VFD short time rating vs. motor starting current 3

[Power0020]

I was looking at Schneider Altivar 71 Plus catalogue and noticed they mentioned the maximum current for 60 s per 10 minutes to be 1138 A and nominal current is 759 A for 400 kW units.

I wonder if I need to check the motor starting current (6.9x651 = 4492 A) in case of DOL starting, I believe the VFD can handle this motor starting softly but nothing is mentioned in the catalogue about that.

Any clue?

 

[itsmoked]

By definition you will not have DOL starting with a VFD so why is a DOL starting current something you have to consider from the VFD's point of view? VFDs will ramp the frequency and hence the current as they accelerate the motor. I think typically they stay down around 3X FLA for everything they need to do. Fear not.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Power0020]

Thanks Keith, if we go with 3X FLA this is about 1953 A, if the motor is accelerating slowly,I will be worried with this limit of 1138 A for 1 minute......

 

[itsmoked]

Hi Power;

First, my 3X comment is what seems to be an occasionally reached maximum I see with drives, not the typical starting current. The typical is less than the maximum.

Secondly, A drive that's limited to 1138 for a minute is probably rated considerably higher than that for, say, 10 seconds. Is your motor really taking more than 10 seconds to come up to speed? 10 seconds is a loooOOOOoong time, though not unheard of.

Thirdly, If you're really taking one minute to accelerate the motor you likely are down in the 1X region of virtually no surge current so the 1138A limit is not even up for consideration.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

You don't have to worry about the DOL starting current. But we can't guess at the suitability of the VFD without knowing the motor load.

On a typical pump or fan application the current can be kept below 100% FLA as the motor is started.

 

[krisys]

The VFD driven motors would draw typically about 110 - 120% of rated current during the acceleration. I am sure, that the inverter current limit will be set much below 300%. If the motor-Load inertia is large, it would take longer time to accelerate. Still there is no problem. As the risk of motor heating will not be there. Because the starting current of the system is comparable to a marginal overload current. Under such marginal overloading condition, the motor can run even for few minutes without overheating.

 

[jraef]

Some back story. When starting DOL (Across the Line for us), most of that starting current is not producing torque because the stator frequency is much much lower than the rotor frequency, so the power factor is extremely low and most of the current is reactive, not active. Yet when considering DOL or RV starting issues with regard to current, current is current and cannot be ignored.

But a VFD is very very different in how it ENERGIZES the motor. The VFD is capable of altering the voltage AND frequency together. This means that the stator frequency can be kept in line with the rotor frequency in terms of producing torque with as much of the current as possible. So for example at running speeds, the frequency differences can be kept at slip speed, but for stating purposes, it can allow the motor slip to increase to the Break Down Torque (BDT) point of the motor at any time. In most Design B motors, BDT is actually HIGHER than Locked Rotor Torque (LRT), 220% vs 160% of FLT. However when starting DOL, you only get LRT at first, BDT only occurs after you already achieve about 80% speed. So because a VFD can give you LRT or even BDT immediately, it can actually accelerate an AC motor FASTER than DOL in many cases. In addition, since the current is mostly active, not reactive, current percentages will follow torque almost 1:1. So in getting 200% torque for acceleration, you will only pull 200% current.

So when a VFD provides an Overload Current rating in percent, Constant Torque rated drive will say 150% current for 60 sec., but also give you s second value, usually 200%, for something like 3 seconds. What this means is that the VFD can make your motor deliver LRT for 60 seconds and BDT for 3 seconds if necessary. A good Vector capable drive can deliver that BDT at as low as one Hz, so essentially immediately, which can be very helpful for overcoming "stiction".

Bottom line, you do NOT need to consider a motor's starting current when selecting drives, just the motor name place FLC and the nature of the load.


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