Replacing star delta motor starter with Variable frequency drive

Thread Starter

mztrordinary

Joined May 25, 2016
54
Hi Guys

As the headline suggests I am looking at replacing some of the controls in a very, very, very old control panel. One of the things i am changing is the star delta controls of the main motor on the machine with a VFD. Obviously with the star delta there are the 3 contactors so I am just checking in with some of the more modern experts , as it has been a while since I have worked on star delta) don't worry I am also qualified just looking for some input or friendly pointers.
I have sourced a VFD which will do the job and matches all of the specs on the motor, power, amperage voltage ect. The motor is a 6 wire motor and I have attached a pic of the terminal box. the run contactor is fitted with thermal overload and has the 3 phases running out of the panel and up into a roof space which i have no access to. The isolator for the motor has six wires coming in and 6 wires going out which has confused me a little.
 

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6 leads in a star-delta motor means it is wound for a single voltage. 2 leads for each phase. 6 leads rather than 12, the green/yellow equipment ground and the presence of a terminal block suggests to me this a European motor. 240/415V? Any particular questions you're looking to get answered? Star-delta motors are wound to start in the star configuration and run in delta, so all you really need to do is permanently connect the motor in delta and gut the old contactors. Be aware that part-winding motors also exist (though they're rare) and it's possible to confuse the two if you're not paying attention.

https://acim.nidec.com/motors/usmotors/TechDocs/ProFacts/Starting-Methods/Part-Winding
 
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MaxHeadRoom

Joined Jul 18, 2013
23,104
Star delta motors with six terminals usually have the six terminals for ease of converting one to the other, either the top three are strapped, (Star).
Or three vertical straps, (Delta).
Just identify the three windings/pairs and connect in delta, if using on the original AC power service.
If a very old motor, I would suggest a 3ph choke between VFD & motor.
Max.
 

Thread Starter

mztrordinary

Joined May 25, 2016
54
Star delta motors with six terminals usually have the six terminals for ease of converting one to the other, either the top three are strapped, (Star).
Or three vertical straps, (Delta).
Just identify the three windings/pairs and connect in delta, if using on the original AC power service.
If a very old motor, I would suggest a 3ph choke between VFD & motor.
Max.

OK well from the image on the left hand side going from top to bottom the phases are u v w. I am away from the machine now and will have to check the other phases on the right hand side tomorrow.
But are you saying that I will only need 3 of these phases once identified to run my motor through the VFD once I connect it in delta just like a normal 3 phase motor?
 

Thread Starter

mztrordinary

Joined May 25, 2016
54
6 leads in a star-delta motor means it is wound for a single voltage. 2 leads for each phase. 6 leads rather than 12, the green/yellow equipment ground and the presence of a terminal block suggests to me this a European motor. 240/415V? Any particular questions you're looking to get answered? Star-delta motors are wound to start in the star configuration and run in delta, so all you really need to do is permanently connect the motor in delta and gut the old contactors. Be aware that part-winding motors also exist (though they're rare) and it's possible to confuse the two if you're not paying attention.

https://acim.nidec.com/motors/usmotors/TechDocs/ProFacts/Starting-Methods/Part-Winding
Just on the below
 

Thread Starter

mztrordinary

Joined May 25, 2016
54
Great guys thanks. Just remembering that there are also terminals at one end of the motor opposite end to where the fan and cowl areit looks like a break but it is not a brea I've never seen it before on newer motors I will post photos in the morning and after I check continuity between those and the terminal box phases. Unless you guys have ever seen something like it. I know its hard to tell until I put pic up
 

Thread Starter

mztrordinary

Joined May 25, 2016
54
Hi guys I have attached some photos of the connections with the cover removed. As you can see from the first image the three phases on the left are coming in from the terminal box from the 3 phases on the RIGHT hand side from the image I attached of the terminal box previously. Those phases go on to the terminals as you can see in the second attached image and then around to the terminals on the last image. All these terminals have continutity between them and the 3 terminals inside the connection box on the right hand side on the motor. I would assume these are my three phases for running the motor?
 

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This is a wound-rotor induction motor. Very expensive. Usually seen on overhead cranes. The leads extending from those brushes form a secondary circuit, much like the secondary of a transformer. I think I can see the resistor grid for it in the top right of the first picture.

What exactly is this motor powering? If this is a critical application I wouldn't touch it with a 10 foot pole unless you're been through specific training for this particular sort of system. Someone could have a really bad day if e.g. the hook on a crane goes down instead of up because the motor doesn't develop enough torque anymore after you've altered the installation.

 
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Thread Starter

mztrordinary

Joined May 25, 2016
54
This is a wound-rotor induction motor. Very expensive. Usually seen on overhead cranes. The leads extending from those brushes form a secondary circuit, much like the secondary of a transformer. I think I can see the resistor grid for it in the top right of the first picture. What exactly is this motor powering?

