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Benefits of motor starter contactors in medium-voltage applications

With both power circuit breakers and contactors able to do the work of starting and stopping a motor, which option makes the most sense?  Here's an example of how applying a fully rated 800 A starter instead of a 1200 A circuit breaker, not only right-sizes the equipment for the application but also translates to other benefits: reduced footprint, lower chopping currents, higher endurance, and faster clearing times. 

Full-voltage starters are the most commonly used method of MV motor control.  Also known as across-the-line starters, this equipment applies line voltage to the motor in a single action via contactors rated to connect and disconnect motor in-rush currents.  An assembly of motor controllers under a common bus is often called a motor control center (MCC).

On the other hand, a MV circuit breaker is a power distribution protection equipment designed to interrupt high levels of short circuit current.  While these devices are commonly found installed in assemblies called switchgear, at times, they've been applied in motor control applications.

Let’s look at common application considerations when choosing a motor starting method including: size of equipment, endurance, impact on other equipment, and safety. Here is the upshot—the motor starter (contactor + overload relay) solution comes out substantially ahead in each of these parameters. 

The optimum way to start medium-voltage motors

With the ability to pair a fully-rated 800 A starter with an 800 A induction motor, there’s an opportunity for an optimum solution to starting and stopping MV motors. Applying a fully rated 800 A starter instead of a 1200 A circuit breaker, not only right-sizes the equipment for the application but also translates to other benefits: reduced footprint, lower chopping currents, higher endurance, and faster clearing times. Using an electrical motor starter with a contactor provides significant advantages:  

  • One-third less footprint and clearance requirements
  • Lower chopping current mean reduced likelihood of adverse impact on downstream equipment
  • 20 times higher mechanical and electrical endurance and 50 times operations between maintenance intervals
  • Faster clearing time which further enhances safety
MVCA 800A left 600x925

Reduced footprint, clearances and front accessibility

Presently used MV MCCs, measure about 36 inches wide by 30-inches deep and only require a front clearance of 36 inches.  They are front accessible and commonly available in a single-high 800 A or a two-high 400 A starter configuration, which can be installed against a wall, back to back, or even in a corner.

Medium-voltage switchgear on the other hand, require rear access, are 96 inches deep, and have clearance requirements of 70 inches at the front, 32 inches on the side, and 36 inches at the rear.  This increase footprint and additional access requirements can become costly, particularly when the equipment is installed in locations such as prefabricated electrical houses where the cost per square-foot is very significant.  

For example, a typical MV MCC lineup with ten 400 A starters will measure 180” in width and would require about 83 square-feet of floorspace, including front clearances.  Utilizing circuit breakers in MV switchgear for the same application, would more than triple the overall required footprint and clearances to 253 square feet.  This increase in footprint, clearances, and overall cost is not necessary specially for motors below 6000 horsepower (HP), which can be controlled by a fully-rated 800 A contactor-based starter.

MVA to MVC with cable tray above 600x400
Medium-voltage switchgear close-coupled with medium-voltage motor control

Lower chopping currents reduces adverse impact on motor

When a vacuum interrupter opens, the level of current produced by the arcing between the interrupter’s contacts almost instantly falls to zero instead of following a nominal current zero decrease of the fundamental 60 Hz waveform.  This sudden interruption in current is defined as current chop and depending on its magnitude and frequency, it can cause serious insulation degradation over the lifetime of a motor.

By design, vacuum interrupters in contactors have a lower interrupting rating and are constructed to operate more frequently than the ones used in breakers. Vacuum interrupters in contactors are manufactured utilizing different metallurgy, which allows them to exhibit a lower chop current (about 1 Amps) than the chop current produced by a breaker interrupter (about 5 Amps).  See below.

MVCA chop current 600x291

1 and 5 ampere chopping current waveforms

Depending on the characteristic impedance of the electrical system, the transient voltage created by the chopping current could be detrimental to equipment insulation.  The waveform image presents the transient voltage created by a 1 and 5 amp chopping current in a simulated circuit with a characteristic impedance of 3,000 ohms.

As shown, the 1 per unit overvoltage condition created by the 1-amp current chop of a vacuum contactor, is well within the rating of most MV insulation systems and will not compel design engineers to use surge arrestors. Vacuum interrupters in circuit breakers on the other hand, have a 5-amp current chop increasing the voltage transient to levels above four times the system voltage. These higher levels of transient voltages often require the use of surge arrestors as a mitigator to protect insulation systems.

MVCA transient 600x291
1 and 5 ampere voltage transient waveform (Z0 = 3000 ohms)

What is a contactor?

A contactor is an electrically controlled switch used for switchging a power circuit with a lower power level control circuit.  Learn from our experts at Eaton's Experience Center to understand contactors, how they compare to circuit breakers and the role they play in motor starting and other applications. 

