Motors

[Knobhead]

Squirrel cage motors.

Should have done this first really, but I’m not the best organised person.

This shows an exploded view of a motor to give some idea of what goes where.

View attachment 12926

The motor can be star or delta connected.
If needs be to allow star delta starting. (See other posts)

View attachment 12927

The basic windings are brought out to the terminal box. Shown here as Delta to the left and Star to the right.

View attachment 12928
Delta terminal box

View attachment 12929
Star terminal box


The rotating field is created by the 6 coils for a 2 pole motor by the rotation phase rotation of the three phase supply

View attachment 12930

View attachment 12931

The motor gets it’s name from the construction of the rotor windings. Take away the rotor laminations you have a squirrel cage.

Transformer induction induces a fixed field on the rotor cage. The cage rotates in an attempt to catch up with the rotating stator field. A forlorn hope as there has to be a certain amount of slip to give the induced current to give rotation and torque.

Early motors had copper bars brazed to copper end rings. Later the copper was replaced by aluminium injection to form a solid casting.
The cage could be in several forms.

A standard cage that would give normal acceleration and torque
View attachment 12932

Double cage where due to the higher flux density on the inner bars at high slip would give twice the torque on start up. It had the down side of higher inrush current.
View attachment 12933

Modern cages are a highbred of both. With aluminium injection these are easier to make now.
View attachment 12934

There are a few companies now doing copper injection for the cage.

As for the motor being constant speed, I’m afraid that’s a myth. As load increases more slip is needed to produce the torque. The motor name plate gives speed at a % of loading.

 

[nickblake]

i always get confused with motors star and delta and how , have no problem fixing faults with them then again i am a tad odd , nice post tony

 

[snowhead]

Very simplified way to remember for newcomers to 3 ph motors;

Star = Start

Delta = Drive

 

[Strima]

Very helpful Tony, cheers mate.

 

[ukylou]

Re: Motor bearings

Thanks Tony very interesting

 

[Bobster]

Was braught up in another thread but here's some bumf on artificial 3phase motor connections.

Now this is taken from my little motor bible and is the only way I've seen in person a 3ph motor run on single phase.

This is another way of connecting it up, now I've only ever read the theory about this. Never seen it in the real world. It's called a steinmetz connection.

I'll write a post with the theory explained when I can spend some time and edit it properly, rather than on my phone.

 

[lemau]

Hi all,Topic for motor and starter is common topic for all electrical person..i would to share some post about electrical motor and motor starter. I hope it can help for more understanding about motor and starter

 

[Knobhead]

Simplified the drawing for you. Also made it so the start P/B doesn’t carry the full starting current and the main contactor can’t close with the switch in the run position.

View attachment 13926

 

[Knobhead]

Primary resistance starter

Primary resistance starter

I’ve only ever seen two of these, they were used on lift motors.

As you can see from the drawing they are pretty simple. On start up contactor K2 closes allowing a reduced voltage to the stator windings, after a short time delay contactor K1 closes and K2 drops out.
They suffer from being limited to X number of starts per hour due to heat build up.

View attachment 14202

View attachment 14201

 

[i=p/u]

its all comming back to me.....

 

[Bobster]

Re: Primary resistance starter

Primary resistance starter

I’ve only ever seen two of these, they were used on lift motors.

As you can see from the drawing they are pretty simple. On start up contactor K2 closes allowing a reduced voltage to the stator windings, after a short time delay contactor K1 closes and K2 drops out.
They suffer from being limited to X number of starts per hour due to heat build up.

View attachment 14202

View attachment 14201

Is this not just effectively a liquid starter?

Any advantage to this design?

 

[Knobhead]

Liquid starters are normally used on slip ring motors where isolation of the phases isn’t as important. There’s no reason a liquid starter couldn’t be used as a primary resistance but it would need major re-engineering to make it safe. Instead of one earthed tank for the electrolyte, three separate insulated tanks would be needed. There was a company in the 70/80’s that made Statormatic starters, the same company made Vapormatic starters for slip-ring motors.
View attachment 14208
Slip ring rotor
View attachment 14207
Stator

 

[Bobster]

Delt with a lot of liquid starters at the cement plant I used to work, usually with 3.3kv drives, had a few long days when the brush gear flashed over with carbon dust, we did try to blow them oh weekly but production often superseded maintenance.

I was just thinking about the therory behind it, am I right in thinking a star delta starter would be a cheeper way of setting this up? I take it the resistor bank is used for the same reason the liquid starters are used? Never seen that set up before.

 

[Knobhead]

The primary resistance has the beauty that it’s flexible as to the % of torque at start up. Star/delta is 1/√3 only.

The beauty of a slip ring motor is it’s high starting torque, something you wouldn’t get with start/delta. A cement mill would just sit there and not move in star. On top of that even at √3 of the DOL starting current (say 6X FLC/√3 = 3.46X FLC) the 3.3 OCB would trip on instantaneous overload. The cement mills I’ve worked on ranged from 900 to 6000HP, I wouldn’t like to be around when the breaker opened. I was stood in front of a 3.3 ACB when that tripped due to a slip-ring flashover, I near had to go and change my underpants.
For heavy starting loads you can’t beat slip-ring motors.

Some I’ve worked on were variable speed using the Kramer system. They started using a liquid starter. Once at full speed the Kramer control would kick in and regulate the current in the rotor, the recovered energy being fed back in to the 11KV system via an inverter and step up transformer.
View attachment 14214

 

[Bobster]

We had a system very similar to that tuning one of the kiln firing fans, it was an abb multidrive invertor 2Mva, that used a resistor bank to start and also managed to recover energy from the drive.

Never got my head around how that worked, it was only on site a couple of months before I left. Was delivered in a steel container as a plug and play system, it weighed 27tonnes.