Linear Non Linear Loads.

[Fordean]

In a hospital. THey have carts that require charging. Are the Charges linear or non linear. Each cart uses 5.1 amp @ 120 v.

I plan on installing 8 carts on a 10/4 cable fuse on a three pole 20 or 25. Just wunder if I should consider a oversized Neutral. Most per circuit will be 3 outlets (5.1 x 3= 15.3 amps) per phase max.

 

[Dennis Alwon]

My understanding is that charging stations are highly non linear loads.

I found this on the web

In order to promote new energy electric vehicles (EVs) it is necessary to build a large number of Evs charging stations. (EV) chargers are highly non-linear load in power systems, they present a potential problem to power system in the forms of excessive harmonic currents and poor power factor. In this paper, individual EV charger model has been set up on the basis of analyzing actual data from a demonstration station in Shen Zhen city. By using Matlab as a simulation tool, we analyzed harmonic current of individual EV charger and individual EV charging station meanwhile the model of V2G was also described. The results show that the 5,7 harmonic ratio is beyond the scope of the national harmonic standards when the EVs charging station connected to the power system. Some measures and harmonic control strategy to improve the loading factor of power systems have been proposed. The research methods and results can also be use for charger suppliers, design units, power grid companies, research institutes.

 

[kwired]

A very basic power rectification circuit will likely have little harmonic distortion, but will not be that great of a steady DC source.

The equipment you are talking about likely has some significant harmonic distortion.

 

[fmtjfw]

My understanding is that charging stations are highly non linear loads.

I found this on the web

Apples and Oranges. The chargers on the carts are basically UPSs with big batteries

 

[Besoeker]

In a hospital. THey have carts that require charging. Are the Charges linear or non linear. Each cart uses 5.1 amp @ 120 v.

I plan on installing 8 carts on a 10/4 cable fuse on a three pole 20 or 25. Just wunder if I should consider a oversized Neutral. Most per circuit will be 3 outlets (5.1 x 3= 15.3 amps) per phase max.

Unless they are very sophisticated chargers and, at the rating you've given I suspect they are not, there will be harmonic currents.

 

[steve66]

A very basic power rectification circuit will likely have little harmonic distortion, but will not be that great of a steady DC source.

The equipment you are talking about likely has some significant harmonic distortion.

A basic rectifier circuit (diode followed by a capacitor in parallel with the load) only conducts during a very short part of the cycle, causing high harmonics.

Offhand, I can't visualize exactly what will happen to the neutral current when it has current from 8 differnt chargers on 3 different phases. Harmonics normally add, but these are occuring at different points in the cycle.

Basically, the 5.1 rated amps (I'm assuming that is input amps) will be compressed into something like 50 amps that lasts 1/10 of a cycle each cycle. (I'm just guessing here - it could be 25 amps that lasts 1/5 of a cycle, or 100 amps that only lasts 1/20 of a cycle, or anything else that multiplies out to an average of 5.1 amps.)

I would upsize the neutral just to be safe, and hope they have some type of filtering better than just a diode input. Otherwise, you may be drawing 150 amp peaks on each phase.

 

[Jraef]

90% of electronic devices now are using an SMPS (Switched-Mode Power Supply), even for chargers. SMPSs definitely produce harmonics. Some however, especially in medical equipment, will have filters built-in and may present a relatively linear load to the supply.

 

[suemarkp]

With 10-4 cable it seems like you have plenty of neutral. From what I've heard of harmonic loads, your worst case average power on the neutral with triplen harmonics would be about 2X of what is on the ungrounded conductors. With the neutral, the 125% sizing rule doesn't apply (you just take it at 100%). So if you take the 15A load, use 2X for harmonics, you're fine with #10 wire. You could probably even push the wire to its 75C rating if necessary, as wire nuts and panelboard neutrals are rated at 75C or more.

 

[kwired]

A basic rectifier circuit (diode followed by a capacitor in parallel with the load) only conducts during a very short part of the cycle, causing high harmonics.

Offhand, I can't visualize exactly what will happen to the neutral current when it has current from 8 differnt chargers on 3 different phases. Harmonics normally add, but these are occuring at different points in the cycle.

Basically, the 5.1 rated amps (I'm assuming that is input amps) will be compressed into something like 50 amps that lasts 1/10 of a cycle each cycle. (I'm just guessing here - it could be 25 amps that lasts 1/5 of a cycle, or 100 amps that only lasts 1/20 of a cycle, or anything else that multiplies out to an average of 5.1 amps.)

I would upsize the neutral just to be safe, and hope they have some type of filtering better than just a diode input. Otherwise, you may be drawing 150 amp peaks on each phase.

A basic rectifier is nothing more than a diode, a full wave bridge is a little more complicated version but is still nothing more than diodes. A diode conducts one direction only, but will conduct for the entire half cycle when voltage is proper polarity for current to flow. A full wave rectification circuit utilizes the entire voltage wave and conducts full current both directions of the cycle also. Where does the harmonics come from in this? You have half a wave of current that is in phase with the same half wave of voltage.

When you introduce high speed switching semiconductor devices that can switch on or off at any place in the cycle is when you start introducing harmonics, you can get distorted current waves that mean little or nothing in relation to the voltage wave.

Someone correct me if I am wrong.

 

[suemarkp]

I think its the capacitor. With a full wave bridge power supply and capacitor, the DC output voltage hovers at 1.41 times the AC RMS voltage (the capacitor basically charges up to the peak voltage, not the average). So little current flows along the voltage wave except near the peak where a bunch flows to keep the capacitor charged up.

 

[mivey]

I think its the capacitor. With a full wave bridge power supply and capacitor, the DC output voltage hovers at 1.41 times the AC RMS voltage (the capacitor basically charges up to the peak voltage, not the average). So little current flows along the voltage wave except near the peak where a bunch flows to keep the capacitor charged up.

You are correct. During the cycle, the voltage decays on the capacitor due to the load draining the stored energy. The current pulses happen on the voltage rise when the AC voltage exceeds the remaining capacitor voltage. The capacitor then begins to absorb energy again. After the peak, if the AC voltage decreases faster than the capacitor voltage decays, the source current drops back to near zero.

The rectifier is the diodes alone. The capacitor is the added filter.

 

[kwired]

You are correct. During the cycle, the voltage decays on the capacitor due to the load draining the stored energy. The current pulses happen on the voltage rise when the AC voltage exceeds the remaining capacitor voltage. The capacitor then begins to absorb energy again. After the peak, if the AC voltage decreases faster than the capacitor voltage decays, the source current drops back to near zero.

The rectifier is the diodes alone. The capacitor is the added filter.

But doesn't supplying a linear load help stabilize the input wave forms, where supplying a non linear load could distort them even more?

If the load is linear it will drain some stored energy from the capacitor, but is also drawing energy from the line at certain points in the cycle, and the capacitor is charged every half cycle instead of every cycle, all of this happening in a regular cycle pattern. With non linear loads isn't current being switched on/off at any point in the cycle, sometimes creating some pretty strange looking and sometimes high level current waves? Then if you add three of these types of current waves together in the neutral of a wye system you can get a conductor that is overloaded because none of those current waves cancel each other out, like they do for a linear load?