Do I need a medium-voltage starter?

[adamscb]

Forum,

I'm designing the electrical system for a project in our plant. I plan on feeding a 4160v, 1250hp motor from a breaker inside 4160v switchgear (the sole use of this breaker is for the motor). My question is this: is there anywhere in the code that states I'm required to install a medium-voltage starter between the motor and the breaker? The breaker is of the thermal-magnetic type, so the motor is protected properly. Could I use the breaker as a means of starting/stopping the motor? Please keep in mind that I'm a new engineer out of school, so this is a legit question. I'm not trying to get around using a starter to save money or anything. Your input is appreciated.

 

[big john]

There's no such animal as a thermal-magnetic medium voltage breaker, so you need to double-check what the protection actually is on this circuit and evaluate whether the settings are suitable for that load.

That said, there's no reason a breaker can't be used in that application, but if this thing cycles frequently, as in every couple of days or more, it's very hard on the breaker and often leads to malfunctions. That's where a contact is valuable in that they can handle a far greater duty cycle than a breaker will.

Just be aware that whatever option you install, you need to have a method of putting a physical air-gap in the circuit because vacuum interrupters are not an effective means of safety isolation.

 

[templdl]

I've.

I've.

Forum,

I'm designing the electrical system for a project in our plant. I plan on feeding a 4160v, 1250hp motor from a breaker inside 4160v switchgear (the sole use of this breaker is for the motor). My question is this: is there anywhere in the code that states I'm required to install a medium-voltage starter between the motor and the breaker? The breaker is of the thermal-magnetic type, so the motor is protected properly. Could I use the breaker as a means of starting/stopping the motor? Please keep in mind that I'm a new engineer out of school, so this is a legit question. I'm not trying to get around using a starter to save money or anything. Your input is appreciated.

How would go about it if this were a 480v system?
Would you feed the motor using a branch aircuit breaker and then control and protect the motor using a combinatin motor starter.
Same with 4160v. You couid feed the branch circuit using a VCB but CV would accomplish it with a fused load interrupter switch which is less expensive. Then control and protect the motor with a MV motor starter such as an ampgard starter which has a vacuum contactor controlled be a solid state programmable relay. The vacuum contactor is capable of being racked out for isolation.
The are are a number of MV starter manufactures but I'm more familiar with the ampgard.

 

[Jraef]

What you are talking about is using a MV breaker as a Manual Motor Starter. If you can adjust the thermal trips into the required range for proper motor overload protection, it can be done. It's not a great idea however. Assuming a vacuum circuit breaker, they are not designed, mechanically, to be used like a contactor. But certainly, if you turn the motor on and off only a few times per month or year, not really a problem. I see it done all the time on large seasonal pumps that turn on in the summer and off in the fall.

There is also a phenomenon called "current chop" when using vacuum interrupters. Vacuum contactors, because they are DESIGNED for that purpose, are specially designed to minimize current chop, vacuum circuit breakers are not. So again, if turning the motor on and off frequently with a vacuum breaker, you might experience premature motor insulation failure n

http://www.danaherspecialtyproducts...Clark/Service_and_Support/JC Current Chop.pdf

 

[adamscb]

There's no such animal as a thermal-magnetic medium voltage breaker, so you need to double-check what the protection actually is on this circuit and evaluate whether the settings are suitable for that load.

That said, there's no reason a breaker can't be used in that application, but if this thing cycles frequently, as in every couple of days or more, it's very hard on the breaker and often leads to malfunctions. That's where a contact is valuable in that they can handle a far greater duty cycle than a breaker will.

Just be aware that whatever option you install, you need to have a method of putting a physical air-gap in the circuit because vacuum interrupters are not an effective means of safety isolation.

I disagree. There are medium voltage breakers out there that can supply both thermal (overload) and magnetic (short-circuit) protection. Hence the name thermal-magnetic breaker.

 

[Tony S]

Have a look at http://www.pbsigroup.biz/protection-relays.php?id=79

 

[big john]

I disagree. There are medium voltage breakers out there that can supply both thermal (overload) and magnetic (short-circuit) protection. Hence the name thermal-magnetic breaker.

That's just unusual terminology to describe MV protection.A true thermal magnetic breaker is a low-voltage molded case, so it's confusing to hear it applied to MV breakers controlled by relaying.

 

[petersonra]

I don't work with MV stuff all that often but most of it I have worked with used some kind of breaker with motorized operators on them instead of motor starters.

 

[zog]

I disagree. There are medium voltage breakers out there that can supply both thermal (overload) and magnetic (short-circuit) protection. Hence the name thermal-magnetic breaker.

