On 7/23/16 4:41 PM, Richard (Rick) Karlquist wrote:
On 7/23/2016 3:25 PM, Roger (K8RI) on TT wrote:
I'd definitely want limit switches. They are good insurance.
73
Roger (K8RI)
Actually, limit switches are NOT good insurance. They
don't protect against the contactor sticking. The
contactor used is the same kind that air conditioners
use, and air conditioner contactors are famous for
sticking. Read about that on the internet.
What you need for insurance is a local (on the box)
power switch that cuts the power to the MOTOR. The
power switch built into the control box only cuts
the power to the coil of a relay that in turn cuts
the power off to the COIL of the contactor. I had
to add a real power cutoff switch to the box.
And this is why the electrical code requires a motor disconnect within a
certain distance of the motor. When I was working for the effects
company building a big tornado machine, we ran into this: we had
cabinets full of fancy variable speed drives, but had to have a
(expensive) three phase disconnect box rated to interrupt locked rotor
current (typically 7x running current) next to each 10HP motor.
There's even more rules if there are significant inertial loads on the
motors.
I have had a contactor stick twice. Fortunately, I
was standing next to the control box monitoring
when it happened and was able to pull the big switch.
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