How to calculate needed inverter power?

1. Feb 15, 2013

Farkane

Hi all ,

I m rookie engineer and i just faced a big problem for me.There is a inverter which has ;

750V DC input voltage
3x400 V AC outpu voltage
10 kVA continuous power
12.5 kVA 30min power

And ı have appx 10kw loads. Is my inverter enough to feed that load?

Even if it can feed how can i prove that ?

2. Feb 15, 2013

Bobbywhy

Farkane, Welcome to Physics Forums!

You have not given enough information to expect a useful response. Please try to put your requirements into one of these:

Here are a few Uninterruptible Power Supply (UPS) Power Requirement Calculators:

http://www.csgnetwork.com/upssizecalc.html
http://www.tripplite.com/en/products/selectors/ups/index.cfm
http://www.refurbups.com/UPS-Sizing-Calculator

If you do not get satisfactory answers, then do return here and post your specific questions and doubts.

Cheers,
Bobbywhy

3. Feb 17, 2013

Farkane

I am working as a trolleybus maintenance engineer. So our auxiliary inveter has appx 10 kW loads when trolleybus is running and as you know trolleybus has 750V DC catener line. Aux inverter input voltage is 750V DC and output voltage is 3x400V AC.It has 10kVA continuous power.Inverter with its 3x400V AC feeding that 10kW load. My point is , how can i determined that inverter power is enough to feed how much load? I need something that prove to my directors.

Thanks

Farkane

4. Feb 17, 2013

I assume the inverter is feeding a motor? In that case the the motor is probably rated with a power factor (PF), cos phi. If the motor is rated 10 kW, that is 10kW mechanical output. Take into account the losses you get electrical active power of, lets say 11kW. And with a power factor of 0.8 you get apparent power of S=13.75kVA. That is higher than the nominal rating of your inverter. Take note that this is just some random numbers just to illustrate the point.

Maybe you don´t need full power when running on aux. power, or maybe just intermittent during acceleration. In such a case, the inverter might do the work. This can be controlled. Any how, the inverter needs to be secured with fuses and controls in order for it to operate inside its ratings.

How is the Trolleybus industry going?

5. Feb 18, 2013

Farkane

You can assume that inverter is feeding auxiliary units like air compressors,steering pumps etc. There is already a motor inverter in the vehicle. According to this informations how can
i calculate inverter need power?

6. Feb 18, 2013

Farkane

Btw trolley sector is pretty good. Sector is reliable and environmental so that most of the municipalities prefer trolley system instead of street tram. Also street trams are more expensive than trolley.

7. Feb 18, 2013

Ok, but the approach is the same. Air compressors and pumps are driven by electric motors. But the loading of these are probably more constant than traction motors.

In order to accurately dimension the inverter you need to know accurately the maximum load that can occur. As you have stated, the inverter can supply 10 kVA constantly, and 12.5 kVA for 30 min.

If this is a factory installation you probably can find the nominal rating of the appliances and circuit in the documentation.

Another important point is the short circuit capacity of the inverter. That is, it must be able to provide enough power to trip the circuit breaker during a fault.

8. Feb 19, 2013

Farkane

As far as i understand my inverter continuous power is the significative factor. Also aux. equipments are working randomly they are not working continuously except steering pump. So that my 10kVa will be enough for my loads. What you think about my idea?

9. Feb 21, 2013

Farkane

Can someone show me the formula of callculating inverter load power? I need some calculation than i can easily understand