Reversing Rotation of a 3-Phase Motor

[lranso1]

First, I'm new here!!! Second, I've got a 400HP, 480V, wye-delta configured, 6-lead, 3-Phase motor. This motor is started by an allen-bradley SMC-FLEX soft starter.

Currently, I've got incoming lines A,B,&C connected to L1,L2,&L3 on the soft start, respectively. T1,T2,T3,T4,T5&T6 on the soft start are connected to lead 1,2,3,4,5,&6 on the motor, respectively.

My question is, can I reverse the rotation of the motor in any way other than switching 2 of the incoming lines? Can anything be switched on the motor side of the soft start to achieve reverse rotation?

 

[wrsharper]

I don't really comprehend your second paragraph but since I have recently installed a soft starter on star/delta 500HP motor I'll put in my two cents.

Simply configure the motor in delta and connect the three leads to the soft starter output. Set the soft starter overload current to motor data plate FLA. Set start time for your particular load. I was successful in using the same start time as the old star/delta with contactors configuration. There is no easy way that I know of to reverse motor direction other than reversing any two soft starter output tap leads.

Please give reasons for your need to reverse the 400HP motor and solutions may pop up.

 

[wrsharper]

I searched the web and found your soft starter. I have never seen a soft starter designed for "inside the delta" with all of the connections made at the bottom of the soft starter. From looking at the wiring diagram, the installer may not be aware that the motor has half of its windings energized when the motor is turned off since the "inside the delta" connections are wired straight through and hidden inside the soft starter from the line side to the output side.

If you search this site you will find lots of negative comments about "inside the delta". If your soft starter has a current rating of the FLA of the motor then I would not use "inside the delta" and connect the motor configured in delta to only T1 and T2 and T3 on the output of the soft starter.

Due to wiring limitations, if you must use inside the delta then I still do not know of an easy way to reverse direction of the motor.

 

[LionelHutz]

Changing the line side connections is the only solution in that application. If you change the motor connections the starter will not control the motor correctly.

I have no problems with the inside-delta connections but I would strongly recommend an undervoltage trip equipped breaker with the trip coil connected to a "fail safe" fault relay on the starter. The "brains" of the starter should be alive before energizing the fault relay and allowing the mains to be energized. Otherwise, a bad SCR can go undetected causing damage to the motor or load as it single phases until the breaker trips. You want the starter ready when the breaker is closed so that it can quickly detect a problem and trip the breaker back off.

 

[DickDV]

Lionel, with all that extra paraphenalia added to the "inside the delta" softstarter, I believe you just made a good case for using an "outside the delta" starter instead.

The only real motive I know of for using inside-the-delta is to save a little money (the current rating of the softstarter is lower) but, with all the other stuff, it probably isn't cheaper either.

You've persuaded me!

 

[LionelHutz]

Well, it's up to you to decide. And there really isn't a big price difference for anything below about 500A or so. But when you're talking about >600A motors the inside delta connection starts to make sense. Talk about ~1000A to 1700A motors and inside delta has a huge size and price advantage.

I've heard the complaints about motors being burned down by a line connected soft-starter when 2 SCR's failed so to me there's no advantage either way. I just see inside delta typically used on large chillers and single phasing a chiller is a bad idea. Often, the newer big breakers are smarter and will quickly trip on the current imbalance.

 

[jraef]

Lionel,
While I agree with you on principle, to my mind there is a significantly larger risk of motor damage from inside the delta connections compared to standard connection of a soft starter. The biggest problem is not that the concept isn't sound, but rather the execution is carried out without enough though because of improper marketing based primarily on price. Let me explain this position.

What you say about motors being damaged by having multiple shorted SCRs in a standard connection is true, but when connected inside the delta it takes only one shorted SCR to cause motor damage because the other side of each complete circuit is always available without that fault contactor. Another often missed aspect to that configuration is that the SCRs must be rated for a higher PIV level than many what manufacturers use because the peak voltages are no longer in wye, they are strictly delta. I know that your favorite manufacturer knows this and uses higher PIV rated devices, but the popularity of this concept in IEC countries where 400V designs are common has lead to their use in North America where 480 and 600V systems push that concept over the edge. For the IEC users, the fact that the manufacturers are building to suit N. America means they end up with higher PIV ratings than they technically need as standard, but unless the soft starters are designed for 690V applications, this means that in 480V and 600V systems where the 400V designs are used, the SCRs are then MORE likely to fail. Again, technically an issue that could be addressed with better education of the users or for manufacturers to automatically use 1800PIV rated SCRs in all products. But in practice the extra cost of 1800V SCRs is pointless in IEC countries so competitive pressures keep them out of most off-the-shelf products, plus very very few salesmen know this issue properly and end up selling products that put the end user at risk.

The other practical problem is that the proper connection pattern for a motor using this technology is unique to this specific starting method. No other starting technology can or does use it. That means that unless an installing electrician has been specifically trained on it or has access to AND is inclined to read and follow the printed instruction very specifically, they are more likely to make a mistake in connecting it, which could easily lead to soft starter and/or the motor damage. For that reason I never recommend it to anyone other than large OEMs with their own specifically trained service group such as the elevator industry or some refrigeration machinery manufacturers. To my mind it's folly to expect "Joe-Bubba the 'lectrician" to understand this somewhat complex issue and I have been proven right about that time and time again.

