24v starters

Status
Not open for further replies.

mbeatty

Senior Member
Location
Illinois
Here is the situation

We installed a conveyor system with 15-24v IEC contractors

10 of the contractors are switched only in alarm
5 are switched pretty heavy.

The 5 that are switched fault out allot some times every 15min

They sent out surge protectors, installed them, no help
Changed card from Triac to relay output, no help
Tonight pulled out the break out board and point to point wired the starters.

Oh installed another 240w power supply wired in parallel ,no help

our company stance has been to go 120v

the vendor sent out there programmer and he has disabled the alarms and the operators say the motors just stop now.


NOW my Question is this a programming problem or is there a problem with switching the 24v that heavy?

In the past few years, we have been switching to 24VDC contactors in our products and have experienced some similar problems (most during testing, some in the field). Some of the 24VDC contactor coils have a heavy inrush which can cause faults if the power supply is not up to the task. We have increased the power supply capacity to resolve the issue. Just a thought.
Mark
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
I hate 24 volt contactors because voltage drop affects them badly. I was called to a plant where they had "shredders" that all had a 6' cord on them. They had 10 or so of these machines, all the old ones worked fine, all the new ones worked intermitantly. What I found was, if the machine was plugged straight into the wall, all would work fine. No problem. But if you unplug the machine, and plug it into a 50 or 100' cord, that would cause enough VD on the primary side of the control xfmr, that it would not produce voltage high enough to activat the starters. All the 120 volt machines worked flawlessly, all the 24 volt machines were extremely unpredictable. Sometime the coils would pull starter in, sometimes not.

Slight tangent based on this post. I think there needs to be a differentiation here between 24VAC and 24VDC. What was described above was undoubtedly 24VAC and I too think that is a fools folly to use it, because yes, VD is a big problem.

But I have been using 24VDC now for a few years and I love it. The thing is, you CANNOT get cheap on your power supply unit. Not all PSUs are created equal, and my experience is that the cheapest Chinese ones are the worst... go figure. Most of the European PSU designs, even if made in China or India as most things are now, have significant over load capacity and a very wide input voltage range. So for example, I have a project right now with 208V input but the machines are going to China and they have line voltage fluctuations of up to 30% throughout the day! The original design used a cheapest model PSU and when the voltage dropped to 200V, the thing just turned itself off. I have been replacing them with Pulz and Siemens 240V power supplies, they can take a VD down to 85V and maintain the output between 23 and 25V, plus they can deliver 150% rated current continuously (with an increase in temperature however). The contactor dropout problems went away.
 

mcclary's electrical

Senior Member
Location
VA
Slight tangent based on this post. I think there needs to be a differentiation here between 24VAC and 24VDC. What was described above was undoubtedly 24VAC and I too think that is a fools folly to use it, because yes, VD is a big problem.

But I have been using 24VDC now for a few years and I love it. The thing is, you CANNOT get cheap on your power supply unit. Not all PSUs are created equal, and my experience is that the cheapest Chinese ones are the worst... go figure. Most of the European PSU designs, even if made in China or India as most things are now, have significant over load capacity and a very wide input voltage range. So for example, I have a project right now with 208V input but the machines are going to China and they have line voltage fluctuations of up to 30% throughout the day! The original design used a cheapest model PSU and when the voltage dropped to 200V, the thing just turned itself off. I have been replacing them with Pulz and Siemens 240V power supplies, they can take a VD down to 85V and maintain the output between 23 and 25V, plus they can deliver 150% rated current continuously (with an increase in temperature however). The contactor dropout problems went away.

Thanks Jraef, My post was extrememly generalized and undoubtedly had a "barely big enough" control xfmr. I have seen plenty of 24 volt systems work flawlessly, so I really should not have wrote that I hate them, especially since this problem is actually the only problem I've had with them(barring normal instances)
 

GeorgeB

ElectroHydraulics engineer (retired)
Location
Greenville SC
Occupation
Retired
The thing is, you CANNOT get cheap on your power supply unit. Not all PSUs are created equal, and my experience is that the cheapest Chinese ones are the worst... go figure. ... I have been replacing them with Pulz and Siemens 240V power supplies, they can take a VD down to 85V and maintain the output between 23 and 25V, ...
I commission industrial equipment, and the Siemens 3 phase in are superb. Those I see are rated for 200-600 in. The most recent startup had 40 amp output with units about 5*5*10 inches ... and they were adjustable from 24-28. I usually set them at about 26 at the supply for "average" VD.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100705-1400 EST

Some general comments based upon comments above.

