Loading limit of number of 10 HP Irrigation Pump Sets on 25 kVA Transformer.Impact of Inrush Current and starting current .

[ShyamSundarS]

In Rural Agricultural Feeders there is a practice of using lower rating transformer for Irrigation Pump set.

In such case observed 25 KVA Transformers are used for loading 10 HP IP sets. The maximum rated current of 25 kVA is 33 A.

There is a possibility of IP set current reaching 18 or 20 A based on the Voltage .

So wnated to place this subject for discussion with respect to the motor maximum current for one 10 HP and what would be the condition when two 10 HP are connected to one 25 kVA transformer .Will it reach a overloaded condition.

Also what will be the impact of inrush current and motor starting current on the 25 kVA Transformer.

[MathieuGuillot · Accepted solution]

Hello,

Thanks for this very practical question. I am afraid it's not possible to give a general answer. 

A 10HP (7,5kW) standard 4P induction motor will have a full load current around 15,5A at 400Vac  

A 25kVA transformer can withstand a permanent current of 36A 400Vac. So on the paper a 25kVA transformer can supply two 10HP pump with 15% margin.

In practice overload is a question of temperature and duration... so it's difficult to give an answer without considering ambiant T°, duty of motors, possible voltage variations ... 

Regarding risk of overolad during motor starting  again it depends of how long last the start, how often it happens, what is the time between two starts ... what is the ambiant temperature...  Transformer can withstand temporary overload, especially liquid immersed. The transformer's manufacturer shall be able to provide such information.

So I can imagine two cases:

- Either the transformer is given and its caracteristics should be known and it's possible to check that it can withstand permanent operation and starting of 2 10HP IP according to ambiant T°, load variations ...   

- Either it's a new installation and the transfomer shall pe specified to operate with 2 10HP pump in a given T° and to withstand the starting charactéristics of the pumps. (Number of start, frequency of starts, starting current, duration ..)  

I hope it helps. 

Mathieu GUILLOT

[ShyamSundarS]

Dear Mathieu GUILLOT,

Thank you for the appreciation of the practical problem.

Yes this issue got evolved during third party inspection of 25 kVA Transformer feeding IP sets .

The IP sets will be switched on twice in a day.

The transformer rated Voltage is 11/0.433 kV. The Voltage drop in the feeder is in the range of 400 V to 350 V.

The ambient in Bangalore Rural is in the range of 20 degree Celisius to 29 Degree Celesius.

I wanted to understand what would be the starting current for a 10 HP Motor and its duration, 

Thank you Mathieu

[MathieuGuillot · Accepted solution]

Hello

Starting current and duration can be calculated with some software. I am afraid SE is not able to provide you a simple tool to do so but you may find some on the web.

To have an accurate calculation you need to know the torque of the pump, at least the type of relationship between the torque and the speed (quadratic, linear ...), its inertia and the characteristics of the electric motor as well. (Starting current, starting torque inertia...)

Regards

[Delcho_Penkov · Accepted solution]

Hello, such a small motor can have very high inrush current. Therefore the problem of starting will be the voltage drop imposed on the motor already running in parallel. I suppose that the design is standardized, as in the beginning of the message it is said to be a typical practice. Otherwise motor starting may need to be performed with a progressive starter, soft-starter or even to check with VSD, where you can potentially benefit from energy saving.

Best regards,