Motor Starting With a Disconnect Switch

[gene jameson]

A farmer-customer wants me to install two electric motors to run irrigation pumps. The two 7-1/2 HP, 1-phase, 230 volt motors will be run on seperate circuits. Each circuit is about 250 ft long, using underground feeder to serve each well.
My question has to do with safely starting, and stopping these motors. The customer has heard that this can be done using a fused disconnect switch. Of course he likes this idea because these switches will only cost him a tenth of the cost of a motor starter. I have asked around and found that this not such an uncommon practice.
Never the less, I am very apprehensive, considering the extremely high starting current involved.
Can you all help me with some perspectives: (1) Can these motors be safely operated, in this environment, with disconnect switches? (2) Do you know of dangerous failures that have occurred using disconnects for this use? (3) Will the fuses in the disconnects provide suitable motor (and farmer!) protection?
I appreciate your advice on this.
Thx, Gene

 

[iwire]

Re: Motor Starting With a Disconnect Switch

Gene this is not really a direct answer to your question, but IMO needs to be asked.

How do you plan on providing the required motor overload protection?

 

[jbwhite]

Re: Motor Starting With a Disconnect Switch

some motors have internal overload protection. others require you to provide the overload protection using a motor starter.

you will need a disconnect either way. unless the pump is next to the panel.

 

[rbalex]

Re: Motor Starting With a Disconnect Switch

From the 2005 NEC, this addresses the general issue:

430.111 Switch or Circuit Breaker as Both Controller and Disconnecting Means
A switch or circuit breaker shall be permitted to be used as both the controller and disconnecting means if it complies with 430.111(A) and is one of the types specified in 430.111(B).
(A) General The switch or circuit breaker complies with the requirements for controllers specified in 430.83, opens all ungrounded conductors to the motor, and is protected by an overcurrent device in each ungrounded conductor (which shall be permitted to be the branch-circuit fuses). The overcurrent device protecting the controller shall be permitted to be part of the controller assembly or shall be permitted to be separate. An autotransformer-type controller shall be provided with a separate disconnecting means.
(B) Type The device shall be one of the types specified in 430.111(B)(1), (B)(2), or (B)(3).
(1) Air-Break Switch An air-break switch, operable directly by applying the hand to a lever or handle.
?

In addition to the reference to 430.83 above, the switch itself must also comply with 430.108 & 430.109 and be ?in-sight? (see the definition in Art 100) as required by 430.102. (Read all of 430, Part IX carefully).

Several dual-element type fuses can provide both motor over-load and short-circuit/ground fault protection.

Edit Add: added the reference to the 'in-sight' definition

[ December 04, 2005, 11:19 AM: Message edited by: rbalex ]

 

[Leitmotif]

Re: Motor Starting With a Disconnect Switch

Two or three years from now he will complain becauese the knife blades are all burned up. Then you gotta replace the switch. Penny wise,,,,,

Granted they are not big motors but I have seen 200 and 300 amp disconnects blow out.

Its a cost of business spend the bucks and do it right.

Dan Bentler

 

[ramsy]

Re: Motor Starting With a Disconnect Switch

During disconnect, I understand the inductive kick from a 5-horsepower motor can produce over 1,000 volts, before the switch contact blows over. This may shorten the life of the switch.

 

[TOMWELDS]

Re: Motor Starting With a Disconnect Switch

And with what Ramsy just said, a set of contacts for a contactor (motor starter) would be alot cheaper to replace than a new safety switch.

 

[dlhoule]

Re: Motor Starting With a Disconnect Switch

It would seem to me that it would be more efficient to install Starters controlled from a central location rather than having to get to each well individually to start motors.

Also; IMO in most cases, it is not a good practice to rely on a disconnect for starting and stopping of motors. I have seen a 60 amp disconnect have the cover blown right off.

I don't want to be anywhere near one when it blows, and if it is going to blow it will more than likely happen when starting or stopping the motor.

 

[bthielen]

Re: Motor Starting With a Disconnect Switch

I think there is a safety issue with using disconnect switches as motor starters. If I'm not mistaken, part of the design of a motor starter is the speed at which the contacts close and open. I was taught that one should never stand in front of a cabinet and also to turn away when throwing the disconnect switch due to the potential for explosion. It is much safer to open and close a low voltage switch controlling a motor starter's contactor coil from a safe distance. With all that said, I wonder how many farmers or other non-electrical persons are aware of this hazard. Providing a means to expose one to these hazards only adds to the danger.

Bob