Pleading ignorance. Difference in induction motor vs generator? 1

[fastline12]

I am sure there might be some basic adjustments to optimize one or the other like bearings and such, but I was sort of lead to believe there are quite a few differences here? It was my understanding that these are effectively the same thing, just depending on which side of synchronous rotation the rotor is on? Leading or lagging? If I have brought an elementary question, please forgive me!

 

[TugboatEng]

The stator are essentially the same but a generator has a wound rotor while an induction motor does not.

 

[davidbeach]

At the most basic level the difference between a motor and a generator is that in a motor the electrical system supplies energy that is converted to torque to turn a mechanical load while in a generator the mechanical system supplies torque to the shaft resulting in the production of electrical energy.

Given that, many forms of small (single phase mostly) electrical machines do not meaningfully convert mechanical energy to electrical energy. Generally most 3-phase machines can be either motors or generators, some performing both roles multiple times per day. The standard 3-phase induction motor can function as a generator if driven above synchronous speed and supplied with an external source of excitation (vars). A synchronous machine has a rotor with a field and does not require the line to supply excitation.

A synchronous generator can generate both Volts and Watts while an induction generator can only generate Watts, the Volts have to be supplied by an external system.

DC machines are a different animal, but follow that basic definition.

 

[crshears]

OP seems to be asking about the differences between an induction motor and an induction generator; if that is in fact the question, what davidbeach said; if not, TugBoatEng is pointing in the right direction...fastline12, please clarify OP for us.



CR

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

 

[fastline12]

My fault for any confusion. I am asking about specific design differences between an induction motor and an induction generator. I realize an induction motor will in fact generate power, but if one was put to work specifically to generate, how would the design change.

I did look up the rotor wound or dual wound motor and seems to be most common with variable rpm situations as it can generate power in a much wider range than a wireless rotor.

Just for consideration (no, I don't have a specific application), if an induction 'motor' is driven at near continuous synchronous speed, would there really be any practical difference to a generator? Wave form, efficiency, generated available power, etc?

 

[TugboatEng]

There is no magnetism in the rotor of an induction motor so if it's driven it simply won't do anything except act as a flywheel. That's why generators have a wound rotor. DC current through the wound rotor magnetises it so it will generate voltage in the stator as it rotates.

 

[waross]

A standard induction motor will function as an induction generator. Many do. An induction motor powered crane lowering an overhauling load, a down hill conveyor with an overhauling load, the list goes on.
Sorry tug but an induction generator does not need a wound rotor.
There may be designs that optimize induction generation, particularly some wind turbine induction generators.
I have set up induction generation demonstrations for students.
In one school lab, I was able to demonstrate induction generation and with a resistor bank in parallel with the motor, the motor, acting as an induction generator continued to power the resistor bank when disconnected from the line.
(The motor was being driven over speed by a DC motor.)
In both cases I used available induction motors over driven by DC motors.
In different school lab, I was able to demonstrate induction generation. With the equipment available I was not able to continue to power the resistor bank when the induction generator was disconnected from the line.
Given the wide range of standard and special designs of rotor squirrel cage windings, I would expect some designs may be more efficient than others for regeneration.
Hopefully one or more of our motor design Gurus will have some information as to optimizing motor design for use as induction generators.

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

 

[davidbeach]

As Bill said, sorry tugboat but your wrong. I've seen utility scale induction generators that are simply squirrel cage machines. As I said, they can't produce voltage, but given an excitation source they can certainly generate Watts. An induction generator with a wound rotor requires far more support equipment than a simple squirrel cage induction generator, though there are reasons to go that way now and then.

Traction elevators may be the most familiar example of a squirrel cage induction generator/motor. Consider an empty, or lightly loaded, elevator. Every time it goes up the "motor" is a generator and power is returned to the supply system or burnt off as heat and every time it goes down the motor has to function as a motor. Heavily loaded it's the opposite.

 

[fastline12]

In tugboats defense, I think he simply means that you cannot simply connect a motor to a load and spin the shaft and make power. You have to excite the coils, but I understand that.

 

[davidbeach]

Not really, he's essentially saying that an induction generator must be a wound rotor induction machine while in the real world there's nothing that stops an interconnected squirrel cage machine from being a generator.

 

[waross]

Hi David. In the lab, I mechanically connected an induction motor to a DC motor. I connected a resistor bank in parallel with the motor.
I energized the motor and the resistor bank. Then I powered the DC motor and increased the speed until the induction motor was acting as an induction generator and exporting power to the grid. I forget the details but I had a lot of instrumentation connected including a Wattmeter.
We saw power being returned to the grid. Then we opened the connection to the grid. The load current provided the excitation and the induction generator continued to power the resistor bank.
Note that the purpose of the demonstration at that time was not to demonstrate induction generation but to impress upon the students the fact that induction motors can and will back feed into a short circuit.
I understand that with some induction generators the residual voltage in conjunction with suitable capacitors will boot-strap and build up voltage without outside excitation. This is something for us both to watch for credible supporting information.

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

 

[jraef]

To answer your specific question, I do not believe there is a physical or electrical difference unless someone markets it as such. I have seen several grid tied wind generators made using standard 400HP induction motors taken off the shelf as 300kW induction generators. I work with a small OEM of low head hydro generators (for irrigation ditches and such) using a standard 40HP motor as the generator, again grid tied, so the grid supplies the excitation. There is nothing special about the motor they use and they even market it saying that users can replace the "generator" with any available induction motor.


" We are all here on earth to help others; what on earth the others are here for I don't know.
" -- W. H. Auden

 

[oldfieldguy]

The last induction generator I dealt with was a hundred HP Induction motor on an expander in an air liquifaction plant. When the plant was started up, ti was fed 480 volts and ran as a normal induction motor. At eh unit came on line and the process took over, the expander went from being a pump to being a turbine, over-speeding the motor, providing watts (real power) back into the system while drawing vars from the system for excitation.

It was plain 'off-the-shelf' induction motor.

We added metering to measure power in or out. Overload protection was a standard thermal overload block a magnetic-only motor circuit protector.

old field guy

 

[DanDel]

An induction generator is simply an induction motor which has been brought up to speed the typical way from a starter circuit and then driven slightly over synchronous speed by a prime mover attached to it's shaft.