Thermal Overload Pre Trip?

[Renton]

Hi Guys I have a mixing machine that breaks down semi solid blocks into liquid form. The problem is that the operators need to add water to the mix to allow the blocks to be broken down. If they add too little water the machine trip on the overload (rightly so) but there is no real indication that the motor is overloading. I have tried making the operators add a definate quantity of water per block added but operators being operators tend not to take much notice. So my question is, is there a product available which can do the following- 1.Have a set of contacts for a pre trip alarm that with operate at a given current below the trip point? (so operator knows he needs to add more water) 2. Maybe has the ability to stop the mixing process after a determind amount of pre trip alarm with out reaching the full overload current. I did think of putting an invertor inline and using the current setting on that as the pretrip and keeping the thermal overload as the FLc trip, but most invertors i have seen need a physical reset to over come an current trip. Some one suggested an electronic overload but i canot find one to saticfies these requirements. Any help would be apprieciated

[Ken Moore]

There are lots of current monitors on the market, some with analog outputs, and some with dry contacts. One possible solution is to purchase a current monitor with dry contacts, when the mixer gets to the preset level of current, the contacts will change state. You can use this contact closure to sound a buzzer and start a timer, if the timer times out, drop out the motor starter hold in circuit. First impression suggest that one of the smart relays would work well for this, such as the LOGO,or TECO, or PICO.

[TConnolly]

Look at http://www.loadcontrols.com/ There are units available with analog outputs and adjustable alarm relays. You can adjust it to trigger an alarm at any desired motor load level. Its more reliable than current sensing alone. Under the application notes there are some examples of using them in mixing applications.

[Renton]

Many thanks for the replys guys The load control units look like they could be the way forward Thanks again

[paulengr]

Normal procedure for a current indication is to get a current transformer (CT). There are many sources, including the "usual industrial suppliers" like Grainger, McMaster-Carr, or MSC. There are two types you can get. An ordinary CT will have a range printed right on it such as "100:5" or "20:1". Most current meters will read either 0-1A, or 0-5A. So if you want to read current on a motor that is in the range of 0-5A, great. Otherwise, if it's up to say 80A, then get a 100:5 CT, which will convert the current so that 100A is equal to 5A, and 80A will be 4A. There are two basic types of CT's. One is the traditional donut which is much more rugged than the other. The other type is a "split core". With these, with the equipment shut off, you can take the two halves apart, slip it around the motor feeder, and then bolt it back together. This helps if taking the wiring apart is going to be a major chore. Most people install them in the starters though so this isn't usually a problem. The second type of CT is an electronic one that converts the signal from whatever range it is designed for into a 4-20mA current loop signal. Some are self-excited and some require external excitation. Although these tend to be more convenient with PLC's, I've found them to be less reliable than an actual current loop meter with redirected 4-20mA outputs. The last thing you need is a meter with a relay. Two names that I've used extensively are Red Lion and Simpson. Both offer several different meters. Pick one with dry contact outputs to trigger your alarm. If you want to send the actual current to a PLC or a strip chart recorder, then pick one with 4-20mA redirected output (not needed if you bought the fancy CT with current loop conversion built in). Either way, I'd also suggest the strip chart recorder or some other trending system. This shows everybody visually what's happening and ends all the silly arguments. One warning with CT's if you are not familiar with them: Do NOT EVER disconnect the leads from a CT while the line it is connected to is energized. If you are lucky, it will only throw sparks and may put a hurt on you. Quite often it also burns out your CT. ALWAYS with CT's, the trick is to take a jumper and purposely short out the output leads before disconnecting the CT from other equipment. Finally, there may in fact be a management problem going on here. If the equipment stops working, what is the operator supposed to do? If it's go to the break room, then it's not that the operator's don't necessarily care. It's that they see an opportunity to take a break any time they feel like it. I would suggest that the production supervisor makes them all aware that the rule is that you have to clean tanks, sweep floor, shovel something out...anything that is at least as much or more work than putting blocks and water into a mixer. And then enforce it. If you put a loud bell on it to where a production supervisor comes running over every time it goes down, it will soon give everyone the impression that this is a bad thing. If not, human nature says that the operator will screw up the mixer no matter how many bells and whistles you attach to it. As to not being obvious why it went down...check the prints on your overload block or check into buying a different one. Frequently they have both a NO and an NC contact on it. NC is used obviously when you wire the starter to trip the starter out. The NO contact can be used to at least turn on a light/buzzer whenever the overload trips.

[Renton]

Hi Paul Yes you are right it is really a management issue, the trouble is there is a fairly high staff turnover and staff rotation in the process, and keeping up with the training can sometimes be difficult. I did not explain it very well but there is clear indication of a trip but no indication that it is being over loaded (ie straining motor) so the first thing the guys know is that it has tripped buy then its too late and they have to drag a fitter of of something to reset it. The main problems areas are the initial mix whan its breaking down the solids and later in the day as it is also a heated process and some of the moisture will tend to evaporate stiffening up the mix, hence the need for a pre trip alarm. I like the idea of a CT + ammeter with a set of contacts as this will give a visual indication of the viscosity of the product as well as an alarm if no body is in the area. I can not say i have seen them with outputs (but maybe because i never looked) so i will look into the cost against a load monitor. Thanks for taking the time to reply Regards