Move it with a VFD - Part 2 |

Friday, April 23, 2010

Move it with a VFD - Part 2

As we saw last time, a VFD can often be used in a motion application where some high torque and variable speed is required. If we apply them properly they are one of the best motor controllers in industry. The key statement there is 'if we apply them properly'.

To apply one correctly we really need to understand the load requirements. Basically, we need to ensure we can supply enough current for the motor so it can produce as much 'torque' as needed.Torque may be a new term for you in the motor world. Generally, we talk about horsepower in the motor world but it relates to torque and that's what we need to discuss in the VFD world.

Torque is a force that produces rotation. It is commonly measured in units of lb-ft (pound-feet). It is the force necessary to cause an object to rotate... Think of a merry-go-round (carousel ride). To get it to rotate by hand you would need to push it with a certain amount of muscle. That 'certain amount of muscle' would be the 'torque'.

If you tighten a bolt with a small wrench it will be difficult. Use a bigger (i.e. longer) wrench and it will be easier to tighten. For example, to tighten the bolt to 50 lb-ft you would have to apply 50lbs of force to a 1ft long wrench. On the contrary, you would have to onlyapply 1lb of force to a 50ft long wrench to tighten it the same.

Many (most?) machinery loads are constant torque loads. Conveyors,compressors, etc are some good examples. Variable torque loads are generally most pumps and fans. A VFD is usually specified for variable torque loads because they can help save energy but they are used forconstant torque loads too.

Peak torque efficiency comes at a certain volts per hertz ratio. As long as our VFD stays at this ratio we can develop the rated torque.For example, let's assume we're using a 460 volt source at 60 hertz.The ratio is therefore:
volts / hertz
460v / 60hz
7.667 v/hz

As long as the ratio stays in the above proportion we can supply the rated torque. So, if the volts fall we'll need to decrease the frequency to maintain the same ratio.

Horsepower is torque with the addition of time and distance. So, it's the torque generated to move something a given distance in a given period of time. It's related to torque by the formula:

HP = (torque * speed) / 5252

For example, if we have 30 lb-ft of torque developed in a motor rotating at 1500rpm we get:

HP= (30 * 1500) / 5252
HP= 45,000 / 5252
HP= 8.57

For reference, if we exert 1 hp we can move 550 lbs in 1 second. If we are just holding the 550 lbs still, we are not exerting any horsepower.There MUST be movement...

No movement would mean no torque which would mean no horsepower. See?
To specify a VFD we need to understand the torque requirements of the load. We need to consider:
1- The torque required to start the motion
2- The torque required to bring the load to speed in a certain period of time
3- The torque required to keep the load moving
4- The torque required when the motor acts like a brake to slow down the load
5- And the peak torque needed by the load

Most folks will use an ammeter to measure the current under all the load conditions listed above and find the peak current drawn. The VFD is then sized to the motor based on the maximum current draw at the peak torque levels plus an extra margin. Notice that the VFD is not specified based on the horsepower ratings of the motor. It's a torque thing..

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