VFD Operation Curve
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winnie
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- Springfield, MA, USA
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- Electric motor research
The graphs are in 'per unit' values. Add a couple of zeros and you have 'percent'. So '1' on the X axis means '100% speed', and '1' on the Y axis simply means '100% rated torque'.
The blue line is the rated torque of the motor/VFD combination. Torque remains constant as speed changes. This means that the motor/VFD combo can provide full rated torque from 0 to 100% speed.
The red line is the maximum permitted overload. You can overload to 240% of rated torque except at low speed where the peak overload is reduced. This limit is probably set by semiconductor cooling. You can only overload like this for a very limited amount of time.
The green line is power output. Since power is torque times speed, the upward slope is obvious. Full torque at 0 speed is still 0 power output, full torque at full speed is full power output.
-Jon
The blue line is the rated torque of the motor/VFD combination. Torque remains constant as speed changes. This means that the motor/VFD combo can provide full rated torque from 0 to 100% speed.
The red line is the maximum permitted overload. You can overload to 240% of rated torque except at low speed where the peak overload is reduced. This limit is probably set by semiconductor cooling. You can only overload like this for a very limited amount of time.
The green line is power output. Since power is torque times speed, the upward slope is obvious. Full torque at 0 speed is still 0 power output, full torque at full speed is full power output.
-Jon
At medium to high speeds the motor losses don’t matter a whole lot and you can achieve peak torque. At low speeds motor performance drops off. Even if you double the VFD size. But you still have 200%+ peak torque.
If this is a factor switching to a PMDC or synchronous or reluctance motor eliminates the issue albeit you have to look at the design all over again. Usually either it’s a minor issue or just up the motor size.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
The one flaw I see in that chart is that the green line is not LABELED as "Power". I agree with Jon that it is showing power (kW or HP), because the values displayed track what I know, but it might have been helpful to the uninitiated if whomever made that chart were to have been more clear.
If one were to continue that graph to the right, you would have also seen that as you keep increasing speed (the X axis) beyond the rated speed of the motor (the yellow), the blue line would start to go DOWN, and the green line would remain flat.
Side note; the red line showing 240% as an overload capability is a bit optimistic, or the chart is only reflecting the MOTOR capability. Most VFDs will either Current Limit or trip off line at 200%in 2-3 seconds. You MIGHT get a brief by spectacular spike of 240%, but it's not that realistic.
This is where you get into “normal” vs “heavy” duty which is the VFD manufacturers thresholds and in some cases you need to consider “extreme” duty. Rock crushers and crusher screens, plugging operations, and servo motors are in the category where the motor might be say 10 HP but you need a “30 HP” rated VFD. Even if you fix the VFD sizing you still have to realize that integral fan cooling (CFM) varies with the square of speed and running into the peak torque region for any length of time is a thermal problem even with an external blower cooled motor. But there are lots of applications where this is normal and reasonable. Whenever you try to work in these “beyond nameplate” zones though there are lots of things to consider. For instance sure in the power limited/field weakening condition (above 60 Hz) you lose torque because you can’t push voltage. But another major issue is that on larger motors you will hit a beating speed limit issue. Of course better bearings can overcome this. I have seen servo motors with 300 Hz base speeds!
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