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High current with softstarter


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#1 ÉlPontus

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Posted 05 June 2006 - 10:24 PM

Hello! Nice forum smile.gif

I am comparing the currents in the start of a motor that controls a fan. I am going to test DOL, star-delta, softstarter and a Frequency converter. It is a 3-phase motor on 5.5kW, 400V, and takes about 7.3 ampere when it is running.

I have so long only tested DOL and the softstarter, but when I look at the current and the voltage on the softstarter I get a bit confused. It is a Siemens Sirius softstarter, model number 3RW3024-1AB14

https://mall.automat...08-&jumpto=6008

I have read that the soft starter is supposed to reduce the current and the torque. But when i look at the currents my meter shows about 50 ampere, and have a peak about 63 ampere. Thats only about 15 ampere less than the DOL-start. So it doesn't look like it is reducing the currents...? Anyone have a clue why it is like that?


And my second question; Isn't the softstarter suppose to work like in this picture?:

IPB Image
(from a ABB-pdf)

My current looks a bit like it:

IPB Image

But what about the voltage, that is supposed to look like the picture from ABB? Can anyone explain what I see in this picture unsure.gif . After I while the lines comes closer and closer to finally look like a regulary sinus-wawe.

IPB Image



#2 kens

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Posted 05 June 2006 - 11:00 PM

Hello,
this type of softstarter is controlling on a Time/Voltage ramp. Have you tried to adjust your settings ie initial voltage and start time? If you play around with these you may see an improvement. What is the nameplate FLC of the motor? The currents that you mentioned seem a little high (Around 6 x FLC) for starting a fan however this type of softstarter does not look at the currents so you cannot set a starting current only a ramp time. Also as it is a two phase controller the currents will be higher than with a three phase controller.
I will let the more clever people answer your second question.
Ken
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing

#3 marke

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Posted 06 June 2006 - 06:24 AM

Hello ÉlPontus

The ABB diagrams are idealistic digrams showing a single phase control of a resitive load. They illustrate the concept of phase control, but not the detail.

The wave forms that you have posted are reasonable and realistic.

The soft starter that you have tried is an open loop timed voltage ramp soft starter and the foltage will ramp up to full voltage at a preset rate. It may also be two phase control, I'm not sure.
You must set the ramp time so that the starter reaches full voltage after the motor reaches full speed, otherwise you will have a high start current.
A fan is a relatively high inertia load so you need to set a long ramp time. If you can, set the start voltage to about 50% and the start time to around 45 seconds. By playing with the settings, you can alter the start characteristics.

The minimum start current is a function of the minimum start torque required by the load, and the ability of the motor to convert amps into torque. I would expect the start current for a fan to be around 400 - 450% compared to around 650% for DOL start.

Best regards,

#4 ÉlPontus

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Posted 06 June 2006 - 12:43 PM

QUOTE(kens @ Jun 6 2006, 01:00 AM) View Post

Hello,
this type of softstarter is controlling on a Time/Voltage ramp. Have you tried to adjust your settings ie initial voltage and start time? If you play around with these you may see an improvement. What is the nameplate FLC of the motor? The currents that you mentioned seem a little high (Around 6 x FLC) for starting a fan however this type of softstarter does not look at the currents so you cannot set a starting current only a ramp time. Also as it is a two phase controller the currents will be higher than with a three phase controller.
I will let the more clever people answer your second question.
Ken

I´m not sure what FLC is, but if it is the normal current that it is supposed to take according to the nameplate, I think it is about 8 Amp. But it could also be 12 Amp unsure.gif Will look on it tomorrow... But according to my meter it says 7.3 Amp, so it should be 8 Amp..?

marke:
I think the current looks nomal, but what about the voltage? Why is it just short lines in different lenghts here and there? Is it conducting and not conducting severel times in one period? not just two times like in the ABB-example (and my own current-wave??)

