120v Normal Closed Relay Testing

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221110-2100 EDT

dw85745:

Some basic information to work from.

A single phase motor fed from a single phase source will not self start mechanical rotation. It will just sit nearly still, but you will feel or hear it vibrate. It can be started by some external applied rotation, a rope and pullly, or something else to momentarily drive the rotor.

To electrically start a single phase motor you need some electrical scheme to produce some limited sort of rotating magnetic field. Thus, at least for starting purposes the motor needs to be at least a 2 phase motor. There are various ways to accomplish this.

The circuit shown by ptonsparky consisted of two phase shifting circuits. One circuit is a capacitor and another coil that is not the main power coil. This circuit is present both during starting, and running. The second circuit adds more capacitance in parallel with the first capacitor ( more starting torque ) just during starting ( in some fashion with some time delay to opening ) .

I don't believe this normally closed contact in series with the starting capacitor is voltage controlled from something relating to starting voltage unless the relay contact also has a time delay function.

.
 
gar said:
221110-2100 EDT

dw85745:

Some basic information to work from.

A single phase motor fed from a single phase source will not self start mechanical rotation. It will just sit nearly still, but you will feel or hear it vibrate. It can be started by some external applied rotation, a rope and pullly, or something else to momentarily drive the rotor.

To electrically start a single phase motor you need some electrical scheme to produce some limited sort of rotating magnetic field. Thus, at least for starting purposes the motor needs to be at least a 2 phase motor. There are various ways to accomplish this.

The circuit shown by ptonsparky consisted of two phase shifting circuits. One circuit is a capacitor and another coil that is not the main power coil. This circuit is present both during starting, and running. The second circuit adds more capacitance in parallel with the first capacitor ( more starting torque ) just during starting ( in some fashion with some time delay to opening ) .

I don't believe this normally closed contact in series with the starting capacitor is voltage controlled from something relating to starting voltage unless the relay contact also has a time delay function.

.
Click to expand...
The delay function is in how long it takes for the cemf to build up. (I think that is the term). The potential relay is very common in well motors, air conditioning, and refrigeration. I've also seen it on older higher HP table saws. Delta brand IIRC.
 
221111=0016

ptonsparky:

See my thread

1-phase induction motor starting current

160215-2133 EST Many tend to lump motor starting current into the term inrush current. The starting current to an induction motor is not comparable in form to the inrush current to a cold incandescent bulb or a transformer core driven into saturation. In a motor there is a moderate air gap in...
forums.mikeholt.com
 
gar said:
221111=0016

ptonsparky:

See my thread

1-phase induction motor starting current

160215-2133 EST Many tend to lump motor starting current into the term inrush current. The starting current to an induction motor is not comparable in form to the inrush current to a cold incandescent bulb or a transformer core driven into saturation. In a motor there is a moderate air gap in...
forums.mikeholt.com
Click to expand...
Okay, I'll look it over again.

Not sure how it relates to the common use of potential relays in single phase motors and this thread.
 
Ok. Got it.

Nice to see how little the voltage must increase to open the contacts. It's no wonder those relays may be position sensitive.

I replaced the old mechanical potential relay in the above table saw with a solid state. The originals were no longer available. It took some effort to adjust the time of drop out.
 
221111-1530 EST

I don't know why one would use a voltage sensitive relay to control the dropout point for an induction motor start winding unless the voltage to that relay was from a generator on the motor shaft where the output voltage was proportional to motor speed. Motor current is easily available and does a good job of indicating when the motor is up to a reasonable speed.

.
 
gar said:
221111-1530 EST

I don't know why one would use a voltage sensitive relay to control the dropout point for an induction motor start winding unless the voltage to that relay was from a generator on the motor shaft where the output voltage was proportional to motor speed. Motor current is easily available and does a good job of indicating when the motor is up to a reasonable speed.

.
Click to expand...
Because it works?
We both know I won't be able to explain it.
Maybe one of the motor guys will jump in.
 
Here's how I see it: Any motor when starting draws a lot of current. That current goes down as the motor comes up to speed. With the start winding in series with the capacitor(s), the voltage is divided between the two. At first the start winding has a low impedance (inductive reactance) while the capacitor(s) have a higher impedance (capacitive reactance). Sooooo, there is a greater voltage drop across the capacitor(s) and less across the start winding. The voltage across the potential relay is the same voltage across the start winding because they are connected in parallel. The voltage across the potential relay is not high enough to energize the relay until some point as the motor comes up to speed and the current decreases. At that point the voltage is divided more evenly between the start winding and the capacitor. There is then sufficient voltage across the potential relay to energize and for the contacts to switch state.
 
221111-1725 EST

TwoBlocked:

Get yourself a memory scope, a current transformer, and a voltage probe. Add to this a motor speed sensor proportional to motor shaft speed. Then also someway to sync the scope to turn on time of the motor. Observe your waveforms, and compare those observation with your verbal description.

.
 
gar said:
221111-1725 EST

TwoBlocked:

Get yourself a memory scope, a current transformer, and a voltage probe. Add to this a motor speed sensor proportional to motor shaft speed. Then also someway to sync the scope to turn on time of the motor. Observe your waveforms, and compare those observation with your verbal description.

.
Click to expand...
I'll leave that to you.
 
221112-0933 EST

TwoBlocked:

Yes, it is quite interesting how the capacitor fed coil has a substantial voltage increase when the motor reaches near full speed. Certainly enough to be detected. I did not try to analyze, observe, how this varies with supply voltage, or motor load.

.
 
dw85745 said:
Thanks both for replies.
petersonra:
I've been hesitant to take a voltage across the contacts as IMHO, that would be like trying to take the voltage on a wire. Am I wrong on this?
I have a sketch, will try and see if I can scan in and post. Late here so tomorrow.
Click to expand...
The better analogy is that the closed contacts are like a wire, so no voltage.
The open contacts are like a cut wire, so the voltage will depend on what is connected to either side.
 
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