Control circuits and autotransformers

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Sparko6079

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A design has been proposed to use an autotransformer to produce 120VAC control voltage in a commercial clothes dryer.

NFPA 79 electrical standards for industrial machinery article 9.2 (control circuit voltages) this limits controls to 120VAC.

Is it true that this 120VAC must not be derived from an autotransformer?

Was the following incorporated into article 9.2?

"9.1.1.1 Control transformers shall be used for supplying the control
circuits. Control circuits shall not be derived from autotransformers."

Thanks and best wishes in the New Year
 
Not sure why you'd use an autotransformer and not just a regular control transformer. Seems like a long way around the barn, and weird at that. At what voltage will the dryer be served?
 
Control circuits and autotransformers

Hi Marc,

The gas burner requires 120VAC and has a 4 amp glowbar for ignition. If the dryer power is to be 120VAC no transformer is needed. If 208VAC or 220VAC will be the supply a 350VA step down transformer is needed. A 350VA transformer cost is about $40. An autotransformer cost is $15 less.

Thanks

John
 
Control circuits and autotransformers

Hi Marc,

Yes, that is what is done when a neutral is available. Those stacked double dryers at coin laundrys use L N L with effective 120VAC to each dryer. Single dryers where a neutral is available is also not an issue.

The transformer is needed where 208VAC, 240VAC is available without the neutral. (Line - Line) Some dryers are ordered with 200VAC single phase electric heat. A 208-220VAC coil in the contactor works fine and is acceptable for export to Europe. Same order to the U.S. requires the control voltage dropped to 120VAC.

For steam heated dryers a 220VAC solenoid valve is used for export models but for the U.S. the code appears to require a transformer and a 120VAC coil.

So I would like to know if autotransformers are allowed.

Thanks
 
Sparko6079 said:
A design has been proposed to use an autotransformer to produce 120VAC control voltage in a commercial clothes dryer.

NFPA 79 electrical standards for industrial machinery article 9.2 (control circuit voltages) this limits controls to 120VAC.

Is it true that this 120VAC must not be derived from an autotransformer?

Was the following incorporated into article 9.2?

"9.1.1.1 Control transformers shall be used for supplying the control
circuits. Control circuits shall not be derived from autotransformers."

Thanks and best wishes in the New Year
Click to expand...

I am not sure that a commercial clothes dryer is covered under NFPA79 anyway.
 
Control circuites and autotransformers

Control circuites and autotransformers

Hi Bob,

I believe the company has always designed under the code. Perhaps this is due to all gas dryers having CSA certification. I suspect CSA might require that designs follow the code.

Bottom line is that I must hit the books. (Both NEC and CSA) What I have gathered here will help in knowing where to look in the code.

From my CSA contact I was told the transformer was not necessary at all because CSA doesn't specify a max control voltage. Also because the dryer control opens both legs when the dryer stops the design is safe. I am not ready agree with my CSA contact. Please correct me if I am wrong but NEC clearly states 120VAC is the maximum allowed for control voltage.

Thanks again
 
Sparko6079 said:
Please correct me if I am wrong but NEC clearly states 120VAC is the maximum allowed for control voltage.

Thanks again
Click to expand...
I don't know where this requirement is, I know its not Art 430 on motor controls.
 
Sparko6079 said:
Hi Bob,

I believe the company has always designed under the code. Perhaps this is due to all gas dryers having CSA certification. I suspect CSA might require that designs follow the code.

Bottom line is that I must hit the books. (Both NEC and CSA) What I have gathered here will help in knowing where to look in the code.

From my CSA contact I was told the transformer was not necessary at all because CSA doesn't specify a max control voltage. Also because the dryer control opens both legs when the dryer stops the design is safe. I am not ready agree with my CSA contact. Please correct me if I am wrong but NEC clearly states 120VAC is the maximum allowed for control voltage.

Thanks again
Click to expand...
I doubt that CSA would require a piece of equipment to meet a code that does not apply.

NEC allows any voltage you want (for the most part) as a control voltage, and in fact line voltage is pretty common, especially for larger starters.

<added> I went and looked at the online version of NFPA79-2007. It would appear to effectively ban starters using line voltage coils.
 
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petersonra said:
I went and looked at the online version of NFPA79-2007. It would appear to effectively ban starters using line voltage coils.
Click to expand...

You should be able to to have line voltage coils but you would need them to be controlled by a circuit of 120V or less (i.e. use an interposing relay).
 
Control circuits and autotransformers

Bob,
I must say I am sure you are correct on CSA. I went cover to cover on CSA and found no mention of NEC. Also as the CSA inspector told me, no mention of a 120VAC limit on control voltage.

240VAC coils would only be considered on electric heat contactor and steam valve solenoid coils.

"I doubt that CSA would require a piece of equipment to meet a code that does not apply."

I looked over the preface to a copy of NEC 2002 and it seems to agree. I only notice a mention of residential appliances.

Please post the link to the online NFPA79-2007. I am sure, as you pointed out, the code does not apply inside a commercial dryer. Perhaps I can get this from the site and have it in writting if any of this comes into question again. CSA requirements should be all that needs to be followed.

Thanks and

Best regards

John
 
Control circuits and autotransformers

The link to NFPA 79 - Electrical Standards for Industrial Machinery is certainly appreciated as answers to all questions are in the regs.

120VAC is the maximum control voltage.

Transformers such as isolation transformers can be used to develop the control voltage. Autotransformers are not allowed.

As to whether or not the regulations apply to commercial dryers I would say they do. The company had dryers purchased by WA state correctional rejected because of lack of door ground bonding. We had to send out mod kits to satisfy the inspectors. Door bonding requirement is listed in NFPA very much as it appeared in the inspection report from WA.

There is a lot to be said for doing the job right the first time. I think I have the ammunition I need. By this time tomorrow I predict I will be working for a company committed to doing the job right or out of a job.

Thanks again

John
 
John,

I managed to delete your PM before replying.

IMO, the NFPA79 120V limitation for control ciruits is addressing the voltages that are brought to the pilot devices (i.e. limit switches, external interlocks and cover mounted pushbuttons). Because you are using 24V with an interposing relay for all of your external control you are meeting the standard even if the coil voltage of the motor "controller" is >120V. The NFPA 79 definition of control circuit (3.3.20) relates to the "logic" performance of the machine.
 
Control circuits and autotransformers

Jim,

I did not mention other items in series with the 120VAC or 240VAC part of the circuit. Between the controller 240VAC heat signal and the oven contactor there are three switches.

Oven high temperature limit switch.
Exhaust high temperature limit switch.
Air flow switch.

These items add logic to the performance of the control and thus part of the control. Under 3.3.23 I believe these would be considered control devices and thus make the circuit a control circuit. "Protecting the machinery" All three protect the machinery.

If these three items could be moved to the interposing relay circuit perhaps the issue could be resolved. This cannot be done because opening the interposing relay circuit removes power from the blower. This would have the system not operating as intended.

Thanks again
John

For those interested, I inherited this design from others that have left the company within the past couple of months and just as CSA is doing an approval audit. I am overseeing the audit with very little experience with the design and obviously dealing with code compliance. The help here is greatly appreciated.
 
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