Fuses on Starter Control Transformers

Status
Not open for further replies.
D

djcrzysounds@yahoo.com

Member
Location
Scranton, Pa
Never had this happen.... a 240VoltPrimary 120Volt Secondary Transformer for inside a Motor Starter Control Cabinet is drawing more then the rated fuse installed by the factory. I've never used the brand that was purchased. It was provided by the fitters for a 'Vaporizing Pump'. Box has its own buttons for Start/Stop and an E-Stop. However there is an additional Start/Stop switch I installed. Unit worked fine when it was installed, however 2 days later I get a call that it isn't working. I checked the fuses and both on the primary side were blown. I put the ammeter on it and replaced the fuse with a 2Amp instead of the 1 Amp in there and it was drawing app. 1.4 Amps. I do not have a VA rating for the transformer and I have not heard from the Control Box Manufacturer yet.

I've never seen this before... Any suggestions or just install a 1.5 or 2 Amp fuse and it will be fine?
 
VA of the XFMR and conductor size are the considerations when selecting fuses, even for a little 500 VA control transformer. I would suspect your XFMR is bad. 500VA and #16 wire? 2A fuse OK.
 
No one has added any load have they ? On occassion, I've seen folk looking for 120v have enough knowledge to know they can find it on the load side of the control tansformer, but not realize the limits and add blowers, pump motors, etc.
 
Augie... There is no real additional load other then a pilot light. The only thing I was thinking of, was the pilot light and the 200' run to the additional start/stop station was the extra draw. I do not know for sure. However there was one more factor I didn't consider and that was 'unqualified nit-wits' with their paws in the enclosure. I know when I left it was latched, however all you need is a slotted screw driver... so you never know.
 
Are there fuses on the secondary side?

Was any equipment added on the secondary side? You could also have a bad contactor coil on the secondary side that has gone bad.

Starting to sound like a bad transformer or wrong one installed by accident.

Sounds like a fun one to troubleshoot.
 
OK verbal schematic. Incoming 3 #8s for 120/208V 3 Phase. Off the fused disconnect comes 2 wires to a fuse block GLUED to the front of the transformer(This fuse block hides some of the nameplate). Fuses were 1 Amp Dual Element/Delay. I put in 1.5 Amp fuses and the problem hasn't happened for 3 days now(we'll see how this week goes)... Load side of fuse block feeds transformer. Sec. Side of Transformer is 120V. And goes to another Fuse. Rating not avail. Due to the fuse not having a label.. Then from the fuse it goes to a terminal block. Hope that might eliminate any confusion for anyone.
 
If you cant pry the fuse block off to see the nameplate, I'd just fugure the primary fuse should be about 1/2 the size of secondary since it's a 2 to 1 ratio. Transformers like that usually short when the go bad. You could probably go to a 3 amp and still be ok.
 
S'mise said:
If you cant pry the fuse block off to see the nameplate, I'd just fugure the primary fuse should be about 1/2 the size of secondary since it's a 2 to 1 ratio. Transformers like that usually short when the go bad. You could probably go to a 3 amp and still be ok.
Click to expand...

After looking through the drawings that came with the controller, I found that the Primary fuses are 1Amp and the Secondary is a 15Amp(Yes 15 don't see how that works but from what you say, I think 1.5Amps is a safe rating to use until I can better troubleshoot the control circuit)

iwire said:
That is not going to add any load at all.
Click to expand...

Correct, except for the additional pilot light that was added to the system. Which I did not mention (My apologies). I think it will be good for right now. Thanks everyone. I just can't believe they account for an additional starter location but not for an additional pilot light
 
djcrzysounds@yahoo.com said:
Never had this happen.... a 240VoltPrimary 120Volt Secondary Transformer for inside a Motor Starter Control Cabinet is drawing more then the rated fuse installed by the factory. I've never used the brand that was purchased. It was provided by the fitters for a 'Vaporizing Pump'. Box has its own buttons for Start/Stop and an E-Stop. However there is an additional Start/Stop switch I installed. Unit worked fine when it was installed, however 2 days later I get a call that it isn't working. I checked the fuses and both on the primary side were blown. I put the ammeter on it and replaced the fuse with a 2Amp instead of the 1 Amp in there and it was drawing app. 1.4 Amps. I do not have a VA rating for the transformer and I have not heard from the Control Box Manufacturer yet.

