120v control transformer on 3 Phase motor circuit

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MD84

Senior Member
Location
Stow, Ohio, USA
I have a 3 Phase 480V motor fed from a 20A cb powering an exhaust fan. There is a 120V motor operated louver that needs powered when the fan is running.

We are thinking of using a 50VA 480V:120V transformer connected to two lines of the motor circuit to power the louver motor operator.

Do I need an ocpd for the transformer? Are there any other considerations with this solution?
 

JFletcher

Senior Member
Location
Williamsburg, VA
I don't know. Never done anything like that before.

I think the circuit you have only needs primary protection, but something about the set up is bugging me.

Good question I hope some others chime in.

Wouldnt he need secondary side OCPD? If the fan faulted, say drew 60A current, the primary side would only see 15A, not enough to trip a 20A breaker.

OP, is it feasible to change the 120V louvre motor to 480V?
 

MD84

Senior Member
Location
Stow, Ohio, USA
I am stuck with the louver motor at 120v. The fan is running at 480 and is not going through the trf so I think the 20A cb is fine there for a short. It does go through a motor starter with ol protection.

From what I have found so far I may be able to size primary protection at 500% so a 0.5A fuse would be acceptable.

I was hoping to find an exception since this 50VA transformer is somewhat power limited. If primary ocpd is necessary that is OK. We are trying to keep cost down however since this is a surprise change we are having to facilitate. We originally were under the impression that these 5 exhaust fans did not need the trf.
 

MD84

Senior Member
Location
Stow, Ohio, USA
I don't know. Never done anything like that before.

I think the circuit you have only needs primary protection, but something about the set up is bugging me.

Good question I hope some others chime in.

It is bugging me some too. The service is 480y/277v. The motor circuit is three wire. I was concerned the single phase 480v load would cause some imbalance. I assume it is OK though and the imbalanced current would return on the other ungrounded conductor(s)?
 

MD84

Senior Member
Location
Stow, Ohio, USA
Wouldnt he need secondary side OCPD? If the fan faulted, say drew 60A current, the primary side would only see 15A, not enough to trip a 20A breaker.

OP, is it feasible to change the 120V louvre motor to 480V?

Just to clarify. The fan motor is separate from the louver motor. Then fan will operate on 480 just fine. The issue is that it will not move any air if the louver motor is not energized with 120V. The louver motor opens the louvers and then uses spring return once power is removed.
 

Smart $

Esteemed Member
Location
Ohio
I am stuck with the louver motor at 120v. The fan is running at 480 and is not going through the trf so I think the 20A cb is fine there for a short. It does go through a motor starter with ol protection.

From what I have found so far I may be able to size primary protection at 500% so a 0.5A fuse would be acceptable.

I was hoping to find an exception since this 50VA transformer is somewhat power limited. If primary ocpd is necessary that is OK. We are trying to keep cost down however since this is a surprise change we are having to facilitate. We originally were under the impression that these 5 exhaust fans did not need the trf.
You should power this circuit through auxiliary contacts.

Your 480:120 xfmr is required to have protection. At 50VA puts it under 2A on primary. Being a 2-wire secondary you can use primary only protection... but it cannot be greater than 300%, which is 50VA÷480V×300%=0.3125A.

Are you certain you only need a 50VA xfmr? What's the rating of the louver motor?
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
You should power this circuit through auxiliary contacts.

Your 480:120 xfmr is required to have protection. At 50VA puts it under 2A on primary. Being a 2-wire secondary you can use primary only protection... but it cannot be greater than 300%, which is 50VA÷480V×300%=0.3125A.

:thumbsup:
 

Ingenieur

Senior Member
Location
Earth
The circuit will be zero seq balanced but line magnitudes/phase angles will differ

what starts the fan?
120 v control circuit?

the 300% primary protection is fine
using the contactor aux contacts or an interposing relay is a good idea
 

Smart $

Esteemed Member
Location
Ohio
... There is a 120V motor operated louver that needs powered when the fan is running.
...
Motor operated louvers typically require open and close limit switches and motor reversing controls. You have not mentioned any, so I want to make certain these are covered also. If you open the louver using fan motor ON as an enabler, how does the louver close when the fan motor is OFF.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
a fair number of 50VA xfmrs will be class 2 power sources and may well be internally protected thus not needing any secondary OCPD.

I think more information is needed for the details but I don't have a major issue with the basic idea.

This is a pretty common setup for motor controls where one will use a small xfmr on two of the incoming power leads to supply control power for the motor contactor.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Motor operated louvers typically require open and close limit switches and motor reversing controls. You have not mentioned any, so I want to make certain these are covered also. If you open the louver using fan motor ON as an enabler, how does the louver close when the fan motor is OFF.

it is common for small actuators like what the OP describes to use an impedance protected gear motor which is constantly energized to hold the louvers open and a strong spring return to "unwind" the motor and close the louver or damper when the motor is deenergized.

A transformer which is also impedance protected against secondarhy short circuit (like a doorbell transformer) might need only a secondary fuse of a size which protects the wiring. That is, much larger than .3A.
The question is whether such a transformer is available for 480V input.
It may be necessary to use a transformer larger than 50VA just to make the fusing possible with a standard size.
 

Ingenieur

Senior Member
Location
Earth
My guess it is probably a small geared actuator with spring return
like a Belimo
they only draw 10-20 VA to open and zilch/spring to close

lol just saw the previous post
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
You can buy any motor starter with a 50VA CPT pre-wired by the factory. You typically don't even need 50VA for the starter coil, but that's as small as they come anyway. So just tap off of that secondary, run it through an aux contact of the starter and be done with this. Seems like a lot of hand wringing over a simple issue.
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
We at the water utility do that all the time. 480 fan, 120V control transformer to run spring return damper motor and contactor which is controlled through line voltage (120V) thermostat. Contactor is the biggest 120V load....damper is minimal. Most are set up with 2 primary (480V) fuses and a secondary (120V) fuse. Size would depend on current draw of the contactor. But....both primary and secondary fuses are cheap protection against a shorted transformer or load. Keeps the smoke inside the wires so the water techs don't whine about the smell. Most control transformers we use are not exactly bullet proof and do fail occasionally. Code is minimum requirements. Adding additional fusing is your choice.
 

Ingenieur

Senior Member
Location
Earth
You can buy any motor starter with a 50VA CPT pre-wired by the factory. You typically don't even need 50VA for the starter coil, but that's as small as they come anyway. So just tap off of that secondary, run it through an aux contact of the starter and be done with this. Seems like a lot of hand wringing over a simple issue.


Hand wringing or harmless banter?
kind of why we asked about the starter and control V
I assumed he did not have 120 available

does tapping off the starter control ckt/transformer violate any listings/approvals? Ul, nema, ?

I say screw the actuator
if the damper is on a ducted discharge get a counter-balanced back draft damper
:)
 

meternerd

Senior Member
Location
Athol, ID
Occupation
retired water & electric utility electrician, meter/relay tech
Hand wringing or harmless banter?
kind of why we asked about the starter and control V
I assumed he did not have 120 available

does tapping off the starter control ckt/transformer violate any listings/approvals? Ul, nema, ?

I say screw the actuator
if the damper is on a ducted discharge get a counter-balanced back draft damper
:)

Done all the time. Solenoid or motor operated valves, control relays, indicating lights, SCADA inputs, etc. Pretty much everything except the motor itself. You just have to be aware of total load on the transformer. If it's illegal, I should be in jail. I'm pretty sure NEMA and UL are manufacturer related.
 
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