Does my motor need a second capacitor?

Question

First things first - I am a complete novice. Thank you in advance for your help.

I bought a motor second hand, though it has never been used. My aim is to have it run my table saw.

What I know:

• As it is currently wired, I've been able to work out the it will run in a clockwise direction (I will change this)
• I know where to put the earth

What I don't know:

• Why does my motor only have one capacitor? All the others seem to have two? Do I need to do anything about this?
• Why does it have a circuit breaker?
• And most importantly, where do I put the neutral and live wires? I have seen one diagram with the the live going into the circuit breaker and the neutral being connected to the U1 point, but the connections diagram doesn't suggest this.

I know that verions of this question have been asked but I've not been able to find the specific example of a single capacitor/ circuit breaker motor.

Thanks again.

Answer 1

Why does my motor only have one capacitor?

Single-phase induction motors that have two capacitors have a higher torque capability when starting and accelerating. The starting capacitor is larger and thus allows a higher current in the starting winding and a greater phase shift of the current in that winding. However, the capacitor and starting winding are not designed to carry that current continuously. The capacitor is disconnected by a centrifugal switch or some other means when the motor approaches the full operating speed. High starting torque is not required for all types of loads, so motors are also offered without the high starting feature to save the expense of the high starting-torque feature.

Motors that have only one capacitor are called permanent-split-capacitor or PSC motors. They are suitable for fans and centrifugal pumps. Those loads are easier to start. A PSC motor could be used for a saw if care is taken to prevent starting the saw with the blade touching the work piece.

The image below shows percent speed vs. percent torque capability due to to the running capacitor and the starting capacitor. At the switching speed, the torque capability transfers from the initial starting curve to the running curve. For a motor with only a running capacitor, the curve starts with the lower running capacitor curve and continues with no transition.

where do I put the neutral and live wires?

There are two "line" connection points marked on the diagram. There is no preference as to which line is "live" and which is "neutral."

Why does it have a circuit breaker?

The symbol that is similar to a circuit breaker is a built-in over-current protective device. It is probably self-resetting. If it opens, it will eventually re-close when the motor is cooler.

Answer 2

Why does my motor only have one capacitor? All the others seem to have two?

You probably mean: Why this motor has only one capacitor, while other saw motors have two?

A: Because the saw operates also in no-load condition, while this motor is intended to be start and run with load always present. Motors with two capacitors have one start and one run capacitor. The start capacitor is disconnected when motor reaches certain amount of speed by centrifugal switch, which your motor doesn't have. Running this motor as saw is not recommended as the capacitor would blow sooner or latter.

where do I put the neutral and live wires?

On connection marked as LINE, while the PE wires goes to the earth mark.

Why does it have a circuit breaker?

Probably is the thermal switch, for overload protection.

The circuit breaker has to be installed on the supply side, that's where the connecting cable comes from.