Induction generators normally need to operate connected to a larger system, so for a directly connected induction generator the system frequency is (near enough) constant. The machine speed will vary slightly with power output. If you're trying to coax some output from the machine using capacitors or similar and it is not attached to a larger system then yes, frequency will vary with speed. In this situation whatever output voltage you have will also be speed-dependent.
This is in regards to a small (35-50kW) wind turbine my company is looking at putting up to reduce electrical costs. The unit (Endurance E-3120) uses an induction machine and can produce electricity onto the power grid without using inverters. So I guess I was wondering how it would be able to maintain the 480VAC @ 60Hz as the wind is not constant and the rotor would spin at different RPM's. I don't fully understand the concept of slip, but I'm guessing the faster the gen spins with regard to the grid frequency, the more negative slip there is. I would also guess that torque required to spin the generator above synchronous speed becomes greater and greater as RPM increases.
I have a poor understanding of AC systems in general. 3 phase, induction, power factor and rotating magnetic fields make my brain hurt.
Repeating roughly what Scotty said:
If you are connected to a large grid, the grid establishes synchronous frequency.
Generator speed will be slightly higher than sync speed (by an amount slip*sync speed)
Slip is proportional to real load (for a given terminal voltage and a given grid frequency).
I'm having a hard time understanding this windmill concept. Is it correct to say that if the generator is geared to produce a certain slip at certain RPM, then it will produce power above that slip and consume power below that slip when connected to the grid?
It will produce power whenever the slip is negative (above synchronuous speed) and it will consume power if the wind isn't strong enough to keep it above synch speed. Wind turbines are therefore disconnected from grid when wind is low.
Without a grid tie inverter it is possible that the machine you're considering is a dual-fed induction generator, although in that size range I'll be surprised to see a DFIG.
For a standard squirrel-cage induction machine, most people are used to thinking of it as a motor, so let's start there.
At speeds near synchronous speed, the relationship between torque and slip is very nearly linear. Let's take a common 4-pole motor for 60Hz operation listed at 1740rpm at rated torque. 1740rpm is 29rps, or 58Hz on a 4-pole motor. The slip is 2Hz at this rated torque. If the load torque were increased to 150% of rated, the motor would decelerate, increasing the slip and therefore the generated torque, until the slip hit 3Hz at 1710rpm and the motor generated 150% of rated torque, balancing the load torque. Similarly, if the load torque were reduced to half of rated, the motor would accelerate, decreasing the slip until it reached 1 Hz at a speed of 1770 rpm, generating only 50% of rated torque to balance the load.
This relationship continues on the other "side" of synchronous speed. If the machine is mechanically driven with a torque equivalent to the rated torque, it will have a -2Hz slip frequency, so the electrical frequency on the rotor will be 62Hz and the mechanical speed will be 1860rpm. Driven mechanically at 150% of rated torque, it will get to -3Hz slip and 1890rpm. Or at 50% of rated, the slip will be -1Hz, and 1830rpm.
In all of these cases, it will be taking power from the grid (for the motoring cases below synchronous speed) or putting power into the grid (for the generating cases above synchronous speed) at 60Hz. All of this assumes that the grid is "big" (to use the technical term) compared to the machine, so the action of the machine has minimal effect on the frequency of the grid.
An induction motor works well with a hydro, diesel or steam prime mover but is totally unsuitable for direct grid connection of wind power. A dual fed induction motor works well with wind. A standard induction motor with a capacitor bank for excitation may be used as an induction generator with wind power if the output is rectified to DC and then inverted back to AC.
While a normal induction motor works well as a generator directly connected to the grid with several types of prime mover, wind power is not on that list.
Well, with an over-sized turbine and shedding a lot of wind most of the time it may work with wind but at that point it makes more sense to use a type of generation that can handle more peak power.
Bill
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"Why not the best?"
Jimmy Carter
In my mind what works best for small wind is a PM Synchronous machine running wild into a rectifier/inverter. Some form of brake and/or feathering mechanism is necessary for high speeds, but otherwise it can run at any speed and produce power. Inverter takes care all the interconnection issues.
Be aware that, on average worldwide across many years of operation, a wind turbine will AVERAGE only 21-23% rated load factor: That is, if the wind turbine is rated at 100 Kwatt, over a year's time, it will average produce only 23 Kwatt. (Somedays, you'll get all 100 Kwatt. Followed by 4 days of 0.0. Two days of 46 Kwatt, followed by 4 days of 0.0 )
Get the point? Trying to "buy back" the purchase price of the turbine based on the saved power is unlikely. By the way, you WILL have to purchase the transformers and wiring and controllers all sized for that 100 Kwatt potential output, because on some days some of the time, it will be producing 100%. Until the wind goes too high, then it shuts down to prevent damage.
davidbeach, that's a logical supposition. There are designs that do just that and many complexities are overcome. You might be interested to know what the disadvantages are: the primary disadvantage is that a PMG suffers from "cogging" at 0rpm. Unfortunately the blades of horizontal-axis wind turbines are optimised to spin due to lift, not drag, so are poor performers at 0rpm - exactly when a bit of oomph is necessary to get a PMG going. One way of overcoming this is to sense the wind, and actually drive the PMG up to some useful speed if the detected wind is high enough. This requires a fair bit of extra control and is inefficient in inconsistent winds, but it can be done. Secondary disadvantages are the reliability, weight, cost and availability of suitable PMGs - induction motors are a dime a dozen in comparison.
The Endurance does indeed fall into a category called "fixed speed" wind turbines, in that power is only generated when the blades are spinning in a relatively narrow RPM range. As turbines get larger, this actually less of a problem than you might think. That's because the efficiency of the blades have a relationship of their own to the wind speed, based on the "tip speed ratio" (TSR). This broadens the range of wind speeds for which the blades remain effective. Additionally, wind is highly volatile itself, and constantly matching blade speed to wind speed in any efficient way is a complicated procedure. Instead, the larger wind turbines actually rely on their mass to smooth the volatility, and then pitch the blades according to the average wind speed to optimise the TSR. That's why even the large DFIG based turbines are effectively fixed speed wind turbines.
I say all this as the developer and proponent of a "variable speed" small wind turbine based on an induction generator. There are definitely some advantages to be had, particularly at the smaller end of town but there are some significant challenges too.
All I know is the larger wind machines are so complicated that even the manufacturers can't develop a model for the generator. If it were up to me I would not allow an interconnection until after a model was provided to the protection engineers.
For small wind machines, even an inefficient generator can be used to produced power in a managable range. And I say inefficient to mean not optominized for any factor other than first cost.
At the distribution level, it dosen't matter to the power company as long as the power quality to the other customers is not impacted, and the total generation on the feeder does not impact the operation of protective equipment, and personal safety issues are addressed. The billing people and adminstration are a different matter, and will change from company to company.
There is no mention of the protection system or control system. I can only speak of steam turbine driven alternators. The governor will sense the drop in speed as the load increases. It should alter the pitch (open the valve) of the blades to speed up the shaft and thus keep the speed back to synchronous speed plus a bit. If the governor reaches a point where it can no longer add more pitch (open the inlet valve), it will be open to stalling. Now the control takes over and lowers the voltage differential to nearer the grid voltage. This is achieved by lowering the excitation voltage to the rotor. With a lower differential voltage, the current flow will reduce and less power exported. With less power being exported, the torque is removed and the shaft speeds up again. The governor sees the rise in speed and reduces the pitch (closes the valve) to slowing down again.
Speed = governor control.
Power = voltage control.
Voltage control is achieved by altering the excitation.
