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# Motor Performance as a Generator

## Motor Performance as a Generator

(OP)
Hello all,

I have an AC motor with a pretty straight forward torque/rpm curve. However I would like to estimate braking torque that can be applied by the motor. I am not so much concerned with how much power is regenerated but wasn't sure how the motor performance differs when used for braking. I read that regenerative braking is ineffective at low speeds because speed and output voltage for a generator are proportional, but the braking torque is still the same? (i.e. the motor still brakes with the same forces it just cant convert it to energy bc low voltage)

Thanks,
Corey

### RE: Motor Performance as a Generator

A 1760 RPM three phase induction motor will brake effectively from 1800 RPM to about [1800 +(1800-1760)] or 1840 RPM.
It will be regenerating near full load current at 1840 RPM.
At 1880 RPM the torque and current will be about twice the rated torque and current.
The synchronous s[peed of an induction motor depends on the number of poles and the frequency.
When you drive the motor over the synchronous speed, it regenerates and brakes.
You may drop the frequency with a VFD. All it takes is money.
The amount of braking will depend on the capacity of the breaking resistor and/or whether the VFD has regenerative ability.
The torque is proportional to the current.
If you try to take more than the rated torque, either motoring or braking, the motor will overload and overheat.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Motor Performance as a Generator

#### Quote:

The amount of braking will depend on the capacity of the breaking resistor and/or whether the VFD has regenerative ability.

Hi Bill,

Your mixing of spellings [braking/breaking] reminds me of the first time I saw some documentation come across my desk reporting on the commissioning plan for a new "air brake switch"; I was both amused and annoyed, and advised the originator that air brake switches might possibly be found on trains or transport trucks, but that we use air break switches.

I was reported on and got chewed out for being disrespectful.

Then others began duplicating that plan, editing it to match their circumstances; as a result my utility now has a nice mix of both air break switches and air brake switches in use.

I said nothing, having no desire to follow my earlier admonition with opening myself up to having a second one reamed.

Long live political correctness!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

### RE: Motor Performance as a Generator

.
.
Now I can't get this out of my head. Is it possible that too much braking may lead to too much breaking?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Motor Performance as a Generator

It could, and for sure we don't want to brake anything...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

### RE: Motor Performance as a Generator

give us a brake, guys.

=====================================
(2B)+(2B)' ?

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