Braking the motor when power lost and come back

khushin

Member
Location
INDIA
Occupation
Electronic engineer
Hello,

I have one problem related to dynamic braking whenever power lost, and after sometime power restore,
i need to apply brake because otherwise motor will take long time to stop.
In this case i will read the speed of the motor through hall sensor once power restore, and synch My IGBTs to that speed and than i decrease the speed to apply quick brake,

My problem, whenever i synch the IGBTs with the current speed only during power on, there is a whistle sound coming from the motor. how to avoid this sound.
During normal braking i am not facing this problem(that means when i apply break, power is not lost) because in this case IGBTs are already sync with the curret speed.
Becuase once power restore, without opening the IGBTs gates i cannot apply quick brake.

If any inputs please let me know....
 

paulengr

Senior Member
What you are saying is very confusing because you are mixing terms with different motor types.

Is this a synchronous motor, DC shunt wound, AC induction, or something else?

There are three issues/solutions.

Sometimes an internal fan makes a whistling or buzzing sound as the blades go by a baffle. If you shut off the drive and coast to stop and you still hear it as the motor winds down, it’s an internal problem. Check for bearing issues just in case.

Second is drive may excite a mechanical resonance. Slowly adjust frequency up and down. If it is just one frequency program a “jump” frequency to avoid that speed. See drive manual.

Third if it is AC and you are using field oriented control or even PWM (that went out years ago), or LCI, the issue is that you are hearing the switching going on in the audible range since switching frequencies are typically 1-8 KHz. So we have three choices. First and most expensive is install sinus wave or common mode choke filters to clean off harmonics. This helps but is quite expensive. Second and easiest is adjust carrier frequency. The lower it is the more obnoxious the noise. Increase it but read your manual carefully as you can damage motors and/or lose performance with higher switching frequencies. Third and quickest option is turn in space vector modulation if your drive has it. Using sensor less vector mode helps too but requires more setup. All three of these last options are not eliminating the noise, just changing the tone or making it a hiss instead of a squeal.
 

khushin

Member
Location
INDIA
Occupation
Electronic engineer
Thanks for your reply. Sorry for my English and expanation.

For your confusion, i will give some clarification.

I have developed my own drive board with atcive PFC and using AC induction motor 450watt (MAX RPM 20000)
This is special applicaiton for centrifuges.
My carrier fr = 16KHz, so i donot have audible noise problem. or no issue for motor running or braking problem..

Now problem is, when motor running at 20000 rpm with rotor on motor shaft and suddenly power goes and after few seconds power restore,
coast to stop of the motor will take 20 minutes to complete stop, so to avoid this i need to apply break, but because power just left and back again,
IGBTs need to sunc with the currnet speed to get dyanmic brake, whenever i open or sync the IGBTs at running speed i got some strang/chuuuu.. kind of sound from the motor for few seconds still i am able to stop motor. My question how we can sync the IGBTs in software to avoid this kind of sound.... becuase once the power restore motor is running at very high speed...
 

paulengr

Senior Member
Let’s take a simple case of a V/Hz system for a minute. Let’s also decrease output voltage to a low value, say 10-20 V. Now if the motor frequency matches the output frequency current is nearly zero because the motor is greatly underfluxed. If we decrease frequency so that it is below the motor speed this causes it to draw current to create a negative torque to slow down and match the drive frequency. DC bus voltage rises as it becomes a generator, not motor. The lower this frequency is the higher the current is, up to a point where it falls off. Above frequency we see current rise as the motor is motoring but since voltage is so low the motor won’t actually accelerate.

So what you do is start at maximum frequency but output MV frequency. When current picks back up you went to far. So the lowest point is the hardest. Now switch into running but the starting current and
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The other option is to maintain power to your control logic so that you never stop and restart your modulation, and thus never have the transient with power restart.

-Jon
 

khushin

Member
Location
INDIA
Occupation
Electronic engineer
Yes now i have solve the issue barking works fine and smoothly without any noise
Thanks
 

khushin

Member
Location
INDIA
Occupation
Electronic engineer
Hello Jon thanks for your suggestion but i am using 2 MCU and maintaing power always is little bit difficult
Anyhow i have slove this problem it was very tricky in software to sync the IGBTs once the power restore.

