Holding Torque with a VFD and AC Induction Motor
Holding Torque with a VFD and AC Induction Motor
(OP)
Can I produce holding torque with an AC induction motor with a Powerflex700 VFD
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Holding Torque with a VFD and AC Induction Motor
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Holding Torque with a VFD and AC Induction MotorHolding Torque with a VFD and AC Induction Motor(OP)
Can I produce holding torque with an AC induction motor with a Powerflex700 VFD
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RE: Holding Torque with a VFD and AC Induction Motor
RE: Holding Torque with a VFD and AC Induction Motor
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Holding Torque with a VFD and AC Induction Motor
Thanks gents
RE: Holding Torque with a VFD and AC Induction Motor
yours
RE: Holding Torque with a VFD and AC Induction Motor
RE: Holding Torque with a VFD and AC Induction Motor
Thanks
RE: Holding Torque with a VFD and AC Induction Motor
You are possibly describing a vector drive as compared to a scalar drive the "standard" VFD. They can hold loads all day long. With a bunch of conditions which we cannot guess about with your meager info.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Holding Torque with a VFD and AC Induction Motor
Generally, AC drives are not good at positioning in the sense that they have no way of keeping track of where they are or where you are telling the load to move. Servos do that routinely. Having said that, it often possible to get acceptable servo-like action with a precision AC drive following a positioning controller in a PLC or stand-alone. Some would call that a single axis controller.
Hope that helps
RE: Holding Torque with a VFD and AC Induction Motor
This is why we have been asking you to give more details. As I said early on, lots of "ifs".
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Holding Torque with a VFD and AC Induction Motor
RE: Holding Torque with a VFD and AC Induction Motor
RE: Holding Torque with a VFD and AC Induction Motor
If you tell a machine designer that this drive can hold the load like a mechanical brake - and he takes your words for it - you are both in for a very unpleasant surprise the day (or night) you get a power outage.
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
"I wish I'd
t a k e n
T H E
S T A I R S !!!!!!!!!!!!!!!!!!!!!!!"
Nites.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Holding Torque with a VFD and AC Induction Motor
RE: Holding Torque with a VFD and AC Induction Motor
I had a motor driving a cryogenic(liquid nitrogen) pump at 4600rpm and we need in inverter to run the motor at 77Hz (This had saved a mechanical gearbox).
The pump is required to operate at -150deg.C and it is only required to be started intermittently (once in a few days). Before the pump can be started, we need to cool down the pump from atmospheric temperature to -120deg.C. Before the pump is cooled down to -120deg.C, we don't want the pump rotor to rotate or else the pump seals will be damaged. Therefore we applied brake on the motor through DC current from the inverter. This braking function is available in the inverter control menu.
RE: Holding Torque with a VFD and AC Induction Motor
However, the DC holding torque (current) level is adjustable as you wish. Base on our experience, the DC holding current level we set is not high and I hardly feel very much warmth on the motor body during DC hold, I don't think the winding insulation will fail in the event of cooling fan breaking down.
I don't have a Powerflex 700 drive but I believe electrical braking should be a standard feature in that drive.
If you download Powerflex 700 User Manual from Allen Bradley website:
http:
I am sure you can find more details about Stop/Brake Stop/Brake mode.
RE: Holding Torque with a VFD and AC Induction Motor
It is also true, as the poster above mentions, that the ABB ACS600 in DTC mode, can hold a load at zero speed. But you must be careful since this is only true for a couple of seconds. After that, the drive looses control of the motor torque and will release the load. Addition of an encoder will fix that but, as has been stated many times already, this is not a suitable brake technique for safety purposes. For that, trust only a spring-set mechanical brake.
RE: Holding Torque with a VFD and AC Induction Motor
That's the first honest explanation of the holding torque issue with DTC drives I have heard from either ABB or competitors, and now I understand. A number of ABB marketing people leave out the "couple of seconds" issue, and customers have experienced some disastrous failures as a result. On the flip side, a lot of their competitors only mention the part about it losing the model, allowing people to believe that it can't do it at all. Your explanation puts the two incongruous stories together and makes much more sense.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Holding Torque with a VFD and AC Induction Motor
I did that when the DTC was introduced many years ago. The "stand soldiers" immediatley switched the drive off. They told me that it was too risky - the motor could start spinning when someone was holding on to the wheel and someone could get hurt.
I refrained from asking them if they didn't trust their system - running away just like that doesn't sound very good. I think that it would have been too tough on them.
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Holding Torque with a VFD and AC Induction Motor
See my contribution above. It has limited control from the first millisecond. But you can only see it after a few seconds of creeping.
