Induction motor acting as generator

[LMAO]

Question has come up regarding induction motor directly connected to utility (no VFD). Basically, motor periodically become gen as result of descending load. Utility company is questioning whether the power is usable. To me it sounds pretty straightforward: power generated by the motor is real and has to go somewhere so we should have real power back in utility. Utility company wants us to prove motor regenerating without an active front end (AFE) VFD has the same effect as the one with AFE VFD. I know it can be verified using a power meter but I think it is about some tax credit or something... not sure.
I am going to go back to my school books to review basics of induction generators but meanwhile I thought I post it here to get your feedback.

 

[Jraef]

Question has come up regarding induction motor directly connected to utility (no VFD). Basically, motor periodically become gen as result of descending load. Utility company is questioning whether the power is usable. To me it sounds pretty straightforward: power generated by the motor is real and has to go somewhere so we should have real power back in utility. Utility company wants us to prove motor regenerating without an active front end (AFE) VFD has the same effect as the one with AFE VFD. I know it can be verified using a power meter but I think it is about some tax credit or something... not sure.
I am going to go back to my school books to review basics of induction generators but meanwhile I thought I post it here to get your feedback.

Same? Not really, but "similar". With an A-T-L connected motor being overhauled by the load, the motor will put energy back into the line, up to the limits of what the motor can do, but ONLY when the overhauling load drives it at super synchronous speed. So for example if you have a 4 pole motor with a 1750RPM slip speed, the only time it is effectively regenerating will be at >1850RPM. At that point is is producing negative torque; it is attempting to RETARD the load, up to and including the limitations is has. Too much overhauling and the motor loses it's negative torque capability, the load runs away AND the motor overloads. Not enough overhauling and the motor either coasts as an additional inertial load, or at some point, starts motoring again; consuming power.

The only thing about this that an AFE drive does for you is this; the AFE drive can keep the motor in regen at ANY load speed. For for example if the load speed is 1600RPM, the VFD simply gives it a speed reference output of 1550RPM. If the load speed drops to 1550RPM, the VFD puts out 1500 then when it drops to 1500 the VFD drops to 1450 (non scaled examples, in reality the VFD matches the motor's slip percentage). So it KEEPS the motor in an overhauling condition regardless of the load speed. In addition, as the motor approaches and OL condition in regen, the VFD can just back off on the negative slip and keep it from having to shut down completely (again, within the limits of its capacity). Without the AFE drive, you have no ability to do any of that.

Frankly I'm a bit shocked that your utility has to ask this question, the concept is the same for Induction Wind Generators, and they have no inverter involved on most of them. They control the overhauling load by slipping wind off of the propeller through changing the angle of attack, but the motor is connected A-T-L.

 

[LMAO]

Same? Not really, but "similar". With an A-T-L connected motor being overhauled by the load, the motor will put energy back into the line, up to the limits of what the motor can do, but ONLY when the overhauling load drives it at super synchronous speed. So for example if you have a 4 pole motor with a 1750RPM slip speed, the only time it is effectively regenerating will be at >1850RPM. At that point is is producing negative torque; it is attempting to RETARD the load, up to and including the limitations is has. Too much overhauling and the motor loses it's negative torque capability, the load runs away AND the motor overloads. Not enough overhauling and the motor either coasts as an additional inertial load, or at some point, starts motoring again; consuming power.

The only thing about this that an AFE drive does for you is this; the AFE drive can keep the motor in regen at ANY load speed. For for example if the load speed is 1600RPM, the VFD simply gives it a speed reference output of 1550RPM. If the load speed drops to 1550RPM, the VFD puts out 1500 then when it drops to 1500 the VFD drops to 1450 (non scaled examples, in reality the VFD matches the motor's slip percentage). So it KEEPS the motor in an overhauling condition regardless of the load speed. In addition, as the motor approaches and OL condition in regen, the VFD can just back off on the negative slip and keep it from having to shut down completely (again, within the limits of its capacity). Without the AFE drive, you have no ability to do any of that.

Frankly I'm a bit shocked that your utility has to ask this question, the concept is the same for Induction Wind Generators, and they have no inverter involved on most of them. They control the overhauling load by slipping wind off of the propeller through changing the angle of attack, but the motor is connected A-T-L.

Fair enough, but what happens to the energy released by the falling load? So for example, a 100kg load dropping 10m results in 9800 joules of potential energy lost by the load. If it doesn't go back to utility where does it go?

 

[wwhitney]

So for example if you have a 4 pole motor with a 1750RPM slip speed, the only time it is effectively regenerating will be at >1850RPM.

If the rotor is being driven by an external mechanical source, why wouldn't it start generating as soon as the speed exceeds 1800 RPM? I thought the no-load slip of an induction motor represents the torque required to overcome the internal mechanical frictional losses, i.e. when the rotor is spinning the load is never really zero.

Cheers, Wayne

 

[wwhitney]

Fair enough, but what happens to the energy released by the falling load? So for example, a 100kg load dropping 10m results in 9800 joules of potential energy lost by the load. If it doesn't go back to utility where does it go?

First it goes into accelerating the rotor. Once the rotor is going fast enough, it starts generating electricity, so some of the energy goes back to the utility. The rotor will keep accelerating until the mechanical power consumed as a generator matches the input mechanical power from the falling weight, at which point all of the energy is going to the utility (ignoring the heat losses in the generator).

Cheers, Wayne

 

[Jraef]

If the rotor is being driven by an external mechanical source, why wouldn't it start generating as soon as the speed exceeds 1800 RPM? I thought the no-load slip of an induction motor represents the torque required to overcome the internal mechanical frictional losses, i.e. when the rotor is spinning the load is never really zero.

Cheers, Wayne

It does technically start immediately, but current equates to negative slip, so if the load is turning it at 1801PRM, the amount of current going back to the utility is negligible, consumed by resistance for the most part. That's why I used the term "effectively"...

 

[wwhitney]

It does technically start immediately, but current equates to negative slip, so if the load is turning it at 1801PRM, the amount of current going back to the utility is negligible, consumed by resistance for the most part.

Thanks for clarifying. I was uncertain because 1800 - 1750 = 50 RPM and 1850 - 1800 = 50 RPM, so I thought maybe you were saying the two instances of 50 RPMs were related. That is, that the 50 RPM represents some electrical losses that have to be overcome in either case.

Cheers, Wayne

 

[junkhound]

Frankly I'm a bit shocked that your utility has to ask this question

+1, poco must be having lawyers in the engineering office now ?