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Author Topic: Autoformers, anyone?  (Read 1661 times)

Mike Sokol

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Autoformers, anyone?
« on: October 16, 2017, 04:34:02 pm »

OK, this is for the RV world, not pro-sound. But I need your input to help un-obfuscate the psycho-babble of the Hughes Autoformer marketing department. Of course, an autoformer is just a buck-boost transformer in "boost" mode with some sort of relay to add a 10% or so boost when the line voltage gets below some threshold. But the marketing language and video implies that the low-voltage of an RV park can damage ALL of your appliances including an Apple switch-mode computer power supply that's rated for 90 to 250 volts, plus it will make AC motors pull more amperage (true for AC-DC motors used in circular saws and drills, but not AC motors IIRC). Plus they dismiss the obvious problem of increasing the amperage coming though the pedestal circuit breakers which will cause them to trip long before rated power is getting into the RV. Please review their site for marketing babble and let me know what you find. It's a great lesson in really understanding how low voltage affects available power.

See the text and link below.
https://hughesautoformers.com/autoformer-university/why-do-you-need-an-autoformer/

Why Do You Need An Autoformer?

Hughes Autoformers are designed to increase voltage to your RV and help eliminate low voltage damage to your appliances. Unlike a boost transformer, the ‘sense circuit’ in the Autoformer will adjust the output based on the load demand. For this reason you can run additional appliances on a 30-amp input. For example, a coffee pot and microwave each draw 1200 watts. Add wattage for the converter and/or a refrigerator – about 800 additional watts – and now you have 3200 watt demand. If you are only getting 100 volts from the supply, the maximum wattage would be 3000 watts. In this case, the Hughes Autoformer will boost voltage to give you 3600 watts to your RV!

The Autoformer output will self-adjust depending on the demand. With the increase in the voltage to the RV (through the Autoformer) the amperage demand will be lower and the overall performance will be greater. Your appliances will operate smoothly and efficiently without premature wear and damage to motors and compressors. With an operation range of approximately 94 to 125 volts input, the Autoformer will boost your RV voltage to safe & efficient levels.

Appliance failure can be costly, as well as frustrating and inconvenient. Many AC motors burn out due to higher-than-rated current draw caused by low voltage. This wasted current could be better used to operate another appliance at the same time the AC is being used. With low voltage you generally can’t run anything else without risk of damage.

« Last Edit: October 17, 2017, 02:35:58 pm by Mike Sokol »
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John Roberts {JR}

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Re: Autoformers, anyone?
« Reply #1 on: October 16, 2017, 05:45:34 pm »

OK, this is for the RV world, not pro-sound. But I need your input to help un-obfuscate the psycho-babble of the Hughes Autoformer marketing department. Of course, an autoformer is just a buck-boost transformer in "buck" mode with some sort of relay to add a 10-volt or so boost when the line voltage gets below some threshold. But the marketing language and video implies that the low-voltage of an RV park can damage ALL of your appliances including an Apple switch-mode computer power supply that's rated for 90 to 250 volts, plus it will make AC motors pull more amperage (true for AC-DC motors used in circular saws and drills, but not AC motors IIRC). Plus they dismiss the obvious problem of increasing the amperage coming though the pedestal circuit breakers which will cause them to trip long before rated power is getting into the RV. Please review their site for marketing babble and let me know what you find. It's a great lesson in really understanding how low voltage affects available power.
Apparently extreme low (or extreme high) voltages can damage appliances, I do not know all the details besides the obvious.
Quote
See the text and link below.
https://hughesautoformers.com/autoformer-university/why-do-you-need-an-autoformer/

Why Do You Need An Autoformer?

Hughes Autoformers are designed to increase voltage to your RV and help eliminate low voltage damage to your appliances. Unlike a boost transformer, the ‘sense circuit’ in the Autoformer will adjust the output based on the load demand.
marketing babble... autoformers are a type of transformer. The only variable output is from adding boost winding in series, when a threshold voltage drop is detected.
Quote
For this reason you can run additional appliances on a 30-amp input. For example, a coffee pot and microwave each draw 1200 watts. Add wattage for the converter and/or a refrigerator – about 800 additional watts – and now you have 3200 watt demand. If you are only getting 100 volts from the supply, the maximum wattage would be 3000 watts. In this case, the Hughes Autoformer will boost voltage to give you 3600 watts to your RV!
Bad math? 100/120 x 3200 resistive load = 2666W   

If autoformer steps 100V up to 120V they should only get the 3200W nominal in the appliances.

