Motor Starter vs Thermal Overload Relay

[Mark.H]

Hi All,

Given the impressive amount of knowledge gathered in my first topic, I really wsnted to open a new one.

The broad question is:
What's the differenxe between Circuit Breaker, Motor Starter and Thermal Overload Relay when protecting a motor?


I can see that at first glimspse they seem to be doing the same work: disconnecting a circuit in the event of overload caused by an ovee current. However, whilst the first one has fixed ratings, the other two can be adjusted or calibrated more specifically according to the load. Also, I can see that, regardless, upstream a Motor Starter (and contactor) OR a Thernal Overload Relay (and contactor), I would always have a Circuit Breaker for safe disconnection, Although I have seen cases where the configuration Motor Starter+Contactor was no protected by Circuit Breaker (no idea why).

So, when is it safer (or appropriate) to use a Motor Starter + Contactor and when a Contactor + Thermal Overload Relay?
Youtube and random websites are not clear about thiz matter.
Thank you All

 

[FreddyNurk]

Circuit breaker has contacts rated to break the prospective fault current to be expected. Generally can't close by itself, unless fitted with additional accessories.
Contactor generally only has contacts rated to break load current, not fault current. It is opened and closed by an external device, including a thermal overload.
A thermal overload is a device that provides a signal or indication of when an overload condition is reached, to be able to take some action (generally to open a contactor).

Its possible to blur the definitions by using a circuit breaker with a thermal element, to allow opening and closing via the circuit breaker using a motorised device, as well as to use a contactor with an upstream fuse.
Some newer devices are all in one, they offer fault current breaking capability, adjustable overload settings and a means to switch the motor on and off. Generally they're rated appropriately for all requirements.

edit: To add to that, motor starter in that context is likely a contactor plus the user interface to start and stop the motor. Some overloads also offer additional protection such as loss of phase, some don't.


EDMS Australia

 

[waross]

A circuit breaker protects a circuit.
There are two aspects of protection:
Overload and short circuit.
Typical breakers are thermal-magnetic devices.
The thermal trip has an inverse time characteristic.
This provides overload protection and allows short time overloads.
The greater the overload the faster the breaker will trip.
The magnetic trip provides instantaneous tripping in the event of a short circuit.
It will typically become active at 5 to 10 times the rated current.
That is; A 15 Amp breaker will trip instantly at between 75 to 150 Amps.
That is not to suggest that the trip point is that variable but rather that the manufacturer will have chosen a setting between 75 Amps and 150 Amps.
A motor starter is a device that is used to start and to protect a motor.
It includes a thermal overload device of some sort.
It may be manual or automatic, using a magnetic contactor.
Many small motors, freezers and refrigerators have an internal thermal trip. These are motors that are generally cord connected and may be controlled by a switch or thermostat but do not have a motor starter as such.
Back to the starter.
The starter will have a thermal overload relay if it is a magnetic starter.
A manual starter will have a thermal trip that trips the switch off directly, similar to a circuit breaker with some differences:
The overload trip will be adjustable.
There are several methods.
Some are adjustable over a range of motor currents.
Some use bi-metal strips. These may be heated by interchangeable "heaters" for different current ranges.
At one time, melting alloy overloads were common. Each heater was good for a range of only a few Amps.
Electrical shops in industrial plants would have a cupboard full of assorted rated overloads.
At one time magnetic solenoid coils with oil filled dash-pots were used for overload protection but I haven't seen one of those for about 30 years now.
Electronic overload relays are becoming the standard now.

I mentioned the two functions of a circuit breaker, overload and short circuit protection.
Short circuit protection must be provided ahead of every circuit or feeder.
For motor circuits the overload protection may be provided at the starter end of the circuit.
For motor circuits, over rated breakers may be used to allow for motor starting surges.
The oversized breaker will provide short circuit protection.
The overload device will provide the circuit with overload protection.

Combination starters and MCC buckets.
A combination of a Motor Circuit Protector and a motor starter in one enclosure or MCC bucket may be used if the combination has been tested by the manufacturer.
You must buy these as a complete unit.
You may not field assemble one of these.
The Motor Circuit Protector, MCP, looks like a breaker but it does not have a thermal trip.
It has a magnetic trip for short circuit protection and overload protection is provided by the overload relay in the starter.

The codes refer to Over-Current and Over-load protection.
Over-current protection is short circuit protection. I have used the term short-circuit protection to lessen the chance of confusing you with similar terms.
Magnetic motor starters.

The upper part is the contactor and the lower part is the overload relay. The starter in the center does not have the heaters installed. It uses the melting alloy type of heater.

Here is a single phase manual starter with a melting alloy heater installed.

Here is a three phase, manual starter.

I have just skimmed the surface here.
There are a lot of different types and features.
Others may add more.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

In the UNITED STATES, the term "starter" describes a motor starting device that includes motor overload protection. So a CONTACTOR plus an OVERLOAD RELAY is a STARTER. If a circuit breaker or fused disconnect is provided in the same assembly it is a COMBINATION MOTOR STARTER. If you have just a contactor without an overload device, it is not a STARTER, at least in the US.

It is possible to use a thermal magnetic MCCB or a low-voltage power circuit breaker as a motor control device as well. There are also MANUAL MOTOR STARTERS used on small motors that look much like a standard light switch but incorporate a motor overload device.

Outside of the US, I have no idea.

 

[waross]

I agree David (dpc).
The same in Canada.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Shanaya]

The NEC electrical code includes thermal overload safety for all 1/3 HP and above motors, so circuit breakers and fuses need to be oversized to allow the motor to start up current, which is around 6 times the full-load current. We are permitted to over-size the breaker or the fuse so as not to have any nuisance or blown fuse.

But now the breaker is too big for the cable, and it's terrible! This is where protection against thermal overload (heaters) comes in. They are only above the motor 's full-load running amps, normally 125 per cent. They don't worry about high starting amps, but they have the ability to respond and shut down the overloaded motors. And that's the key: to shut down the overloaded motors until the motors burns.

Smaller motors use the Klixon switch, same use, but without the motor starter combo.

Bi-metallic strip heaters are not affected by high ambient heat, some of them are customizable.

I love thermal overloads. Ok, I think they're interesting.

 

[Mark.H]

Thank you all for your answers. I made up my mind on what I need to do for my project