Motor Regeneration with VFD

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mull982

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I have had issues with several of my VFD applications in the plant where they were tripping due to regenerative voltage feedback from the motor with some high inertia loads. I have looked into several solutions including braking resistors, dc braking etc... Someone has recently told me that by putting the VFD closer to the actual motor this would help alleviate some of the regeneration. I'm not sure that I buy this because I cannot see why this is true. I would tend to think just the opposite that with the dirve further from the motor the regen voltage would be less simply due to the resistance in the cable and associate voltage drop. Can anyone comment on this?

The same person also mentioned that using VFD cable would help alleviate the regen voltage. I always understood that the VFD cable was a cable simply to help with heating due to the harmonics and irregular waveforms from the dirve. Can VFD cable help eliminate regen voltage?
 
The problem is that as the motor reduces its speed it increases the voltage on the DC bus of the drive. The only ways to handle this are: to length the time that the motor is slowing down (increase the deceleration time) or to bleed off the increased voltage by using braking resistors, or in fancy drives by adding a re-gen package and putting the voltage back into the AC supply, or even in multi-drive systems by sharing the DC bus between several drives.

VFD location and output cables are not factors in this problem.
 
080716-1012 EST

mull982:

The rotating system, motor and its load, have kinetic energy that has to be removed to slow down the rotating elements. The rate of change of energy is power.

Regenerative braking would mean that you take energy from the rotating system and store it some other place. For example a battery or capacitor.

If you dissipate that energy in a resistor, then it is not regenerative in the true sense of the word, and I would prefer to call it dynamic braking.

Typically VFDs or Vector drives have an internal DC supply with a moderate size capacitor for filtering and absorbing some regenerative energy.

What is usually done is to add a resistor connected thru a relay in parallel with the capacitor and shunt the capacitor when the capacitor voltage rises too high. This is the least expensive way from a component cost point of view to handle random high deceleration cycles, such as on CNC machines.

If you have a very repetitive task, then from an energy standpoint a larger capacitor bank or a battery would be advantageous because the regenerated energy will be used on the next cycle to accelerate the system. This could be a big saving in energy cost.

Also if you had many machines, such as CNCs, one large DC supply with either a large capacitor bank, and/or battery feeding many machines, then the energy demands of all the machines would average and probably require a smaller central DC supply system than the sum of doing the same job individually in each machine.

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Further explanation:

A motor only regenerates under 2 inclusive conditions;
  1. The motor's windings are excited, i.e. there is energy applied to them which maintains the magnetic field in the stator and rotor.
  2. The frequency of the regenerated voltage coming back from the motor is higher than the applied frequency, i.e. your VFD is applying power at 51Hz and the inertia in the load is still keeping the motor spinning at a speed that creates a frequency of 52Hz.
Without BOTH of those conditions, the motor cannot regenerate for more than a second or two (due to residual magnetism). What happens is, you have the Decel function enabled and when you turn the speed command down or off, the VFD applies a lower frequency to attempt to cause the motor to slow down faster than if it were to coast. But the rate at which the VFD is trying to decelerate that motor is faster than the motor will actually slow down because of the inertia. The VFD is therefore applying energy to the windings which keeps the magnetic fields active, but the applied frequerncy is now lower than the motor's virtual frequency and the motor becomes an induction generator. The energy from it pumps back into the VFD's DC bus and has no place to go, so the VFD trips to protect itself.

So the easiest way to prevent your VFD from regenerating is to simply turn off the Decel function if you don't need it! Most VFDs come with the Decel function enabled as a factory default and most people don't bother to change that. If you set the VFD to "Coast to Stop" (or some such similarly worded option), the transistors are turned off when not needed and the motor will not regenerate for more than a second or two because it is not being continuously excited. Another option, mentioned in a previous post, is to extend out the Decel time to be a lot longer than the Coast-to-Stop time. That way, the applied frequency is never lower than the motor's virtual frequency, thus no regen.

If for some reason you do need the Decel function, then for a high inertia load you MUST have the Dynamic Braking option (braking resistors and a DB module in most cases). A Line Regenerative VFD is another option but is essentially twice the price of a standard drive because it does that by having another VFD that fires the regenerated energy back into the supply line. Shared DC bus systems are fine if you have multiple motors on a machine, but not applicable to one-drive applications.

DC Braking will NOT do anything for you with regards to regen problems, in fact it may make it worse. DC Braking is something entirely different and is only for low speed use at the very end of a braking cycle. Forget that as an option in your case.

Your "friend" is totally wrong on this issue by the way. The lead length as well as the VFD rated cable issues are related to a different problem, called "Standing Wave Generation" and has to do with damage to the motor winding insulation for the most part. Neither issue has anything to do with your VFD's DC bus tripping on OV. He is apparently mixing up the issues in his mind because they both have the word "generation" in them, pay no attention to him.
 
All – keep it simple. OP is referring to a VSI system, non regenerative

What is the difference between an induction generator and an induction motor? Perhaps the direction we cut flux?

On a VSI, OV is either the rotor exceeding stator velocity connected to the VSI output or a power bump on the VSI input.

Solution is probably easy, once OP explains application ...
 
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now we are talking about a fault that normally occurs in the stop sequence..could he be talking about a load that with a change in velocity due to external circumstances buy either removing to much such as a clump of unmixed batching in a dump. this would cause a surge in the motor and that could cause the fault as well..
 