3200W @ 120V is 2.66 A... The 100V autoformer primary while stepping up, will draw 3.2A (again 3200W load) No power is is created or destroyed by the transformer.  If anything autoformer will have lower resistive losses than a straight step up transformer but that difference should not be significant. (it will be significant to cost of the step up device).


Quote
The Autoformer output will self-adjust depending on the demand. With the increase in the voltage to the RV (through the Autoformer) the amperage demand will be lower and the overall performance will be greater. Your appliances will operate smoothly and efficiently without premature wear and damage to motors and compressors. With an operation range of approximately 94 to 125 volts input, the Autoformer will boost your RV voltage to safe & efficient levels.
as you noted the primary current will increase by the ratio of voltage step up. So 100V stepped up to 120V will draw 20% more current from 100V primary (all else equal).
Quote
Appliance failure can be costly, as well as frustrating and inconvenient. Many AC motors burn out due to higher-than-rated current draw caused by low voltage. This wasted current could be better used to operate another appliance at the same time the AC is being used. With low voltage you generally can’t run anything else without risk of damage.[/color]

Auto formers are smaller and cheaper (less copper and less iron for same output power). The only downside is that the secondary is not galvanically isolated from primary, but for this application who cares?

JR
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Kevin Graf

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Re: Autoformers, anyone?
« Reply #2 on: October 16, 2017, 05:46:31 pm »

Their description is not what I think of as an autotransformer. I think of a single winding with several taps.
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Stephen Swaffer

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Re: Autoformers, anyone?
« Reply #3 on: October 16, 2017, 06:32:28 pm »

Power = volts x amps that much is simple-pick your point in the circuit and that law will apply so....

If I have a 30 amp breaker in the pedastal and 100 volts at the pedestal, I can pull 3000 watts of energy from that pedestal-no matter what "magic" device I place after that pedestal.

If I have a resistive load, then yes, bumping the voltage to the load up will indeed increase it's wattage-simple ohms law.  That may indeed be a useful function-perhaps improving the function of a coffee maker, etc.

If voltage is too low on a motor, it may indeed increase the current draw since a motor depends on reverse induced EMF to limit current flow, so yes bumping the voltage up here may also be an improvement.

If the OCPD is located in the RV-say you feed the Autoformer from a 50 amp 120 volt nominal/100 volt actual supply, then indeed the Autoformer could allow you to use the full 3600 watts your RV can consume.

In practice, you can probably pull an extra amp or 3 from a 30 amp circuit-so you just might actually get 3200-3300 watts out of a 30 amp 100 volt measured circuit.
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Mike Sokol

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Re: Autoformers, anyone?
« Reply #4 on: October 16, 2017, 07:11:09 pm »

If voltage is too low on a motor, it may indeed increase the current draw since a motor depends on reverse induced EMF to limit current flow, so yes bumping the voltage up here may also be an improvement.

IIRC The increase in current from lack of Reverse EMF is a problem primarily with AC/DC motors with brushes, hence why you can burn up the brushes on a circular saw on a really long extension cord. I had always assumed that this effect was much less for AC only motors, and the low voltage was mostly a problem during startup when the starting cap is drawing a huge amount of current and the motor doesn't come up to speed quickly enough to open the centrifugal starter switch. Or am I missing something?   
« Last Edit: October 17, 2017, 08:50:39 pm by Mike Sokol »
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Frank Koenig

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Re: Autoformers, anyone?
« Reply #5 on: October 16, 2017, 10:31:19 pm »

IIRC Reverse EMF is a problem primarily with AC/DC motors with brushes, hence why you can burn up the brushes on a circular saw on a really long extension cord. I always assumed that this effect was must reduced for AC only motors, and the low voltage was mostly a problem during startup when the starting cap is drawing a huge amount of current and the motor doesn't come up to speed quickly to open the centrifugal starter switch. Or am I missing something?

