How to Choose Power Transformer Solution?

Industrial control power transformers convert high input voltages to lower levels required for control circuits and devices in industrial systems. Acting as the backbone of these systems, they supply critical power to motor controllers, relays, solenoids, and other industrial equipment.

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Selecting the right transformer for your system involves a thorough understanding of your application’s requirements and environmental conditions. This guide provides a short technical breakdown to help you make the right choice.

Understanding Industrial Control Power Transformers

Industrial control transformers, also known as control power transformers (CPTs), are designed to provide a stable, low-voltage power source for control circuits. Unlike standard transformers, they are engineered to handle the inrush currents associated with electromagnetic devices, ensuring that the secondary voltage remains within acceptable limits during such events. This stability is vital for the reliable operation of control components.

Applications

• Motor Controllers: Provide stable voltage for relays, contactors, and motor starters.

• Industrial Machinery: Power control circuits in CNC machines, robotic systems, and conveyor systems.

• Automation Systems: Enable operation of programmable logic controllers (PLCs) and sensors in automated production lines.

• Relays and Solenoids: Enable reliable actuation of electrical switches and mechanical systems.

• Lighting Circuits: Supply low-voltage power for control systems in industrial lighting applications.

Types of Industrial Control Power Transformers

• Step-Down Transformers: Reduce high input voltages to a lower, usable level for control circuits. Commonly used in applications requiring 480V to 120V conversion.

• Isolation Transformers: Electrically isolate the input and output sides to enhance safety and protect sensitive equipment.

• Auto-Transformers: Compact and cost-effective for specific applications but lack isolation between circuits.

• Standard Industrial Control Transformers: Designed for general-purpose applications, these transformers provide reliable performance for most control circuits.

• Encapsulated Transformers: Sealed in a protective resin, these are ideal for environments exposed to dust, moisture, or corrosive materials.

• Multi-Tap Transformers: Offer multiple primary or secondary voltage configurations, providing flexibility for systems requiring different voltage inputs or outputs.

• Energy-Efficient Transformers: These transformers minimize energy loss, often featuring optimized core materials and windings for reduced operational costs.

Transformer Selection: Factors to Pay Attention To

To ensure a transformer meets the demands of your industrial control system, specific technical parameters must be evaluated. These factors directly influence the performance, efficiency, and reliability of the transformer within your application.

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1. Voltage and Current Ratings
   • Primary Voltage: This is the input voltage that the transformer will receive from the power source. It must match the available supply voltage in the facility.
   • Secondary Voltage: The output voltage required by the control circuit. Common secondary voltages include 120V, 24V, or other levels as dictated by the control devices.
   • Current Rating: The transformer must supply sufficient current to all connected devices without exceeding its capacity.
2. Frequency Compatibility
   • Ensure that the transformer’s frequency rating matches the system’s operating frequency, typically 50 Hz or 60 Hz, to prevent issues such as overheating or reduced efficiency.
3. Load Requirements
   • Inrush VA: The initial surge of current when devices are energized. Transformers must handle this without significant voltage drops. Inrush VA is calculated by summing the inrush VA requirements of all devices that will be energized simultaneously.
   • Sealed VA (Steady-State VA): The continuous power required to maintain device operation after startup. This is determined by adding the sealed VA requirements of all devices in the circuit.
4. Environmental Considerations
   • Temperature: Transformers should be rated for the ambient temperatures of the operating environment to prevent overheating.
   • Humidity and Dust: In environments with high humidity or dust, transformers with appropriate enclosures (e.g., NEMA-rated) are necessary to protect against contaminants.
   • Ventilation: Adequate ventilation must be ensured to dissipate heat generated during operation.

Steps to Select the Right Transformer

Translate system requirements research into action by evaluating voltage, load, and environmental needs to select a CPT that meets demands.