A DC relay, contactor, or solenoid does not have a large inrush current. Unless there is a capacitor across the contactor coil the initial current will not exceed the steady state current which is V/R. Before a solenoid is energized the inductance of the coil is a lot lower than after it is fully seated. As a solenoid closes its inductance goes up.

Experimentally with an AB #2 starter(AC) but operated with DC and a coil resistance of 40 ohms, a power supply voltage of 60 VDC, and vertical orientation I can just pull-in the relay most of the time. The current curve rises to about steady state, then at about 100 MS there is a dip from the inductive effect from the plunger reaching its seated position, and a moderately quick rise to steady state.

There is a major disadvantage to using a DC solenoid vs and AC unit. This results from the need for a large current to initially pull-in the solenoid, and after pull-in the need for a much lower holding current. This is a property of the magnetic circuit. With DC excitation the pull-in current is equal to the holding current unless some other means is used to modify the current. In an AC solenoid the inherent characteristics automatically lower the current after pull-in. Thus, much less coil power dissipation.

On the other hand if the thermal design of a DC solenoid is such that the continuous high power dissipation can be handled, then it is burnout proof when stuck in any position. A normally designed AC solenoid when stuck part way to the seated position may very well burnout.

You will note at 60 V and 40 ohms the power dissipation in the coil is 90 W. Very high for that little coil.

See my post numbered #6 at
http://forums.mikeholt.com/showthread.php?t=123694&highlight=contactor+pull-in
This provides some information on the AB #2 contactor.

.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100705-1617 EST

ptonsparky:

Create a model as follows:

A 10 ohm load that can be switched (S2) on and off from the following capacitor. A 100,000 mfd capacitor, a 1 ohm resistor from the + side of the capacitor in series with another switch (S1), to the + side of a 10 V battery. All negative terminals connected together.

Initially S2 and S1 are open, and the charge on the capacitor is 0.

Upon closing S1 the voltage increases exponentially from 0 to 10 V, and the current from the battery decreases exponentially from 10 to 0 A.

Next open S1 and the capacitor voltage remains at 10 V. Then close S2 and the capacitor is discharged exponentially from 10 V to 0. Initially the current is 1 A decreasing to 0 A. The discharge rate is 10 times longer than the charge rate.

The time constant of charging 1*0.1 = 0.1 second. Thus the voltage or current changes about 63% in one time constant. The discharge time constant is 1 second.

Next start with a fully charged capacitor and S1 closed. Close S2. Instantaneously the capacitor can not change voltage. Thus, initially no current from the battery. Thus, no inrush current from the battery to the capacitor. The capacitor voltage will drop from 10 V to 10*10/11 = 9.0909 V. The battery current will never exceed 10 - 10*10/11 = 0.9191 A.

No. There is no inrush current as a result of the load. If the load is non-linear, then the problem is more complex. A DC solenoid does not cause an inrush current greater than its steady state current.

.
 

mbeatty

Senior Member
Location
Illinois
The nice thing about DC is that there isn't an inrush current. Measure voltage at the coil when powered and tell us what it is.

My recent experiences have shown that some DC coils have a high "PULL-IN" current and low "HOLD-IN" requirement. I have dealt with several manufacturer's and have done testing. Wouldn't "PULL-IN" be considered the same or similar to inrush?
Just asking, as I want to know.
Regards,
Mark
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
100714-1339 EST

mbeatty:

"Inrush", "Pull-in", and "Drop-out" refer to quite different measurements.

"Inrush" generally refers a large initial current to a load when a constant voltage is applied to the load. See photo P1 at my web site http://beta-a2.com/EE-photos.html . This is an illustration of inrush current when voltage is applied to the bulb when the voltage is at its peak. Photo P2 shows a much lower peak when turn on is nesar a zero crossing.

A relay, solenoid, or contactor is an inductor with some series internal resistance. Look up the differential equation analysis of a simple battery, switch, series resistance, and inductor. If the initial conditions are that the switch is open,and there is no current flowing in the inductor, then just after the switch closes there is still no current flow. Following the switch closure the current rises exponentially to a steady state value of Iss = V/R .

"Pull-in" current is the current required to pull in the armature. Greatest current to move the armature is usually when armature is maximum open. This current has to be below the steady state current referenced above.

"Drop-out" current is the current level after a relay has been pull in that you must be below for the relay to drop out. This current has to be below the "Pull-in. If not oscillation would occur

.
 

RETRAINDAILY

Senior Member
Location
PHX, arizona
Thanks for all the Replies and info :)

The Conveyor Manufacture has said this was the fist time dealing with the new low current starters and sent out replacement contractors with standard 24v coils and it all works fine now.

So we will send them back to AB they want to check them out.
 
Status
Not open for further replies.
Top