Detecting Motor Rotation when Motor is not Energized

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We have a 350hp motor connected to a 480V VFD. We recently have found that due to the location of the motor a fan upstream is pulling a draft and causing the load on this 350hp motor to spin and thus causing the motor to spin. This motor and load spinning is causing mechanical problems (suprisingly no electrical problems yet) when this motor is spinning and then started while spinning.

We are trying to determine a way to detect if this load and motor is rotating (due to upstream draft) prior to starting the motor with the vfd. The only thing that I can think of is putting a shaft encoder on the shaft of the motor? Witout a shaft encoder, and with the motor not energized is there a way for the drive to be able to detect that the motor is turning? For instance is there enough residual magnetisim remaining in the motor when it is de-energized for the drive to detect a forward or reverse rotation? Maybe with some sort of sensorless vector setup??

What happens if this load is spinning forwards due to the draft and is then started while spinning in the direction of rotation? Will the current drawn be less since the motor is already running at a higher rpm point on the motor speed/torque curve?

What about if the motor was spinning in the reverse direction when started? I have heard that the motor would draw more than LRC due to the fact that it would be operating with a negative slip. Would the current seen be much larger than LRC? This is what I would think you would see with a DOL start but have no idea what you would expect to see with a VFD start.

Is there a function on the drive similar to DC braking that would lock the motor in place when not running.

Flychatching is for another purpose and would not solve the problem. it would speed up your drive in the reverse direction.

There is a function in some drives that is specifically designed for cooling tower fans and they detect the backfeed from the motor. They continuously inject a variable current to counteract the airmovement accross the blades and that is sufficient to stop the fan from moving in either direction.

A shaft sensor may not be able to tell the direction, only that there is movement.
 

Jraef

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The VFD feature you want to look for is called an "anti-windmilling brake". Several of the better VFDs on the market have this feature. It uses a common feature in almost all VFDs called a "DC Injection Brake", but instead of braking the motor when you release the run command, it brakes it when you apply the run command, then when it detects zero speed, it begins ramping in the proper direction.

Almost all VFDs now provide a standard DC injection brake feature, but only a few have the modified anti-windmilling brake. However I have successfully "fooled" drives without it into doing what I needed by using a smart relay. I set up 2 ramp profiles in the VFD that can be called via digital inputs wired to the smart relay; ramp 1 being "anti-windmill" and ramp 2 being "normal". On first start, the smart relay triggers ramp 1 which tells the drive to "start" the drive at 15Hz and immediately stop it, which triggers the standard DC injection braking. Then when I get a zero speed signal from the drive, the smart relay triggers the ramp 2 profile that allows normal operation. After the motor has been shut down for more than a few minutes (beyond coast-to-stop time), the smart relay resets to begin the routine next time. If you don't already have the VFDs it's a lot easier to just buy one with that as a standard feature, but if you have to retrofit, this is a less expensive way to get it rather than toss the existing drives. However, evaluate the likely other benefits of replacing the drives, i.e. age, efficiency, other beneficial features etc. It may be better to replace.

The drive can only detect motor speed when it is applying power to the motor. It will essentially count the rotor bars passing through the field it establishes with the DC injection by virtue of the current disturbances they create.

Also just FYI, flying restart technically does not change the current profile of the motor starting, it only affects the time it takes to get to full speed. But when you are using a VFD, the VFD is always in full control of the current profile anyway, so there really is nothing to worry about with regards to starting into a spinning load. I don't know of any VFD on the market any more that does not offer flying restart. It used to be difficult to implement on older Current Cource Inverters, but on modern PWM drives it's a no-brainer.
 

mull982

Senior Member
The VFD feature you want to look for is called an "anti-windmilling brake". Several of the better VFDs on the market have this feature. It uses a common feature in almost all VFDs called a "DC Injection Brake", but instead of braking the motor when you release the run command, it brakes it when you apply the run command, then when it detects zero speed, it begins ramping in the proper direction.

So you are saying that this "anti windmilling feature" is similar to DC injection braking except it injects the the DC when the run command is given? Does DC injection work by simply injecting a DC current into the motor thus creating a steady DC field in the stator and lock up the rotor or prevent it from spinning?


