Small boost transformer ?

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electricblue

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I've never used these. I need to boost voltage from 208 to 220 for a 16 amp 1PH motor. Is the primary wire size going to be the same as the secondary size? If so, I can use the primary OCPD for the motor and wire straight to the motor.:?
 
I've always found it weird that so many people think they need buck/boost transformers for things that are within range of nominal voltage espessially when I see it on tanning beds~?
Seems to me that all your doing is adding unwanted losses.
 
mopowr steve said:
I've always found it weird that so many people think they need buck/boost transformers for things that are within range of nominal voltage espessially when I see it on tanning beds~?
Seems to me that all your doing is adding unwanted losses.
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We changed the service on a building because the owner insisted his tanning beds needed 240V. I looked at the label on the bed and sure 'nuff it said 208/240V. Whatever. We worked all night and I got overtime.
 
ActionDave said:
We changed the service on a building because the owner insisted his tanning beds needed 240V. I looked at the label on the bed and sure 'nuff it said 208/240V. Whatever. We worked all night and I got overtime.
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Depends on whether the tanning lamp ballasts are truly dual voltage or if the lamps will be operating at reduced output on 208.
To him time in the beds is money and if he can run more people through in each day the extra expense might well be worth it to him.
 
electricblue said:
I told them it would be fine on 208 but the salesman sold them on longevity at 220. So hey it's money
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It is money so go for it, but for future reference whether that longevity is worth the price depends on a few factors. The most impact will be determined by actual supply voltage, 208 nominal may run anywhere between 198 and 216. If it is on the high side (and many but not all utilities do deliver on the higher side of nominal, but load conditions can bring that down) then you are already sort of close to low end allowed for 240 volts.

Motors can take a wider voltage variance from their nameplate rating and still perform well with little efficiency impact compared to some other loads. How close the motor runs to its output rating does have some impact though, as well as amount of run time it typically sees. One that runs continuously near full load rating may be more worth adjusting input voltage then one that is lightly loaded and sees limited run time.
 
Back to the original question. The primary can be smaller wire, but since both the primary and secondary transformer wiring are in the same box (or transformer included junction box), then there is no primary wiring outside of the box, so the practical answer is NO. Confused? The feed to the motor connects both to the primary and secondary leads of the transformer. If the circuit already had a 1 HP motor, everything else should remain the same. I always recommend a "starter" or other overload protection for large motors. But, I've seen many with just short circuit protection.
 
Frank DuVal said:
Back to the original question. The primary can be smaller wire, but since both the primary and secondary transformer wiring are in the same box (or transformer included junction box), then there is no primary wiring outside of the box, so the practical answer is NO. Confused? The feed to the motor connects both to the primary and secondary leads of the transformer. If the circuit already had a 1 HP motor, everything else should remain the same. I always recommend a "starter" or other overload protection for large motors. But, I've seen many with just short circuit protection.
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The buck/boost is what NEC calls an autotransformer. One side of the line (for simple single phase like the OP has) has a portion of the transformer in series with it to either buck or boost voltage, how much change and in which direction is made depends on how the multiple taps of said transformer are connected. The other side of the input line does connect to a portion of the transformer coil but also connects directly to one of the motor input leads.

In the OP's case where we are boosting voltage the supply side conductors will have higher current then the load side conductors. This will be the opposite if we were bucking voltage.

Overcurrent protection does need some attention. Though most smaller single phase motors already do have some sort of protection integral to the motor. But for those that don't you size it as you normally would if it is on the load side of the transformer - based on motor rated current.

If you happen to place protection on the supply side of the autotransformer then you need to take the current difference on each side of transformer into consideration when selecting setting.
 
Yes. For a typical 1 HP motor at 230 volts, the motor draws 8 amps. The secondary of the transformer must pass this 8 amps, so the primary will draw .8 amps (using the typical 10% boost, a 240 to 24 volt transformer as an example). So the current from the "panel" is 8.8 amps. Which, by the way, is the typical design current for a 1 HP motor on 208 volts.

This is why I said the primary leads of the transformer can use smaller wire, but not any wires feeding from the source to the secondary or motor! So, unless the primary is fed separate from the motor wiring, no, same size wire. And the OCP would be the same rating. The overload setting would change depending on if it was before or after the transformer.
 
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