Capacitors

[Waxman]

My electrician has the idea to install a capacitor on my HP pump's motor.

What does everyone think of the cost benefit analysis of installing a capacitor to reduce initial drain on startup?

 

[MikeV]

If its a 25 hp motor, it would take one heck of a capacitor. A lot of air compressor motors have capacitor start motors, but usually 3-5 hp. I don't know of a 25 hp capacitor start motor.

 

[MEP001]

I've never heard of a capacitor-start 3-phase motor at all. It's usually one or the other.

Is your electric billed at a flat rate or is it based partly on peak usage? If it's the latter, the usual way to reduce start-up load is with a VFD. If it's the former, there will be no savings at all.

 

[Waxman]

It's partly based on peak demand.

The advice was from my electrician who wired the carwash.

He said a capacitor was less costly than a VFD.

 

[RykoPro]

I agree with MEP, never seen a cap on a 3 phase motor.

 

[Greg Pack]

Is he possibly referring to TVSSs? They are essentially a bunch of capacitors. Some people are claiming energy savings when using them, which appears to be controversial ,to say the least.

 

[Waxman]

Is he possibly referring to TVSSs? They are essentially a bunch of capacitors. Some people are claiming energy savings when using them, which appears to be controversial ,to say the least.

He said Capacitor. Maybe he didn't realize it was 3 phase?

I have to talk to him more when he completes my blower shutdown project.

 

[washnvac]

If you do not need to control speed, then use a soft start. I just replaced all my dryer motor startes with an Allen Bradly SMC-3 soft starter. These are way less costly that VFDs. Plus it gives you lots of start up options like: currentlimit (% of FLA) of 150%, 250%, 350% or 450%, how many seconds to full start, initial torque, kick start, and soft stop. I added a second automatic to a 5/1. (So now it is a 4/2, with 10 vacuums) I replaced (4) 15 hp dryer, (2) 15 hp hydraulic pump, and (6) 7.5 hp dryer starters with these. I only have 400 amp service to this site, and it was going to cost $35k total to bring in 600 amp. The 15 hp starts were $1k each, the 7.5 hp were $600. Ofcourse, the naysayers said I had to have 600 amp service to run this. Been up since end of December with no problems. My utility transformer is 416 amps. On the businest days when both sets of dryers running, I am at 403-408 amps, with 525 spikes. But dryers are only running 75 seconds or less. A bit long winded there, but one of these would save you money is the bottom line.

 

[ken-pro]

I have a customer with 3 phase power at a 4/1 who installed these on every 3 phase motor in the building.

They are a special capacitor that wires in parallel to the 3 phases of power feeding the motor.

The theory is that since the motor is a very large inductor, the capacitance is supposed to balance out the inductance, allowing the motor to "Look" more efficient to the power grid (Making the AC power factor closest to unity)

These links:
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powfac.html
http://en.wikipedia.org/wiki/Power_factor
describe the theory.

From what I understand, if your power factor is unity (1.0) then your power meter will show the least possible current usage. In theory this should help no matter how your power is billed.

My customer has only had them installed 3 months, so it is too soon to tell if they work or not. They were recommended by his electrician.