Motor controllers
- Thread starter Ksm
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I believe if there are no motor overload protective device in the motor than a controller is necessary.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
You are required, per Article 430, to have a means to protect the motor and circuit, and to disconnect it from the line. You also SHOULD have a means of controlling the motor, but it's not required. But whatever you do use, then has to follow the rules in 430. For example, if you use the disconnecting means as the way to control it, the disconnecting means must be rated to start and stop motors. Everything else beyond that is subject to the specifics of the motor, installation and desired outcome.
Also, what would be your perception of a motor controler? It could be a simple as a manual starter toggle switch type with an overload or a mechanical push button with an overload.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Yep, as long as whatever it is, it has a motor start/stop rating equal or greater to the motor being controlled. Even many of the little "ice cube" relays have some sort of motor rating. It just needs to have it shown on the label.
kwired
Electron manager
- Location
- NE Nebraska
A motor controller also is not required to open all lines to the motor, just whatever is necessary to interrupt current flow. Interrupting two of three phase conductors is sufficient to interrupt all current in a typical three phase motor, or interrupting one line in a 240 volt single phase motor circuit also gets the job of controller done.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
As long as there is still unning overload protection in all ungrounded conductors.
Agreed,however, I remember way back when ,when only two overloads were required.I believe it changed to all 3 phase's in the mid to late 60's,but you know I can't remember why.Thinking about the OL's being located between the main starter contacts and the motor winding what would it hurt.Two of 3 phase would be open so single phasing couldn't occur,,wait can it single phase on 277V? If so that might be the reason,thinking maybe a failed main contact in the phase without an OL could let it single phase on 277V,ut then I'd need a ground fault on one of the other phase's at the same time,,,,okay need a smart guy to bail me out of this memory failure one more time.
dick
Open Neutral
Senior Member
- Location
- Inside the Beltway
- Occupation
- Engineer
Assume a short to ground on the unprotected phase. What protected the building & occupants?
I guess they better have good rubber soled shoes and can run fast.
kwired
Electron manager
- Location
- NE Nebraska
I'm not really sure the reason. I don't believe short to ground is the reason for a couple reasons.
1. only one overload is required for a single phase motor, if there would be a short to ground in the unswitched lead it would bypass overload device.
2. overload protection for magnetic starters does not open motor circuit it opens the control circuit. If there would be a short to ground in an unswitched lead the overload sensing device will still open control circuit but will do nothing to open that unswitched lead - outside of burning up the element in the overload device if current level should get high enough to do so.
Putting overloads in all three phases does provide better protection.
The concept of only providing two overload elements worked fairly well. If motor is receiving good voltage on all three phases and is overloaded they will protect it. If you lose one phase the current in the other two will go up on most loads and the motor is still protected. If you lose two phases the current stops flowing just like it does with a controller that switches two lines.
That concept still applies for the situations where fuses provide the overload protection. If you blow one fuse - motor will single phase, current in remaining two phases will go up and a second fuse will blow which stops the motor, but with one lead still energized.
ptonsparky
Tom
- Occupation
- EC - retired
I just put in an adder for 4 exhaust fans and dampers on a project. I figured starters, disconnecting means, the whole bit. Dang. They show up with thermal protection and disconnects. I feel so guilty. Dampers start the fans after they become full open so that was a bit different from smaller units I have done in the past
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
It is about shorts to ground. in the olden days (when I started in this industry) when only two phases of 3 were required to have OL protection, there were a lot of fires caused by arcing ground faults in motor circuits because the ground occurred in the unmonitored phase. If the resistance was high, the added current on the other 2 phases may not be enough to cause them to open the control circuit or to cause the breaker to trip or the fuse to blow. I remember when the change happened (1978 or 79 I believe), there were a number of white papers put out by the motor control mfrs about the need for the change. Many, like AB, Sq D. and Furnas took the opportunity to completely redesign and modernize their starter designs, most of which are the ones we still see today. The others just added a 3rd pole to their OLR for a while and redesigned later. I don't think GE ever did though.
Re:#1: If it is 115VAC single phase, you only need one OL device, because the other leg is already "grounded", being the neutral. If it is 230V single phase, you DO need 2 OL devices, one in each UNGROUNDED conductor. You only need to SWITCH one leg, but you need to MONITOR both legs for OL.
kwired
Electron manager
- Location
- NE Nebraska
Table 430.37 only requires one overload unit for single phase motors. It does require it to be in the ungrounded conductor if there is one conductor that is grounded.
I can see some validity for the change to three units based on what you have mentioned. I still think that would be pretty rare circumstance, and could even happen with three elements. Most of the time a ground fault is taking out the fuse or circuit breaker, unless there is a poor equipment grounding conductor. If you have a fault mid winding - it may be more useful, but your motor is toast anyway, it is just a matter of limiting fault current traveling through grounding paths for extended time more than protecting the motor.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Huh, funny how the mind works. I could have SWORN it said an OL in each ungrounded conductor. It doesn't, you're right. It says a FUSE in any ungrounded conductor. So in the imortal words of Rosanne Rosannadanna on SNL, "Never mind".
I do remember the blast of notices back in the late 70s when we had to exchange all of our motor starters with 2 OLs per phase to ones that had 3 per phase. We were using a lot of A-B starters back then and we had to swap out the Buletin 709s for the new 509s. it was all about grounded shorts in the unprotected leg. We also used a lot of Furnas starters and had to change the Class 40 starters for the "Innova 45" version for the same reason. Good times, good times...
But then again, I just proved that my memory is flawed too, so take it for what it's worth I guess...
Yea thats funny is right ,you seem positive in was the late 70's so my late 60's is wrong now I got to try and remember what happened in the late 60's,,sure is tough to get old ain't it?
dick
kwired
Electron manager
- Location
- NE Nebraska
The change took place before my time in the trade, but I swear I seen conversion kits on the shelf in the spare parts room at a industrial site I used to do work at for adding the third overload to the Furnas starters.
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