The drive can only detect motor speed when it is applying power to the motor. It will essentially count the rotor bars passing through the field it establishes with the DC injection by virtue of the current disturbances they create.

So you are saying that without a physical sensor such as a tach the drive can only detect speed with power is applied to the motor? I guess then there is not enough residual magnetism in the motor when it is stopped to create the field necessary for counting the rotor bars as you mentioned? If the point of the DC injectioni braking is to lock the motor then how would this DC injected field be used to count the rotor bars and thus the speed if the motor is supposed to stop when "injected"?


Also just FYI, flying restart technically does not change the current profile of the motor starting, it only affects the time it takes to get to full speed. But when you are using a VFD, the VFD is always in full control of the current profile anyway, so there really is nothing to worry about with regards to starting into a spinning load. I don't know of any VFD on the market any more that does not offer flying restart. It used to be difficult to implement on older Current Cource Inverters, but on modern PWM drives it's a no-brainer.

I know a motor that has power removed and then has power re-applied while there is still a field in the motor will cause the motor generated voltage to be out of sync with the source voltage and thus cause a huge current spike when power is re-applied to this spinning motor.

However what about a motor that is spinning at some speed, who's field has totally decayed? Are you saying that applying power to this motor at an other frequency than its spinning at will not cause a current transient? Can you explain what happens from a torque and current standpoint when this occurs?

We have another drive that seems to trip when a similar instance occurs.
 

gar

Senior Member
090030-1238 EST

Can you detect that an induction motor is rotating when there is no excitation to the motor?

Yes.

There is a sufficient residual magnetic field to induce a voltage.

I just put my Fluke on the terminals of my lathe 1/2 Hp single phase motor and by hand rotated a few RPM and saw maybe 15 MV, and with no rotation the output was zero.

Is this a reliable method for mull982 I do not know. Can I detect direction? Maybe. Can I detect speed? Maybe very roughly.

If you really have an interest, then experiment and see what you can learn.

.
 

mull982

Senior Member
Did we ever determine whether the idling blower turns in its running direction?

We opened up the equipment and verified that the blower is turning in the correct direction when drafting or spinning. Therefore the motor and load were spinning in the correct direction of rotation when the VFD energized.

I'm guessing that the motor was spinning however the drive stated its ramp at 0HZ when started thus stopped the motor suddenly and caused mechanical damage of the coupling.
 

mull982

Senior Member
I've researched the "Anti-Windmill", and "Flying Start" technologies discussed above and my understanding of them is as follows:

Anti-Windmill protection is very similar to DC braking except the DC current is injected when the start command is given to the motor as opposed to when a stop command is given. The DC current will bring the motor to a stop, and once the drive detects the motor is stopped it will then start its ramp to the setpoint speed. The drive will detect when the motor is stopped by using the same magnetic field created by the injected DC current and using this field to look for current disturbances as the rotor bars pass through this static field established in the rotor.

When DC current is injected into the startor of the motor it creats a static magnetic field in the air gap. This magnetic field then induces a voltage in the rotor and thus a current in the rotor due to the rotor being shorted. (Heres the part I'm not sure of). This current induced in the rotor is in the opposite direction of the rotation of kenetic energy of the rotating mass???? Because it is in the opposite direction the kenetic engergy from the load is dumped across the resistance of the rotor thus slowing the motor down???

Because this injected current is a DC current there is no phase shift between the stator field and the field induced in the rotor thus keeping the rotor field in phase with the stator field and not allowing rotation of the motor. This is different from AC current in a motor who's magnetic field established in the rotor is offset 90degrees from the stator field due to the inductive reactance of the rotor. This offset is what allows the rotation of the rotor.

A Flying start is used by a drive to match the drive output frequency to the frequency at which the motor is spinning. In order to detect the speed at which the motor is spinning the drive injects a small amount of voltage (AC or DC?) into the stator in order to establish a field. It then used this field similar to detection with DC injection to look at current disturbances caused by the passing of the rotor bars through the field. I'm not sure if it can detect forward vs reverse direction? Once it knows the speed of the motor, the drive outputs the equivelent frequency thus allowing the drive to catch the motor to get in sync with it and ramp up the the setpoint speed. This allows a smooth electrical and mechanical transition. I'm still not sure what happens electrically when the drive is started from 0Hz while the motor is still spinning without using a flying start?

