Manage load shedding

**CLIVE-TRIANGLE** · 15-Apr-15, 06:03 AM

The EPS is no good as a UPS, it switches too slowly. So the UPS as expected does go to it's battery. However I would have thought the once the EPS has switched, it would be happy. Alas no.

Dave, the EPS is a modified wave form, so perhaps that's why.

Once this happens the downstream UPS will switch back to normal mode if it is happy with the power that's being delivered to it by the upstream UPS.

That is what is not happening.

**Justloadit** · 15-Apr-15, 08:01 AM

So when the battery of the UPS is flat, then the EPS will continue maintaining power for the duration of the charge of the EPS, should not be a problem then, until power is returned..

**CLIVE-TRIANGLE** · 15-Apr-15, 08:12 AM

Originally Posted by Justloadit

So when the battery of the UPS is flat, then the EPS will continue maintaining power for the duration of the charge of the EPS, should not be a problem then, until power is returned..

That's what I was hoping for, but it doesn't happen. I have even disconnected the supply from the EPS to the UPS, then reconnected. It's simply as if it's not there. The other devices, not connected to the UPS but to the second outlet on the EPS, work fine. Could it be that the UPS rejects the modified sine wave of the EPS?

**AndyD** · 15-Apr-15, 11:03 AM

That is the most likely possibility, the 'modified sine' output of the EPS is not up to scratch. You could try reversing the UPS and EPS so that the EPS is downstream but if the output of the EPS is poor quality I'm not sure if this would have adverse effects on the items it's running.

**Justloadit** · 16-Apr-15, 07:35 AM

Andy, not necessarily, today's equipment are made with SMPS, to maintain low cost, which work of a DC signal. The internal feedback loops with in the UPS, adjust the PWM, so that the voltage required by the equipment is maintained, this is the reason that a SMPS is rated between 95-265V. These SMPS's will work with a pure DC signal, so the fact that the supply is a modified sine wave will have no effect on the equipment. The incoming supply is first inserted through a bridge rectifier to convert to a pure DC signal, which it then uses to supply the equipment.

The only time that a "pure" sine wave is required, is when there are inductive loads involved, such as an electric motor, be it a fridge, pool pump, water pump, microwave, fluorescent lights which use a starter and a choke, very cheap LED lamps which use RC as part of it's PSU.

TVs, decoders and most modern printers will work quite happily with a modified sine wave.

The reason that inductive loads and motors require a sine wave is due to 2 reasons, the first being that the square peak of the incoming wave is on for too long, which causes the iron in the motor to overheat because the motor was designed for a shorter peak. The 2nd reason, is the rise time from zero to peak is vertical, which changes the characteristics of the inductor filter circuit and excessive current will flow causing overheating.

**CLIVE-TRIANGLE** · 16-Apr-15, 09:24 AM

Then I'm back to square 1.

**Dave A** · 16-Apr-15, 01:18 PM

Can you post the make and model number of the UPS, Clive?

**Justloadit** · 16-Apr-15, 01:40 PM

I would follow AndyD's idea, of placing the EPS after the UPS.
So Power fail, UPS continues till battery flat, then EPS continues from there. When power returns, UPS battery charges, EPS battery charges, and load supplied by mains.

**AndyD** · 16-Apr-15, 09:49 PM

Originally Posted by Justloadit

Andy, not necessarily, today's equipment are made with SMPS, to maintain low cost, which work of a DC signal. The internal feedback loops with in the UPS, adjust the PWM, so that the voltage required by the equipment is maintained, this is the reason that a SMPS is rated between 95-265V. These SMPS's will work with a pure DC signal, so the fact that the supply is a modified sine wave will have no effect on the equipment. The incoming supply is first inserted through a bridge rectifier to convert to a pure DC signal, which it then uses to supply the equipment.

The only time that a "pure" sine wave is required, is when there are inductive loads involved, such as an electric motor, be it a fridge, pool pump, water pump, microwave, fluorescent lights which use a starter and a choke, very cheap LED lamps which use RC as part of it's PSU.

TVs, decoders and most modern printers will work quite happily with a modified sine wave.

The reason that inductive loads and motors require a sine wave is due to 2 reasons, the first being that the square peak of the incoming wave is on for too long, which causes the iron in the motor to overheat because the motor was designed for a shorter peak. The 2nd reason, is the rise time from zero to peak is vertical, which changes the characteristics of the inductor filter circuit and excessive current will flow causing overheating.

What you say is true but a power supply with active PFC and/or harmonic filters will often run warmer if the power is modified sine. This may or may not be an issue. Realistically, as you say, it's unlikely the appliance will suffer immediate damage unless is poorly designed in the first place but the lifespan of the device could be shorter running on a modified sine supply due to the higher component temperatures.