Yes I am referring the the DC bus Overvoltage faults that occur on the drive when a high inertia load causes a motor to regenerate. Thanks for all of the informative resoponses
 
080717-0757 EST

ptonsparky:

A Google search for VSI system produced this result
http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel2/608/4487/00178181.pdf?temp=x
So VSI is the acronym for Voltage Source Inverter whatever that really means.

http://www3.interscience.wiley.com/journal/105057062/abstract?CRETRY=1&SRETRY=0
Limited additional information.

http://sciencelinks.jp/j-east/article/200611/000020061106A0261393.php
Sort of unrelated but may be of interest on how to get maximum power from a solar array.

http://en.wikipedia.org/wiki/Inverter_(electrical)
This reference at least gives an indirect definition by comparison with CSI which is a current source inverter. To find this particular spot in the discussion search for the string --- voltage source inverter ---.

Too many acronyms are used in these discussions, and thus clearness of meaning can be lacking.

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A _voltage_ source is a device which tries to maintain the same output terminal voltage no matter what the loading. If you open circuit a voltage source, then the output terminal voltage is simply the device rating, and the output current is zero. If you short circuit a voltage source, then the output current gets extremely high.

A _current_ source is a device which tries to maintain the same output current no matter what the loading. If you open circuit a current course, then the output terminal voltage gets very high, possibly arcing (a loaded 'current transformer' is a good example of a current source). If you short circuit a current source, then the rated current simply flows.

A voltage source inverter has an internal capacitor bank that maintains a roughly constant DC voltage. This capacitor bank is connected to the output terminals via the switching transistors, so that the output terminals develop a pulse approximation to the desired output voltage waveform.

A current source inverter has a large internal _inductor_ that maintains a roughly constant DC current. This current flow adjusts slowly with load, but does not adjust at the rapid switching frequency. This current flow is coupled to the output terminals via the switching transistors.

A slight disagreement with Jaref: DC injection braking will work at both high and low speed. It may not be used at high speed because of motor heating, but it will work, and if the controller is capable of it might help the OP with his problem. When you are slowing a system down, the kinetic energy has to go _somewhere_, either into friction, a braking resistor, regen back to the line, or blowing up the capacitor bank. DC injection braking puts this energy into the motor as heat, and thus may be a bad idea when the motor is at high speed.

-Jon
 
I have worked on a film line where some of the drives were regenerative, that is they would put the power back feed back into the grid. They cost a "bit" more but they do work. They also used to be called 4 quadrant drives. Resistors are limited and you loose the energy. It the motor runs regenerative often go with the regen drive.

RC
 
I agree if your load is the culprit on this then you need to buy a regenerative drive..it will put the excessive power back on the line..if it then trips you will need to physically put some sort of resistance on the load to help control the excessive momentum..
 
080717-1205 EST

winnie:

What does the following statement mean?
A _voltage_ source is a device which tries to maintain the same output terminal voltage no matter what the loading. If you open circuit a voltage source, then the output terminal voltage is simply the device rating, and the output current is zero. If you short circuit a voltage source, then the output current gets extremely high.
Click to expand...
What is the definition of a load? Is a generator a load? I suggest it should be included. Do you allow current (energy) to flow into a voltage source? Again you should. This would be the typical definition of a voltage source in an equivalent circuit. Is a series pass voltage regulated power supply a voltage source? Can you back feed current into it?

The post by Ranch did not clarify anything with the use of VSI since we really do not know what this means.

A typical low cost variable frequency drive will have rectifiers feeding a filter capacitance (limited energy storage), and followed by the inverter. This system has the capability of large energy flow toward the load, but very limited capability to receive energy from the load. Is this VSI? This is the crux of the problem. Because of the input diodes energy flows in one direction only and that is from the power line.

By making the capacitor bank sufficiently large and/or using a storage battery one can solve the reverse energy problem without the waste of dumping the energy into a resistor.

My point at the start is that the term VSI needs clarification in relation to the problem of this thread.

A drive is regenerative if you have a place to put the energy and reuse the energy. Regeneration does not of necessity require feeding back to the grid.
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I was simply trying to differentiate the generic concept of a voltage source from the generic concept of a current source.

In an _ideal_ voltage source, the output terminal voltage would be constant; no matter how much, or which direction, the current was flowing.

A battery is a rough approximation of a voltage source; current can flow out of the battery (discharge), or into the battery (charge), but the voltage stays _very roughly_ the same.

A common inverter with an input rectifier, capacitor, and output bridge is also a very rough approximation of a current source. The output voltage stays relatively constant as the load is increased.

However as you note, this approximation to being a good voltage source falls apart quite quickly if the drive regenerates.

-Jon
 
winnie said:
...
A slight disagreement with Jaref: DC injection braking will work at both high and low speed. It may not be used at high speed because of motor heating, but it will work, and if the controller is capable of it might help the OP with his problem. When you are slowing a system down, the kinetic energy has to go _somewhere_, either into friction, a braking resistor, regen back to the line, or blowing up the capacitor bank. DC injection braking puts this energy into the motor as heat, and thus may be a bad idea when the motor is at high speed.

-Jon
Click to expand...

I'll give you that. I would never use DCIB from a VFD for high speed braking because when a load is spinning fast, it takes a lot of DC current to create any effective braking torque. Most VFDs are not designed to put out the high DC current for an extended length of time. So I concede that it is possible, just not advisable.

Nice explanation of VSI and CSI by the way. I used to tell people the difference between a CSI drive and a VSI drive is that you could submerge the motor being run by a CSI drive and it would likely survive (I have seen it), whereas if you did that with a VSI (PWM) drive, the drive would likely be toast. On the other hand if you did lose the motor for some other reason, the VSI could be connected to any replacement motor and back up and running with little effort, whereas the CSI drive needed to be impedance matched to it. And heaven help you if someone opened the circuit on your CSI output cables!
 
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