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Holding Torque with a VFD and AC Induction Motor
As you can see, there are numerous methods of 'holding' a motor shaft, whether it be for a few seconds or until smoke appears due to too high an injected DC current. The main point is that if your application requires a brake that, if power failed and gravity took over, then what would you require to happen? As already mentioned, no matter how good the supposed performance, it means nothing when the main source of power has gone.
Mechanical wear or not, most brakes are designed for failsafe and this is considerably more failsafe than the VFD.
RE: Holding Torque with a VFD and AC Induction Motor
Now, if there was an encoder that could tell the drive where the shaft is, than it would be easy - and works very well. That's why you always need an encoder in hoisting applications.
But the DTC people (sometimes) claim that it does not need an encoder to keep a hanging load. So, there you are. A motor without encoder. A torque. A motor model that is valid at some temperature - but not for a wide range.
Producing torque means rotor current. Current means heat and heat means increased rotor resistance, which means that the motor model isn't valid any more. In bad cases, the creeping changes to a fast rotation and that's where the drive loses control and the whole thing goes berserk.
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
You may recall Gunnar, before the advent of the digital signal processors, Siemens actually used the bang-bang technology now know as Direct Torque Control but moved on from that about 15 years ago or so.
RE: Holding Torque with a VFD and AC Induction Motor
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
A couple of questions,
With an encoder used as you describe, do I understand that as the model becomes imprecise, the shaft will turn a small amount. The encoder responds to this small movement with the appropriate adjustment to hold the shaft almost motionless.
Also, what is the effect of changing the load on a stopped hoist motor. That is, people getting on or off an elevator or a concrete bucket being loaded or dumped?
Is a position encoder mandatory for these applications and is it good practice to add a mechanical brake?
Thanks
Respectfully
RE: Holding Torque with a VFD and AC Induction Motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Holding Torque with a VFD and AC Induction Motor
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
Respectfully.
RE: Holding Torque with a VFD and AC Induction Motor
While there seems to be some delight here is bashing DTC, I have yet to find any sensorless vector system to compare to it. As long as you use it within it's limitations, very good performance results and I have built my reputation on it for the last eleven years. Of course, the marketing people tend to get carried away with their claims and that does no-one any good, in my opinion.
Just for an example of the capabilities that I see in DTC, I have about two dozen hydraulic pump (aircraft) test cells operating from 8000 to 20000 rpm and from 50 to 500hp that routinely must go from no load to full load with a 3ms torque loop update and with a total speed error of not more than 10rpm +/-. Properly commissioned and tuned, this is acheivable without an encoder on the motor.
I'm satisfied that this is remarkable performance and have no trouble selling it day after day.
RE: Holding Torque with a VFD and AC Induction Motor
Quote:
2104 DC CURR CTL
Selects whether DC current is used for braking or DC Hold.
0 = NOT SEL
. Disables the DC current operation.
1 = DC HOLD
. Enables the DC Hold function. See diagram.
. Requires parameter 9904 MOTOR CTRL MODE = 1 (VECTOR SPEED)
. Stops generating sinusoidal current and injects DC into the motor when
both the reference and the motor speed drop below the value of parameter
2105.
. When the reference rises above the level of parameter 2105 the drive
resumes normal operation.
2 = DC BRAKING
. Enables the DC Injection Braking after modulation has stopped.
. If parameter 2102 STOP FUNCTION is 1 (COAST), braking is applied after start is removed.
. If parameter 2102 STOP FUNCTION is 2 (RAMP), braking is applied after ramp.
2105 DC HOLD SPEED
Sets the speed for DC Hold. Requires that parameter 2104 DC CURR CTL = 1 (DC HOLD).
2106 DC CURR REF
Defines the DC current control reference as a percentage of parameter 9906 (MOTOR NOM CURR).
2107 DC BRAKE TIME
Defines the DC brake time after modulation has stopped, if parameter 2104 is 2 (DC BRAKING).
Unquote.
I have seen the DC HOLD working at my pump as long as the speed reference is set to below 100RPM (this is my setting), and is not limited to short duration. I can meaure the DC current being injected into the motor winding as long as the motor is under "DC Hold" condition.
And ABB gave a warning notes in the Manual: "Injecting DC current into the motor causes the motor to heat up. In applications where long DC Hold times are required, externally ventilated motors should be used."
RE: Holding Torque with a VFD and AC Induction Motor
Our discussion on the capabilities of DTC, on the other hand, involve the drive controlling motor torque at zero speed as a brake. That's where the short term limit is encountered unless an encoder is involved.
