Experience related to AC motor operation with or without encoder 3

[panter]

Daer experts ,
I am involve on a project to replace a 28 DC motors 15 kW with AC motors and I am interested for yours experiences related to the maximum starting torque of an AC motor in open and closed loop configuration .
As it is about drives that in winter conditions sometimes have big problems with starting and measurements that indicate that DC motors can not start mixer drive with two maximum motor torques .
We have two different offers that guarantee that the same AC motor can overcome it one with and another without speed feedback so it gives us a headache .

Thanks in advance .

 

[TugboatEng]

Encoderless performance varies widely from drive to drive. That could be one explanation for one supplier being able to meet the requirements without feedback.

 

[lukin1977]

are the AC motors 15kW also? How many poles?

 

[jraef]

AC motors can produce Locked Rotor Torque (LRT) when first energized that it is usually 150-160% of Full Load Torque (FLT), but the actual MAXIMUM torque that an AC motor can produce is called Break Down Torque (BDT) and depending on the Torque-Speed curve design of the motor, that can be anywhere from 150 to 300% of the motor Full Load Torque rating. Most common motors are Design B because they are the most energy efficient now. Design D motors offer the most torque initially, but at full load, the penalty is increased losses and most people don't want to deal with that. But depending on the nature of your machine, that might be a consideration, i.e. it doesn't run for a long time, but when it is needed, you need the high torque.

Design B motors will be capable of 200-220% FLT as BDT. VFDs that are capable of Flux Vector Control (FVC) will allow that motor to deliver BDT for between 2-3 seconds at any moment, including Startup, so using an FVC capable drive can overcome the 150% LRC limitation. Whether that is enough to overcome your cold weather problem is another story. If you need BDT for longer than 3 seconds, you may have to significantly over size the VFD, and you might need to consider the effect that might have on the motor shaft (I learned that one the hard way once...).

Then from there, SOME VFDs can provide FVC without an encoder (called "Encoderless FVC), others (most) must have the encoder, because from a dead stop, the VFD needs to know where the motor rotor is with relation to the winding poles in order to make the right calculations and adjustments. That's what the encoder does, although Encoderless FVC uses some more advanced current detection systems to do it fast enough to not need the encoder and that capability just varies by brand / sophistication etc.

There is a slightly lower capability of drive control capability called "Sensorless Vector Control" (SVC) that can also provide higher torque on start-up, but typically 150% (or Locked Rotor Torque) not the full BDT value because until the rotor is actually moving already, SVC is basically just an "educated guess". I would be leery of those that claim 200% STARTING torque using SVC.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Design D motors are excellent for high inertia loads subject to short time high torque demands.
Shears and punch presses with large flywheels are often driven by design D motors.
The actual cut of punch is mainly powered by the inertia of the flywheel.
The high slip capability of the design D motor then accelerates the flywheel up to speed and the motor idles until the next work cycle.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[FacEngrPE]

Your starting torque requirement is a direct result of the cold viscosity of whatever you are mixing. You can either heat the mixing tank with "heaters" or with the mechanical energy of the mixing paddle.

I suspect that if the motor has enough torque to turn the paddle at all, it will eventually turn at the speed you want, if you can keep it from tripping (I²*t protection).

One way to do this is to specify the motor must be able to produce 100% design torque at zero speed for however long you want (can be continuously). You will get the motor in a frame 1 or two sizes larger than usual for that power, and most likely a blower. As you do not need the motor to act as a position-er, sensor-less vector control is likely good enough. This seems over kill for a mixer.

 

[itsmoked]

Sensorless Vector can't keep a motor stable at sub 1 RPM can it? How could it with zero back emf - the thing being read instead of using a sensor. You usually need a couple of RPM, like about 3 to get feedback.

Furthermore starting the motor turning in SV is the usual Hail-Mary 'make a guess' and correct it once something starts moving.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

Keith, that is an accurate assessment.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RedSnake]

I have some similar discussion at work right now.

Not really shore if I do understand this sentence.
As it is about drives that in winter conditions sometimes have big problems with starting and measurements that indicate that DC motors can not start mixer drive with two maximum motor torques.

The torque requirement would be the same with a AC motor.

My take on this is to replace it with a DC motor.
The big advantage with this is that they have a constant torque in all speeds.
And they are very easy to regulate to a specific speed if they are equipped with a taco meter generator or a puls sensor.
Even if new AC motors and drives can do the job, there is less effort doing it with a DC motor and drive.

There are more things to take into account with a AC motor/drive as Jraef explained.

Best Regards A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[jraef]

The down side to larger DC motors and drives is the required brush and commutator maintenance, which as the older workforce familiar with that ages out, is becoming a major headache for users because the new kids replacing them have, in many cases, never seen a DC motor and don't know anything about them. I'm going through that right now with a client that has some 50HP DC motors and drives on an old machine that have failed due to lack of maintenance. The old dudes who did that for years all retired and the guys they were able to hire to replace them were not aware that they had to do PM on the DC motors. So we are replacing the DC drives and motors with AC motors and vector drives.

PMDC is of course an option if you can afford it, but in that case, AC motors with Vector Control drives are likely less expensive and more readily available now.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[RedSnake]

Jraed # The down side to larger DC motors and drives is the required brush and commutator maintenance.

Done maintenance on the DC drives for many years, not difficult to learn and does not take long to do either.
Maybe I am getting old :-(
But I agree there is a lot of skills that seems to disappear this days.

We bought a reserv motor to one of our DC drives it wasn't that expensive.
And the new DC drives have the same functionality's as AC Servo so that way it isn't that different.

I am not shore if PMDC is brush less DC motors, there seems to be a lot of different namnes and types of motors.

BR A




“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein