Starting Large 480 VAC motors DOL with Switchgear Breaker

[nhcf]

We are looking at project specs which are specifying new 480 VAC switchgear that will control several larger HP (125 - 300 HP) motors directly across the line using the switchgear breakers and not via an MCC with motor starters/contactors. I am more familiar with motor starter construction - what are the special considerations/specifications/options that should be considered when ordering LV circuit breakers that will be used to directly start larger motors? Are the breaker requirements the same as for non-motor feeders? Is motor OL protection normally just factored into the trip curve of the electronic trip unit?

What are the benefits of starting directly from the breaker vs. specifying a NEMA starter? Cost? Size? Other?

Thanks for any insights/references.

 

[dpc]

This is commonly done for facilities which run continuously. Large baseload power plants, in my experience, are often configured this way, putting all motors above 100 hp on low voltage power circuit breakers rather than motor starters. The biggest issue is the number of breaker operations. If the motor is being started 20 times/day, it would likely be better to use a large starter. Contactors are rated for many more operations than LVPCBs, at least on paper. The main argument against the large contactors is reliability. A quality LVPCB should be more reliable and less trouble-prone than the large contactors. It takes a lot of energy to pull in and hold those large contacts, whereas a breaker is generally going to be mechanically-held and energize to trip. A NEMA Size 6 starter is a monstrous, noisy, heat-generating beast.

The breakers will require a reliable source of tripping power, but if there is already a battery system, this isn't a big issue. The digital trip units in modern LV breakers can be configured to provide adequate motor protection. Motor protection relays could be added if desired. The breakers are generally drawout and can be quickly swapped out in case of failure.

I suspect the LV power circuit breaker approach will be more expensive than starters, and the switchgear requires front and back access generally. The circuit breaker approach definitely works - I designed hundreds of them back in the day.

 

[nhcf]

Thanks for your response, this is consistent with my thinking. All of the motor applications where I've seen this design have been older (50+ year) installations and it was not clear to me that it was still a prudent design approach. Loads will not be stopped / started frequently, and 125 VDC power is available for control power. I was most unsure of achieving required OL protection, which you've confirmed is can be achieved with ETU (or motor relay).

 

[che12345]

I fully agree with the learned advice by Mr. dpc.
1. In all cases the breaker is latched in closed position mechanically. where a contactor requires the coil to be energized continuously to hold it closed.
2. Although contactor can be fitted with a mechanical latch which latches on closed position, but it needs a short pulse to release the mechanical latch to unlatch. Otherwise the contactor will remain closed; the motor/load would automatically energized/run upon power resumption; which is prohibited (dangerous).
3. With the breaker installed with an under-voltage release, it can be wired to trip open automatically upon voltage failure; similar to a contactor.

Che Kuan Yau (Singapore)

 

[LionelHutz]

There really aren't any benefits to that approach, except it's "just" a circuit breaker.

125-300hp really isn't very large for motor starters.

The heat and power requirements of a contactor or soft-starter aren't that bad. The size 7 contactor we sell takes 1000VA to pull-in and 17VA to hold. You could power it with a 300VA transformer. A soft-starter would require a 2-300VA.

Power circuit breaker sections are magnitudes more expensive than MCC sections and they take up much more room.

There are MCCBs available with motorized operators, if you want a simple breaker on-off to control a motor. They would need an overload or protection relay.