I suppose that you could say that DC injection is a non-synchronous method of forcing zero speed where the use of DTC or any other sensorless vector attempt at zero speed is a synchronous or near-synchronous forcing of zero speed.
The important point in all this is that neither is acceptable for safety stopping and holding purposes.
RE: Holding Torque with a VFD and AC Induction Motor
"Producing torque also needs movement. A true flux vector drive is zero torque at zero speed."
I can't agree with this at all. True flux vector drives can produce full torque at zero speed. For flux vector drives, there is nothing special about zero speed. They compute a slip frequency proportional to the desired torque, and produce an electrical frequency equal to this slip frequency plus the mechanical frequency (rotor speed).
However, as several have pointed out, a shaft sensor is almost certainly required to detect zero speed well enough to really hold position. Most "sensorless vector" drives use back EMF to estimate velocity, and this goes away near zero speed. (There is a lot of interesting research into injecting high frequencies into the motor and observing the response to get true position control without a shaft sensor, but I haven't seen good commercial implementations yet.)
I will also agree with the assessments of ABB DTC here. Great technology for many purposes, but occasionally oversold by their marketing people.
Curt Wilson
Delta Tau Data Systems
RE: Holding Torque with a VFD and AC Induction Motor
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
rather than disagreeing, I think you are agreeing. Is there any point of full torque at zero speed? There is a point to full torque at any change from zero speed.
The accuracy of vector performance at low speeds is the question and the ability for drives to perform the vector calculations at frequencies <4 or 5 Hz is questionable. Most have to revert to speed control at frequencies sub 4Hz.
RE: Holding Torque with a VFD and AC Induction Motor
RE: Holding Torque with a VFD and AC Induction Motor
"Is there any point of full torque at zero speed?"
Yes there is! It is vital for crane/hoist applications, as some have mentioned above. The cranes that lift the Space Shuttle on and off the special 747 to take it from a California landing back to Florida use flux-vector controlled induction motors, and you can be darn sure that NASA cares about full torque at zero speed!
There is a vital distinction to be made here between flux vector control with a shaft sensor and flux vector without a shaft sensor (so-called "sensorless vector"). Nothing in the flux-vector control algorithms using a shaft sensor "falls apart" near zero speed, so they are fully capable of maintaining full torque at or near zero speed. And if a position shaft sensor (encoder or resolver) is used, a position loop can be closed to hold true zero speed with zero steady state error in the speed (because the position loop provides integral velocity control).
However, the flux vector control algorithms without shaft sensors run into two problems near zero speed. First, the back EMF they use to detect speed gets very small, so the signal-to-noise ratio gets horrible. Second, they have no position sensing capability to provide the automatic zero steady-state error in speed that a position sensor provides.
Curt Wilson
Delta Tau Data Systems
RE: Holding Torque with a VFD and AC Induction Motor
While I don't doubt your experience, you must understand that we are talking about very disparate applications. Holding a pump impeller still is VERY different from suspending a Boeing 747 from 4 hoists in mid air and releasing the mechanical safety brakes! I have done that with Flux Vector Drives using encoder feedback and a torque proving feature found (for a long time) only in one particular version of Yaskawa drives (Electromotive) and a few other brands modified with Delta Tau front ends (CSWilson's company). Since then, many other companies have perfected torque proving (by a variety of names) and it is widely available and very proven in the field. Whatever the hype, DTC or DC Hold will NEVER be safe enough to accomplish that. ABB came in with their ACS600 with DTC at the time we were doing that project and begged Boeing for a chance at it. We lifted a 40 ton test weight, set the safety brake, released the brake, and the DTC drive dropped the load. End of story, they never were given a 2nd chance. The Electromotive drives held it locked in place, you could not even detect movement when the mechanical brake released.
Note to sed2developer. I can (from the above experience) attest to the validity of "full torque at zero speed" with an encoder feedback.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Holding Torque with a VFD and AC Induction Motor
Dave
RE: Holding Torque with a VFD and AC Induction Motor
You want to use an AC drive for a simple positioning application. As everyone has pointed out you'll need to have encodder feedback to the drive for it to be able to hold the load at a position accurately. I have recently been playing arround with an ABB ACS800 drive with the position software installed. The motor is 11.5kW with forced cooling and a 1024pulse encoder (two chanels and zero pulse). The positioning accuaracy is fantastic, and in that particular application we are loading up the positioning drive/motor with an identical motor on a regen drive in torque control. It was very entertaining to have the position drive hold a position and then dial up a torque on the regen drive. There is no observable movement of the motor shaft, yet the drive displays were showing lots of Amps and matching torque values, as you'd expect.
