Induction motor as an isolation transformer and harmonic filter

[Frank Bourbeau]

I have a 12-lead 5 hp motor that I want to use as a rotary isolation transformer. This transformer will serve as a 1:1 isolation transformer and also as a harmonic filter to clean up the distorted primary voltage. To accomplish this, I used the low voltage connections specified on page 5 of the EASA handbook but with certain connections omitted to create two isolated delta windings. This resulted in the following connections:

Pri. 1 U1&U2 Pri. 2 V1&V2 Pri. 3 W1-V2
Sec. 1 U5&U6 Sec. 1 V5&V6 Sec. 3 W5-V6

The unloaded motor runs with normal current and near-synchronous speed when connected per the EASA table (U1&U2&U5&U6, etc.). However, when the windings are connected in two isolated deltas, the motor draws high current and runs slowly when ac voltage is applied to one of the deltas.

I thought the motor would run as a normal motor with only one of the two delta windings energized. Can anyone explain why it didn't?

Frank Bourbeau, Enerpro Inc.

 

[GoldDigger]

Was there any mechanical load on the motor when tested?
Was there any electrical load on the "secondary" when tested?

I believe that the normal use of the 12 lead motor construction is to allow connection as a wye for reduced voltage starting or a delta for full torque running.
If only one of the two deltas is connected and the other delta is left disconnected (the delta winding not closed) the motor will operate at half the current and half the magnetic field used for normal running. This will reduce the available torque and will result in higher slip while unloaded.

But by closing the second delta winding you are not creating a simple fixed core transformer, and the apparent result is that current is flowing in the non-driven delta which is opposing the magnetic field created by the driven delta, causing the high current and low torque that you are measuring. The motor will also be overheating severely because of excess current in both delta windings.

One problem may be that, like a rotary synchronous single to three phase converter, your configuration will not work unless the motor shaft is already turning near synchronous speed (low slip).

I would try this experiment:

1. Make the three jumper connections, J1-J3, with a three pole contactor.
2. Close the contactor.
3. Start the motor and wait for it to stabilize at its no load speed.
4. Open the contactor.

Chances are good that after opening the contactor with the motor at full speed you will see the expected output voltage on the non-driven delta. But I make no predictions about how it will react to load on that winding.

I deduce from your employment at Enerpro and the Enerpro logo on the drawing that this is an original concept rather than an adaptation of a proven technique.
It is quite possible that you do not have a sufficient grasp of motor operation to be designing such a product.

 

[ActionDave]

I am 100% intrigued by this. A motor at it's deepest level is a rotating transformer, but a motor does not have a primary and secondary windings, it has poles and groups. Never ever heard of a rotary isolation transformer. Why would anybody want one?

 

[GoldDigger]

I am 100% intrigued by this. A motor at it's deepest level is a rotating transformer, but a motor does not have a primary and secondary windings, it has poles and groups. Never ever heard of a rotary isolation transformer. Why would anybody want one?

In theory, the reason one would want one is that *when properly configured* it acts like a motor generator, and therefore isolates poor power quality at the input and provides clean power at the output. The rotating mass provides the filtering.

It is, however, also like a motor generator in that you have to keep the output disconnected until it is up to speed.

In a standard transformer the coupling is through mutual inductance in a fixed core, and the rotary filter does not have that simple coupling. Think of a rotary phase convertor as possibly the closest analog.

Sent from my XT1585 using Tapatalk

 

[Besoeker]

I am 100% intrigued by this. A motor at it's deepest level is a rotating transformer, but a motor does not have a primary and secondary windings,

A wound rotor IM does. Google ISK.
But I wouldn't remotely consider using it as an isolation transformer.

 

[Frank Bourbeau]

Your Start/Run Suggestion

Your Start/Run Suggestion

9-18-08:23

There was no load on the secondary when I tried to start the motor with the secondary delta isolated from the primary(motoring) delta.

This morning I did as you suggested:

1. Started the motor with the two delta windings in parallel.

2. Opened a contactor to disconnect the "secondary" delta from "primary"(motoring) delta.

3. The motor current did not change when the secondary was isolated from the primary (problem solved!)

Next week I will put a SCR rectifier (lagging power factor) load on the secondary to find how much VA the secondary can supply.

Thanks for your help.

Frank Bourbeau

Was there any mechanical load on the motor when tested?
Was there any electrical load on the "secondary" when tested?

I believe that the normal use of the 12 lead motor construction is to allow connection as a wye for reduced voltage starting or a delta for full torque running.
If only one of the two deltas is connected and the other delta is left disconnected (the delta winding not closed) the motor will operate at half the current and half the magnetic field used for normal running. This will reduce the available torque and will result in higher slip while unloaded.