My recollection is that conventional induction motors suffer with under-voltage*. So called "universal" motors (inductance-matched, series wound, brush-commutated motors), as used in portable tools, vacuum cleaners, etc., do fine with low voltage. They just go slower.

*There are exceptions, such as "torque motors", as used to tension the tape in tape recorders back in the day, which are induction motors designed to run at variable voltage to supply variable (~velocity independent) torque.

 -F
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Marc Sibilia

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Re: Autoformers, anyone?
« Reply #6 on: October 17, 2017, 07:19:08 am »

My recollection is that conventional induction motors suffer with under-voltage*. So called "universal" motors (inductance-matched, series wound, brush-commutated motors), as used in portable tools, vacuum cleaners, etc., do fine with low voltage. They just go slower.

*There are exceptions, such as "torque motors", as used to tension the tape in tape recorders back in the day, which are induction motors designed to run at variable voltage to supply variable (~velocity independent) torque.

 -F

Induction motors running a fixed mechanical load will draw more current on low voltage.  The speed of the motor (before accounting for slip) is determined by the frequency, not the voltage of the AC.  The mechanical power to the load doesn't change much because the speed doesn't change much.  When the voltage drops, the torque at the same exact speed drops, but since the mechanical load is essentially the same, the motor can't keep up and slows down just a little bit.  This increases the slip (the difference between the synchronous speed and the actual speed of the motor, e.g 3570 rpm on a nominal 3600 rpm 4 pole motor).

The current drawn by the motor is a very strong function of slip, so a slight reduction of speed causes a large increase in current.  It will slow down just enough to generate the torque it needs to match the load.  Electric motors are pretty efficient devices (except shaded pole motors), so if the mechanical load is constant power because of the relatively constant speed, the electrical input power needs to be constant.  When voltage goes down at nearly constant speed, current goes up.

With universal (AC/DC brush type motors) speed is proportional to voltage, so reducing voltage reduces speed, mechanical load, and current.
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Mike Sokol

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Re: Autoformers, anyone?
« Reply #7 on: October 17, 2017, 08:19:05 am »

Induction motors running a fixed mechanical load will draw more current on low voltage.  The speed of the motor (before accounting for slip) is determined by the frequency, not the voltage of the AC.  The mechanical power to the load doesn't change much because the speed doesn't change much.  When the voltage drops, the torque at the same exact speed drops, but since the mechanical load is essentially the same, the motor can't keep up and slows down just a little bit.  This increases the slip (the difference between the synchronous speed and the actual speed of the motor, e.g 3570 rpm on a nominal 3600 rpm 4 pole motor).

The current drawn by the motor is a very strong function of slip, so a slight reduction of speed causes a large increase in current.  It will slow down just enough to generate the torque it needs to match the load.  Electric motors are pretty efficient devices (except shaded pole motors), so if the mechanical load is constant power because of the relatively constant speed, the electrical input power needs to be constant.  When voltage goes down at nearly constant speed, current goes up.

With universal (AC/DC brush type motors) speed is proportional to voltage, so reducing voltage reduces speed, mechanical load, and current.

Interesting. So my understanding is that the reverse EMF caused by the self-generator action of the AC/DC (or DC) motor is what limits the winding current as the rotational speed reaches some maximum design limit. But I think you're saying that pure AC motors actually change their impedance based on the slip angle speed of the rotor as compared to the rotating magnetic field. So if you added another motor to spin the first motor in phase with the line frequency, then the winding impedance would increase, and thus draw significantly less current. Am I thinking about this correctly?   
« Last Edit: October 17, 2017, 03:04:10 pm by Mike Sokol »
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John Roberts {JR}

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Re: Autoformers, anyone?
« Reply #8 on: October 17, 2017, 09:47:19 am »

Their description is not what I think of as an autotransformer. I think of a single winding with several taps.
That description is what happens when a non-engineer marketing type gets involved. An engineer probably told him the design features, then he put lipstick on it.

JR
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Frank Koenig

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Re: Autoformers, anyone?
« Reply #9 on: October 17, 2017, 12:50:27 pm »

Induction motors running a fixed mechanical load will draw more current on low voltage.  The speed of the motor (before accounting for slip) is determined by the frequency, not the voltage of the AC.

Marc, exactly right and nicely explained. Thanks. --Frank
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