This is my understanding of the two techniques from the brief amount of research and information I could find on the subjects. I may be way off :)
 

Jraef

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So you are saying that this "anti windmilling feature" is similar to DC injection braking except it injects the the DC when the run command is given? Does DC injection work by simply injecting a DC current into the motor thus creating a steady DC field in the stator and lock up the rotor or prevent it from spinning?
Yes it is a DC injection scheme, so that creates a non-rotating magnetic field in the stator, which in turn creates a magnetic field in the rotor that is rotating counter to the direction the rotor is spinning, which is what brings it to a stop.

So you are saying that without a physical sensor such as a tach the drive can only detect speed with power is applied to the motor? I guess then there is not enough residual magnetism in the motor when it is stopped to create the field necessary for counting the rotor bars as you mentioned?
Detection via residual magnetism is a horse of a different color; it's something you do by applying a meter. The VFD is not "looking" at anything when it is in the off state, it's only connection to the motor is the transistors and if you turn them on to try to see anything, they will be emitting as well. So once it is on, it is far more reliable for it to energize the windings so as to count pulsations from the rotor bars than rely upon residual magnetism, which by the way varies greatly from motor to motor and in fact, some motors have nearly zero.

If the point of the DC injection braking is to lock the motor then how would this DC injected field be used to count the rotor bars and thus the speed if the motor is supposed to stop when "injected"?
The DC injection is just setting up that stationary field, primarily for the purpose of stopping the motor but since it is already doing that, monitoring for current pulsations as the rotor bars cut the lines of force is just a simple and reliable way to watch the rotor speed. It doesn't interfere with the braking action.

I know a motor that has power removed and then has power re-applied while there is still a field in the motor will cause the motor generated voltage to be out of sync with the source voltage and thus cause a huge current spike when power is re-applied to this spinning motor.

However what about a motor that is spinning at some speed, who's field has totally decayed? Are you saying that applying power to this motor at an other frequency than its spinning at will not cause a current transient? Can you explain what happens from a torque and current standpoint when this occurs?

We have another drive that seems to trip when a similar instance occurs.
Of course there is still a current transient when power is applied, at any speed. Think about it, when you apply power to a motor that is standing still, you STILL have a difference in the applied frequency and the motor frequency, it just so happens that the motor frequency is 0!

But I think I understand what you are asking. To get it clear, realize first that there are two "transients" that occur when you start a motor; Inrush and Starting current. Inrush is only the very brief spike in current that is a result of establishing the magnetic field in the stator and rotor, before the back-emf inhibits it. The magnitude is limited only by the resistance of the wire itself, the magnetic permeability of the laminated steel and the air gap between the stator and rotor. It decays very quickly, usually in less than 2 cycles. This is what does not change, regardless of whether the rotor is spinning or not. Starting current is the amount of current, OVER TIME, that is pulled as a result of the high slip that occurs between the stator rotating field and the rotor's rotating field. So if the rotor is already spinning, there is less slip differential and that current magnitude ends up being slightly lower and lasts less time because it takes less time to get to full speed.

So to put that together, when you energize into a spinning motor (assuming it's own field had decayed), there will still be "Inrush" as the magnetic fields are established, which will be just as high as if the motor were standing still. Once those magnetic fields are established, you start dealing with slip differential. So a motor at standstill is at 100% slip, you get locked rotor current magnitude, which decays as the motor speeds up and the slip differential decreases. If the motor is already moving at lets say 80% speed, you get less slip differential and therefore less magnitude of the current transient. That curve is very steep however, so anything less than about 60% speed is not going to show much difference in magnitude, just duration.