It is powering a mixing vessel which was used to mix large amounts of bread dough. The motor runs into a gearbox and then it is shafted into the machine.
 
Oh, so they used a wound-rotor motor because mixing blades in dough is a really tough load to start. You'll definitely want to find a way to preserve the existing resistor grid and it's controls. It may be possible to configure the auxiliary outputs on the VFD to cut in or out the appropriate amount of resistance for a given speed range. You may also want to program a minimum speed so the motor won't stall if set too low. The big concern is the amount of starting current that your VFD can source. With the mixing blades stuck in a big ball of dough, the motor is going to demand a significant amount of current right from the outset.
 
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Thread Starter

mztrordinary

Joined May 25, 2016
54
Oh, so they used a wound-rotor motor because mixing blades in dough is a really tough load to start. You'll definitely want to find a way to preserve the existing resistor grid and it's controls.
I thought that by installing a VFD I could achieve this using very slow ramp times. This mixer now mixers 500kg of powder instead of bread dough although the load is still tough its not as tough as it used to be.
 
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I'm not an expert with VFDs, but I recall there are two voltage control schemes for them. One is a linear voltage-frequency ratio and the other results in a proportionately higher voltage at lower frequencies to improve low speed torque. You'll want to be sure your drive is capable of the latter. V/F vs Vector, I think? Others will be able to shed much more light on this.

Bottom line I'd use every trick at your disposal to maximize starting torque to make sure you don't end up having to come back to this installation later. Vector control combined with an auxiliary output to cut in/out the resistors once the drive reaches it's minimum programmed speed - with a grain of salt.

 
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Thread Starter

mztrordinary

Joined May 25, 2016
54
I'm not an expert with VFDs, but I recall there are two voltage control schemes for them. One is a linear voltage-frequency ratio and the other results in a proportionately higher voltage at lower frequencies to improve low speed torque. You'll want to be sure your drive is capable of the latter. Others will be able to shed much more light on this.
Ok thanks for the input. I'll talk to my motor rewind guy at some stage and see what he has to say about it. He's a long time in the game and will give me solid advice.
 

MaxHeadRoom

Joined Jul 18, 2013
23,104
If using a wound rotor induction motor with a VFD, you would just need to short the outputs of the slip rings, i.e. turn it into a normal squirrel cage induction motor and control the RPM with the VFD instead of variable resistors across the rings.
The sensorless/vector VFD is used in precise control methods such as machine spindles etc, the Freq/Hz versions are used in Fan applications etc.
Max.
 
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Thread Starter

mztrordinary

Joined May 25, 2016
54
If using a wound rotor induction motor with a VFD, you would just need to short the outputs of the slip rings, i.e. turn it into a normal squirrel cage induction motor and control the RPM with the VFD instead of variable resistors across the rings.
The sensorless/vector VFD is used in precise control methods such as machine spindles etc, the Freq/Hz versions are used in Fan applications etc.
Max.

I always get the shivers when someone mentions shorting things out on a motor control circuit lol
 

MaxHeadRoom

Joined Jul 18, 2013
23,104
I was referring to the slip ring output of the rotor/armature, the simple variable resistance motor controller takes the output and gradually introduces a decreasing resistance to the generated 3ph output of the rotor, this start the rotor at low rpm, until the controller resistance is down to zero and maximum rpm.
Usually the 3ph output is rectified, then you only have to introduce resistance to a two conductor supply.
If you short the output, the it is at maximum torque and rpm.
Then the 3ph in can be produced from a VFD, in order that the input supply can be varied.
Max.,
 

Thread Starter

mztrordinary

Joined May 25, 2016
54
I was referring to the slip ring output of the rotor/armature, the simple variable resistance motor controller takes the output and gradually introduces a decreasing resistance to the generated 3ph output of the rotor, this start the rotor at low rpm, until the controller resistance is down to zero and maximum rpm.
Usually the 3ph output is rectified, then you only have to introduce resistance to a two conductor supply.
If you short the output, the it is at maximum torque and rpm.
Then the 3ph in can be produced from a VFD, in order that the input supply can be varied.
Max.,
Ah ok sorry I understand now thanks Max very helpful
 
If you short the output, the it is at maximum torque and rpm.
Maximum *running* torque, not starting torque, correct? The point of introducing resistance is to increase starting torque is it not? One would have no need to reduce starting torque on a dough mixer by paying extra for a motor into which they can insert resistance. Quite the contrary.
 
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MaxHeadRoom

Joined Jul 18, 2013
23,104
If you had a open circuit to the slip rings, the motor current would be almost nil, and the rotor would remain stationary.
You increase torque by decreasing resistance.
Max.

A wound-rotor motor, also known as slip ring-rotor motor, is a type of induction motor where the rotor windings are connected through slip rings to external resistance. ... When the motor reaches full speed the rotor poles are switched to short circuit. During start-up the resistors reduce the field strength at the stator.
 
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