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Any Sparks about? Ohms law

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Flynnwood

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The question at the end is a yes/no :D

We've had a large gas fired heater fitted at work. Four large air inlets (one each side), with one large heat outlet - with heat being pumped out by a large fan. It's a big heater.

This started blowing a 10 amp fuse inside the heater.

On the 2nd visit by the engineer, the manufacturer told him to cover up 75% of the air inlet, to reduce the amps down to 7.8 (which it did). I saw it with my own eyes on his meter.

This whole approach seemed odd to me though (and the engineer), so my research led to this:

"This equation, i=v/r, tells us that the current (i), flowing through a circuit is directly proportional to the voltage (v), and inversely proportional to the resistance (r). So if the voltage is increased, the current will increase." ( I get that bit)

"But if the resistance is increased, the current will decrease."

My question: Why would/how covering up 75% of the air inlets actually reduce the resistance in the 'actual fan', or is it some by-product? (I don't get that bit, despite the measured reduction).

Why a manufacturer would make such a heater this way (with 4 large intakes that could cause too many amps to blow internal fuses) is beyond me. I won't mention the manufacturer, as I would like purely impartial electrical advice if possible.

Thank you.
 

HappyHacker

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i would imagine that covering the air inlet has reduced the amount of air the fan shifts and therefore the amount of work the motor has to do thus reducing the amps.

Does not make sense that the unit blows fuses when used as designed! Is sufficient air shifted/heated to keep the area at the right temp?
 

Eric The Viking

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It's more complex than just Ohm's law, as the induction of the motor introduces a phase shift (current and voltage peaks don't coincide over the AC cycle).

I have a feeling that the less air is actually being moved, the faster the fan turns, and in turn the less it is loaded. That means the motor is closer to free-running. I assume that in turn means it draws less current, but I'm struggling to understand why.

Even if that's right it's a daft way to design the thing.

Someone who knows (or can actually remember stuff) will be along in a minute...
 

CHJ

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Ohms law is not the prime factor relating to the excess current flow.
The motor is no doubt an induction motor and hence it needs to spin at its design speed to provide the back EMF (acts as its own generator) to counteract the inrush current consumption that the windings resistance alone wants to take.

If the units work as built in other locations I would get your electrician to check the power supply feed to ensure the wiring is man enough for the start up current draw.
 

Yojevol

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The answer to your basic question - why does the current reduce when the air flow is restricted? is simple to explain an it has nothing to do with the electrical resistance or the type of motor installed. The heater is a closed system which has an energy input and a work output which must match. If the work output is restricted by reducing the airflow the energy input must reduce accordingly. The energy input is measured in terms of volts and amps The volts remain constant . Therefore the amps must reduce in order to match the reduced load.
In this case if the airflow is restricted and the heating elements carry on as before, the temperature of the output air is going to rise. That doesn't sound a good idea to me.
Brian
 

jimmy_s

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Big space heater so probably a centrifugal fan to produce a decent static pressure. If you reduce the inlet area into the fan inlet the flowrate of the fan is reduced. Used to be used to control flowrate in variable air volume air conditioning systems - inlet guide vane control.

The motor then has to do less work. The motor is most likely an induction motor. Current in the motor windings is proportional to slip (difference between rotor speed and Synchronous speed). Slip reduces as motor has to do less work on air as there's less going through the fan impellor so current reduces.
 

Flynnwood

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@ Yojevol "In this case if the airflow is restricted and the heating elements carry on as before, the temperature of the output air is going to rise. That doesn't sound a good idea to me."

Interestingly, the temperature measured on the first ducting output did increase by around 30%.

@ CHJ "The motor is no doubt an induction motor and hence it needs to spin at its design speed to provide the back EMF (acts as its own generator) to counteract the inrush current consumption that the windings resistance alone wants to take.

If the units work as built in other locations I would get your electrician to check the power supply feed to ensure the wiring is man enough for the start up current draw."

Again interesting, (I had to look up back EMF - thanks), the manufacturer specs an internal 10 amp fuse, the engineer fitted a 12 amp on his 2nd visit after fitting the 75% reduction on the intakes :shock:

@ jimmy_s "Current in the motor windings is proportional to slip (difference between rotor speed and Synchronous speed). Slip reduces as motor has to do less work on air as there's less going through the fan impellor so current reduces."

Many thanks to everyone that replied. Any further comments on the above would be most welcome :)
 

Sideways

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Here's what I suspect is happening :
First see the typical power / current / torque curve for an AC electric motor, lifted from the electrical academia website

electrical academia.jpg


In normal conditions, the system should have been designed so that the motor running a little below max horsepower, current is a bit below maximum and you are to the right of the peak torque, maybe 1 tick mark left of the 1800 rpm in this graph.

Start to block the air inlet and the fan will have to work harder, current goes up, torque increases, speed doesn't drop much yet and power goes up. Everything you already expect as you ease to the left but still at the right hand end of the graph.

If you block 75% of the air inlet, I expect the issue comes from the fan.

Have you ever used a handheld blender and felt how it behaves when the blades can't pull the liquid through properly ? You get cavitation. The blades begin to race without effectively moving the liquid and the gadget stops working properly. Blocking the inlet makes this happen but I'm pretty sure that blocking the outlet by the same amount wouldn't have the same effect.
If this happened in your heater, you're effectively stalling the airflow, the fan is spinning even faster but no longer working properly, it isn't properly loaded and the motor will actually speed up. You'll go very close to the synchronous speed where the graph plunges down to the right in this zone the current falls too.

So, this isn't really about ohm's law. The manufacturer uses an industrial AC motor becuase that's sensible. They're efficient, simple and long lasting. Designing an extraction system is not an amateur job, they have to choose duct sizes, fan curves, motor curves, etc to work together so that each component is operating in the right place on it's performance curve.
If the heaters have started blowing fuses, then it suggests that something has changed - typically with age, wear or whatever. Maybe there wasn't enough tolerance or spare capacity designed in at the start, but it could also be a telltale that maintenance is needed

Have the fan blades become loaded with debris ?
Are the filters blocked ?
Are you using a new brand of filter with different air resistance ?
Are the motor or fan bearings worn out (they have a finite life) ?
Has someone meddled with the system - blocked a duct up or add an extra outlet ?
If it's single phase - is the motor capacitor ageing ?
...

Cheers
 

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