Choosing the Right Coil Voltage for Your Siemens 3TF48 Replacement

The coil voltage of a Siemens 3TF48 replacement has to match the control circuit that feeds the coil, which is often a different voltage from the motor supply. BRAH Electric lists its B3TF48 replacement contactors with a 24VAC, 110/120VAC, 208VAC, 240VAC or 480VAC coil, and the characters after the dash in the order number show which coil a listing carries. If the contactor is sound and only the coil has burnt out, BRAH also lists 3TY7483 replacement coils for the 3TF48 from 24VAC to 480VAC.

This guide gives both lists, then covers the three things to confirm before you order: the control voltage, whether it is AC or DC, and the frequency.

3TF48 coil voltage at a glance

• Read the full order number from the nameplate. 3TF4822-0AK6 and 3TF4822-0AV0 are the same contactor with different coils.
• Measure the control voltage at the coil terminals, or read it from the coil and the control drawing. Do not take it from the motor voltage.
• The BRAH part number is the Siemens number with a B in front: 3TF4822-0AP6 becomes B3TF4822-0AP6.
• Every contactor and coil in the tables below is listed with an AC coil. For a DC control circuit, call (855) 355-2724 with the part number.
• A failed coil can be replaced on its own with a B3TY7483 coil of the same voltage.

BRAH 3TF48 replacement contactors by coil voltage

The table lists the BRAH replacements for the 3TF48 in the catalogue, sorted by coil voltage. Each is supplied complete with the control coil shown. All seven are 3-pole, 600V contactors with 2 NO / 2 NC auxiliary contacts, so the coil is the only thing that separates one row from the next.

Three points about reading this table:

• The suffix goes with the coil. On the 3TF48 listings, -0AC2 is listed at 24VAC, -0AK6 at 110/120VAC, -0AM1 at 208VAC, -0AP6 at 240VAC and -0AV0 at 480VAC.
• The two numbers with no suffix, 3TF48 and 3TF4822, are listed with a 110/120VAC coil. If your nameplate has a suffix, order by the full number so the coil is not left to chance.
• The 75A and 60HP figures are catalogue values. Match the replacement to the nameplate of the contactor you are removing and to the motor nameplate, and do not treat the catalogue amperage as a motor rating for your application.

For the other 3TF sizes and how the same suffixes are listed across the range, see the guide to Siemens 3TF contactor replacements. All sizes are under replacement contactors.

3TY7483 replacement coils that fit the 3TF48

The coil in a 3TF48 is a separate part with its own Siemens number, 3TY7483, followed by the same style of voltage suffix. The catalogue describes each BRAH coil below as a magnetic control coil for Siemens World Series 3TF48 and 3TF49 contactors and motor starters, supplied as an assembled unit with control wiring terminals.

Changing the coil alone makes sense when the main contacts are in good condition and the contactor moves freely by hand with the power off. It is also the way to correct a contactor that was supplied with the wrong coil. The coil list includes 277VAC, so if your control circuit is 277VAC, call (855) 355-2724 to confirm the contactor and coil combination before ordering. Coils for other contactor sizes are under magnetic coils.

If the coil is not the only thing wrong, the guide on what to do when Siemens 3TF48 parts are unavailable shows the BRAH part for each component of the contactor.

How to confirm the control voltage

Use at least two of these sources and make sure they agree.

• The coil itself. The voltage and frequency are marked on the coil. On an installed contactor the marking can be hidden, so check it when the old part comes out.
• The order number. Compare the suffix on the nameplate with the table above.
• The control drawing. The schematic shows where the coil circuit is fed from: a control transformer, a separate control supply, or two of the line phases.
• A measurement. Measure across the coil terminals, A1 and A2, while the control circuit calls for the contactor to close. This is a live measurement and belongs to a qualified person using a meter rated for the circuit.

A label on the panel door is not proof. Panels are modified over the years and a control transformer may have been changed or re-tapped. The voltage at the coil terminals is the one that counts.

AC or DC: check before you choose a voltage

A 24V control circuit can be AC from a transformer or DC from a power supply, and the number alone does not tell you which. AC and DC coils are built differently and one cannot stand in for the other. Set the meter to AC and then to DC at the coil terminals, and check the drawing for a transformer or a DC power supply feeding the control circuit.

The contactors and coils in both tables above are listed with AC coils. If your 3TF48 runs on a DC control circuit, call (855) 355-2724 with the full part number.