DickDV,
In reference to your comment about "the only reason..."
The elevator industry actually uses them for retrofit because their technicians are NOT necessarily savvy enough to reconnect a star-delta motor in straight delta, so they use this configuration (with the fault contactor) because the 6 motor lead connections (by specific design) are exactly the same as what they remove. The product they use is called a\n "elevator soft starter" and is different only in the very specific instructions and labeling that accompanies the devices which is matched to the original installation drawings of the elevators.

 

[waross]

I have a question about reversing. In a normal delta connection:
The end of the "A" Phase winding is connected to the start of the "B" Phase winding.
of the "A" Phase winding is connected to the start of the "B" Phase winding
The end of the "C" Phase winding is connected to the start of the "A" Phase winding.
Reversing all windings is not the same as reversing the line connections.
If you reconnect the end of the "A" Phase winding to the start of the "C" Phase winding, the end of the "C" Phase to the start of the "B" Phase winding, and the end of the "B" Phase winding to the start of the "A" Phase winding. will the motor reverse?
If this works, it would allow two, three pole contactors, inside the delata to reverse the motor.
Any suggestions from you fellows in rewind shops?
respectfully

 

[LionelHutz]

jraef

The extra risk is why I recomended a trip coil in the breaker.

The SCR's in line connected starters will see line-to-line voltage levels during acceleration so I don't consider the SCR voltage rating for the inside delta connection anything special. Using SCR's rated for less than 3x the line voltage is pushing the devices voltage rating with either connection.

As for the connections, the very commonly used Y-delta starter has the 6 motor leads coming back into the cabinet. Why would it be much different for a soft-starter? Put the 6 motor terminals on the soft-starter and properly label them and it's simple to wire. I've never seen anyone do it wrong yet.

At any rate, The answer to the origional post is that reversing the motor requires changing 2 of the line side connections.

 

[HCBFlash]

Well, I'm just a 'lectrician, but in my experience (getting systems installed and functioning correctly, and red-pen-ing engineering drawings and doc's), reversing connections to the T1-T2-T3 terminals at the starter, NOT changing the wiring of the motors T-leads, will work and work properly.

Many(most?) solid state drives I've worked with in the not too distant past will function and "rotate" the same regardless of the incoming phase rotation. It's as if there were a rectifier and inverter of some sort inside them things!!!

Please let us all know how this works out for you! and don't forget faq731-376

 

[baxtersdad]

The standard method to change direction on a six lead motor is to interchange two of the three phases. With six leads, you have to do it in two places. You would interchange T1-T2 and T6-T4. This will give you the result you seek.

 

[jraef]

HCBFlash,

Just FYI, what you are thinking of is a VFD, which does convert the AC to DC, then back to AC. So with those devices it's true, input phasing is irrelevent. Also, you would never need to use Y-Delta starting when you have a VFD anyway and the VFD can reverse the motor electronically, so essentially nothing about this thread was pertinent to VFD applications.

Solid State soft starters however do NOT convert, so the output phasing is always the same as input phasing. To complicate matters more, some older SS Soft Starters cannot have their input phasing reversed because of the way they do their line sensing.

 

[jraef]

The standard method to change direction on a six lead motor is to interchange two of the three phases. With six leads, you have to do it in two places. You would interchange T1-T2 and T6-T4. This will give you the result you seek.

Without checking the validity of this, why would you do it like that anyway? It would take 2 extra contactors instead of one.

 

[waross]

Hi jraef;
Hopefully this installation has a line contactor and not just a feed breaker and a soft starter.
If you reverse "Inside the delta", you may be able to use smaller contactors.
I understood the original post as asking if there was another way to reverse the motor without reversing the incoming leads.
That said, I don't like the idea of reversing "Inside the delta" anymore than you like using soft starters inside the delta.
Respectfully

 

[jraef]

Good point. In fact, I wasn't thinking right anyway. It's 2 extra contactors no matter what.

New job, big company, not enough sleep because of learning all the new three-letter-acronyms.

My appologies.

 

[wrsharper]

I did not submmit my real concern with inside the delta with all six motor taps connected to the bottom of the soft starter with the three straight through conductors hidden from the electrician.

I transfered to the electral side of the field from the electronic side 15 years ago after working in electronics for 22 years. My first electrican job had a jack leg electrician do a motor starter wiring prior to my arrival. I knew that contactors and motor starters had the "live" line side connected to the top of these devices and the "dead" load side connected to the bottom. All of the motor starters where I worked at had this correct configuration except for one that had the motor tap wires connected to the top of this motor starter and the line to the bottom. I was requested to reverse the motor direction to unjam a screw. I got a good hit from 480 when I tried to reverse the live bottom wires of this motor starter that was deenergized.

My point is that without an additional contactor to disconnect the soft starter a electrician may learn the hard way that three connections are live on the bottom of this type of inside the delta soft starter when the starter is not turned on. If a disconnect contactor is not used upstream of the soft starter then there should be a warning on the soft starter itself because this is new to most of us.

There is a move in industry and at my work place to make mechanics into jack leg electricians which in my opinion only makes this type of soft starter dangerous.

 

[jraef]

wrsharper,
While I share your concern, it must be mentioned that technically, it would be a major safety violation in just about any jurisdiction for anyone to have been working on any motor leads without a safety disconnect being open and locked open.

That said, what you experienced happens a lot anyway.