The pot would typically be used to control motor speed. The only reason I could think you would need it is if it is to control the speed of the motor for hydraulics or maybe the gearbox/transmision. Depending on the motor that is in the lift you could adjust the rpm to work best for the weight of the vehicle etc...
Other than that I don't see a reason for it.
Scratch that, i just saw the diagram, the amp adjust is the motor overload setting.
Such as, 5 hp motor will pull appx 15amps at full load, you adjust the "amp adjuster" to trip the overloads at the setting to keep from burning the motor up.
So yes, you need it to protect the electric motor.
Also, from the factory this lift was 3 phase, did the owner switch the motor out to a single phase motor? It sounds like he did but i am guessing here. To break both hot legs going to the motor you will need to put in a Double pole Single throw switch. It may be fine but if the motor ever rotor locks it could cause some damage to the wiring etc... It would be safer to have some sort of overload on it.
Take a pic of the starter/contactor, motor, and pushbuttons, If i can see what you have i could probably tell you if the single phase contactor has built in overload protection.
If you can get me a picture of the magnetic contactor, I would say that the knob is adjusted to max seeing as how it is broke off.
If you get me that I can hunt down the manufacturer and then see if i can find a replacement or at the very least get a picture of a new one so you will know where in the range of that knob needs to be to keep the motor protected.
As it is now he has bypassed the contactor and is running the motor off a switch. The only protection you have is the breaker in your panel. It will work fine as is but if you want to protect the motor etc... you will need to put a new starter in it and wire up the switch to control the coil in the starter.
The breaker in your panel is sized for the complete circuit, say 20amps, the overload on the starter/contactor is to protect the motor. The overload would be set at say 15 amps to trip. If there is no overload the breaker in the panel would eventually trip depending on what it is but it may be too late thus burning the motor up.
See if there is a CED, Rexel, or Graybar electrical distributor in your area. They should have a Manual motor starter. Ask them for a 2 pole manual starter, then you will need to find out the full load amp on the motor unless you know the horsepower. (the dist. should be able to size the overloads based on HP and voltage)
As pook said, you are probably fine, but at the least change that switch out to a Double pole single throw so that you are turning off both hot leads to the motor. This will get you by until you have more time to go back and install a starter.
A starter is nothing more than a contactor with an overload built in or bolted onto the load side of the contactor. Some are manual and require no control circuit others are magnetic and require a control circuit to energize a coil that magnetically pulls the contacts together.
The easiest for your app would be a manual starter.
If it is a small motor, pulling no more than 15 amps. see if you can find a GE or equal. (i sell ge, so i have the part numbers)
GE Part# CR101H1
this is a 2 pole manual starter
You will need a heater/overload as well
Depending on the load, Say 14.8amp the part # would be
CR123H171B this is just an example since i don't know exactly what motor you have.
Total cost should be no more than $60 to $70
This is as cheap as you can go but still protect the motor.
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