small 3ph. motor starting circuits

[biofueler]

what is the rule of thumb for the size of a 3ph. (480V) motor that requires a starter? I'm assuming I can start 1-2hp motors with a simple relay energizing a 3-pole contactor circuit. The relay is controlled from an 'off/run/start' switch. I have used these types of circuits to send power to an mcc to start motors, but was wondering what size can be started with a simple contactor?

 

[Skogsgurra]

There are contactors that can start 400+ kVA motors. It is not a question of what the motor or contactor can take, more a question of what the grid can tolerate and what the mechanical parts of the installation can withstand. DOL is seldom (never, I would say) a problem for the standard induction motor, but voltage drop and exploding couplings are. There are also loads that cannot take a DOL start. Many conveyors and large fans are such loads.

There used to be rules that motors above a certain size couldn't be started direct on-line, but they are not common today. The reason was simply that voltage drop was too big in those grids with low SC capacity so that other loads would drop out and the flicker was annoying if starts were frequent.

Gunnar Englund

http://www.gke.org

 

[biofueler]

"DOL"?

 

[Skogsgurra]

Direct On Line - starting without any other devices than a switch or a contactor.

Gunnar Englund

http://www.gke.org

 

[BigWallyB]

Not sure if this is what you are asking but 1 and 2 Hp motors at 480v only draw 3-6 amps FL. There are 3ph switches (under 5 Hp) and drum controllers (if reversing is required) that can be used at site rather than running control wiring.

Life is what happens while we're making other plans.

Wally

 

[Paulusgnome]

You can start motors this small with a simple 3-pole contactor. Commonly-sold DOL starters for this size motor will normally use exactly this.

It is a good idea to fit a thermal overload relay. Most contactor manufacturers sell overloads that fit directly to the contactor to facilitate this.

Note that contactor manufacturer's data books will typically spec the contactor for Type 1 coordination : this means that ion the event of a locked rotor fault the overload/contactor combination will protect the motor, but only at the expense of the contactor contacts which will usually need replacing after such a fault.

 

[bklauba]

For a quick reference, pickup a copy of the "Ugly's Electrical References", sold by almost all electrical supply houses. On page 40 - 41 (at least in the printing I keep with me), there is a table entitled: Full-Load Current and other data, Three Phase Motors, which gives the HP, motor current, recommended breaker size, STARTER SIZE, heater size, wire size, and conduit size. A lot of info on one table, and very useful.

Also, you might consider a solid state overload instead of thermal. These are electronic, rather than electro-mechanical, and thus age very gracefully. They also can detect single phasing, and shutoff quickly, rather than toast the motor or contactor (as Paulusgnome pointed out) is a concern. Most of these that are on the market (regardless of whose name is on them, i.e., A-B, Cutler-Hammer) are made by Sprecher & Shuh.

BK

 

[biofueler]

the ugly's table shows starters being used all the way down to 1/2 HP motors. Anything below 2HP uses a '00' starter according to that table.

FWIW, I was thinking of using the circuit on pg 44.