Motor to motor load test with VFD

[cork1025]

I'm trying to understand something that I'm being asked to help supply motors for. The customer wants two 600HP motors. One motor to run off their VFD and the other motor to run off the incoming 3 phase power. That part doesn't really seem to be an issue.

They also want to regen the power from the second motor. Everything I have read says that you need to have two VFDs for this to work. Option A is to tie the two VFDs DC BUS together. Option B is to buy a regen VFD to put the regen power back on the grid. Without one of those options, I'm not sure how they are planning on using regen power.

Does anyone here have any experience or thoughts? The customer is pretty sure that they can just have two motors and one VFD and be able to regen power without the power company knowing...up to 80%. Everything I've read says the contrary. Other people I've talked to and what I've read online says they are not right. But just trying to get some more insight.

Thank you.

 

[waross]

Is this an over unity scheme?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cork1025]

I'm sorry but I don't know what that is.

 

[waross]

They can use one motor with VFD to drive the other, grid connected motor overspeed. It will then regenerate most of the power used back to the grid. To what purpose?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cork1025]

They only plan on over speeding the motor connected to the VFD to 63 or 64HZ. You can regen power back to the grid without a regen VFD? How can you do that? I guess I don't understand why people would buy a regen VFD if they don't need to?

 

[BrianPetersen]

An induction motor driven beyond synchronous speed - direct connection to the line, no VFD - will act as a generator and feed active power back to the grid.

 

[itsmoked]

As stated, overdriving => generating.

Why they might want the other motor and the VFD is that the VFD can control how much overdrive (power export) is occurring. Secondly the excess power they have may be electrical and not rotational so the meed to convert it to rotational somehow to allow it to turn the overdriven motor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Over unity is power for nothing, perpetual motion, motor driving a generator powering the motor, and using a VFD to drive a motor over-speed so as to regenerate power back into the grid.
If this is to try for cheap or free power, then yes you will be able to regenerate the equivalent of almost 600 HP. Looks great until you remember that you still have to buy the power to run the VFD and the driving motor.
This is a good technique to save a lot of power when testing motors, however it works best if the motor under test drives the VFD motor and the VFD is a regenerating type. If you will be testing a lot of motors the saving in energy compared to braking the motor and wasting the energy as heat will soon pay for the extra cost of the regenerative VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Any induction motor can be an induction generator if you have the following: excitation of the windings, so that the electromagnets are active, and something driving it faster than the synchronous speed. The VFD driven motor running at 64Hz satisfies condition 2, and being connected to the grid satisfies condition 1.

The purpose remains an issue. It will take more energy to run the VFD driven motor at 64Hz than the grid connected motor will be capable of generating back. So there will be a net consumption of energy and no real work performed. So again, why?


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[LionelHutz]

You need a regen VFD if you want to load a test motor that is not driven by a VFD. You do this to test motor controls that are not a VFD or because you don't want to introduce the non-60Hz harmonics to the motor under test.

I doubt they'll need 3Hz or 4Hz difference between the motors. The 3 test setups we have will fully load the test motor with <1Hz difference on 2 systems and just over 1Hz difference on the third. We use a regen setup so that the test motor can be used to test any type of motor control. But in the past we have used a VFD on the test motor and just line connected the load motor.

 

[waross]

2% slip will give a speed of 1764 RPM but both the driving and the driven motors will have slip so a 4% frequency difference is reasonable.
If the motor under test is a lot smaller than the drive motor then the required frequency difference drops towards the 2% figure.
If the motors are rated in the 1740 RPM range rather than the 1760 RPM range the required frequency difference will be greater.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

We have 2 x 60hp motors rated 1770rpm & 1740rpm, or 90rpm total slip = 5%. So, in theory you would expect it to require 3Hz to fully load, except in practice full load happens around 1Hz.

We also have 2 x 150hp motors rated 1770rpm so the total slip is 3.3%. In theory this would require 2Hz to fully load except in practice it happens a little over 1Hz. Oddly enough, these motors require a larger frequency differential compared to the 60Hz motors even though their combined nameplate slip is lower.

 

[cork1025]

Sorry I was on the road Thursday/Friday...

Anyway, I guess I'm not fully understanding this. The motor they have connected to the grid needs to be over sped before it becomes a generator, right? So if it's running 60HZ off the grid and the other motor/VFD is running at 64HZ, it would overspeed it by maybe 2 or 3HZ. How much regen power is that? It doesn't seem like a lot. And how will that save them money if they don't have it connected to a regen drive?

