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brianafischer

DC Motor Control and Overload Protection

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I am currently investigating 12 VDC motor control. The control panel has a 3-phase 480VAC drop and the goal is for ON/OFF control of a 12 VDC 20 amp max continuous motor. The duty cycle is a minimum of 5 seconds ON/OFF. I have seen many DC motor control designs that incorporate a simple high-current relay such as this with no overload protection. My concern is the lack of an overload for the motor due to the frequent switching. Almost all AC motor designs incorporate motor starters with overloads. How can this achieved with DC motors, and what are your suggestions for motors operating at 12VDC? Thanks!

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OK, this is a bit confusing. First off, this sounds like a small universal motor, something you'd use on your fishing boat from a battery. It's very expensive compared to a comparable AC motor. It will have just two leads and you simply fuse one or both legs with a fuse straight out of the NEC sizing guidelines. If not, then I'm confused. You're talking about using DC but without a drive. I can't think of any application where I'd use DC over AC without a drive. With drives, DC is somewhat of a pain in the rear. Not sure why you want to bother. The motors are more expensive both to replace and maintain. The three advantages previously held by DC motors were better speed/positioning control, much lower cost drives, easier control above base speed, and easier to "link" drives for torque sharing (do a series/parallel wiring arrangement). With the advent of IGBT's and both sensorless flux vector and encoder-based vector drives, all of those advantages have disappeared. About the only remaining advantage (which is beaten out by AC servo motors, especially with DDM) is less cogging at very low speeds. If you still insist on doing anything with DC, call your local motors/drives guy (Baldor, Square-D, Allen-Bradley). They can easily tell you exactly what you need. DC in general is a bit tricky in this area. The drives come with all the motor protection you need. You can fuse all 4 legs. However, there's a serious issue with the fields. Depending on what happens in the event of a motor failure, it can be pretty nasty. If you have an unchecked field loss, the motor goes into field weakening and will go to whatever RPM it can reach before it burns up or flies apart. I'm very familiar with the fun of DC drives because my plant has a solid mixture of DC, AC, pneumatic, and hydraulic drives. Other than the small amount of AC equipment and some of the pneumatics, the rest of the stuff is nothing but a very expensive pain in the rear to maintain compared to the pneumatic or AC equivalent. The only reason I haven't gutted all of it is the cost of doing it.

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I would like to avoid this at all costs, but a simple fuse may not stop a DC motor from running away!

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It requires all 4 leads (armature and fields) to be brought out separately to detect the problem, or some sort of "speed switch". The way that field loss is normally detected is based on monitoring the current draw on the fields. Doing it is fairly complicated so it's pretty much in the realm of a drive.

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i have done many automotive projects (assembly and test stations) with 12VDC motors (seat, windows, mirrors...). first of all - these motors ARE cheap (even too cheap - but that's ok considering what are they meant for and how much they have to run). you don't want to switch those on very often - or for very long. the other end of spectrum are continuous duty motors (like windshield wiper, fuel pump etc.) for switching we only use DC relays (pretty much the same with one you posted link to) since they are much cheaper and better for this job than contactors. as far as circuit protection goes there are different things that can be done, all depending on situation (can be fuse, breaker or electronic such as in drive or PSU for example). bottom line is time delayed overcurrent protection and current limiting if possible (this can be done in different ways). we use much bigger power supplies, 40 or 60A, sometimes up to 100-120A. only in rare cases motor current (stall) will exceed 20-25A even if you use big PSU and large conductors in test cable. this is because wires in the wireharness of the assembly are so small (#18 to #16 at best). to reduce voltagedrop everything is mounted close to load (PSU, overcurrent protection and relays). for example if it is assembly line, lowest end protection is used - since motors are only powerd briefly (few seconds) just to align parts so assembly can be done. circuit breaker is good enough (carefull here - check the rating and specs). since in either case there is PLC controling motor function, it is also commonly used to monitor how long one motor has been running (retentive timer) to prevent it from overheating (protect motor - and more importantly operator). to reduce cycle time of the station it used to be common to increase PSU voltage but there are problems with that. Stall current is going to be much bigger (cheap solution is series resistor - properly sized of course). this became hardly pratical in recent years because majority of products come with one or another type of electronics so voltage must be limited to certain value (usually 14.5V) so PSU units with remote sensing are right choice. for high end (like test fixture) it is required to monitor and record (graph) number of parameters such as voltage, current, torque etc. in most cases there will be lab class power supply (like Xantrex etc.) and since those are nice devices, they have brains, analog inputs and outputs, communication etc. this means it is possible to see fast changes in current etc. in other words PLC can stop the motor much earlier in case of overcurrent but this doesn't replace overcurrent protection devices. For example if it is test station and power supply doesn't have analog outputs to indicate current (like any decent lab PSU), we use shunt with mV/V converter. Current transducers with hal sensor (they have hole to pass wire through just like in current transformers) are cheaper but have poor accuracy and response time which makes them nearly useless in such applications. for example if you run 80% current (such as stall) and then turn off load, it will take considerable time for reading to settle( some 2 seconds to drop to 10% reeading and good 5sec to get near 0%). shunt and transducer get this in milisecond but this is not cheap (~ $500 per motor). if you are looking for bargain there are chips from Allegro that do pretty much the same and code $5-10. I have some but didn't have time to test them.

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12 DVC motor is hard to control by a contactor, what I recomment is using a solid state relay with the corresponding fuses size.

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I am specifying a power supply to provide power to two 12VDC fan motors. These may be switched on at the same time. The motor has the following inrush current curve: The voltage needs to be maintained at 12V over the following motor start-up current 80A @ 20msec, 55A @ 40msec, 35A @ 140msec, 25A @ 220msec I see that the PSP-600-12 has a 12VDC, 0-50A output. The Overload Protection is 105% - 135% (67.5A) constant current limiting. Any advice/opinions?

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i guess this is one motor... any reason to maintain voltage at 12V? this psu seam to hold up full load current (50A) for 20ms. after that you can count on lower voltage on output due to current limiting (even with single load)

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Thanks for the reply. You are talking about the "Output Parameters": "setup rise time" and "setup hold time" here, correct? Don't you mean the PSU can hold overload current (67.5A) for 20 msec? Isn't this extra power coming from an output capacitor that will drain after 20msec? The product page for the fan is here and the data sheet is here. This fan is used in a system that will use ON/OFF control (with a minimum safe ON time for me to determine) that will be used to control the temperature. I know there are easier ways of doing this, but the purpose of this configuration is to simulate automotive conditions. I would like to provide adequate power to the fans so the start-up of the fans does not derate the life (which could occur if the voltage dropped too low during inrush due to current limiting). My current plans are to use two of the PSP-600-12 series in parallel which could support up to 135 amps. Is this overkill? Edited by brianafischer

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