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Open Circuit Voltage and Short Circuit Current
Numerous kinds of alternators and generators are often sold which might not be appropriate at all for use in a wind generator. Most often, the real power capabilities of an alternator are obscured by wild claims about open circuit voltage (OCV) and the short circuit current (SCC). Stop being fooled! This article will describe what open circuit voltage and short circuit current, and explain why they are important for designing a wind turbine that produces optimal power in real world wind speeds.
A 3-phase generator produces “wild” Alternating Current (AC) that is converted to Direct Current (DC) by a 3-phase bridge rectifier as shown in the diagram below. The resulting rectified DC voltage can be used to charge a battery, feed a grid tie inverter (GTI), or can be used with a DC heating element to heat air or water. In most cases, small wind generators are used to charge one or more batteries which are commonly referred to as a battery bank.
Figure 1: A flow diagram of a wind turbine generator charging a 12 volt battery.
There are two fundamental characteristics of a generator that need to be known to determine if it will be a good candidate as part of a wind turbine. The first is Open Circuit Voltage and the second is Short Circuit Current.
Open Circuit Voltage for Wind Turbines
Generators spinning with no load (nothing attached to the rectifier) generate an Open Circuit Voltage (OCV). This voltage is proportional to the RPM of the generator and is fairly linear, meaning there are a fixed number of RPM’s per volt. As the rpm of the generator increases, the voltage increases.
For example, if the OCV of a generator is 12 VDC at 100 rpm, then at 200 rpm the OCV of the generator will be roughly 24 VDC. This suggests that when you double the RPMs, you are doubling the voltage.
In designing a wind turbine, it is important to match the RPM performance of the generator to the RPM performance of the rotor blade set covering the normal 8 – 30 MPH wind speed range.
As an example, let’s design a good OCV for a wind generator that is going to be used to charge a 12 volt battery bank. You want the OCV of the generator to produce about 12 VDC in about 7-8 mph winds in order for the wind turbine to start charging the 12 volt battery bank.
Unfortunately, many generators or DC motors that look good at first glance require much higher RPM to reach 12 VDC than what a good set of wind turbine blades will develop. As an example, let’s assume you are using 35-inch blades (like our WindGrabber Blade Series) and hit 250 RPM in a 7-8 mph wind. If the 35 inch blades can do 250 rpm in 7-8 mph winds, then we know that our generator must produce an OCV of 12 VDC at about 250 rpm. If we design the generator to produce 12 VDC at about 250 rpm and use the 35 inch blades, then we will have a wind turbine that produces the correct OCV to properly charge a 12 volt battery bank!
As a side note, usually small ~1000 Watt wind turbines operate in the 200 – 800 RPM range. Yes, it is possible to add gearing, but just like a geared bicycle, the blades become harder to turn, and higher wind speeds are required to get them spinning and to keep them spinning. There is no free lunch. It is generally better to select a generator that can generate 12 VDC at a fairly low RPM and not to use gearing.
Before you read on, here is a video which shows the open circuit voltage vs. RPM of our Windtura 750 generator:
And before we start talking about short circuit current, watch this video of the short circuit current vs. RPM for our Windtura 750 generator:
Short Circuit Current Applied to Wind Turbines
So let’s say we found or designed a generator that can produce an OCV of 12 VDC at the correct RPM’s for our blade set. The next question is, “Can it produce sufficient current (amps) as RPM increases?” This is where Short Circuit Current (SCC) comes in. In this case, the DC Positive and the DC Negative outputs of the bridge rectifier are tied together (shorted), and an ammeter is used to measure the current as the RPM increases. A test of up to 800 RPM on a powerful motor stand is sufficient in most cases to determine what the SCC curve of a generator is. Just like in the voltage case, there are some generators that simply do not produce much current at 800 RPM to be considered good candidates for use in a small ~1000 Watt wind turbine. (If you forgot why we are using the number 800 rpm, it is because that is about the maximum rpm that a small 1000 Watt wind turbine is going to hit in high winds. We don’t care about the SCC of a generator at 5000 rpm because a wind turbine will never reach an rpm that high)
It is important to note that the MAXIMUM current a generator can develop is when it is short circuited. Once a battery or some other load is added, there is more resistance in the circuit, and thus there will be less current that can flow. In practice, the most current you will ever see from a generator in a real world application is about 50-70% of the SCC in a well designed system.
Below is the OCV and SSC for the current Windtura 750 (2012). Some improvements may happen in the future so don’t consider this chart to be anything other than a snapshot of the current state of our PMA. The gas driven test stand in the two videos above was used to generate this data.
Figure 2: OCV vs. RPM and SSC vs. RPM for the Windtura 750 Generator
You can see that the OCV reached 12V at about 140 RPM and rose at a constant rate relative to RPM (approximately 10.4 RPM/volt or about 0.1 volt/rpm). The SCC was about 18 amps at 140 RPM and peaked above 50 amps at the higher RPMs. This is exactly what a good candidate generator for a small wind turbine looks like: The OCV of the Windtura 750 approximately doubles when the rpm of the generator doubles. And the SCC continues to quickly increase in the 200-800 rpm range!
In practice, when matched with a set of our WindGrabber blades, the Windtura 750 Generator can produce 35-40 amps in high winds (27-30 mph). On a 12V battery system, this translates to about 500 watts delivered to the battery bank, and on a 24V system over 1000 watts is delivered.
Tip: Don’t Get Tricked by Dishonest Sellers
CAUTION – One trick used by some suppliers is to advertise fantastic levels of watts by taking the OCV and multiplying it by the SCC! This is incorrect. Don’t fall for it!
You know from what you read above that the OCV test was under no load, and that the SCC test was with the turbine shorted. You can’t do both at the same time, but these suppliers hope you don’t realize that. In a true battery system, the voltage will be held to the 12-14.5V range with a 12V battery bank and 24-29V for a 24V battery bank.
As long as the voltage produced by the wind generator is slightly higher than the battery voltage, current can flow, and you want as much current to flow as possible. The true delivered power (Watts) is this voltage (the battery bank voltage) multiplied by the measured current.
This voltage is NOT the OCV and the measured current is NOT the SSC! At this point, we no longer care about OCV and SCC as those tests were simply used to determine if a candidate generator would be a good choice for use in a small wind turbine.
A Final Note: Stator Losses
Stator Losses: Power (Watts) is calculated by the square of the current (amps) multiplied by stator resistance (Ohms). If the stator has a high resistance, even let’s say 3 ohms, at 10 amps it will consume 10 amps x 10 amps x 3 ohms = 300 watts! This is 300 watts that will not make it to your battery or GTI, and the stator will also get very hot! It is clearly very critical to use a low resistance stator and to use heavy gauge wires throughout your whole system. Low resistance will allow the most current to flow and the most Watts delivered to your load (battery bank, GTI, heating element, etc).