Motor starter or just a disconnect?

[mike9999]

Newbie question:

Still confused on this matter.

Why does some motors need starters and some just a disconnect. How do you know when a motor needs one?

 

[bphgravity]

All motors have a controller and require a disconnecting means (save for a few exceptions). The type of controller and/or disconnect (one device can serve as both) is purely a design consideration as long as it meets the specific issues outlined by the code such as location, rating, ect...

 

[mike9999]

Clarification

Clarification

I know all motors need a disconnect. Except for some exceptions that you mentioned which I am not aware of. My question is more on when do we need a starter and when we do not need a starter. Bigger motors I would suspect most likely need one, but where do we draw the line sizewise, designwise, etc.

 

[ptonsparky]

This depends again on the application and the customer. Single motors under 5hp that are used occasionally, manually started we may use a fused disconnect. Frequent starts, 480V, and larger hp we use starters because they will last longer and IMO are safer.

 

[cpal]

Try reviewing part VII of article 430 for motors (disregarding article 440)

Starters are in a sence a controller (predetermined sequence of events)

430.81 (A) allowes the BC OCPD as the controller 1/8 Hp or less

(B) allows cord cap and receptacle as the controller 1/3 Hp or less.

A disconnect will function to some degree as a starter on/off

but it may not fullfill all of the requirments of a controller From a Code prospective under some conditions a disconnect can be the starter (minimum requirments) As Byran mentioned it is a design consideration and not all situations will apply equally.

 

[FNCnca]

Are there disconnects that are listed as load break devices or motor starters?

 

[Bob NH]

Are there disconnects that are listed as load break devices or motor starters?

Contactor = Starting Relay without overload protection or disconnect

Starter = Contactor + Thermal Overload Protection

Combination Starter = Starter + Disconnect (usually with overcurrent protection)

If you have a small motor that contains its own thermal overload protection, you can use a contactor without the overload protection. You still need a disconnect and OCPD.

I have seen some manual starters that include manual disconnect, magnetic trip overcurrent protection, and thermal overcurrent protection all in a "block". An example is the Telemechanique GV2ME series that goes up to 63 Amps in the largest size.

 

[FNCnca]

From a Code prospective under some conditions a disconnect can be the starter (minimum requirments) As Byran mentioned it is a design consideration and not all situations will apply equally.

I guess my last post was more directed at the quote. I did not think that any disconnects were listed as starters.
I do know that in my company, you better not use a disconnect as a load break device except in the case of an emergency.

 

[mike9999]

Thread is very informative. Thanks.

 

[Jraef]

I guess my last post was more directed at the quote. I did not think that any disconnects were listed as starters.
I do know that in my company, you better not use a disconnect as a load break device except in the case of an emergency.

To answer your earlier question (and this one), No, a disconnect cannot be listed as a "starter" because of the definition of a "starter", but Yes there are disconnect switches that are rated for motor starting duty. Or rather, most disconnects are so rated, although the rating may be for a motor load far smaller than the amp rating of the disconnect. In most catalogs for disconnect switches they will provide you with an amp rating, and then separately, a motor load rating in HP or kW. There are a few really cheap disconnects that do not have a motor rating, but they are actually somewhat rare because it just means the manufacturer was too cheap to even try!

That said, just because you CAN do it, doesn't mean it is OK to do it. You still need to have ALL of the necessary protections in your motor circuit, including running overload protection. That can be very difficult to attain on larger motors using just fuses and a disconnect. An Overload Relay provides far superior protection, but needs something that it is capable of opening in order to work. An OL relay cannot open a disconnect switch, but it can drop out the electromagnetic coil of a contactor. So the combination of an OL relay and a contactor is called a "starter". Once you have the electromagnetically controllable contactor in the circuit, you can also use it to start and stop the motor through other automatic means, i.e. automation.

A "manual motor starter" still has the overload relay and can disconnect the motor under an overload condition, but it does NOT include the ability to automatically turn a motor on by external means, hence the term "manual". They are also typically limited to small motor controls where the overload disconnect action can be accomplished using springs and such. Above about 50HP, the mechanical forces necessary to keep the contacts from forcing each other apart under normal load make mechanical motor starters difficult to make and use.

 

[mkgrady]

Newbie question:

Still confused on this matter.

Why does some motors need starters and some just a disconnect. How do you know when a motor needs one?

If the motor has internal thermal protection it does not need a starter. If the motor is thermally protected it will say so on the nameplate. If the motor gets too hot it will shut itself off.