Buck Boost Transformer

[dvcraven052256]

I am thinking of using a buck-boost step-up transformer to feed some 480V equipment. I only have 240/3 available service in this space. Relative to the motors I am feeding although small (2) motors at 5amp load each. Do I need to be concerned about inrush currents when choosing a xfmr. size?

Just curious I have not use a BB xfrm much.

Thanks Dan Craven

 

[augie47]

Standard buck and boost transformers are limited to a 10% changes I believe.

 

[winnie]

Forget about buck/boost or step up. Your question is about transformer impedance.
You need to consider transformer impedance and motor starting current. Will the supply voltage drop too much when the motor is starting. This depends heavily on the motor and the type of mechanical load.

Autotransformers are available for large voltage changes, e.g. 208V to 600V. See:

3-Phase Autotransformer 600-480-400-240-208v

3-phase autotransformer designed to convert a range of different voltages in both step-up and step-down operations. This unit has a wide range of applications.

store.maddoxtransformer.com

 

[hillbilly1]

Standard buck and boost transformers are limited to a 10% changes I believe.

I think it’s more like 20%, but yeah, not good for this application.

 

[hillbilly1]

Forget about buck/boost or step up. Your question is about transformer impedance.
You need to consider transformer impedance and motor starting current. Will the supply voltage drop too much when the motor is starting. This depends heavily on the motor and the type of mechanical load.

Autotransformers are available for large voltage changes, e.g. 208V to 600V. See:

3-Phase Autotransformer 600-480-400-240-208v

3-phase autotransformer designed to convert a range of different voltages in both step-up and step-down operations. This unit has a wide range of applications.

store.maddoxtransformer.com

Or just a plain step up transformer.

 

[__dan]

The motors might be internally configurable for 240V 3ph. It's common for 3 ph motors to have multi 240 / 480 Voltage ratings with winding connection changes. You would also have to change the overload protection setting and probably control circuit Voltage (taps). Then take a second look at what's left of the load on the 480 to see what's necessary.

 

[hillbilly1]

Forget about buck/boost or step up. Your question is about transformer impedance.
You need to consider transformer impedance and motor starting current. Will the supply voltage drop too much when the motor is starting. This depends heavily on the motor and the type of mechanical load.

Autotransformers are available for large voltage changes, e.g. 208V to 600V. See:

3-Phase Autotransformer 600-480-400-240-208v

3-phase autotransformer designed to convert a range of different voltages in both step-up and step-down operations. This unit has a wide range of applications.

store.maddoxtransformer.com

Those seem to be popular overseas, but in the states most just use step ups. We don’t have extreme gaps in voltages like overseas.

 

[topgone]

The motors might be internally configurable for 240V 3ph. It's common for 3 ph motors to have multi 240 / 480 Voltage ratings with winding connection changes. You would also have to change the overload protection setting and probably control circuit Voltage (taps). Then take a second look at what's left of the load on the 480 to see what's necessary.

Or, have the motors rewound for 480V, whichever is cheaper!

 

[garbo]

The highest secondary voltage I think that I ever saw was 24 volts on buck boost transformers but they might make some with a 48 volt secondary ( two 24 volt windings ) . So like Winnie and others mentioned forget about using buck boost to double or halve the supply voltage. Purchase a step up transformer. If you have motors reconnected or rewound from 480 volts to 240 volts you will have to replace the starter heaters or hopefully adjustable ampere overload block will have a high enough setting. The motor amps will be double the 480 volt amps when going down to 240 volts.

 

[kwired]

Forget about buck/boost or step up. Your question is about transformer impedance.
You need to consider transformer impedance and motor starting current. Will the supply voltage drop too much when the motor is starting. This depends heavily on the motor and the type of mechanical load.

Autotransformers are available for large voltage changes, e.g. 208V to 600V. See:

3-Phase Autotransformer 600-480-400-240-208v

3-phase autotransformer designed to convert a range of different voltages in both step-up and step-down operations. This unit has a wide range of applications.

store.maddoxtransformer.com

Or just a plain step up transformer.

I would think they would be similar in cost to step up/down transformer as well. They should have similar winding content for similar kVA rating.

 

[Besoeker3]

Usual Brit here. As far as I call we do not use buck/boost transformer. Is there any need ?

 

[hillbilly1]

The highest secondary voltage I think that I ever saw was 24 volts on buck boost transformers but they might make some with a 48 volt secondary ( two 24 volt windings ) . So like Winnie and others mentioned forget about using buck boost to double or halve the supply voltage. Purchase a step up transformer. If you have motors reconnected or rewound from 480 volts to 240 volts you will have to replace the starter heaters or hopefully adjustable ampere overload block will have a high enough setting. The motor amps will be double the 480 volt amps when going down to 240 volts.

32 is usually the highest on a single.

 

[hillbilly1]

Usual Brit here. As far as I call we do not use buck/boost transformer. Is there any need ?

Used frequently in the states for raising 208 to 240, or vice versa.

 

[winnie]

I wasn't saying not to use a buck/boost. I was saying that buck/boost vs step up is not the important question. The OP asked if they needed to think about motor starting current in transformer sizing, and I answered that they would need to evaluate that by looking at transformer impedance and the resulting voltage drop.

'Buck/boost' is a term used to describe an autotransformer circuit built using a step down transformer, say a 120/240:12/24 transformer where you place the low voltage coil in series with the high voltage coil to get a small voltage change. The bulk of the power flows directly from source to load, with only a fraction going through the transformer. This let's a small kVA transformer boost voltage for a large kVA load.

Autotransformer setups are also available for large voltage changes such as 240:480V or 480:600V. I posted an autotransformer which could take 308V to 600V.

I would expect a 20kVA 240:480 autotransformer setup and a 10kVA 240:480V step up transformer to have about the same mass, so there is some benefit to the autotransformer approach.

I would also expect the autotransformer setup to have lower impedance if the mass were the same.

An autotransformer might not work with the supply grounding.

But if the grounding is correct and the impedance acceptable, I wouldn't hesitate to use an autotransformer.

Jonathan

 

[Besoeker3]

Used frequently in the states for raising 208 to 240, or vice versa.

But does it actually buck AND boost ?

 

[zbang]

Well yes, but not at the same time. Right-side-up it boosts, upside-down it bucks.

 

[LarryFine]

 

[Besoeker3]

Well yes, but not at the same time. Right-side-up it boosts, upside-down it bucks.

So it does it actually buck AND boost ? That was what I was expecting. Sort of........

 

[LarryFine]

So it does it actually buck AND boost ? That was what I was expecting. Sort of........

It all depends on the connections.

 

[wwhitney]

An autotransformer might not work with the supply grounding.

The principal limitation in the NEC is 210.9 / 215.11, which require that "the system supplied has a grounded conductor that is electrically connected to a grounded conductor of the system supplying the autotransformer." With some exceptions that don't apply when boosting 240V to 480V.

So if the OP has a 240V 3P3W ungrounded delta (not too likely), that rules out autotransformers. If it's a 240V 3P3W corner-grounded delta supply, then using two 240V:480V boost transformers for the ungrounded legs would comply with the above sections.

If it's a 240V 3P4W delta with a center-tapped conductor that is grounded, then I'm not clear on whether the above text would allow you to create a 3P4W 480V delta in which one conductor is grounded, but not actually used by the load, or if that would be prohibited.

Cheers, Wayne