Motors

[Knobhead]

I’ve asked Lenny to set up a new sticky thread for motors and their control. I hope others will help with diagrams and theories for the benefit of others.

 

[Knobhead]

It’s going to take some time to put together. I want to start with basic motors and control but then going in to some more complex motors and starters.
I’ll apologise for the control drawings, they will be line drawings, not little blocks showing wire A in to terminal B. your going to have to use some thought to transpose them to your application.

The motors and starters I want to cover are:

AC:
Squirrel cage
Wound rotor
Dual speed
Dahlander (dual speed)
Schrage (the motor in my avatar) (variable speed)

DC:
Series
Shunt
Compound

Starters:
DOL
Star Delta (open transition)
Star Delta (closed transition)
Auto transformer (open transition)
Auto transformer (closed transition Korndorfer)

It’s not going to be a definitive guide, but based purely on experience so at times you may have to bear with me.

Other members of the board will give better information on modern soft start and VFD’s than I can.

 

[Silly Sausage]

Courtesy of The silva.foxx, there's a very good link in here...
http://www.electriciansforums.net/e...stop-start-contactor-overload.html#post381564

 

[gerard]

Hi,

See attached file on various motors - DC, AC, Universal, etc. and their characteristics. It's not perfect (the spelling!) but it will help if trying to choose a motor. Hope this helps.

Regards,View attachment 8737

 

[Knobhead]

Motor windings

For R, Y, B read Brn, Blk, Gry I’ve not gone decimal yet, plus it’s quicker to type.

The most basic motors will have just three terminals in the TB. How it’s connected internally isn’t an option. The plate rating will state one voltage so other than rotation there’s not a lot to worry about. These are for direct on line starting only.

View attachment 8907View attachment 8908
Regarding rotation the convention is connect R, Y, B to A, B, C the rotation will be clockwise looking at the shaft. (Works 99% of the time).

Most motors will have six terminals A1 B1 C1 top row C2 A2 B2 bottom row. 1 being the start of the winding, 2 the end.

View attachment 8906
Some will be dual voltage others suitable for star/delta starting.

Dual voltage will have two options, typical will be 230V delta 400V star. Some you may come across are:
400/230
415/240
433/250
690/400
750/433
(I’ve only ever come across the higher voltage ones a couple of time in 35 years).



Motors suitable for star/delta starting will usually just give one voltage for delta. If the supply is up to it they will start direct on line.

View attachment 8904View attachment 8905

 

[Knobhead]

OK I know this is in electrical thories but I wanted to put it in “motors”
Squirrel Cage Motors.

On starting the rotor is stationary so it draws what is called locked rotor” current. This can be 5 to 8 times the FLC. The current is regulated primarily by the resistance of the winding with a small inductive element. As speed increases the inductance increases while the resistive element remains the same.



Running light the motor turns at near it’s synchronous speed causing high inductance in the stator windings and therefore low current. The motor cannot achieve full synchronous speed, as a small amount of “slip” is needed to induce current in to the rotor’s squirrel cage. As load on the shaft is increased slip increases inducing more current in to the squirrel cage, which in turn reduces the inductance of the rotor allowing more current to flow through the windings.

Synchronous speeds for an induction motor is governed by the number of poles in the rotor. This is give by Hz X 60 / No. of poles. So at 50Hz the synchronous speed for a 2 pole motor it will be 3000RPM.

 

[Knobhead]

Re: Autotransformer starters

View attachment 9101

There are two types of autotransformer starters and to add to the confusion they are wired the same. The sequence and the transformer construction are different.

The standard starter is open transition where at the change over from start to run there is a brief moment when no power is supplied to the motor. This can cause mechanical stress to the motor and load.

The Korndorfer starter is closed transition, so no loss of power at change over. Because of this the transformer has to be a bit more “beefy”. But because there is no break in the supply to the motor there is no mechanical stress.

Standard starter, open transition:
contactors 1 and 3 close. Giving a reduced voltage to the motor
time delay
contactors 1 and 3 open
small time delay (no power to motor)
contactor 2 closes. Giving full power to the motor

Korndorfer starter, closed transition:
contactor 3 closes
small time delay
contactor 3 closes. Giving a reduced voltage to the motor
time delay
contactor 3 opens. The transformer is now acting as a choke
small time delay
contactor 2 closes. The transformer is now shorted between it’s incoming and % tappings. But as it's the same potential bewteen the two points there is no current flow
small time delay
contactor 1 opens.

As can you see the sequence for the Korndorfer puts stress on the transformer where it has to withstand acting as a choke. So for small motors it’s uneconomical.

 

[Knobhead]

Re: Dahlander Motors

I hadn’t intended to cover these so soon but the thread about a roller shutter door prodded me to go ahead. I’ve only ever got involved with 2 of these motors, and in all honesty I hope I never see one again!
The motor for the roller shutter door would be drawing 2.

The Dahlander motor is a two speed motor that offers a 1 : 2 speed switching. It is a specially wound pole switching motor requiring a special dual speed starter.

The Dahlander motor is wound as a star motor or a delta motor with centre taps on each leg of the winding. The motor is connected in star or delta for one speed and in "double star" for the other speed.The starter requires three contactors (five for reversing), a speed 1 main contactor and a speed 2 main contactor and a star contactor that can differ in it’s connection depending on the motor layout. Two over load protection relays are required, one in series with each of the main contactors.

Dahlander motors can be wound for constant power, variable torque or constant torque applications.A constant power Dahlander motor is would as a delta motor, and a constant torque motor is wound as a star.I’ve done the drawings showing reversing starters as this is what I used to work on.
View attachment 9135

View attachment 9134
This is a simplified version of the above.
View attachment 9142

View attachment 9133

 

[Knobhead]

Correction to post#7

Korndorfer starter, closed transition:
contactor 3 closes
small time delay
contactor 1 closes. Giving a reduced voltage to the motor
time delay
contactor 3 opens. The transformer is now acting as a choke
small time delay
contactor 2 closes. The transformer is now shorted between it’s incoming and % tappings. But as it's the same potential bewteen the two points there is no current flow
small time delay
contactor 1 opens.

 

[paul wild]

Thats great keep em coming guys
regards
paul

 

[Knobhead]

Star delta overloads

View attachment 9231

The motor needs overload protection via an overload unit, with a S/D starter the O/L’s can be placed in the circuit in two different positions. With a DOL starter the setting is simple, just set at the FLC of the motor.

Method “B” is the preferred method as it provides the correct O/L protection during starting. The disadvantage is the setting of the O/L unit. In a S/D starter only a proportion of the current flows through the O/L unit, therefore the relay has to be set at 0.58 (1/√3) of the FLC. Not quite so simple.

At times there are advantages to method “A”. I’ve used it for starting very heavy inertia loads where there would be a risk of tripping during start up. Some early starters even shorted the O/L’s out all together.

 

[alli]

Thanks for this thread, really interesting.

Do you ever sleep? 03:31 AM

 

[Knobhead]

Insomnia
View attachment 9236

 

[alli]

Sorry to hear that, I guess that's why your drawings are superb.

 

[silva.foxx]

To supplement Tony's SD info Line and Delta contactors are sized at 58% flc and the Star contactor at 33% flc of the motor load.

For example: a 220kw 3ph AC induction motor pulling say, 360 amps and star-delta started. Line and Delta contactors to be sized min. 210 amps and Star contactor at 120 amps. So you'd use 2off 250 and 1off 140A contactors (assuming AC-3 inductive requirement) as opposed to 3off 400A contactors. This saves on both expense and panel space.

On much smaller motors this is less of an issue and you'd typically find the three contactors similarly, if not indentically, sized.