If your application does not require the accuracy of a servo drive system then I would urge you yo consider the postioning drive.
RE: Holding Torque with a VFD and AC Induction Motor
Martin
RE: Holding Torque with a VFD and AC Induction Motor
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Holding Torque with a VFD and AC Induction Motor
now quickly down my tangential path...
Curt, I obviously bow to your far greater knowledge on this subject and was by no means trying to counter your description. It was my description in this thread that was wrong(quite possible) or I'm fundamentally wrong (also quite possible). I was simply trying to say (maybe if I put it the other way round) that 'with' movement in a closed-loop vector drive then the function of generating the active current, and therefore 'holding' the shaft stationary (if your setpoint was zero) will occur. Using your hoist with the 747's as an impressive example, the 747's are trying to create the error in the rotor angle and it is there you find out how well your drive performs.My point earier about zero speed and zero torque was simply (and maybe wrong) that if there was zero setpoint and no error, then certain drives will overflux due to the controller oscillating, effectively looking for an error. So the torque generated is not actually necessary but an indication of the performace of the control algorithm. This is the point where I think, "do I hit submit post and make a complete d*&k of myself", but as you can see I did and therefore the need to learn is a stronger urge than the other.
RE: Holding Torque with a VFD and AC Induction Motor
Not to worry. The whole point of this forum is for everyone to learn (except me, of course...)
Let me say at the outset that I consider a flux-vector controlled AC induction motor using a shaft position sensor to be a real positioning servo drive. I know lots of people who use them as such. The only thing they really give up to what most people consider to be servo drive/motor systems is that they have substantially lower torque-to-inertia ratio, because the rotor moment of inertia is typically 4 to 5 times higher than for a permanent-magnet brushless servo motor of the same power rating.
In our own positioning controllers, only two setup variables need to be set differently for controlling induction motors as opposed to permanent-magnet brushless servo motors. First, induction motors require a non-zero "slip gain" (slip-to-torque ratio), whereas for PM servo motors, this parameter must be zero. Second, induction motors require a non-zero "magnetization current" command (direct current) in order to induce current, and hence, a magnetic field in the rotor. This is not required when the rotor has permanent magnets creating its field.
Now, a positioning servo drive, with either a PM or an AC induction motor, can sit all day at zero position error, whether or not there is an external load. It does not need to hunt. The key is integral gain in the position loop. The integrator can "charge up" so that a torque command is output even in the absence of an error at the moment.
It is a common misconception that servo drives need to "hunt" dynamically to hold position. A good servo system can sit fat, dumb, and happy all day long at zero error if there is a constant load (zero or non-zero). Hunting is usually a sign either of response to changing disturbances or bad setup.
Curt Wilson
Delta Tau Data Systems
RE: Holding Torque with a VFD and AC Induction Motor
RE: Holding Torque with a VFD and AC Induction Motor
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Holding Torque with a VFD and AC Induction Motor
Gunnar Englund
www.gke.org
RE: Holding Torque with a VFD and AC Induction Motor
I wasn't trying to speak terribly precisely, and I used "charge up" as a colloquialism. In an analog system, the voltage on a capacitor typically does charge up to perform the integral function. In a digital system, the number in a register gets bigger.
But "wind up" usually refers specifically to the "charge up" of an integrator past where it can do any good -- that is, when the servo output is already saturated at its maximum magnitude. This can be very problematic, because the very large value in the integrator can cause the servo to overreact as the servo comes out of saturation. Most decent servo algorithms now have some kind of "anti-windup" feature that prevents the integrator from charging up further if the output is saturated.
Skogs clarified the nature of what is happening to create holding torque nicely. At 0 rpm (0 Hz) rotor speed, the stator frequency may be 2 Hz. As far as the rotor electromagnetic dynamics are concerned, this is no different than a 58 Hz rotor mechanical frequency (1740 rpm for a 4-pole motor) with a 60 Hz stator frequency. The rotor "sees" a 2 Hz slip in either case.
Curt Wilson
Delta Tau Data Systems
RE: Holding Torque with a VFD and AC Induction Motor
Okay so the DC isn't actual DC. Thanks for the clarification. Makes sense to me now.
I see your "charge up" as it describes analog/capacitor PIDs actual functioning methodology (analowhat?).
Windup/anti-windup I understand as half my work is coding PIDs into embedded systems. I have fought that war many times/ways.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com