Holding Torque with a VFD and AC Induction Motor 4

[450x]

Can I produce holding torque with an AC induction motor with a Powerflex700 VFD

 

[itsmoked]

DickDV; Can you explain to me why the drive would be able to stop the load for a few seconds then lose control? I can see no logic to this at all! I believe you, but cannot even vaguely picture the why.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Smoked,

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

http://www.gke.org

 

[itsmoked]

I see that skogs but I want the why that was the how.


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ozmosis]

450x
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.

 

[Skogsgurra]

The WHY is simple. You need current in the rotor to get any torque. And if the rotor is at standstill, the only way of getting current in the rotot is to produce a slow three-phase systems that rotates against the torque. If this system rotates too slow, the shaft will move. If it rotates too fast, the shaft will rotate against the torque. Both are bad, but rotating with the torque is less bad.

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

http://www.gke.org

 

[ozmosis]

Producing torque also needs movement. A true flux vector drive is zero torque at zero speed.
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.

 

[Skogsgurra]

Yes, that's right. The DTC modulator, in effect, keeps the flux vector between two tolerance circles. Using a two-point controller technique, aka Bang-Bang.

Gunnar Englund

http://www.gke.org

 

[waross]

Hi Gunnar
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

 

[itsmoked]

Thanks guys, now I see.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Yes waross. Never build any hoisting device with an open loop VFD. And don't forget the mechanical brake. It is mandatory. Not only good practice.

Gunnar Englund

http://www.gke.org

 

[waross]

Thanks skogsgurra
Respectfully.

 

[DickDV]

itsmoked, probably better to go with skogs explanation that to try to contradict him. I've tried that before and found the reaction a bit extreme.

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.

 

[digitrex]

I believe DickDV has misunderstood about DC HOLD and DC braking. These are the texts copied from the ABB manual:

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."

 

[DickDV]

I believe I understand the issues here, digitrex. DC braking can go on for as long as the motor remains under its thermal limit and has no relationship to DTC at all.

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.

 

[cswilson]

sed2developer:

"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

 

[Skogsgurra]

Yes, those are the important words: "occasionally oversold by their marketing people" and that's why DTC gets some beating from technicians.

Gunnar Englund

http://www.gke.org

 

[ozmosis]

Curt
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.

 

[DickDV]

digitrex, I just now see your point about DC Hold and DC Braking. I tend to call both "DC Braking". ABB's intention, I believe, is to refer to DC Braking as dynamic braking as in slowing a load down. DC Hold refers to holding a motor at standstill without any intentional rotation. Both are independent of DTC as I mentioned above and use DC injection which can go on for as long as the motor remains within its temperature limits.

 

[cswilson]

sed2developer:

"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

 

[jraef]

This is one of the more fascinating discussions here I must say. It's what makes this forum so wonderful.

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 measure the DC current being injected into the motor winding as long as the motor is under "DC Hold" condition.

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.

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