Not sure I understand where this is going.If I have a 1750 rpm motor and run it at 900 rpm with a VFD am I at risk of motor damage?Assuming the correct motor and VFD are mated,I really dont see why this would cause damge.The very reason for me wanting a VFD is to be able to vary the speed with out changing belt position on the pulley system.
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VFD Question
- Thread starter gredpe3
- Start date
- Joined
- Feb 8, 2014
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A few random thoughts on VFDs and motors
Most 17xx RPM (1800 RPM nominal) motors will run just fine at twice the base speed. Don't try this with a 3600 RPM nominal speed motor.
Most systems will run just fine from about 15Hz to 120Hz as long as you don’t try to run in constant horsepower mode (increasing torque at reduced speed). There are special motors that will operate from 0 to 240 Hz (or higher). If you need full torque at reduced speed, then you need a forced air-cooled BLDC (Brushless DC) motor. The torque curve on these motors is almost flat from 0 to their rated RPM
Typically, as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed.
Running continuously at very low speed can cause overheating if the torque requirement is high. If the torque requirement is light, most motors will run just fine as low as about 15Hz. Lower than that a standard squirrel cage induction motor starts to get a bit unstable. Operating at greater than the rated speed at high torque requirements can cause overheating also, as well as inverter trips due to over current.
I have heard stories of non-inverter rated motors failing, but in 30 years of working with VFD’s I have never seen it in light use. I have seen inverter rated motors fail when operated in their normal envelope, but this is in continuous heavy industrial use. The last big system install I was involved with had about 700 VFDs/Motors in the system. The ones that did fail, did so in testing, or were defective right out of the box. These were Allen Bradley VFDs and Baldor motors.
I ran my old BP clone with a 2HP Taiwanese motor at about 15 to 120Hz for years without a problem, and it is still running just fine. In most cases I left the belt on the 660 setting. I did change the belt position or switch to low range if I needed more or less speed and/or higher torque for a given operation.
My new (to me) mill has a 3HP Baldor inverter rated motor on it, normally I run between 30 and 90Hz because it is also a variable speed belt. I sometimes use lower settings for light power tapping and reaming rather than shifting into low range. When running at maximum variable speed setting on the belt, I do not run the VFD at greater than 60Hz, just to keep the mechanics in their normal operating envelope.
Most 17xx RPM (1800 RPM nominal) motors will run just fine at twice the base speed. Don't try this with a 3600 RPM nominal speed motor.
Most systems will run just fine from about 15Hz to 120Hz as long as you don’t try to run in constant horsepower mode (increasing torque at reduced speed). There are special motors that will operate from 0 to 240 Hz (or higher). If you need full torque at reduced speed, then you need a forced air-cooled BLDC (Brushless DC) motor. The torque curve on these motors is almost flat from 0 to their rated RPM
Typically, as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed.
Running continuously at very low speed can cause overheating if the torque requirement is high. If the torque requirement is light, most motors will run just fine as low as about 15Hz. Lower than that a standard squirrel cage induction motor starts to get a bit unstable. Operating at greater than the rated speed at high torque requirements can cause overheating also, as well as inverter trips due to over current.
I have heard stories of non-inverter rated motors failing, but in 30 years of working with VFD’s I have never seen it in light use. I have seen inverter rated motors fail when operated in their normal envelope, but this is in continuous heavy industrial use. The last big system install I was involved with had about 700 VFDs/Motors in the system. The ones that did fail, did so in testing, or were defective right out of the box. These were Allen Bradley VFDs and Baldor motors.
I ran my old BP clone with a 2HP Taiwanese motor at about 15 to 120Hz for years without a problem, and it is still running just fine. In most cases I left the belt on the 660 setting. I did change the belt position or switch to low range if I needed more or less speed and/or higher torque for a given operation.
My new (to me) mill has a 3HP Baldor inverter rated motor on it, normally I run between 30 and 90Hz because it is also a variable speed belt. I sometimes use lower settings for light power tapping and reaming rather than shifting into low range. When running at maximum variable speed setting on the belt, I do not run the VFD at greater than 60Hz, just to keep the mechanics in their normal operating envelope.
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- Jun 12, 2014
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I had thought that "as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed" applies to the characteristics of the load, i.e.
VARIABLE TORQUE LOADS:
Load types require reduced torque when driven at speeds less than the base speed of the load. Conversely, such loads may require increased torque when driven at speeds greater than the base speed of the load. These are classified as variable torque loads. Many variable torque loads decrease with the square of the speed. This is characteristic of centrifugal pumps, and certain types of fans and blowers. Typically, as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed.