#5 kens

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Posted 06 June 2006 - 10:58 PM

Hi, sorry about the jargon FLC is Full Load Current which is the nameplate rating of the motor. A 5.5kW 400V motor is probably in the 10 to 12 amp range. The reason that it only draws 7.3 amps is that the motor is not fully loaded by the fan. However when we consider starting currents we should always use the nameplate rating. If the figure on your motor is 12 amps then the starting currents are more reasonable. If your fan is starting without any air dampers closed it is a fairly heavy duty start as along with the inertia of the fan the torque requirement increases as the speed increases. I would expect a starting current of somewhere around 50A. The reason for the higher current (63A) could be that the initial voltage is set too low and the time ramp too short so that the motor is only just starting to move as the voltage increases. When you watch the motor start does it seem to accelerate as soon as the start commences or does it sit and hum before moving? I would suggest trying the settings that marke has given and see what the currents do then.
It can be a bit of trial and error setting up a time/voltage ramp but if you have many of these applications on site it will be worth getting them right.

Ken
An expert is one who knows more and more about less and less until he knows absolutely everything about nothing

#6 ÉlPontus

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Posted 07 June 2006 - 10:10 PM

The FLC on the motor is 9.9A 400V/ 5.7A 690V. I changed some on the two possible values of the soft starter. The best result I got was when I had the starttime on max (20s), and the voltage on 30%. I reduced the currents to about 34A, compared to 50A before. The peaks were on 63A before, forgott to measure that this time ohmy.gif

Later I tryed the start-delta-start and the currents there was only 23A (the meter didnt show any higher peaks) in the start, and when it changed to delta the currents was only getting up to about 12A. This was when I waited for the motor to finish the acceleration, and that happened pretty fast.

So if you only look on the currents, the start-delta seems better..? But the tourqe should be better on the soft starter, I guess?

#7 GGOSS

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Posted 08 June 2006 - 02:39 AM

A star/delta starter provides a better current vs torque relationship than a soft starter however it is generally not suited to applications requiring high levels of starting torque...as a fan normally would.

Having said that, the fact that you achieved a succesful start with the soft starter at 34 amps (or 3.43 x FLC) strongly implies to me that \either the motor is oversized for the application (as has been indicated by the low run current) and/or the motor has 'better than average" start characteristics.

Assuming a Locked Rotor Current of 6 X FLC and a Locked Rotor Torque of 1.4 x FLT, this motor would develop 45% torque with the soft starter limiting current at 3.43 x FLC. With a star/delta starter, we can expect 1/3 FLT at 1/3 LRC whilst the motor is connected in star. Therefore 47% torque at 2 x FLC starting current is achievable.

Actual LRC and LRT details of this motor were no provided, therefore it is impossible for me to say how much torque the fan actually requires. The above however does show, that in cases where the start torque requirement is low or when the motor has a low LRC and a high LRT, a star/delta starter may allow for a succesful start to be achieved at a starting current which is significantly lower than that achievable with a soft starter; current and torque transients aside.

Regards,
GGOSS

#8 kens

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Posted 08 June 2006 - 03:54 AM

Hi, a nice reply from GGOSS. Clearly this load has a fairly low inertia perhaps a direct drive axial fan. I seem to recall from your other posts on soft starters (assuming that they were about the same application) that the fan only has a few starts per day. Clearly there is little advantage in using a SS for current limitation in this case however the SS will avoid the harsh torque transients and also the high current transition from star to delta. If it is not a direct drive on the fan there will also be less wear on the drive belts with a SS.
Ken

An expert is one who knows more and more about less and less until he knows absolutely everything about nothing

#9 ÉlPontus

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Posted 08 June 2006 - 09:41 PM

I have no idea if it is a direct drive axial fan. But the "fan-cabinet" (or what you call it in english) and the motor placement looks like in the nice picture below (the proportion is not right..).

IPB Image



I asked a person on the phone (who hasn't seen my voltage picture) about the voltage wave I was asking about in the first post. He said that it probebly looked like that because I measured between the two controlled phases. If I had measured between one controlled phase, and a zero point/groud, you should be able to see that it is only "cuts" the voltage 2 times/period.. Do you agree with that?

#10 marke

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Posted 09 June 2006 - 08:37 PM

The voltage waveform is what I would expect for a phase to phase measurement. The waveform in the earlier diagram will not be achieved with a soft starter as the power factor of the motor during start is less than unity. The bite out of the waveform is shifted such that the current stops flowing after the voltage zero crossing, not at the voltage zero crossing as shown in the diagram. Additionally, the OFF voltage will not be zero as shown in the diagram due to voltage fed back from the motor during the OFF period.
You are seeing this in you measured waveform, but you are getting the effect of two phases being controlled rather than one.

Best regards,




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