 85 × 64 mm mounting centres with 103 × 93 × 99 mm maximum outline, and BK-1000 at 125 × 123 mm with 173 × 186 × 174 mm; Carel and Leipole publish their own A, C and outline dimensions per size, which differ between makers. [1] [2] [3]

03

Size the transformer for inrush VA, not only continuous VA

Hammond's HPS Imperator control transformer data sheet, published by distributor AutomationDirect, sizes a machine-tool control transformer from three load values: sealed (continuous) VA, inrush VA when relays, contactors and solenoids pull in, and the inrush power factor, for which it recommends assuming 40% when unknown. The nameplate VA must cover the continuous load, and the inrush VA in the regulation table must cover the circuit inrush. If supply voltage varies no more than 5%, the 90% secondary-voltage column applies; if it varies up to 10%, the 95% column; the 85% column gives minimum values for device operation and is for reference only. The range runs from 50 to 1,500 VA, 600 V class, UL listed under file E50394, rated −20 °C to 40 °C ambient, with a 55 °C rise and 105 °C insulation class at 50–150 VA and an 80 °C rise and 130 °C class at 250–1,500 VA. [4]

The BK and JBK series above are rated by continuous VA; ask the supplier for an inrush VA figure at a stated power factor and secondary voltage so the transformer can be checked the same way. [1] [2] [3]

Editorial calculation: a 150 VA unit allows 699 VA of inrush at 90% voltage, about 4.7 times its nameplate rating, but only 512 VA (3.4 times) where the 95% column applies; a control circuit with 600 VA of inrush and a ±10% supply therefore needs the 250 VA size (880 VA in the 95% column).

List every coil's sealed and inrush VA, state the supply voltage variation, and ask the supplier for the regulation table of the exact model before choosing the VA rating.

04

Trace the parts of a control transformer

This is a functional reference built from the sources cited; it is not one maker's design.

05

Fill the specification fields with values for your panel

A control transformer selection needs values for your control circuit. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

06

Send the voltages, loads and cabinet data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Primary: [voltage; tolerance; frequency]
Secondaries: [voltage and VA each]
Load: [total VA; inrush of coils]
Performance: [regulation; efficiency]
Insulation: [class; test voltage]
Mounting: [hole pattern; outline limit; terminals]
Environment: [ambient; altitude; humidity]
Volume: [quantity; approvals]

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