But by closing the second delta winding you are not creating a simple fixed core transformer, and the apparent result is that current is flowing in the non-driven delta which is opposing the magnetic field created by the driven delta, causing the high current and low torque that you are measuring. The motor will also be overheating severely because of excess current in both delta windings.

One problem may be that, like a rotary synchronous single to three phase converter, your configuration will not work unless the motor shaft is already turning near synchronous speed (low slip).

I would try this experiment:

1. Make the three jumper connections, J1-J3, with a three pole contactor.
2. Close the contactor.
3. Start the motor and wait for it to stabilize at its no load speed.
4. Open the contactor.

Chances are good that after opening the contactor with the motor at full speed you will see the expected output voltage on the non-driven delta. But I make no predictions about how it will react to load on that winding.

I deduce from your employment at Enerpro and the Enerpro logo on the drawing that this is an original concept rather than an adaptation of a proven technique.
It is quite possible that you do not have a sufficient grasp of motor operation to be designing such a product.

 

[Jraef]

What is the nameplate voltage rating of that motor, and what did you connect it to? The configuration that you used is for an IEC 12 lead motor connection as a single speed motor using the LOW voltage.

 

[Phil Corso]

Frank...

Current-flow calculation is no different, whether load is symmetrical, or unbalanced, than that of the series-impedance-starting situation!

Regards, Phil Corso

 

[Phil Corso]

Frank...

Further to my earlier comment!

Have you confirmed that that the windings are properly displaced on the stator?

Phil

 

[winnie]

My _guess_ is that you have a 4 pole motor, and given its size the motor is wound with 6 separate 'phase groups' around the stator.

Phase groups on opposite sides of the stator would normally be connected together (either in series or parallel depending on high or low voltage).

By connecting only 3 of the 6 phase groups, my guess is that only half of the stator was energized, rather screwing up the magnetic field structure. Presumably the field structure had much more harmonic content, and on start-up the motor got 'locked' into one of the harmonic rotating fields. By using the full winding to start the motor spinning, you got it up to the point where the fundamental harmonic dominated the torque/speed curve.

If this guess is correct, you _might_ have success if you swap the V5-V6 phase group for the V1-V2 group, spreading the energized coils around the entire stator. If this works then the motor might spin up to full speed without the contactor...or it might spin up to half speed as an 8 pole motor.

Regarding using motors as transformers, I once worked with a guy who had set up a wound rotor machine as a variable transformer. The shaft was prevented from rotating using a gear mechanism that would permit him to change the angle, and the rotor leads were used as the output. By changing the angle between rotor and stator he got variable voltage phase angle, and he used that to get the variable voltage. He ran a motor rewind shop, so re-purposing a motor to build test equipment made sense to him.

-Jon

 

[GoldDigger]

..
..

Regarding using motors as transformers, I once worked with a guy who had set up a wound rotor machine as a variable transformer. The shaft was prevented from rotating using a gear mechanism that would permit him to change the angle, and the rotor leads were used as the output. By changing the angle between rotor and stator he got variable voltage phase angle, and he used that to get the variable voltage. He ran a motor rewind shop, so re-purposing a motor to build test equipment made sense to him.

-Jon

Slight off-topic extension:

Theatrical dimmers have used a variety of mechanisms before the variable autotransformer (Variac TM) entered the picture and was eventually replaced by electronic (phase chopping) controls.
Among them were liquid variable resistance units similar to those used in motor starters, series inductors with a movable core to change series impedance (variable reluctance), and transformers in which either the angle of the primary and secondary coils in an air core configuration or an iron core transformer with a (re)moveable core were used.
These all worked just fine with single phase input.

 

[Andy Delle]

Slight off-topic extension:

Theatrical dimmers have used a variety of mechanisms before the variable autotransformer (Variac TM) entered the picture and was eventually replaced by electronic (phase chopping) controls.
Among them were liquid variable resistance units similar to those used in motor starters, series inductors with a movable core to change series impedance (variable reluctance), and transformers in which either the angle of the primary and secondary coils in an air core configuration or an iron core transformer with a (re)moveable core were used.
These all worked just fine with single phase input.

Yes, the liquid dimmers were called "piss pots"! Back in 1900 through the 1930s they used buckets* full of salt water with movable electrodes as dimmers. I guess it smelled like hot piss! Where was the NEC and OSHA back then

P.S. I'll also bet those were metal buckets too. Hell, that saved you one electrode!

 

[Jraef]

Yes, the liquid dimmers were called "piss pots"! Back in 1900 through the 1930s they used buckets* full of salt water with movable electrodes as dimmers. I guess it smelled like hot piss! Where was the NEC and OSHA back then

P.S. I'll also bet those were metal buckets too. Hell, that saved you one electrode!

Still used to this day, called a “liquid rheostat” and the motor version is a “Liquid Resistance Controller” for use with Wound Rotor Induction Motors.