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#### smoothflow

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Hello guys,
I have a somewhat offbeat question: What would be the maximum power and torque output by using a 5.5 kW VFD and a 2.2 kW induction motor combination. This will be an intermittent duty system with duty cycle of from 10 to 60 minutes.

#### JRaef

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As to power:
The power drawn is determined by the LOAD, not the motor. The VFD cannot "make" the motor draw more power than the load requires, other than a slight loss of efficiency. So second point, the power rating of the VFD is completely irrelevant once it is at LEAST the rating of the motor. Side note to this however; many VFDs will not allow you to set the motor protection to a value less than 50% of the maximum rating of the VFD, so using a 5.5kW VFD on a 2.2kW motor may mean that you must add an external OL relay.

As to torque:
The motor size, in your case 2.2kW, is actually the MECHANICAL rating of the motor, just as we use horsepower. That mechanical raing is a shorthand expression of XXX torque at YYY speed, based on the Across-the-Line (DOL) operation of the motor running at it's full slip speed. So on a motor torque-speed curve, this is the 100% torque value at the natural slip speed, called the Full Load Torque (FLT) and if looking at a basic Design B motor, that is going to be around 3-5% slower than the synchronous speed. If the motor is slowed down even more, torque increases to the point in the torque-speed curve where it is the highest, around 200-220% of FLT. That is called the Break-Down Torque (BDT), because that represents the apex before which a FURTHER drop in speed begins to REDUCE the torque rapidly. So that is the PEAK torque the motor is capable of delivering, but only for a short time. Once at full speed, a motor's current draw percentage roughly equals the torque output. So at FLT, the motor will be drawing FLC, so at 220% torque, the motor will be drawing 220% current, which is an overload condition that if left alone, will eventually damage the motor. But the motor CAN, and does, do this a lot, for short periods of time, as the load changes because the BDT is what the motor develops to RE-accelerate a load after a change.

So answering the question as stated is not as clear as you wanted it to be. The "maximum" power is determined by the load, but cannot exceed the motor rating for more than a few seconds, so you could say that it would be 2.2kW. The "maximum" torque would need to be further defined by a time frame, but if you are interested in CONTINUOUS torque, it would be determined by the motor speed, which you did not provide, but you can calculate it as kW * 9550 / RPM.

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