But VFDs make it more interesting because the applied frequency of the power coming from the drive is not the design frequency of the motor, so the slip differential is relative now. In other words, if the drive can determine what speed the motor is running at, it can find a reasonable output frequency to apply and keep the current at absolute minimal levels. If, however, the VFD does not have that ability to "find" the rotor speed, then it's just a shot in the dark. If your drive is having troubles, either it is an older design that can't monitor rotor speed and do a flying restart, or maybe you just don't have the feature enabled.

And by the way, torque follows current, so what happens in the above is essentially happening with regards to torque as well.

OK, my fingers hurt now...
 

LarryFine

Master Electrician Electric Contractor Richmond VA
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Henrico County, VA
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We opened up the equipment and verified that the blower is turning in the correct direction when drafting or spinning. Therefore the motor and load were spinning in the correct direction of rotation when the VFD energized.

I'm guessing that the motor was spinning however the drive stated its ramp at 0HZ when started thus stopped the motor suddenly and caused mechanical damage of the coupling.
I would think that the VFD would see the already-turning motor as an easier-to-start motor. Weird that the VFD "wants" the notor to be stopped at start-up.
 

Doug S.

Senior Member
Location
West Michigan
I'll admit to skimming, but didn't notice anyone mentioning contacting the manufacture? If it was made after 1980, SOME of the drive manufactures are pretty good about this kind of stuff. Especially if you are buying or may be buying 350hp drives.

Otherwise ( the drive is archaic or no $$$ for the fix ) a low tech option would be a mechanical brake after stand still & no run cmd, ( as mentioned stand still sensors are pretty cheap, dependable, can be safety rated ) release at start sequence.
A mechanical brake applied via power off may also keep the safety comity happy.

My 2?
Doug S.

Edit: My idea may be problematic if the fan never actually stops turning... and injection may be an issue at the bottom end of decel?
 

mull982

Senior Member
O.K. I found some more information about how the flying start works in the drive.

When the flying start feature is enabled in the drive, the drive will output a reduced V/Hz output to the motor which will result in a reduced motor current which I believe is maybe 75% of rated drive current. The drive will output this voltage at a particular frequency which can be selected in some drives or is 60Hz by default in others. The drive will then begin ramping this frequency down or "sweeping" this frequency down towards zero. As the drive is sweeping this frequency the drive will see a small current being consumed by the motor. Once the frequency output reaches the speed that the motor is spinning at the current that the drive sees goes to zero and then quickly reverses direction. The reason the current reverses direction is because as the output frequency passs through the motor speed the motor switches from "motoring" which consumes current to "generating" (re-gen) which produces current in the opposite direction.

Once the drive sees the current reverse direction it recognizes the speed the motor is spinning at and then outputs this frequency to the drive with the full V/Hz to catch this rotating motor. The drive will then ramp the drive to whatever the setpoint frequency is.

If the motor happens to be spinning backwards then as the drive sweeps its frequency towards zero it will not see a reversing current and will ramp to 0Hz. Once it gets to zero Hz it will output a negative frequency and start again sweeping towards zero. As it sweeps in the opposite direction (on the negative side of 0) it will again look for a reversal of current to determine the motor speed. Once the drive recognizes the motor is spinning in reverse it again matches this frequency and ramps the drive towards zero at which point it will begin its ramp towads setpoint speed.

If the drive has a starting frequency for the sweep you must be careful that it is not set too low, or the drive will sweep towards zero and never see the reversal of current and thus not detect speed. If it cannot detect the speed it will inject a DC current into the motor in an attempt to slow it down and then will repeat its sweeping ramp again in an attempt to find the motor speed.

This is how I understand this technique.

The only thing I'm not 100% sure about is how the magnitude of the "sweep" voltage and current effect the motor? Why doesnt this output attempt to produce a torque on the motor? Are these values too low to produce a sufficent torque on the motor and thus the motor continues to spin?

I also dont know what percautions must be taken to avoid DC bus overvoltage issues? If the sweep voltage is produced with the motor still has a residual flux is there a possibility of the this voltage being out of sync with the motor regen voltage and thus trip the drive? Should there be a delay?

Also what if the drive sweep frequency is started below what the motor is spinning at. Is there a possiblility that as the drive sweeps and searches for speed the motor can regen and produce an overvoltge with the field produced by the reduced voltage in the drive?
 
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