help with a fan speed controller

[big-steve]

Hi guys
I'v got a question to do with a fan speed controller. It's a systemair STL-0100-at. I can't get the model of the fan/ motor it's controlling due to the badge being weathered. Bit it is single phase. I am just wondering how these speed controllers work. I was called to a site for the extract canopy not working and the fan is controlled via the said speed controller. The store has recently had a minor refit (DB's upgraded, new boilers ect)
I was called there and found the speed controllers PC board blown and the 10A breaker tripped so changed the speed controller, all working. But I noticed that when the fan was at full speed the motor was pulling 5.4 amps
And when the speed was lowered the current increased to as high as 19.6 amps (lowest speed) and then obviously the breaker tripped. I don't know what size Mcb was fitted before the upgrade but it's a 4mm cable supplying this fan. So my question is.. Is it normal for the current to increase the lower the speed? And could it be that this was overlooked in the re-fit and a 10 amp mcb fitted instead of say a 20? Because it trips the Mcb when the speed is lowered.
Thanks in advance

 

[telectrix]

it sounds as though the speed controller is using power when the fan is at a lower speed. if you're sure it's the right controller and wired correctly. try a 20A type C breaker/ or contact manufacturers.

 

[Marvo]

I am just wondering how these speed controllers work.

They work with phase angle / voltage control (Link).

Systemair fans often use an external rotor motor that's mounted inside the actual impeller. Motors generate the large proportion of their heat in the rotor so because the rotor is external and because it's so efficiently cooled directly in the main air-stream they can basically get away with murder as far as being able to run the motor almost into stall with low efficiency and massive I squared R losses.

Systemair fans also come in some very odd kilowatt ratings and their maximum run currents per kilowatt are often much higher than other manufacturers.

I'd suggest you get the information from the plate on the fan and check the running current is within what is stated there. If the run current is too high you may be using an incompatible speed controller for that particular fan or it may have already suffered damage.

Most external rotor motors of this nature aren't rewindable plus they're supplied matched and balanced to the impeller so more often than not if the motor is tickets then you'll end up replacing the entire fan.

It's also worth noting that Systemair fans usually have a thermal protector (thermistor) on the windings which should be wired back to the speed controller (or DOL starter) to shut it down if overheating occurs.

 

[Darkwood]

Without details of the motor is hard to call what physical size is this fan?

Inline fans don't behave in some cases as you would expect sometimes slowing them down makes them do more work and hence current rises due to air flow issues and cavitation, your old speed board is probably the best point to gage the fan rating - if it is fitted with a 10amp fuse i dare say thats an indicator that for some reason you have a large over current in the fan this can be due to many this for correct connection to motor or bearing issues

 

[big-steve]

The inline fan itself is a systemair and the speed controller is systemair, I can't get the details of the fan motor but I can get the FLC ( that was behind the isolator which was 7.2A. But when the speed controller is turned rite down the current increases and my thoughts are that it was supplied by a 20A MCB before the re fit.

 

[Marvo]

Not all Systemair fans are compatible with all Systemair speed controllers, especially the voltage type. You even get a scenario where a certain model of Systemair fan is compatible with a voltage speed controller only if it has a certain generation motor so it can be a bit of a nightmare.

Bottom line is I think the run current difference between low and high speed is to great. Triple the run current on low speed means there's a high likelyhood of existing motor damage.

 

[Lucien Nunes]

Sounds like some good points above, I don't know the units, but just had a thought: What kind of meter are you using to measure the current? Many DMMs will over-read a chopped waveform such as you will get at low speed from a phase angle control but will be correct on a sine wave at full speed, so the current might appear to increase more than it really does. Obviously the MCB is also indicating the current is too high but I have been caught out with incorrect results before when I forgot which meter I was using to measure a speed controlled circuit.

 

[Marvo]

Good point, that might explain the very large discrepancy in run current between low and high speed.

 

[big-steve]

Yeah that is a good point! It's a fluke t5-1000