hashem said:
Tom,
With reference to TD.08H.17.T.E document, it is quoted that "Operation at rated torque at twice speed means that the motor will be operating at twice its rated horsepower".
Is that achieved for all AC motor, or there are some restrictions? When we can use this method?
Any example?
This has nothing to do with it's being an AC motor. This is just the laws of physics. hp = Tlbft x rpm / 5252. It doesn't matter if it is an AC motor or a hamster wheel - twice the speed at the same torque load means twice the horsepower load.
It is important to keep separate the load requirements, the load capabilities of the engine or motor, and the capacity of the energy delivery system.
The load power is dictated by the rate energy is needed to do the intended job. I could
theoretically raise the Washington Monument with a hamster wheel - it would take a lot of gear reduction, reallly good bearings, and a very long time.
On the other hand, if I actually tried this I would probably exceed the thermal limitations of the hamster. After a few minutes the little bugger would die of heat stroke. Many of the limitations on AC motors are similar - sustained operation above rated power causes the thermal capabilities to be exceeded and the motor will fail from insulation burnout or whatever. You rarely see a motor fail from structural failure (twisted shaft, for example) due to overload.
If I could get a water cooled hamster (you figure out where to put the hose!) I might not have thermal problems. However, the ability of the energy delivery system would probably result in failure. The hamster's system simply couldn't provide enough energy transport in the blood stream to sustain muscular activity for the required duration. He'd stall out.
Similarly, the VFD cannot provide twice the rated power to the motor. The phenomenon Dick is talking about has to do with the limits of the VFD's ability to deliver current to the motor, and the ability of the motor to convert that current to rotating energy. The drive can't produce output voltge higher than what it sees at the input terminals. It can't produce current in excess of the thermal capacity of the internal components. That limits the power it can provide and the current it can provide. These in turn limit how much power the motor can produce and how much torque the motor can create.