1. Assess System Requirements
   • Determine the primary and secondary voltage requirements based on the control devices and available power supply.
   • Calculate the total inrush and sealed VA by consulting device specifications to ensure the transformer can handle peak and continuous loads.
2. Consult Industry Standards
   • Refer to standards such as NEMA ST-1, which outlines performance requirements for industrial control transformers, to ensure compliance and reliability.
3. Evaluate Environmental Factors
   • Physical Environment: Assess the installation area for potential exposure to extreme temperatures, moisture, dust, or vibration. Select transformers with appropriate insulation classes and enclosures to withstand these conditions.
   • Mounting Options: Consider the available space and mounting configuration (e.g., wall-mounted, panel-mounted, or floor-mounted) to ensure compatibility with the installation site.
4. Check for Voltage Regulation Requirements
   • Voltage Drop Management: Choose a transformer with sufficient regulation to maintain a consistent secondary voltage even during peak inrush loads. Poor regulation can lead to unreliable operation of sensitive control devices.
   • Percentage Impedance: Choose a transformer with a suitable impedance rating, as this affects voltage drop and fault current levels.
5. Plan for Future Scalability
   • Consider potential system expansions or increased load requirements when selecting a transformer. Overestimating load requirements slightly can provide flexibility for future upgrades without overloading the transformer.
6. Prioritize Energy Efficiency
   • Transformers with higher efficiency ratings can reduce operational costs over time. Look for units that comply with energy-efficiency standards such as DOE (Department of Energy regulations) or similar regional guidelines.

Best Practices for Integration

1. Perform Load Testing: Before commissioning, test the transformer under expected load conditions to verify performance and stability.
2. Use Proper Circuit Protection: Install circuit breakers or fuses on both the primary and secondary sides of the transformer to prevent damage from overcurrent or short circuits.
3. Ensure Proper Grounding: Grounding is critical for safety and helps protect the transformer and connected equipment from electrical faults.
4. Regular Maintenance: Inspect transformers periodically for signs of wear, overheating, or contamination. Clean and replace components as necessary or invest in professional maintenance services.

Common Mistakes to Avoid

Common mistakes when selecting transformers include underestimating inrush current, leading to voltage drops during startup, and ignoring environmental conditions such as temperature, humidity, or dust, which can affect performance. Oversizing or undersizing the transformer can result in inefficiency or failure, while neglecting compliance with industry standards may cause safety hazards and legal issues.

Buy or Rent Transformers at VCM Solutions

At VCM Solutions, we offer a wide range of high-quality transformers to meet the diverse needs of industrial applications. Our team of experts can help you assess your system requirements and choose the ideal transformer for your specific application.

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Electrolitty

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Choosing a suitable power transformer

« on: November 15, , 08:22:59 pm » Hi,

I am looking for a suitable power transformer (230V primary) to build a small regulated linear power supply that will deliver +15V, -15V and +5V (max. 300mA per rail). The schematics should closely follow those of a vintage Tektronix A power supply.
See https://w140.com/tekwiki/images/a/a9/070--02.pdf  page 48 of 61.

I have been searching for quite a while on the electronic components distributors web pages (specifically the one with the rodent's name) but couldn't find a transformer with suitable values. I am now thinking of simply using two transformers wired in parallel, one with dual (more or less) 18V (with center tap) for plus and minus 15V and a second transformer for the +5V rail.
That is not really elegant but I can't think of a better solution. Perhaps I simply didn't find the right one in the transformer list with + entries.
I thank you in advance for your input.

coppice

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Re: Choosing a suitable power transformer

« Reply #1 on: November 15, , 08:42:23 pm » That market for transformers is not what it used to be, now the majority of appliances are using switch mode supplies. So, the range of off the shelf transformers has shrunk. However, most of the linear transformers in products which need more than a single rail have always been custom wound for that product. It is economic to wind quite low volumes of customised transformers, as long as they use a standard platform. Choose a core and primaries to suit your needs, and they wind the appropriate secondaries on top of that. If you are close to the vendor they used to be happy to make one offs that were not crazily expensive, suitable for prototyping.

jwet

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Re: Choosing a suitable power transformer

« Reply #2 on: November 16, , 02:32:05 am » I think the two transformer design is where you'll end up.  One of biggest advantages that manufacturer have over home builders is custom magnetics.  Most PS's for a lab PS use a main winding of 70V or so center tapped for the main rails, often these will have taps too to minimize power dissipation at lower voltage.  They also have an aux winding for say 15v that they can rectify and tie anywhere and then maybe a small 5v winding to make a negative rail.