CONSTANT TORQUE LOADS
With constant torque loads, the torque loading is not a function of speed. Typical applications are: Traction drives, Compressors, Conveyors, Positive displacement pumps and Hoists. As the speed changes, the load torque remains constant and the horsepower changes linearly with speed. Constant torque loads cause motors to draw relatively high current at low speeds when compared to variable torque applications. This is why the same size drive may have a lower HP rating for Constant Torque applications.
Many VFDs can be set up with VT or CT type settings. Base speed is their RPM at the nameplate motor rated Hz (usually 60Hz): Many 3 phase inverter rated motors are rated for a CT ratio of around 10:1, they can deliver constant torque below their base speed of 60Hz down to around 5-6Hz. AC Vector motors (CT ratio around 1000-2000:1) provide full torque essentially down to ~0 RPM. They loose horsepower in a 1:1 ratio below their base speed, they maintain HP above the base speed to around 2X their base speed. They maintain full torque below base speed (as noted above), they loose torque in a somewhat linear fashion above to 1.5-2X their base speed (non-linear above that). As you mentioned, most manufactures rate their inverter (4 pole) motors to a maximum RPM around 2X their base speed, smaller (2-5HP) vector motors nameplate often go to 4-5K. Yikes. Most of the smaller Vector rated AC motors are TENV, and do not have the usual overheating problems with lower or above base speed of cooling associated with the usual forced air cooling units.
One intriguing idea I was considering for my lathe was to use a 6 pole 1200 RPM vector motor instead of a 4 pole 1800 RPM. A 6 Pole 1200 RPM vector motor provides higher full load torque (~150% of 4 pole) and maintains constant horsepower to about 2X base speed, so a usable working RPM range of 0-2400 RPM. Unfortunately the 6 pole motors are 1-2 frame sizes bigger then their 1800 RPM version, and did not fit in my application.
Great reference I saw posted elsewhere on [FONT=&]Feed your VFD with the right power[/FONT]:
http://machinedesign.com/motorsdrives/feed-your-vfd-right-power
Mark
VARIABLE TORQUE LOADS:
Load types require reduced torque when driven at speeds less than the base speed of the load. Conversely, such loads may require increased torque when driven at speeds greater than the base speed of the load. These are classified as variable torque loads. Many variable torque loads decrease with the square of the speed. This is characteristic of centrifugal pumps, and certain types of fans and blowers. Typically, as the speed decreases, the torque decreases with the square of the speed and the horsepower decreases with the cube of the speed.
CONSTANT TORQUE LOADS
With constant torque loads, the torque loading is not a function of speed. Typical applications are: Traction drives, Compressors, Conveyors, Positive displacement pumps and Hoists. As the speed changes, the load torque remains constant and the horsepower changes linearly with speed. Constant torque loads cause motors to draw relatively high current at low speeds when compared to variable torque applications. This is why the same size drive may have a lower HP rating for Constant Torque applications.
Many VFDs can be set up with VT or CT type settings. Base speed is their RPM at the nameplate motor rated Hz (usually 60Hz): Many 3 phase inverter rated motors are rated for a CT ratio of around 10:1, they can deliver constant torque below their base speed of 60Hz down to around 5-6Hz. AC Vector motors (CT ratio around 1000-2000:1) provide full torque essentially down to ~0 RPM. They loose horsepower in a 1:1 ratio below their base speed, they maintain HP above the base speed to around 2X their base speed. They maintain full torque below base speed (as noted above), they loose torque in a somewhat linear fashion above to 1.5-2X their base speed (non-linear above that). As you mentioned, most manufactures rate their inverter (4 pole) motors to a maximum RPM around 2X their base speed, smaller (2-5HP) vector motors nameplate often go to 4-5K. Yikes. Most of the smaller Vector rated AC motors are TENV, and do not have the usual overheating problems with lower or above base speed of cooling associated with the usual forced air cooling units.
One intriguing idea I was considering for my lathe was to use a 6 pole 1200 RPM vector motor instead of a 4 pole 1800 RPM. A 6 Pole 1200 RPM vector motor provides higher full load torque (~150% of 4 pole) and maintains constant horsepower to about 2X base speed, so a usable working RPM range of 0-2400 RPM. Unfortunately the 6 pole motors are 1-2 frame sizes bigger then their 1800 RPM version, and did not fit in my application.
Great reference I saw posted elsewhere on [FONT=&]Feed your VFD with the right power[/FONT]:
http://machinedesign.com/motorsdrives/feed-your-vfd-right-power
Mark