AC drive setup and control for kiln motor 2

[rockman7892]

We are getting ready to commission a 480V 350hp motor connected to a VFD for a cement kiln application. AC drive is 750hp drive. Although we plan on bringing a drive engineer On-Site to assist with the startup, I wanted to gain a little insight for myself the best way to setup the the drive control mode for this application.

The motor has an encoder, and the control methods avaliable in the drive a V/Hz and FOC (no sensorless vector). I'm assuming for this application we would want to have the drive in the FOC mode, is this correct? I know the kiln will have a very high starting torque so what is the best way to accomplish this? The motor has a maximum running rated torque, however it also has a "1 min" rated torque. What is the purpose of this 1 min rating? Is this a rating to be able to provide full torque a zero speed with the motor datahseet implies.

Just wanted to see if others have had experience with AC drives on a kiln application who would be willing to share their experiences or insight.

Thanks

 

[itsmoked]

Make and model?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

The motor itself is a Reliance Motor

The drive is a Rockwell Powerflex 700S. This is the same drive that had the dual power units I posted a little while back.

 

[DickDV]

Open loop V/Hz control does not manage the motor torque or (as a result), motor speed accurately so you get most of the motor slip speed as error (typically around 2% of base speed). The drive also cannot manage the motor as speed drops below about 4hz.

FOC or Field Oriented Control is capable of very tight torque and speed control because it knows the exact motor shaft speed by way of the encoder. Because of this, the control remains good right down to zero speed.

The result is much better starting torque and less speed error. Definitely the right choice for a kiln application.

 

[Skogsgurra]

Some kiln applications are so unbalanced, especially just after start, that you can get into regeneration part of the rotation. That can make your drive explode (or at least trip on DC link overvoltage) if it isn't designed to regenerate or dissipate braking energy in a resistor bank.

It is wise to consider that before the first start.

Gunnar Englund

http://www.gke.org

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

 

[Thedroid]

A kiln can over run once they have unbalanced rings built up also. I haven't had the chance to work with a modern AC kiln drive yet, just dual motor DC drives.

 

[rockman7892]

I was reading through an application article and it gave some information on sizing a drive for a kiln motor application. It was goverened by the following equation:

VFD Size = (D/F)(Tp/Tthmax)(Pm/nm*pfm)

Where D=Derating factor of motor operating from VFD
F=Factor considering peak inverter current
capability compared to inverters rated current.
Tp=peak torque
Tthmax=continuous motor torque at max oper speed
Pm=motor power
nm=motor efficiency
pfm=motor power factor

Has anyone ever seen this equation or know why it is used for sizing a drive for this application? The motor datasheet says the motor has an OL torque of 250% full load torque with a corrosponding current. The motor curves show these as being a "1 Min" torque and "1 Min" current.

The actual VFD is a dual power unit VFD as mentioned, with each power unit having the following ratings:

Normal Duty: 700Hp
Heavy Duty: 500Hp
Input: ND=792A, HD=735A
Continuous Amps: ND=820A, HD=730A
1 min overload: ND=902A, HD=1095A
2 sec overload: ND=1230A, HD=1314A

Do these drive paramaters look like they match the required paramaters of the motor for the continuous and overload ratings? Is the 1 min overload rating of the drive in order to produce the 1 min torque and current ratings of the motor?

When setting up these dual power units do they both need to be coordinated somehow to supply the same output current?

 

[rockman7892]

DickDV

Why in V/Hz mode can the drive not manage speed and torque below 4Hz? Is this something to do with voltage drop across stator windings? So in V/Hz the drive cannot provide full torque at zero speed?

Is this why a manual voltage boost is used in V/Hz mode for low speeds, whil a vector control will adjust this voltage automatically?

 

[DickDV]

You do have some issues with IR drop in the copper windings at how hz but the main control problem is this: If the speed error is 2% of base speed and base speed is 1800rpm, then 2% is 36rpm.

At full speed (1800rpm) the error is 2% but at half speed (900rpm), the error is 4%. At one-tenth speed (180rpm) the error is 20% (36/180) and at 36rpm the error is 100%.

See what's happening here? Once error becomes about half of the commanded speed, the drive can't distinquish between no slip (no torque) and full slip (full torque) because it gets lost in the error.

Even a precision drive with only 1rpm speed error can have this problem at 1rpm since the signal is all error.

 

[rockman7892]

DickDV

Your explanation on increasing slip error makes sense and I see whats happening. Can you explain how this increased slip error effects the amount of torque that the drive outputs? Specifically relating to the your comment:

"Once error becomes about half of the commanded speed, the drive can't distinquish between no slip (no torque) and full slip (full torque) because it gets lost in the error."

How does this inhibit the drive from providing correct output torque?

 

[itsmoked]

Juicy bit of info there Dick. Thanks.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Dick - 2% of the 'base speed' ? Do the VFD makers put those words in small print ? Thanks for that tip.

Muthu

http://www.edison.co.in

 

[DickDV]

rockman, the answer to your question lies in how an AC induction motor magnetizes its rotor and produces torque in the shaft.

I would kindly suggest that you get a good motor textbook or white paper and note particularly the relationship between slip speed and rotor magnetic intensity (and, consequently, torque).

The drive system is not where your answer lies but rather the motor.

With all due respect, I find this type of question frequently in my seminars and often from drive specialists with a lot of experience. The problem is that they don't understand the motor end of the system---only the drive. Getting up to speed on the motor workings makes the drive a lot more understandable, too.

Good luck and, if, after looking into the motor, you have further questions, I'd be pleased to help again.

 

[rockman7892]

DickDV

Thanks for your help. I understand that the stator field induces a voltage in the rotor and this induced voltage field creates a current in the rotor which is lagging the voltage depending on the reactance of the rotor. This current in the rotor then produces a magnetic field which is 90deg offset from the current angle. The rotor magnetic field then interacts with the magnetic field from the stator which produced the rotor voltage to create a Bnet field which produces torqe in the motor. The difference the magnetic field from the stator and the magnetic field from the rotor is the slip.

Do I have enough of an understanding Do I have enough of an understanding between the manetizing the rotor, and the slip and torque produced in the rotor to understand while slip and speed errors increase at lower speeds.

Your help is kinly appreciated. Any papers or references explaining this or slip compensation would be appreciated.