Here are a few ideas-
1. Rewind transformers yourself.  This can be done but is pretty hard core and is art as much as science.  I've never done it but I've watched videos (instant expert!).  I think the easiest way might be toroidal transformers.  I have "modified" off the shelf toroids type (taken widings off).
2. Find a starving custom transformer guy.  These are rare these days.  There is one near me in Sanford, NC.  He will do big HV transformers for HAM transmitters or big welding transformers.  Its labor intensive and he needs to make a living.  He comes to the Raleigh, NC Hamfest each year and probably sells a few.  He has regular customers that keep him afloat.
3. Buy old broken power supplies.  I have a few around here that I bought for this purpose because they had a nice XFMR.  Mostly getting very dusty but one the list.  They have a three tap winding on a +-60v main that had a bunch of relays taking them in an out originally.
4. Stack up some standard stuff and remember Torioidal transformers- Mouser, Digikey, etc sell these as well as a lot of semi-custom toroids around +-70V or so at 500 VA for reasonable.  Audio amp builders use these and its an active hobby.  Then hack together what you can out of a couple of off the shelf transformers for your control and aux windings.

I really don't like swtichers for lab supplies but its getting harder and harder to avoid them especially with the prices.

Good Luck.

srb

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Re: Choosing a suitable power transformer

« Reply #3 on: November 16, , 03:54:19 am »

Hi,

I am looking for a suitable power transformer (230V primary) to build a small regulated linear power supply that will deliver +15V, -15V and +5V (max. 300mA per rail). The schematics should closely follow those of a vintage Tektronix A power supply.
See https://w140.com/tekwiki/images/a/a9/070--02.pdf  page 48 of 61.

I have been searching for quite a while on the electronic components distributors web pages (specifically the one with the rodent's name) but couldn't find a transformer with suitable values. I am now thinking of simply using two transformers wired in parallel, one with dual (more or less) 18V (with center tap) for plus and minus 15V and a second transformer for the +5V rail.
That is not really elegant but I can't think of a better solution. Perhaps I simply didn't find the right one in the transformer list with + entries.
I thank you in advance for your input.

A Bel-Signal 14A-56-515 would do the job. It is available ex stock from Mouser but is probably not the cheapest around.

It is only rated for 250mA from the +/- 15V outputs but you could easily get the 300mA you need if you substantially reduce the load on the winding for the +5V supply, which is nominally rated at 3A.

MathWizard

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Re: Choosing a suitable power transformer

« Reply #4 on: November 16, , 06:46:47 am » So Tektronic made a PSU based around LM317's ? Wow I expected some big discrete BJT circuits.

For such low power, I'd look at salvaging transformers from stereo's, CD players, etc. A lot of them have center taps. I've made 2 copies/versions of the same vintage 30V-1A PSU. I've used different combinations of salvaged stereo transformers for it. Now I'm using a big multi winding 1, capable of a lot more more than I'm using. « Last Edit: November 16, , 06:54:22 am by MathWizard »

Electrolitty

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Re: Choosing a suitable power transformer

« Reply #5 on: November 16, , 01:11:34 pm » Thank you very much for the answers, they took me a good step further.
As I would like to realise the project without too much delay I prefer to either use two transformers or the solution suggested by srb.
As the power supply connector providing power output has 4 pins (common, +5V, +15V, -15V) one would need to have a combined common lead for the 3 voltages. Considering the schematics of the BEL transformer, is it feasable to tie common of 5V and 15V together? I'm somewhat sceptical. Never did something like this.

jwet

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Re: Choosing a suitable power transformer

« Reply #6 on: November 16, , 05:29:13 pm » Yes- the great thing about transformer secondaries is that that they're totally isolated, you can stack them as AC or DC.  Be careful pulling a load that would put tow dissimilar windings in series but tying things together is fine.