Thermal is one type of overload protection, they do not mean the same thing. Magnetic is one type of short circuit protection, again, not the same thing. Thermal magnetic is a term used to describe the methods used in small LV molded case breakers. Larger (modern) LV molded case breakers and modern LV power breakers do not use thermal magnetic trip devices, they have not been used in power breakers for 40 years.

MV breakers do not have ANY internal protective devices, they rely on external inputs to tell them when to trip from relays.

Regarding your OP, you do not HAVE to use a starter but they exist for a good reason, to start motors. Breakers are not designed for that type of duty and while they can be used in that application they will have many failures and failed parts. My company repairs MV breakers as the core of our business and our customers that use breakers as motor starters usually have to send them in every couple years for complete overhaul and repair. (Hmmm, on second thought maybe I should encourage folks to use breakers as motor starters)

Also, as mentioned, using a VCB presents additional problems from chopping and prestrike failures.

 

[paulengr]

I do MV work all the time. Most is custom gear. I have used a MV vacuum breaker once as a starter. It works, but...I was using an SEL relay so doubling as a breaker/starter settings was no problem. Still had a fused disconnect upstream, too. Remember that if ONE phase welds shut, you can't detect it easily. The basic issue though is vacuum interrupters for breaker duty are rated like 3000 cycles and a crazy high interrupting current rating. The contactor duty interrupters are rated like 30,000+ cycles and a relatively low interrupting limit so typically they are paired with MV fuses and set to trip just as a breaker would.

Also an issue is whether they "latch" or not. Typical breaker service (outside of mining) is to latch even under power loss whereas contactor's do the opposite. The danger here is if you have a fault before the relaying has booted up from power failure and it gets you into using either DC battery chargers or capacitive trip devices, which again may drive a decision to use an under voltage trip. Its not well published but most microprocessor relays have enough "substation duty" capacitors in their switching power supplies to survive low voltage under fault or even total power loss for about 100 ms but its not enough juice to drive a trip coil, thus the CTD. That's the essence of the "controllers" you find in "real" starters. AB's own controller is available as OEM style and works well. Most others (Toshiba, Eaton, and variants) mostly use a small PCB relay board that frankly fails a lot.

So the "breakers as contactor's" worked but production liked to start and stop them A LOT, enough we were approaching the 3K limit on the interrupters but before we got there, lots of springs and PCB relay cards failed prematurely because the manufacturer hadn't done this kind of "accelerated testing" before. In the prior life, cycling was perhaps once or twice a week instead of multiple times per hour. And yes, they were wound rotor motors so starting current was managed. Lesson learned: creative solutions trying to meet crazy unrealistic delivery schedules is flirting with disaster.

 

[templdl]

I do MV work all the time. Most is custom gear. I have used a MV vacuum breaker once as a starter. It works, but...I was using an SEL relay so doubling as a breaker/starter settings was no problem. Still had a fused disconnect upstream, too. Remember that if ONE phase welds shut, you can't detect it easily. The basic issue though is vacuum interrupters for breaker duty are rated like 3000 cycles and a crazy high interrupting current rating. The contactor duty interrupters are rated like 30,000+ cycles and a relatively low interrupting limit so typically they are paired with MV fuses and set to trip just as a breaker would.

Also an issue is whether they "latch" or not. Typical breaker service (outside of mining) is to latch even under power loss whereas contactor's do the opposite. The danger here is if you have a fault before the relaying has booted up from power failure and it gets you into using either DC battery chargers or capacitive trip devices, which again may drive a decision to use an under voltage trip. Its not well published but most microprocessor relays have enough "substation duty" capacitors in their switching power supplies to survive low voltage under fault or even total power loss for about 100 ms but its not enough juice to drive a trip coil, thus the CTD. That's the essence of the "controllers" you find in "real" starters. AB's own controller is available as OEM style and works well. Most others (Toshiba, Eaton, and variants) mostly use a small PCB relay board that frankly fails a lot.

So the "breakers as contactor's" worked but production liked to start and stop them A LOT, enough we were approaching the 3K limit on the interrupters but before we got there, lots of springs and PCB relay cards failed prematurely because the manufacturer hadn't done this kind of "accelerated testing" before. In the prior life, cycling was perhaps once or twice a week instead of multiple times per hour. And yes, they were wound rotor motors so starting current was managed. Lesson learned: creative solutions trying to meet crazy unrealistic delivery schedules is flirting with disaster.

Great and to the point answer!