I've never seen this before... Any suggestions or just install a 1.5 or 2 Amp fuse and it will be fine?
Click to expand...

In some cases, in case of smaller motors, no separate primary protective device is required but the motor SCPD is sufficient to protect the transformer.
The transformers have a high inrush current so fuse with the proper curve must be selected, not just any ordinary fuse.
100-150VA transnformers should be sufficient for most size 1-3 starters. As per 450.3(B) table if the trasnformer rated current is less than 2A and you're only using pprimary protection the fuse rating can be up to 300% of the nameplate current.

430.72 (C) Control Circuit Transformer.​
​
Where a motor control
circuit transformer is provided, the transformer shall be
protected in accordance with 430.72(C)(1), (C)(2), (C)(3),
(C)(4), or (C)(5).​

Exception: Overcurrent protection shall be omitted where
the opening of the control circuit would create a hazard as,
for example, the control circuit of a fire pump motor and
the like.​
(1) Compliance with Article 725.​
​
Where the transformer
supplies a Class 1 power-limited circuit, Class 2, or Class 3
remote-control circuit complying with the requirements of
Article 725, protection shall comply with Article 725.​

(2) Compliance with Article 450.​
​
Protection shall be permitted
to be provided in accordance with 450.3.​

(3) Less Than 50 Volt-Amperes.​
​
Control circuit transformers​

rated less than 50 volt-amperes (VA) and that are
an integral part of the motor controller and located within
the motor controller enclosure shall be permitted to be protected
by primary overcurrent devices, impedance limiting
means, or other inherent protective means.​
(4) Primary Less Than 2 Amperes.​
​
Where the control
circuit transformer rated primary current is less than 2 amperes,
an overcurrent device rated or set at not more than
500 percent of the rated primary current shall be permitted
in the primary circuit.​

(5) Other Means.​
​
Protection shall be permitted to be provided​

by other approved means.


 
weressl said:
In some cases, in case of smaller motors, no separate primary protective device is required but the motor SCPD is sufficient to protect the transformer.
The transformers have a high inrush current so fuse with the proper curve must be selected, not just any ordinary fuse.
100-150VA transnformers should be sufficient for most size 1-3 starters. As per 450.3(B) table if the trasnformer rated current is less than 2A and you're only using pprimary protection the fuse rating can be up to 300% of the nameplate current.

430.72 (C) Control Circuit Transformer.​
​
Where a motor control
circuit transformer is provided, the transformer shall be
protected in accordance with 430.72(C)(1), (C)(2), (C)(3),
(C)(4), or (C)(5).​

Exception: Overcurrent protection shall be omitted where
the opening of the control circuit would create a hazard as,
for example, the control circuit of a fire pump motor and
the like.​
(1) Compliance with Article 725.​
​
Where the transformer
supplies a Class 1 power-limited circuit, Class 2, or Class 3
remote-control circuit complying with the requirements of
Article 725, protection shall comply with Article 725.​

(2) Compliance with Article 450.​
​
Protection shall be permitted
to be provided in accordance with 450.3.​

(3) Less Than 50 Volt-Amperes.​
​
Control circuit transformers​

rated less than 50 volt-amperes (VA) and that are
an integral part of the motor controller and located within
the motor controller enclosure shall be permitted to be protected
by primary overcurrent devices, impedance limiting
means, or other inherent protective means.​
(4) Primary Less Than 2 Amperes.​
​
Where the control
circuit transformer rated primary current is less than 2 amperes,
an overcurrent device rated or set at not more than
500 percent of the rated primary current shall be permitted
in the primary circuit.​

(5) Other Means.​
​
Protection shall be permitted to be provided​

by other approved means.


Click to expand...


Thanks Lazlo... I think I've got more then enough information on this one. 1.5Amps it is:)
 
Status
Not open for further replies.
Top