Capacitor hook up for star delta motor starter
Capacitor hook up for star delta motor starter
(OP)
What is the proper method of connecting power factor correction capacitors to a star delta motor starter? Is there any NEC reference for this (I could not find anything in NEC 460)? The motor in question is 3 phase, 208V, about 500 HP.





RE: Capacitor hook up for star delta motor starter
Because of the potential for a major contribution to a VERY nasty transient when switching from Star to Delta, the best method is to have a separate contactor in front of the capacitors and only switch them on after the start sequence is complete. Tie the output of the contactor to the line side of the starter ahead of the point where the power splits off for the two circuits. Anything else is asking for trouble.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Capacitor hook up for star delta motor starter
Can you elaborate a bit on the reason for a large potential transient during the transistion. Should I be able to measure this with a disturbance analyzer?
RE: Capacitor hook up for star delta motor starter
When the starter switches from the Star to the Delta pattern, it must open the circuit to avoid a short (let's not get into Closed Transition here). In doing so, the motor is left un-powered, if even for a second. During that time there is no torque output, so the load will slow down. This alone means that under the best of circumstances, the motor flux penetration is lost in transition and must be re-established for the Delta run operation, so there is usually as much inrush current, again, as there would have been without starting in Star. but it gets worse. Large motors have significant residual magnetism, so when the motor is re-connected in Delta, it is often still acting as a generator. Now you are connecting a generator to a line that is out of synch! So depending upon where in the sine waves the re-connect is made, there is either a huge current spike or a huge voltage spike, or sometimes one followed by the other. The torque transient that this can create has been known to shear the shafts off of the motor. the voltage transient is capable of shorting out SCRs in other parts of the facility.
Add capacitors to that mix and you are essentially GUARANTEEING that the motor field is being maintained during transition. Most likely those "energy saver" caps were blown the first time they were energized. Not matter though, they were not doing squat towards saving energy anyway, your people were scammed by that salesman.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Capacitor hook up for star delta motor starter
The typical installation would be to connect the caps to 1M because it closes at start and then stays energized during the transition. The caps can still see some transient voltages during the transition as jraef describes.
Raising the power factor will lower the line current draw which will lower the conductor heating a little bit. But, you will never see anything close to 50% savings, not even 5% most likely.
RE: Capacitor hook up for star delta motor starter
RE: Capacitor hook up for star delta motor starter
The problem will be if both open transition is used and the capacitor is connected to the motor terminals.
Open transition generates severe transients on transition. A capacitor on the motor terminals will make these transients more likely and more severe.
To add to jraef's explanation, when the motor circuit goes open during transition, the frequency of the voltage generated by the motor will always drop to a frequency determined by the motor speed. With an induction motor in normal operation this frequency will always be less than line frequency. The window of opportunity for a reclosing in sync is very small, and even if the voltages are momentarily in sync at the instant of closing, the lower frequency will give rise to torque and current transients. Capacitors will make a bad situation worse.
Capacitors on the line side will reduce the starting current slightly, but so little that although there have been papers written on the subject most of us have never seen capacitors used to reduce starting currents. If your capacitors are properly applied they may reduce the starting current by about 7% or 8%.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Capacitor hook up for star delta motor starter
RE: Capacitor hook up for star delta motor starter
BUT...
Take this diagram showing how PFC caps are connected internally and add it (mentally) to the diagram above, connecting the caps between 2M and S.
You will see that, as soon as "S" opens, the capacitors are instantly charged by virtue of the fact that 1M is still closed and the charging current flows through the motor windings to the caps across their internal Delta connection. So the caps are now available to maintain the motor field as it slows down.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Capacitor hook up for star delta motor starter
BUT, we will have an inductor in series with a capacitor on a 60 Hz. circuit. The voltage across the motor windings or across the capacitors or possibly both may exceed line voltage. It will depend on the respective reactance of the motor windings and the capacitors.
This may not produce similar transients to the transition, but I would try to avoid it. If you are able to monitor or record the voltages, you may see some interesting voltages across the caps and the motor windings when S opens.
I would put the caps on the load side of M1 and let them supply a little bit of the magnetizing current as the motor starts.
Nice drawings jraef.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Capacitor hook up for star delta motor starter
RE: Capacitor hook up for star delta motor starter
I'd be more interested in holding their feet to the fire about the supposed "energy" savings, insisting they measure it in kWh over an exactly measured real operating time under the same conditions, not just Amps or even kWh over a different number of operating hours or conditions as may of them try to do.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Capacitor hook up for star delta motor starter
1) Your mileage may vary.
2) Savings may vary according to your PoCo's tariff structures.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Capacitor hook up for star delta motor starter
"Energy" savings is not the same as "cost" savings. Saving energy certainly saves cost, capacitors only save costs if there are tariffs associated with poor power factor. But in this case... "a salesman convinced them to install "energy saving devices" to reduce their energy use by up to 50%!" There is NO WAY that adding capacitors will save 50% energy! Even 50% of their internal transmission losses due to I2R would be a stretch, but those losses are typically so low as to not be worth considering.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Capacitor hook up for star delta motor starter
AMEN brother...
Keith Cress
kcress - http://www.flaminsystems.com
RE: Capacitor hook up for star delta motor starter
On another note, the transition is so quick that the back emf from the motor has no time to decay (capacitors or not). This is why an open transition Y-D starter has such a high current and torque transient. The problem that I see is when the 2M closes the caps will "dump" their energy into the motor contributing to a higher current and torque transient.
RE: Capacitor hook up for star delta motor starter
RE: Capacitor hook up for star delta motor starter
RE: Capacitor hook up for star delta motor starter
The placement of capacitors in relation to the overload sensing is subject to code rules. The caps may sometimes be connected directly to the load side of M1 ahead of the current sensing devices so as to not affect the overload protection. I don't have my code books to hand, but check your code.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Capacitor hook up for star delta motor starter
Rule 2
Size motor overload protection to
account for capacitor-supplied
current.
Overloads are typically set to
measure the total current drawn by
the motor. When PFCCs are used,
they become the source of part of
that current. If the current they
provide isn't registered by the
overload protectors, potentially
damaging amperage can reach the
motor. The simplest way to ensure
that the overloads "see" all current
supplied to the motor is to position
the PFCCs upstream of the overloads
as shown in Figure 2.
If the capacitor connection points
are downstream of the overload
devices, route the PFCC leads
through the overloads as shown in
Figure 3. This assures that the
overloads register both line and
capacitor-supplied current.
The last paragraph describes the installation at the chiller, except the capacitor leads are not routed through the overloads.
You are correct about the code. NEC 460-9 requires that when capacitors are installed at load side of the overloads, the overloads must be resized to account for the reduced currents.
If the capacitors are installed at line side of the contactors, than the code requires a separate disconnect and overload device. The installers moved the cap terminations from line side of the contactors to load side because they did not want to install the required over current protection and disconnects!
So far, the installers have made two attempts and both contain NEC 460 violations.
However, my concern when starting this thread was to explore the potential for the caps to produce damaging transients. At this point, it appears that if they terminate the caps to load side of the M1 contactor and then run the capacitor leads through the CTs, they will have an installation that meets Code requirements and does not contribute any greater transients than the original system.
RE: Capacitor hook up for star delta motor starter
Bill
--------------------
"Why not the best?"
Jimmy Carter