Induction motor with Drives

[Murlinor]

Hello!

When starting a induction motor with drives, one usually start at low frequency and get high output torque, and the inrush current is reduced dramatically compared to DOL start. Can someone explain why the inrush current is reduced?

Thanks

 

[davidbeach]

Slip is reduced.

 

[Skogsgurra]

davidbeach is correct - but a bit short - in his answer.

The high starting current (equal to locked rotor current) when starting DOL is because slip is 100 percent and then gradually decreases down to nominal slip (or lower if motor not fully loaded) as the motor gets up in speed.

When using a VFD, the motor is kept at or just above nominal slip during the whole start. Frequency is increased, usually linearly, at a pace that keeps torque and thus current low.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Murlinor]

Thanks for the answers!

If the load requires a start torque 2 times the nominal, how does the drives manage to produce that torque when the current is limited? I stuck up in the where the torque is proportional with the power dissipation in the rotor.

 

[Skogsgurra]

It can not. Not if the current limit is below current needed to produce double rated torque. You will need to set the limit higher - or use a larger drive.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Murlinor]

Ok, now Im beginning to understand

What is the maximum typical current limit (X*FLC) that a drive can have. If a DOL requires 6*FLC to produce a torque 2 times nominal torque, what will the current limit be with a drive?

 

[Skogsgurra]

It can be a lot less, actually. It is because the DOL start is very "torque inefficient". You do the starting way down the speed/torque curve and get very littel lb-ft per ampere. Using a VFD can reduce the current needed a lot. It also depends on the motor characteristic. A NEMA "A" motor will need perhaps 2.2 - 2.5 FLA to deliver 2 times rated torque. You can only do this for a short time, like during start up.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Murlinor]

Hello Gunnar.

Thanks for the answer. I understand the difference know

"Takker så mye for hjelpen!"

 

[Skogsgurra]

So, where in Norway (or Denmark, perhaps) are you? I am in Karlskoga.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Murlinor]

I'm from Norway and live in Oslo. I work as a consultant actually. I used to work for ABB, mainly with switchgears and high voltage physics. Electrical motors isn't my strongest subject, but it is fun to learn something new!

 

[Skogsgurra]

We are going there to have some kveite soon. Only place you get a decent one. Swedes overcook it.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Murlinor]

Kveite is very good and tasty fish, but I more fan of salmon fish. Anyways you can try gravlaks or lutefisk, typical Norwegian food

 

[Skogsgurra]

..or rakörret..

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Murlinor]

Thanks again Gunar for your help!

Maybe I have more questions in the future!

Thans and take care!

 

[LionelHutz]

To further add to the answer, most motors started DOL will not produce 2x rated torque until they reach >70% speed. Typically, they produce a low of about 120% torque and a high of about 240% torque as they accelerate.

On a VFD, you are always running on the part of the motor curve between the breakdown torque speed (the torque peak) and the synchronous speed (100% motor speed). Looking at the curve, you can see how it is possible to create more torque than a full-voltage type of start since you can use that torque peak at any rpm with a VFD.

If you look at the curve and find the motor torque you want then read the motor current off the curve to find how much current you need. You need a VFD capable of producing this current if you want the torque.