Eliminating start up inrush current

oil pan 4 · 02-13-17, 08:37 PM

My 5ohm 200w resistors came in. So I put them on on my manual soft start experiment board.
I suspected that running the starter with power factor correction capacitors removed would make more heat in the resistors, which it did. Massively more heat. So much heat the resistors had to have at least 2 or 3 minutes of cool down time minimum between starts.
With power factor correction I could start the motor 2 times back to back and it still wouldn't get them as hot as one uncorrected start.

I was also hypothesizing that a slightly different size motor start capacitor may work better starting an unloaded motor on reduced voltage, since capacitor start motors are designed to give maximum starting torque at full voltage.
It has a 600uf capacitor from the factory. I found that with a 420-488uf capacitor (actually a 340-408uf start cap plus an 80uf motor run cap) it did better. So I ordered the closest thing to that size I could find, a single standard size 400-480uf cap.
On reduced voltage starting the motor draws 1 to 2 less amps and comes up to speed faster with the smaller capacitor.

This motor would draw 35 to 40 amps originally, starting with full power and the belt driving the compressor.
Just unloading the motor, removing the belt, starting the motor at full power, it would violently jump to 22 amps.
Unloading the motor, full power starting with the smaller start cap and power factor correction it draws 18 to 19 amps and still dims the lights in the house.
So I believe full power starting draws a lot more amps than what's showing up on the amp meter.
With resistive starting, a smaller start cap and powerfactor correction it peaks at16 amps. It quickly climbs to 16 amps, with no violent inrush. I can see it build to a peak of 16 amps from lower numbers on my amp meter. And most importantly no lights dim upon starting so there is no violent current inrush.

So I have at least cut starting amps in half.

02-13-17, 08:37 PM	#11
oil pan 4 Land owner Join Date: Nov 2012 Location: NM Posts: 1,026 Thanks: 12 Thanked 127 Times in 107 Posts	My 5ohm 200w resistors came in. So I put them on on my manual soft start experiment board. I suspected that running the starter with power factor correction capacitors removed would make more heat in the resistors, which it did. Massively more heat. So much heat the resistors had to have at least 2 or 3 minutes of cool down time minimum between starts. With power factor correction I could start the motor 2 times back to back and it still wouldn't get them as hot as one uncorrected start. I was also hypothesizing that a slightly different size motor start capacitor may work better starting an unloaded motor on reduced voltage, since capacitor start motors are designed to give maximum starting torque at full voltage. It has a 600uf capacitor from the factory. I found that with a 420-488uf capacitor (actually a 340-408uf start cap plus an 80uf motor run cap) it did better. So I ordered the closest thing to that size I could find, a single standard size 400-480uf cap. On reduced voltage starting the motor draws 1 to 2 less amps and comes up to speed faster with the smaller capacitor. This motor would draw 35 to 40 amps originally, starting with full power and the belt driving the compressor. Just unloading the motor, removing the belt, starting the motor at full power, it would violently jump to 22 amps. Unloading the motor, full power starting with the smaller start cap and power factor correction it draws 18 to 19 amps and still dims the lights in the house. So I believe full power starting draws a lot more amps than what's showing up on the amp meter. With resistive starting, a smaller start cap and powerfactor correction it peaks at16 amps. It quickly climbs to 16 amps, with no violent inrush. I can see it build to a peak of 16 amps from lower numbers on my amp meter. And most importantly no lights dim upon starting so there is no violent current inrush. So I have at least cut starting amps in half.