Motor Starter sizing

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Muhammad Umer ansar

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karachi pakisatan
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junior electrical engineer
I have been working on motor starters since few days. I want to know why instantaneous type circuit breakers are used instead of inverse time breakers in starters? and how to distinguish between them.
I m also confused of how to size the contactor and overload for the same
 
Muhammad Umer ansar said:
I have been working on motor starters since few days. I want to know why instantaneous type circuit breakers are used instead of inverse time breakers in starters? and how to distinguish between them.
I m also confused of how to size the contactor and overload for the same
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The thermal curve does not coordinate well with motor starting. The overload performs the overload function. The “breaker” is for short circuits. There are several versions. A magnetic only circuit breaker is called an MCP. It is adjustable. There is also a UL Tyoe F. This is not adjustable but the overload that is often built in is.

Sizing contactors depends on a couple factors. You need to know the application. If it’s plugging or jogging duty (bumping a lot), NEMA has a chart for upsizing, IEC calls this AC4. But with AC4 watch out for the number of cycles it is designed for. Often it is quite low so you may want to purposely “oversize” the contactor. For star-delta it is sized about 2/3rds of normal size since the current is split between two contactors, and the star is even smaller. Otherwise for “normal” cases with IEC use the FLC from the NEC chart and pick based on the voltage and the AC3 column in the catalog. With NEMA use the voltage and HP rating. The overload is sized according to the contactor size and motor FLA. Here you are looking for the current rating and the physical size if you intend on bolting it up directly to the contactor. Often these are available as matched sets. The heater sizing on eutectic overloads is fairly complicated. Just follow the procedure but these are becoming less popular. With electronic or bimetallic overloads you just pick one where the FLC is somewhat in the middle and set the overload to the motor FLA. Bimetallic overloads have about a 2:1 range and electronic ones have about a 5:1. On larger motors you also need to consider CT ratio and burden size you won’t be using an overload directly, and over about 200 HP upgrade to an advanced electronic model size the rotor becomes the thermal limit not the stator so you need stall protection.
 
zbang said:
In motor controls, the motor starter/controller protects against overload, the breaker protects against shorts/high-current faults. The simpler starters use a heater of some sort and they're often simply selected based on the starter size and and the motor power (HP or KW).

https://en.wikipedia.org/wiki/Motor_controller is a good starting point and you can see an example of the selection at https://www.alliedelec.com/m/d/69521217f3c85568364beb47865643ad.pdf
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Oh got it!
One more thing, if MCCB provides both Overload and short circuit protection then why we use separate Overload Relay in Star delta starter in combination with MCCB? Why cant we use Overload feature of MCCB instead of using separate overload?
 
Muhammad Umer ansar said:
Oh got it!
One more thing, if MCCB provides both Overload and short circuit protection then why we use separate Overload Relay in Star delta starter in combination with MCCB? Why cant we use Overload feature of MCCB instead of using separate overload?
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You can, if the overload feature of the circuit breaker just happens to match what you need. Since it rarely does you usually end up having separate overloads to protect the motor and the circuit breaker just protects against short circuit.

In fact there are motor protection circuit breakers available that have an adjustable overload and an unadjustable magnetic trip.
 
Muhammad Umer ansar said:
Oh got it!
One more thing, if MCCB provides both Overload and short circuit protection then why we use separate Overload Relay in Star delta starter in combination with MCCB? Why cant we use Overload feature of MCCB instead of using separate overload?
Click to expand...

Essentially all wiring has a common thermal damage curve but that curve is different for motors. So you can’t substitute. Different shape. And it has trouble with starting currents that look like overload.
 
petersonra said:
You can, if the overload feature of the circuit breaker just happens to match what you need. Since it rarely does you usually end up having separate overloads to protect the motor and the circuit breaker just protects against short circuit.

In fact there are motor protection circuit breakers available that have an adjustable overload and an unadjustable magnetic trip.
Click to expand...

That’s “not” a circuit breaker. When you say breaker that means it has a thermal trip set up for wiring and (UL 489) instantaneous trip. UL 1077 is the other type. Thus falls under the category of supplemental protection. I know...highly nuanced here. Blame UL. It’s called various things (motor circuit protector, manual motor starter, etc.) but it’s a UL Type F device. MCPs (magnetic only adjustable trips) are Listed as components only. They can’t be used under UL 508A (not on the list) or stand alone. They have to be part of a Listed assembly that is inspected such as an MCC. Very frustrating because true circuit breakers don’t coordinate well with motors but that’s the rule.

The overload is usually an outright bimetallic overload relay. I recently took one apart accidentally. The main part is clearly a magnetic trip only breaker. This one was made by LSIS. The overload relay was just a regular bimetallic type built onto the bottom, except it had a little mechanical arm that pushed the trip latch, rather than the typical relay contacts.

The one downside of these is that when it trips on most of them you can’t distinguish between a short (electrical issue) and overload (mechanical).
 
Motor protection circuit breakers are listed under UL508. Type E is just the MPCB. It has an adjustable overload trip and a short circuit trip. Type F is a type E with the addition of a contactor. The biggest downside I see to them is they are not listed for use on 480 V delta power systems. But, they can be used on 480/277 systems.

MCP are UL489 devices with just the IT. MCP can certainly be used in a UL508a listed panel since UL508a panels are a listed assembly, but it has to be used as part of an assembly blessed by UL. Used to be UL blessed a lot of drives with MCPs. Not so much anymore.

ETA: MPCB are also cursed in that size 00 are not UL listed but at least used to be commonly sold in the US, and there is some inconsistency in the AIC rating. Many are rated at 65 KAIC but there are a handful of part numbers in most manufacturers lines that are only rated for 30 KAIC at 480/277.
 
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