Tag: transformer

Have you ever seen a low voltage transformer like the one shown above? It has multiple input (primary) taps for a good reason.

It is common to find 3-phase and single phase equipment rated to operate on both 240v and 208v power. This is because three-phase power can either be 208v leg to leg when the building has a wye type transformer or it can have 240v leg to leg when a delta transformer is in use.

Single phase power (in the US) is almost always 240v leg to leg or 120v leg to neutral because one “phase” of the three-phase transmission line is being used and is wired in the transformer secondary to create two opposite sine wave (180 degrees out of phase) legs of 120v power.

The motors in a unit that is rated for 208v or 240v must all be designed to operate within that range of voltages. However, if the transformer is designed for 240v and only 208v is applied the secondary voltage will also drop below the rating. This can lead to issues with the controls such as chattering relays and contractors. In most cases the system will function normally, but in cases with long runs of control wire (high voltage drop) or sensitive electronic controls it can have a greater impact.

All you need to do when starting up or servicing a system with a multitap primary transformer is to ensure that the primary conductors are on the correct taps.  If you ever find a system where the control voltage is higher or lower than expected by about 10% then you will want to check for an improperly tapped transformer.

— Bryan

The definition of a transformer is a device that changes the voltages in an alternating current circuit.

You may have heard of an autotransformer or a buck and boost transformer and these terms are usually being used for the same type of device just highlighting different aspects. A transformer does not need to be a buck and boost to be autotransformer and it does not need to be an autotransformer to be buck and boost but often the two elements go together.

Autotransformer

The word auto in autotransformer really just means one or single not really “automatic” or “automated” in the way we usually think of it. It is an autotransformer because it only has one inductive (magnetic) winding shared by both the primary and secondary.

Buck and Boost

Buck just means that it decreases the voltage and boost means it increases it. A buck and boost transformer means that it can both increase or decrease the voltage.

What is their application? 

Buck and Boost autotransformers are often used to make small changes in voltage, say from 208v to 240v (boost) or from 240v to 208 (buck). They are usually efficient and inexpensive when only small changes are needed, whereas a traditional two coil transformer is more practical for larger changes.

Most of these transformers will have multiple tap points for different output and input voltages and can often be connected in different configurations to perform a wide range of functions like in the case of the Emerson Sola HD.

One major consideration with an autotransformer is that there is no isolation between the primary and secondary so a failure of the isolation of the windings of an autotransformer can result in the input voltage being applied to the output and component damage. There is also greater likelihood of harmonic and ground fault issues because of this “mixing” of primary and secondary.

— Bryan


A 40 VA transformer is rated for 40 VA or Volt-Amps on the secondary.

For a typical 24 volt secondary this is simply using watts law to calculate amperage.

40 ÷ 24 = 1.666666 ∞ (round up to 1.67 already)

So you cannot place more than 1.67 amps of constant load on the transformer without overloading it.

Many accessories use 24 volt power, such as zoning systems, UV lights, alarms etc…


This common UV light uses 0.65 amps, if you connected to the existing transformer that would leave only 1.01 amps for the system controls.

It is generally a safer bet to either add in an additional accessories transformer or upsize the existing transformer to 60 or 75 VA.

When adding an additional transformer make sure it either has an internal fuse, or add a fuse inline to protect the transformer and connected devices.

— Bryan

I had an old timer tell me that you can never connect two transformers together because they will “fight one another”.

If you are anything like me (and heaven help you if you are), whenever someone says something like that, a cartoon in your head starts playing.

In this case, I imagine two transformers with boxing gloves on duking it out to see which one “wins”.

The truth is you can connect two transformers together so long as you are careful, but you need to know why you’re doing it and then do it properly.

Transformers have a VA (Volt-amps) rating that dictates how many volt-amps (volts x amps, which is watts simplistically but there is a more complicated reason it is called VA in transformers that we won’t get into here) the transformer can handle on the secondary.

Above we show two 75VA transformers with 24V secondary windings.

75VA÷24V=3.125A

So with a 75VA transformer, you can run a maximum of 3.125 Amps, if you needed more power you would need to either go get a larger, more expensive transformer or…. you could connect another identical one in parallel. If you connected two 75VA transformers in parallel you would then have 150VA of secondary capacity which can be necessary in some cases with multistage commercial units or some large accessories.

In this case, parallel simply means connecting the two primary and secondary windings together in the exact same way as we show above… Pretty easy

It is SUPER important to get the polarity exactly the same and use two transformers with identical winding turns in the primary and secondary and identical secondary coil impedance (resistance).

In fact, it is so important that I advise that you only do this if you have two identical model transformers.

To be even safer, connect the primary windings first and check the secondary’s against one another with a voltmeter before actually connecting them to the system. For a typical 24v secondary you can connect the two common wires to ground to act as a stable reference first then check the two R or Hot side leads to one another and then to common. They should read 0v to one another and 24v to common. If you get anything other than 0v from hot to hot then you want to recheck your primary wiring and ensure that they are exactly the same.

— Bryan

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