Higher mechanical and electrical endurance

Motor starters using contactors have extended mechanical and electrical life compared to breaker-based starters. The impact here is substantial. For motor starting applications, contactors can require little, if any, maintenance for decades. In contrast, breaker-based solutions for motor starting could require repeated maintenance  beginning within a year of operation.   

As seen in the table below, vacuum breakers have a mechanical endurance of 10,000 operations with a manufacturer recommended maintenance period of 500 operations.  Vacuum contactors are commonly rated for 25,000 electrical operations before maintenance is recommended and a total electrical life up to 300,000 operation.

Breakers are constructed to stop high levels of current in the event of a fault.  They are excellent interrupters, and as such, they are better suited for power distribution applications instead of motor starting applications where the number of operations significantly increases.  For example, an application where only four motor starting operations are required per day, would lead to maintenance periods of 4 months if a breaker-based controller is applied.  Using the same scenario, we find that the contactor will not require any recommended maintenance for more than ten years.  Breakers can be used in this context, but it’s important to understand manufacturers’ maintenance recommendations in order to implement maintenance schedules that accommodate for this usage. 

Vacuum breaker and contactor rated operations and maintenance

Device

Rated electrical operations

Recommended maintenance operations

Vacuum breaker

10,000

500

400 A vacuum contactor

300,000

25,000

800 A vacuum contactor

200,000

25,000

Enhanced safety: faster clearing times and lower let through currents

Breaker-based motor controller short circuit protection is usually provided by the instantaneous overcurrent setting of the motor protection relay.  Although this type of protection operates with no intentional time delay, it is important to note that inherit delays exist due to relay and breaker operations or total clearing time.   

To obtain the relay operating time, the protection engineer must account for the relay’s output contact and instantaneous protection operating time.  Relays typically have an 8msec contact operation time with a 30 msec maximum pick up time.  This means the relay’s total operating time is about 38 msec or about 2 ¼ cycles.  Once the relay picks up and closes its output contact to trip the breaker, it will take an additional 50 to 83 msec (3 to 5 cycles) in order to fully open its contacts and clear the fault.  Adding both the relay’s and the breaker’s operating times, the total clearing time is 88-121 msec (5-7 cycles).

Class E2 motor controllers utilize a main contactor to make and break load and overload currents, in addition to MV current limiting fuses for interrupting fault currents that exceed the breaking capacity of the main contactor.

Most 400 A MV contactors have interrupting ratings between 6,000 A to 8,500 A; and 800 A MV contactors have interrupting ratings of 7,200 to 12,500A. In order to obtain higher interrupting ratings, current limiting fuses are supplied as backup protection to interrupt and limit short circuit currents higher than the contactor’s rating. The motor starter design must ensure the contactor does not open above its interrupting rating, and instead allow the fuse to clear this fault.

The table below tabulates the minimum and total clearing times as well as the let-through current of common fuses utilized in 400 A and 800 A MV motor starters.  As seen on this table, the higher the levels of prospective short circuit current at the fuse location, the faster the fuse takes to clear and limit the let through fault current feeding the fault.  This current limiting feature clears a fault within ½ cycle or at least 10 times faster than a breakers’ instantaneous protection, greatly reducing the amount of arc-flash energy produced during a fault making it safer for its users.

400 A and 800 A MV starter fuse characteristics

Prospective SC Current RMS (A)

400 A - 24R

800 A - 44R

800 A - 57X

Minimum Melt (cycle)

Total Clearing (cycle)

Peak Let Through (A)

Minimum Melt (cycle)

Total Clearing (cycle)

Peak Let Through (A)

Minimum Melt (cycle)

Total Clearing (cycle)

Peak Let Through (A)

12,000

2

2.6

33,000

15

18

N/A

18

60

N/A

15,000

1

1.2

35,000

6

9

N/A

4.8

16

N/A

20,000

Let through region

< 0.5

37,000

2

4

N/A

1.4

4.7

N/A

25,000

Let through region

< 0.5

40,000

1

2

N/A

0.6

2.4

N/A

30,000

Let through region

< 0.5

43,000

Let through region

1

N/A

Let through region

1.5

76,000

35,000

Let through region

< 0.5

45,000

Let through region

0.9

82,000

Let through region

1.1

80,000

40,000

Let through region

< 0.5

47,000

Let through region

< 0.5

87,000

Let through region

0.8

83,000

45,000

Let through region

< 0.5

50,000

Let through region

< 0.5

90,000

Let through region

< 0.5

86,500

50,000

Let through region

<0.5

51,000

Let through region

<0.5

93,000

Let through region

<0.5

90,000