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I just purchased a secondhand portable band saw (corded) that just isn't reaching the maximum speed. I suspect that the speed circuitry is the culprit and not the motor as that is easier to sabotage.

So assuming that the maximum speed and power that the AC motor alone can achieve being connected directly to AC mains is what the circuitry will have at the maximum dial, can I simply connect the motor raw to the AC and bypass the circuitry? This is also assuming that the AC motor is singled phase AC. I've looked at the service parts manual (I haven't gotten to opening it yet) and it seems that there are only to wires going to the stator, I believe.

So how do I tell if it's safe to simply connect the raw AC mains to the motor itself? Could I simply measure the resistance and inductance of the motor and calculate the total impedance, to make sure it will not exceed 11A that the band saw is rated?

Also, do I have to worry about Back EMF? I have a bench grinder that is simply an Induction AC motor and I know it doesn't have a flyback diode. So do I NOT need a similar diode when running this test?

EDIT:

Come to think of it, the only 2 wires seem to be connected to the rotor, as there are brushes. So it could be a DC motor and it's controlled through PWM. I only have multimeters... So how do I test if it's PWM, especially if the PWM doesn't go negative?

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... can I simply connect the motor raw to the AC and bypass the circuitry? This is also assuming that the AC motor is singled phase AC. I've looked at the service parts manual (I haven't gotten to opening it yet) and it seems that there are only to wires going to the stator, I believe.

You need to check the motor and read the rating plate. If it has a simple speed controller it is unlikely to be an induction motor.

So how do I tell if it's safe to simply connect the raw AC mains to the motor itself? Could I simply measure the resistance and inductance of the motor and calculate the total impedance, to make sure it will not exceed 11A that the band saw is rated?

See above.

Also, do I have to worry about Back EMF?

Back EMF is the voltage generated by a motor due to rotation. Since it opposes the voltage driving the motor it reduces the current drawn by the motor as it speeds up. Its absence at zero speed is what causes the typical inrush current at startup.

I have a bench grinder that is simply an Induction AC motor and I know it doesn't have a flyback diode. So do I NOT need a similar diode when running this test?

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A "snubber diode" will short circuit the mains on negative half-cycles.

Flyback diodes are used on DC motors. If you put one on an AC motor it would provide a short-circuit on one polarity of the mains.

I expect that the motor is a universal motor as these require simple speed controllers.


Update 1

enter image description here

Figure 2. The parts diagram shows the location of the brushes.

Induction motors don't have brushes. It is a universal motor.

You can estimate the starting current by measuring the DC resistance and using Ohm's Law.

As you can see the circuitry is NOT simple as it even has sensors. So maybe it's an Induction AC motor?

The circuitry for a universal motor speed controller is much simpler than that for an induction motor (a variable frequency drive / VFD). It is not an induction motor.

Update 2

Come to think of it, the only 2 wires seem to be connected to the rotor, as there are brushes.

You will find that the rotor is connected in series with the stator winding.

So it could be a DC motor and it's controlled through PWM.

enter image description here

Figure 3. Most likely the motor is a triac controlled AC "dimmer". By adjusting the turn-on point of the power the voltage and current to the motor can be controlled.

I only have multimeters... So how do I test if it's PWM, especially if the PWM doesn't go negative?

Switch the meter to AC volts and monitor the voltage across the motor as the speed control is adjusted. If it is not reaching full speed it may be that it is only getting either the positive or negative half-cycles of the mains due to component open-circuit failure.

It could be PWM controlled...

It is not PWM controlled in the normal meaning of the term. See above.

Probably should be wary of the voltage peaks, huh?

Huh? Just be wary of the voltage. Electrocution is a risk.

Would a diode-capacitor peak detector prove very useful for this?

No.


Update 3

From what limited knowledge I have of motors and driving them, the problem with Phase Fired Controllers is that they don't give a very even drive. It's OK for a very slim range, then it runs very botchy.

That's true in that they typically don't detect droop in speed as the load increases.

That's unlikely as this is a high quality product and like I said even has sensors.

It probably has one sensor to give one or more pulses per revolution. With this simple feedback the controller can compare set-point with actual and modify the phase angle to maintain set speed. Again, this would not be considered complex control.

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  • \$\begingroup\$ A comment is low-key, so I suppose it would be alright to post this link: documents.milwaukeetool.com/54-40-6241.pdf. As you can see the circuitry is NOT simple as it even has sensors. So maybe it's an Induction AC motor? \$\endgroup\$ – Derpy_Merp Nov 15 '17 at 4:16
  • \$\begingroup\$ I knew the answer about the bit about needing a flyback diode. I actually don't know why I even added part. \$\endgroup\$ – Derpy_Merp Nov 15 '17 at 4:21
  • \$\begingroup\$ So, finally, if IT IS an AC induction motor, I can just go ahead as I described above?? \$\endgroup\$ – Derpy_Merp Nov 15 '17 at 4:23
  • \$\begingroup\$ See the update. \$\endgroup\$ – Transistor Nov 15 '17 at 4:40
  • \$\begingroup\$ It could be PWM controlled... Probably should be wary of the voltage peaks, huh? Would a diode-capacitor peak detector prove very useful for this? \$\endgroup\$ – Derpy_Merp Nov 15 '17 at 4:48
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I believe this is a permanent-magnet DC motor. I see no stator wiring connection. There is a magnet ring near the commutator. That is probably part of a speed sensor. The controller could be either PWM or phase control. With a decent speed sensor, I believe armature phase control of the armature of a motor with a permanent-magnet field would provide quite adequate control for a saw. Whether it is PWM or phase control, the motor is totally unsuitable for direct connection to AC. It could run on full-wave rectified AC, but the speed may be too high.

Upon further study, I see that white field wires 26 and 27 are connected to the brushes. That indicates a wound field is connected in parallel with the armature. A universal motor would have the field connected in series with the armature, so I don't believe anything in the first paragraph needs revision.

Rather than take the motor apart, in might be a good idea to see if an oscilloscope can be connected to the brushes through the brush tubes.

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