OT: A Star-Delta wiring for 6-lead motor

A 45 KW motor (440Vac, 3Ø, 60Hz, IFL=73.2 A) is having a contactor (for star & delta) rated at 22KW @ 440Vac and 1-set-of-14 mm2 THHN. (See attached picture).

Clearly the contactor & the wires are undersized, which leads to the conductors being heavily hot during continuous operation.
I think that wire must be at least 22 mm2 or 30 mm2 and the contactor must definitely be replaced.

However, since the circuit breaker is 150AT, should I use a much bigger wire size (say 50 or 60 mm2)? Since the CB is 150AT, I am thinking of using a wire size that matches that ampacity. Also, if this will be the case, I need to change the main feeder.

What do you think?

If the contactor is only rated at 22kW and the motor is rated 45kW.. ya better think twice or pay your insurance premium.
Use the NEC for sizing of starters (contactors), conductors, overloads etc..

[FONT=Arial, Helvetica, sans-serif]Part I. General Requirements[/FONT]

• [FONT=Arial, Helvetica, sans-serif]Full load currents. Use the motor full load current rating (see Tables 430.147, 430.148, or 430.150) when determining conductor ampacity [430.22], the branch-circuit short-circuit and ground-fault protection device [430.52 and 62], and the ampere rating of switches [430.110]. Do not use the current rating on the motor nameplate for this purpose (Figure 430-2). [/FONT]

[FONT=Arial, Helvetica, sans-serif]Note: Graphics not part of this newsletter, they are contained in the textbook, Understanding the National Electrical Code.[/FONT]

• [FONT=Arial, Helvetica, sans-serif] Motor nameplate current rating. Use the motor nameplate current rating when selecting devices intended to protect motors, motor control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start [430.31].[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Motor controller terminals. Connect motor controllers and terminals of control circuit devices with copper conductors, unless identified otherwise. Torque motor control conductors 14 AWG and smaller at a minimum of 7 lb-in. for screw-type pressure terminals, unless identified otherwise. See [430.9], [110.3(B)], and [110.14 FPN].[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Motor locations. Locate motors to facilitate maintenance and provide adequate ventilation [430.14].[/FONT]

[FONT=Arial, Helvetica, sans-serif]Part II. Conductor Size[/FONT]
[FONT=Arial, Helvetica, sans-serif]Some people misapply Chapter 3 ampacity tables when sizing motor conductors, and therefore undersize the motor conductors. Motors have inrush current, which conductor sizing must allow for.[/FONT]

• [FONT=Arial, Helvetica, sans-serif]Single motors. Per [430.22], size motor branch-circuit conductors no smaller than 125 percent of the motor FLC rating listed in Table 430.147 or 430.148 (Figure 430-4). Size the branch-circuit short-circuit and ground-fault protection device per 240.6(A) and 430.52(C)(1) Ex. 1.[/FONT]
• [FONT=Arial, Helvetica, sans-serif]Multiple motors. Per [430.24], size multiple motor conductors as follows. First, multiply the full-load current rating of the highest-load motor by 1.25. Then, add up the full-load current ratings of all the other motors in the group. Add these two numbers. That's your motor load for calculating ampacity. Add any other loads on that conductor, to calculate total conductor ampacity[/FONT]

[FONT=Arial, Helvetica, sans-serif]Source: http://www.mikeholt.com/[/FONT]


[FONT=Arial, Helvetica, sans-serif]He is a recognized authority.[/FONT]

LWCiak,

I will check that out.

By the way, why does the wire multiplied by 1.25 & the protection by 2.5? I have long been wondering.

Just in case you didn't know in a star delta 6 lead configuration the phase currents on the conductors will be motor full load amps / 1.732. So in your case you will see 42 amps on the conductors at 100% load. But that said it works out to 25kW using the general 0.9 cos theta / eff contant, so I agree you are right to be concerened.

theripley said:

That is not the case when metered with ammeter. The reading is 78A.

Click to expand...

That is on the main feeder cables.

The current on the cables to motor will be as HJTRBO said.

Zimmer said:

That is on the main feeder cables.

The current on the cables to motor will be as HJTRBO said.

Click to expand...

Yes, certainly the case. Sorry for the wrong information. I actually deleted my post.

HJTRBO said:

But that said it works out to 25kW using the general 0.9 cos theta / eff contant, so I agree you are right to be concerened.

Click to expand...

What do you mean by this statement?

3 phase power formula for a motor is P(watts) = Vline x Iline x power factor x efficiency x 1.732.
Example: Your motor with 0.9 pf (assumed) and 0.9 eff (assumed) = 440 * 73.2 * 0.9 * 0.9 * 1.732 = 45.2kW
Your star and delta contactors = 440 * 42.3 * 0.9 * 0.9 * 1.732 = 26kW
In your case I just fudged the power factor (cos theta) and efficiency to a general number of 0.9 that I use these days to give me a general ball park figure. You know, the 22kW contactors may theoretically be correct. But as you are describing the entire set up is running at the thermal limit. Which may be ok, but if you are uncomfortable with it then upgrading it won't do any harm. I did this a few years ago to an Atlas Copco air compressor. They customer wasn't happy with the thermal imaging results even though they had been consistent over a couple of surveys. Upgraded the wire and contactors to the next size up and she ran cool as!