Coil frequency: 50Hz or 60Hz

An AC coil is wound for a frequency as well as a voltage. In the coil table, the 24VAC coil is listed as 50/60Hz and the 120VAC, 208VAC, 240VAC, 277VAC and 480VAC coils are listed as 60Hz. That suits a 60Hz supply in North America. If the equipment runs on a 50Hz supply, or was built for export, read the frequency on the installed coil and call with the part number before ordering.

Control transformer and voltage drop

The right coil can still misbehave if the control supply is weak. If a new coil chatters or will not pull in, look at the supply before blaming the coil.

• Control transformer. Check the primary taps against the line voltage, and measure the secondary with the contactor energized. A transformer that is too small for the connected load, or tapped for the wrong primary voltage, delivers a low secondary voltage at the moment the coil pulls in.
• Long control runs. Wire resistance, undersized conductors and several devices sharing one circuit all lower the voltage that reaches the coil. Measure at A1 and A2, not at the transformer.
• Loose terminals. A loose or corroded connection in the coil circuit, including the overload relay contact and any stop button or interlock in series with the coil, can drop enough voltage to make the contactor chatter.

A coil fed with too little voltage may fail to pull the contactor in fully, and a contactor that chatters wears its main contacts quickly. A coil fed with too much voltage runs hot and fails early.

Before you order a 3TF48 contactor or coil

• The full Siemens number from the nameplate, including the suffix, or the 3TY7483 number from the coil.
• The control voltage, confirmed as AC, with the frequency.
• Whether you need the complete contactor or the coil alone.
• The number of auxiliary contacts in use. The contactors in the table are listed with 2 NO / 2 NC.
• The motor nameplate current and horsepower, compared with the catalogue values for the replacement.

BRAH Electric designs its parts in-house and tests them at its Carlsbad, California facility. It is an aftermarket manufacturer and is not an authorized Siemens distributor. Contactors and coils carry a 2-year warranty from the date of invoice. In-stock orders placed before 3:00 PM Pacific ship the same business day. For anything not in the tables, call (855) 355-2724 with the part number, Monday to Friday, 6AM to 5PM Pacific.

Safety when measuring and changing a coil

Confirming the control voltage means measuring a live circuit, and changing a coil means opening the contactor. Only qualified personnel should do either. Take the voltage reading first, with the protective equipment your site procedure requires. Then de-energize the starter, apply lockout/tagout to the power circuit and to the control supply if it is fed separately, and verify the absence of voltage before touching the contactor.

Siemens 3TF48 coil voltage FAQs

What happens if I fit the wrong coil voltage?

A coil rated below the control voltage overheats and burns out. A coil rated above it may not pull the contactor in, or may let it chatter. Neither is safe to leave in service, so stop and fit the correct coil.

Can I use a higher-voltage coil to be on the safe side?

No. A coil is not a part you oversize. A 240VAC coil on a 120VAC control circuit will not develop enough pull to close the contactor properly. Order the coil voltage that matches the circuit.

How do I know which coil my 3TF48 has if the label is unreadable?

Start with the suffix of the order number on the contactor nameplate and compare it with the contactor table. Then measure the voltage at A1 and A2 with the control circuit calling for the contactor. If the two do not agree, find out why before ordering.

Do I need a new contactor if only the coil has failed?

Not always. BRAH lists B3TY7483 coils for the 3TF48 and 3TF49 in six voltages. Check the main contacts and the arc chute while the contactor is open. If they are worn or damaged as well, a complete contactor may be the better repair.

Why does the contactor hum loudly after the coil was replaced?

AC contactors make a faint hum in normal service. A loud hum or chatter usually points to low voltage at the coil, the wrong coil voltage or frequency, or dirt or damage on the faces of the magnet. Measure the voltage at the coil terminals first.

Should I replace the control transformer at the same time?

Only if it is the cause. Measure its secondary voltage with the contactor energized. If the voltage holds steady at the coil rating, the transformer can stay.

References

• IEC 60947-4-1, Low-voltage switchgear and controlgear: contactors and motor-starters. The product standard for electromechanical contactors, including their control circuits.
• OSHA 29 CFR 1910.333, Selection and use of work practices. Covers work on or near energized parts, which includes taking voltage measurements.
• OSHA 29 CFR 1910.147, The control of hazardous energy (lockout/tagout).
• NFPA 70E, Standard for Electrical Safety in the Workplace.