Thanks
 

Barbqranch

Senior Member
Location
Arcata, CA
Occupation
Plant maintenance electrician Semi-retired
Is it that big of a problem to just let the load coast down on those occasions when the power drops out? How frequently do you anticipate that happening while you are in the braking mode?
 

paulengr

Senior Member
Is it that big of a problem to just let the load coast down on those occasions when the power drops out? How frequently do you anticipate that happening while you are in the braking mode?

It’s a centrifuge. Coast down takes almost an hour sometimes. Very, very high inertia. And then there’s the strange scroll drive that runs most of its life in regenerative braking to the point that most just use shared DC bus, no line connection.
 

khushin

Member
Location
INDIA
Occupation
Electronic engineer
It’s a centrifuge. Coast down takes almost an hour sometimes. Very, very high inertia. And then there’s the strange scroll drive that runs most of its life in regenerative braking to the point that most just use shared DC bus, no line connection.
Yes you are right. It is not industrial AC drive which normally runs max 3000-4000 Rpm, my application it runs around 20000Rpm. In laboratory, normally power failure will be rare but if power lost during motor operation, it takes almost 25min to complete stop which is not acceptable by user.
Also DC break not the right soluion to stop the motor. Anyhow, now works really fine and able to stop the motor in 40 seconds from max speed without any noise or problem.
 

khushin

Member
Location
INDIA
Occupation
Electronic engineer
Hello All,

I have one question about VFD, as my application is centrifugation which needs to run motor at 20000rpm maximum.
I have 2 different motors from the supplier.
Motor weight is around 8.5Kg
My switching frequnecy is 16Khz.

My problem:
In one supplier motor, my IGBT MODULE(Power module) not heating means it is within limit 60 degree max.

In second supplier motor my IGBT MODULE(Power module) heating very fatst means 85 degree max.

I am giving in supply to electronics 230vac , 200v 3-phase motor. when i compare both the motors at same speed, motor phase current and input current i.e 230v input supply shows same. Also electronics baord and software same for both motors.

Only difference in two motors are when we measure the resistance between phase normal motor it shows 5.8 ohms
The motor which is heating it shows 4.3 omhs

My question is why it is heating?
Secondthing,
Suppose if i reduce switching frequency to 8Khz how i can use filter to avoid audible noise using filter
How can we selet filter between motor and electronics?

Thanks for the support
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Did you tune the drive to each motor independently? Modern drives must be tuned to the motor, ESPECIIALLY when being used in extreme applications such as this. Differences in the motor equivalent circuit result in different performance of the firing algorithm, so using the tuning parameters of one motor will not work correctly on a different motor.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
Did you tune the drive to each motor independently? Modern drives must be tuned to the motor, ESPECIIALLY when being used in extreme applications such as this. Differences in the motor equivalent circuit result in different performance of the firing algorithm, so using the tuning parameters of one motor will not work correctly on a different motor.
Yes, but the problem here is just braking.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Yes, but the problem here is just braking.
No, he added on a new problem today, motor heating when running using different motors. Not considered good forum etiquette to string onto an existing thread with a new problem for the exact reason you just demonstrated, I should have instructed him to start a new thread.
 

Besoeker3

Senior Member
Location
UK
Occupation
Electrical Engineer
No, he added on a new problem today, motor heating when running using different motors. Not considered good forum etiquette to string onto an existing thread with a new problem for the exact reason you just demonstrated, I should have instructed him to start a new thread.
Thanks, Mr J.
 

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
...
My problem:
In one supplier motor, my IGBT MODULE(Power module) not heating means it is within limit 60 degree max.

In second supplier motor my IGBT MODULE(Power module) heating very fatst means 85 degree max.
...
My question is why it is heating?
Is there a dV/dt filter on the output of the VFD which is getting too hot? If so, make sure that it is connected correctly (i.e., with the shunt capacitors on the motor side).
 
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