Logic tells me (and I'm not an engineer...I think I've previously stated) that in order to get the most out of a system like this that is used to test VFD after VFD every day the setup would be:

Test VFD
600HP motor connected to the VFD
Regen VFD
600HP motor connected to Regen VFD

Then, you could control both motor speeds. If you wanted to simulate a load, you could set motor one at 60HZ and motor two at 15HZ (or whatever you wanted). It would simulate a load AND turn that motor into a generator (maybe it overspeeds it to 30HZ or 40HZ). That regen power is then pushed back to the regen VFD and then put back on the grid. 3-4 tests per day, 1-2 hours per test. I feel like that's the way to perform the best tests AND save $ over time by putting that power back on the grid.

What am I missing here? (not being sarcastic...just someone in sales trying to figure this out and give the customer the best result)

 

[davidbeach]

You simply tell the VFD under test to run at 60Hz and tell the regen VFD that you want it to absorb X HP. It will the create the necessary slip speed to absorb that amount of power.

 

[jraef]

I'm still not clear. What is the PURPOSE of this setup? Are you testing a VFD? Testing a motor? What's the intended goal of the "test"?

If what you are doing is testing a VFD under varying load conditions, then you sue a Regen VFD for the load and do what davidbeach said. The Regen VFD will automatically keep the RELATIVE stator frequency lower than the rotor relative frequency so that it constantly regenerates.

If all you want to do is test the VFD at a fixed 60Hz, but want to have the Regen motor be the load, then just use a sheave/belt connection to the Test motor so that the Regen rotor spins faster than its synchronous speed, but stays connected to the grid so that it stays excited. If you want it at maximum load on the Regen motor, you have the relative rotor speed be the equivalent percentage of the slip speed if it were motoring.

There will of course ALWAYS still be a negative loss in power consumption, you don't get this for free. But using a motor in regen cuts the losses drastically, essentially to just the electrical and mechanical losses in the system in terms of heat from conversion / inversion, friction, winding resistance, iron losses and wind.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[cork1025]

Purpose - Test VFDs. Put them under test for 1-2 hours before shipping them out to customers.

I think it's smarter to vary the load conditions but I can't make that decision for the customer...just my 2 cents.

Now here's where I still don't get it and please forgive me - I don't understand how over speeding the test motor will make that much of a significance of power savings if they really aren't overspeeding it by that much. It sounds like it's possible that this motor will put some power back on the line...but is it THAT much? Wouldn't you get more power savings over time if you set up a REGEN VFD on the motor?

If it doesn't look like I'll ever get this, it's ok to give up on me. This just isn't naturally clicking in my head.

 

[itsmoked]

I think you're missing that you only need to barely overspeed the motor hooked to the grid to have it returning 100% of its rating. You need to overspeed it only between 1~3% of its name plate speed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cork1025]

Yes, That is definitely that part I was missing. From what I was told from others, that wasn't possible. Ok, well that definitely makes more sense.

I guess I don't get HOW that's possible...but maybe that's another discussion.

 

[itsmoked]

It's really easy. The induction motors you use are all about spinning at their design speed. You power them with a fixed frequency of 60Hz and they will spin at their designed RPM. As you load them more and more they will slow and as they slow they draw more power.

The shaft spins more slowly than the theoretical synchronous speed. A 4 pole motor as an example has a synchronous speed of 1800 RPM but the 'fully loaded' speed on the nameplate would typically be 1725 RPM. Notice that difference? The difference is because of "slip" all the "work" is transferred via the slip.

Turning it around you will need to overspeed the motor about the same amount to have the slip dragging the motor shaft faster to "put back" the power of the same nameplate amount into the grid. Hence all the above talk about a slightly higher frequency of the driving motor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

Start with a line connected motor. Say this motor is rated 100kW and has a speed of 1750rpm. This also means the motor is 60Hz rated and has a synchronous speed of 1800rpm. This motor will run at 1750rpm when you put 100kW of load on the shaft.

Now, attempt to drive the motor to 1850rpm. You will have to put 100kW of power into the shaft to reach 1850rpm. There are only 2 places the 100kW can go - motor losses and back into the power grid.

Notice the above - with 100kW at the shaft as either input or output the motor has a slip of 50Hz. The sign just changes.

Now, one way to drive the motor to 1850rpm is by connecting it to another motor operated by a VFD. You then use the VFD to run this driving motor faster than it would run when connected to 60Hz.

Now, say they are identical motors. You have to run the driving motor at 1850rpm to reach 100kW. With a slip of 50Hz, the driving motor has a synchronous speed of 1900rpm. Now, back calculate frequency = 60Hz x 1900/1800 = 63.3Hz.

If you don't really understand the above, then I suggest some Googling on induction motor basics.