I agree with HJTRBO.
The contactors are OK.

The Line and Delta contactors should be rated for 22KW (0.58 IFL) and the Star contactor should be rated for 15 KW (0.33 IFL).

A 22 mm2 cable should be OK (how long is the cable??)

nonuke said:

I agree with HJTRBO.
The contactors are OK.

The Line and Delta contactors should be rated for 22KW (0.58 IFL) and the Star contactor should be rated for 15 KW (0.33 IFL).

A 22 mm2 cable should be OK (how long is the cable??)

Click to expand...

Estimated length is 50 m, including rise/ drop. The current cable installed is 14mm2, which has ampacity of 70A. Reviewing, I think 14 mm2 wire is okay. I am just wondering why it is heating at an alarming temp. I can't seem to figure out.

HJTRBO said:

You know, the 22kW contactors may theoretically be correct. But as you are describing the entire set up is running at the thermal limit. Which may be ok, but if you are uncomfortable with it then upgrading it won't do any harm. I did this a few years ago to an Atlas Copco air compressor. They customer wasn't happy with the thermal imaging results even though they had been consistent over a couple of surveys. Upgraded the wire and contactors to the next size up and she ran cool as!

Click to expand...

Perhaps I can justify it with,"Well, according to the computation is must work fine. Maybe it has something to do with the quality of the wire or the contactor. I think it's better to upgrade it to the next size up." What do you think?

Reduced voltage motor start

this is a reduced voltage starter (star/Delta

check the 'AC' rating of your contactors.
with out looking at a manual
the Star 22Kw rating is AC3 rating but it is switching @220V Star configuration and only when starting the Main contactor is the Load device.
However if you look at either the AC2 or AC1 rating you will find that 45Kw is fine

the Delta Contactor can also be 22KW @ AC3.

at the end of the day ALL 6 wires are broken at the same time when either a stop or a fault.

this is common place with all reduced voltage starters

As has been correctly pointed out, there is nothing incorrect in your component selection (maybe not in the wire either, but i am in N. America so we dont use metric wire sizes).

It may be that the conductors are heating up because someone has extended the transition time too long and the motor is failing to accelerate properly. Star-Delta starting is inflexible in that there is only one amount of current "reduction" (which is debatable in and of itself). In Star, the starting torque and current is reduced to 33% of normal. In the case of current, that is 33% of Locked Rotor Current, which is usually 600% of FLC, so starting current is still 200% of FLC. In the case of torque, that is 33% of Locked Rotor Torque, which is typically 160% of Full Load Torque, so the starting torque becomes about 53% of FLT. So in essence you get half of the motor FLT at twice the motor FLC. If the low starting torque is insufficient to accelerate the load to about 90% speed before transitioning from Star to Delta, the current still spikes to AT LEAST LRC when the transition happens, often significantly MORE, because of a phase shift that takes place which can put the line current out of phase with the motor back EMF. Current spikes of over 2000% are theoretically possible. When that happens, people often attempt to lessen that transition spike by using the ONLY tool available in a Star-Delta starter, the transition timer. They think that by increasing the time before transition, the current will be lower, when in fact it cannot, it can ONLY be higher. There is NOTHING you can do with a Star-Delta starting scheme to change the current values, that is the problem with it. So if it does not work, your only option is to throw it out and try something else. That is why I NEVER specify them. I prefer solid state soft starters, they can always be adjusted to some value tht will work (if properly selected and applied).

In your case if the conductors are over heating and that is your only concern at the moment, make sure your transition timer is set to no more than about 1-1/2 seconds. If your current is still too high after that, then the problem is that this is a misapplication of a Star-Delta starter and there is nothing you can do with it to change that.

what temperature are your cables
take an infrared photo of the system

@jraef it is fine to not use them but you still must use contactors with soft starters. including possibly a bypass contactor

What does worry me is your picture
the DELTA contactor is actually the Main contactor
the STAR contactor is actually the DELTA contactor
where is the STAR contactor

as far as the Motor circuit breaker
what is your wiring standard for 73 amps and 150 amps

iant said:

what temperature are your cables
take an infrared photo of the system

@jraef it is fine to not use them but you still must use contactors with soft starters. including possibly a bypass contactor

Click to expand...

Maybe, maybe not. It doesn't affect the more infinite adjustability however, which is the point of using them over Star-Delta.

What does worry me is your picture
the DELTA contactor is actually the Main contactor
the STAR contactor is actually the DELTA contactor
where is the STAR contactor

as far as the Motor circuit breaker
what is your wiring standard for 73 amps and 150 amps

Click to expand...

You can do Star-Delta with only two contactors, it is done that way in the elevator industry all the time. It is inherently dangerous in my opinion, but perfectly possible. What we think of as the "Main" contactor is eliminated, those 3 conductors go directly from the breaker to the motor, so they are live all the time. But because of at least one of the other contactors being required to complete the circuit, the motor does not run until one of them closes. OEMs often do this just to save the cost of the third contactor, making the argument that it is not unsafe because you are required to open the breaker and lock it before servicing anything anyway. I still hate that scheme.