Variable Frequency Drive drives a 10 HP 3-phase 208V motor, drawing ~20A.
Need to turn off power to VFD (and some accessories: fan and light) using a
What is the difference between a magnetic starter and a contactor? Thermal
protection aside, is there any difference? Yes, the starter may have
auxiliary contacts that can be used for the on/off control circuit.
Are there more differences than the formula "starter = contactor + thermals +
aux contacts" would suggest?
There might well be, Motor starters are purpose designed
contactors that take into account the normal locked rotor
surge current. That is why they are rated in horsepower.
In effect, the contacts are over-sized to live through the
normal starting surges and the abnormal overload currents,
till the overloads trip, including breaking the inductive
load of a stalled motor drawing locked rotor current.
Current rated contactors have different and often lower peak
inrush and interrupting current rating, and possibly lower
power factor current allowances during interrupting,
relative to their normal continuous rating. An exception is
contactors used as part of circuit breakers (substation
breakers, for instance) that must break short circuit fault
current, successfully. Most contactors have fuses for this
function, so they do not have to survive interrupting a fault.
All that said, you are not switching power to a motor, but
to a drive (rectifier feeding a capacitor bank, possibly
through some inrush control mechanism, with little inductive
impulse during interruption), so you need a contactor that
lives through that process for a desired lifetime and number
As I understand it a magnetic starter is supposed to be wired so if
power fails the contacts open and do not re-energize after power
returns. If you want to duplicate this with a contactor wire one side
of the contactors coils to the OUTPUT side of the contactors
contacts. Some means will have to be added to manually turn on the
contactor by physically pushing the contacts in. Once powered it will
stay on until a switch in the coils power line in turned off or power
fails. If power fails the contactor cannot restart because the
contacts are now open and to be physically operated again. All the
above may be dangerous depending on how you go about it. Wouldn't
want to have an exposed contactor so an insulator of some kind will
have to project outside the box it's mounted in. Take this with a
grain of salt, I've never tried it but have seen it done. It may
also violate electrical codes.
By definition, a motor starter consists of a contactor + thermal overload
Basic contactor includes one(1) aux contact normally used as a "Holding
contact" in the control circuit.
The "Contactor" portion may be either manual or magnetic operation.
Either way, the thermal overload relay is designed to trip the contactor in
the event of a sustained overload, thus releasing the "Holding contact".
Auxilary contacts are exactly that, auxilarily devices under control of the
contacto, usually available with 3-4 max..
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