Induction Motor - 110V, 60Cycle, 3Phase, FL=16.4A HELP

[Strostkovy]

A VFD with a filter on it would do what you need. The filter simply being for suppressing inductive spikes that could kill the insulation. If you don't like that approach, you can use any 2:1 (like 480 to 240) transformer after the VFD and filter to reduce the voltage and also use a smaller VFD.

The cheapest approach is likely a 5-10hp capacitor start rotary phase converter and a small transformer like the one mentioned above. It has to be rated to the full current so I think around 6kVA

120V single phase input VFDs also exist, but may be expensive around that power level.

 

[Strostkovy]

Actually, 20A variacs of the cheap Chinese variety go for about $80. I'd use one on each leg of whatever 3 phase source you have. If you can find a second hand 3 phase variac then that's even better, and especially so because you don't need to know what voltage or winding configuration the motor is designed for, as you can just ramp up the voltage until the current hits FLA then back off a skosh.

 

[johansen]

The good thing with fan-type loads is that usually, less speed results in them drawing significantly less torque. Many fan manufacturers let you do reduced-voltage speed control (via triac or autotransformer types), because the increasing slip, decreasing torque, and decreasing voltage cancel out to give you roughly constant current above like 50% voltage.


You can have, for the sake of argument, a 230V supply with 280V on the motor winding and 150V on the cap.

Reduced voltage for induction motor driven fans can work up to about 1/4 to 1/2 hp depending on cooling the motor, as the motors are already less than 50% efficient. Wont work on a 2hp 3 phase motor, you will burn it up.

You really need a variac, watt meter, and capacitor bank to find the optimal run cap for a 3 phase motor to run from a single phase supply.

 

[JST]

The good thing with fan-type loads is that usually, less speed results in them drawing significantly less torque. Many fan manufacturers let you do reduced-voltage speed control (via triac or autotransformer types), because the increasing slip, decreasing torque, and decreasing voltage cancel out to give you roughly constant current above like 50% voltage.

How much of that logic applies to an asymmetric supply like a static converter, I'm not sure.

I'd be tempted to slow it to 40-50Hz to limit noise complaints and improve life while still getting most of the experience.

Phase angles get messy. The capacitor is opposite to the inductance in the motor (magnetising current). The real/resistive current from the load is at 90 degrees to both. Under many circumstances, putting a capacitor in series with the motor winding actually increases voltage across that motor winding, because it's cancelling out the voltage drop caused by the magnetising current, leaving only the resistive voltage drop across the line.

You can have, for the sake of argument, a 230V supply with 280V on the motor winding and 150V on the cap.

These things are not a mystery. There is good solid physics and engineering behind all of it. Given the correct data about the circuit, everything can be calculated for perfect performance.

Normally, in these situations, you do NOT know many, if any, of the important circuit constants. The people who DO know those things, are either long deceased, the records were lost, or the maker will not release the data to some random person who asks. You are kind of stuck with a cut and try, because you cannot get the circuit data you would need to "design" the system.

The issue with capacitors is when you get into resonance with circuit inductance at the mains frequency. At that point the reactances disappear (offset each other), and the current can get very high, as can the voltage. Things begin to change rapidly as you approach the resonance point. What happens in any particular case depends on the circuit type, series or parallel, the values of the "components", etc.

Normally, if you try various capacitors starting with ones much lower in capacitance than needed, as you increase capacitance, you will find the point you want before you get into trouble with resonance.

 

[SomeoneSomewhere]

I've seen reduced-voltage speed control done (with autotransformers) on IIRC a 4HP/3kW fan, and heard of larger. Nothing really changes when you start looking at more power; everything scales together.

That leaves very little voltage left for the capacitor.... any voltage drop in it will reduce the applied voltage and power. So the higher voltage allows the capacitor voltage drop and still allows full voltage input to the third wire. There is also some interaction with the transformer to improve the phase angle.

I agree that you don't want to approach full resonance. My point is that the capacitor does not necessarily provide voltage drop, but in fact voltage gain.

 

[JST]

I've seen reduced-voltage speed control done (with autotransformers) on IIRC a 4HP/3kW fan, and heard of larger. Nothing really changes when you start looking at more power; everything scales together.

I agree that you don't want to approach full resonance. My point is that the capacitor does not necessarily provide voltage drop, but in fact voltage gain.

The voltage "gain" is VERY dependent on load. You have a resonance, and the "boost" depends on the "Q" or quality factor. That goes down with loading. Makes sense, because a given power at a higher voltage is less current.

The resonance boost does not change the energy.... you still do not get out more than was put in. So you will draw more, and incur all the losses that were there to begin with, just increasing the losses due to more current.

If you have the current, fine. But if you are trying to boost after a resistive drop, you do not have the current, and probably cannot "fix" it effectively. If it is an RPC, then you need a larger idler motor.

You do not have control over the basic "Q" of the resonant circuit, because the idler motor etc has a given inductance and resistance. For a lower impedance and the same boost, you would need a lower resistance inductor, but the motor you have is what you have unless you upgrade.

 

[SomeoneSomewhere]

I was considering the static phase converter case, which is similar to single-phase motor theory. Despite the voltage across the capacitor, the voltage across the run and aux windings is similar. Some permanent-split motors and most synchronous motors have identical main and aux windings, and you swap them to reverse direction.