Variable Speed Drives - "Slowing down the motor?" 1

[mjkey]

Hi all,

I have a question regarding Variable Speed Drives.
I understand that it can have full control of the fan motor speed by varying the frequency and modifying the voltage/current. However, I've been wondering if the VSD can actually "slow" down the motor.

Say the fan was running at 100rpm, and we want it to now be running at 50rpm. How does the VSD actually slow it down? Does it just let the intertia of the motor reach the 50rpm? Or does it physically drag/slow down the motor?

Cheers

 

[itsmoked]

You say Variable Speed Drive. There are many types of VSD. I presume you're actually talking about a Variable Frequency Drive (VFD) as your VSD.

If so... E V E R Y T H I N G in regards to VFDs can be changed and adjusted.

So yes. It can slow the motor shaft speed down. It can slow it all the way down to stopped. It can over speed the motor to more than twice its nameplate speed.

That said there are lots of trade-offs and hazards to the motor, connected machines, and possibly humans, if some settings are used ignorantly. For instance, running too fast could cause a motor to explode, or a coupling, or part of a machine, to fragment. Serious consideration and possibly conversations with maintenance or the machine manufacturer should be considered before radical changes are made.

Lots depends on the machine type when changing speed. Sometimes a machine can lose synchronization at a different speed and be jammed or broken.

A fan case like yours doesn't have a lot of consequences but still some. If you exceed the maximum designed fan speed it will shed blades and cause mayhem. Too slow and due to the airflow restrictions no air will move. The fan will essentially just beat at the air. If you slow a motor too much it doesn't get any cooling and can overheat. However, with a fan the load on the motor goes down with the cube of the speed reduction which means the motor quickly gets very unloaded so loss of cooling usually isn't an issue with fans. Other loads that are not fans could well fry the motor in a minute or two by slowing down.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[mjkey]

@itsmoked

Appreciate your response. Yes the VSDs i'm talking about are VFDs used for fans.

I guess my question really is, how do you "slow down" a fan with a VSD? How does it actually work? I understand how you can speed up the fan. But does do you slow it down?

Cheers

 

[MintJulep]

https://www.motioncontroltips.com/faq-what-is-dynamic-braking-and-when-is-it-used/

 

[itsmoked]

Umm.. Maybe I'm not getting your question completely..

VFDs have a control panel and a display. They have settings. Hundreds of settings. There will be one (one of the main in-your-face ones) that states frequency. Normally this is in Hertz and usually runs between 0 and 60Hz (in the US). 60 causes the motor's plate RPM to occur. Likely 1725 or thereabouts. At 60Hz the motor will run the nameplate speed. Half the sixty hertz, 30, the motor will run about 1/2 the name plate speed. Whatever speed your fan is now look at the display for the immediate hertz. Typically if it shows that you can simply press a down arrow somewhere that reduces the Hertz value and the motor will slow accordingly.

How it does this internally? With much, really very, very much magic and raw mathematics supported by amazing electronics power elements. As far as the motor is concerned it slows down because it simply chases the rotating 60Hz magnetic field, except now the field slows down in frequency. The motor slows.. But since the motor is not designed for this slower frequency it would normally draw excessive current and fry in seconds. The VFD magic, however, goes further than just reducing the frequency and also, cunningly, reduces the voltage at the same time in a very tightly controlled proportion to the reduced frequency. This then prevents the barbecue. (abuse as previously stated not-withstanding).

Am I on track yet?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[lukaiENG]

In more basic terms, think about regenerative braking in an electric car. the wheels are slowed down when the EMF generating motion is removed, since the wheels are (initially) still rotating they generate a "back-EMF" in the motor windings which is used to push regenerate some change back in the batteries - the motor becomes a generator essentially.
It's the same concept. Keith's version is more technical, hopefully this analogy helps you on your way.