24 VDC MOTOR - Page 1
Forum section: electronics / beginners

author: jarah
author_rank: Newbie
timestamp: October 28, 2019, 04:38:54 pm
content: I am having trouble with the inrush current on a 24vdc motor burning up brushes every couple months. How do I soft start the motor? It has to be automatic and the FLA of the DC motor is 39. What would be the most cost effective way to auto limit the voltage to the motor on start up? I've heard i could just put a resistor in line but I don't know how to choose a resistor. Also, the motor starts up between 40-60 times a minute during the busy period.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 28, 2019, 05:00:33 pm
content: It depends on a lot; is this a one-of? Probably just buy a cheap drive with accel control. Maybe just a inrush limiter would be enough for your app like this:https://www.digikey.ca/product-detail/en/ametherm/MS35-0R550/570-1023-ND/749863(Sometimes a resistor or inrush limiter is used and then shorted out with a relay for improved efficiency & max torque.)

author: Rigolon
author_rank: Regular Contributor
timestamp: October 28, 2019, 05:00:33 pm
content: Hi,Here is a link that may help you.https://www.eevblog.com/forum/beginners/soft-start-a-d-c-motor/Never tried the circuit suggested by Zero999.Although I think that this circuit is not able to control speed. But if all you wish is to soft start it's the one I would use.

author: rstofer
author_rank: Super Contributor
timestamp: October 28, 2019, 05:06:57 pm
content: One start per second just has to be hard on the motor. Given this start frequency, how soft can the start really be? It's not like it can take 2 seconds to get up to speed.I think if I were doing the mechanics of such a thing, I would let the motor run and control the load with a clutch and perhaps a brake.

author: andy3055
author_rank: Super Contributor
timestamp: October 28, 2019, 06:34:01 pm
content: Quote from: jarah on October 28, 2019, 04:38:54 pmYou may not have installed arc suppression caps from the brush terminals to ground. Most often I have seen small caps like 10/100 pf or so. you can have a range of caps and experiment with them. Some motors have holes in the casing where you can observe the brushes contacting the commutator.

author: max_torque
author_rank: Super Contributor
timestamp: October 28, 2019, 07:11:09 pm
content: What are you doing with the motor that requires it to stop and start so often? Is a brushed DC motor the right choice? ie a stepper or Brushless motor, or even a solenoid (linear "motor") might be a better choice for your application??

author: jarah
author_rank: Newbie
timestamp: October 28, 2019, 08:34:39 pm
content: I am almost positive that we're using the wrong motor but it is what we have and, myself being handicapped by my lack of knowledge and experience with DC power, i am not sure how to change directions. We use it on a vertical lift gate that, during peak hours, will be used around 60 times an hour for about 2.5 hours. Id love to go brush less, or use a step motor, but i don't have enough knowledge to even ask the right questions or get the right motor in to test it on our system. I am also trying to find a DC power supply that is big enough to handle the load of the motor so i can make the batteries just a reserve system. I bought one that was rated for 42amps, about 3 higher than the motor fla, and it will run the motor but it seems to over heat and fail, i am guessing it is overheating because I've never actually seen it fail and I'm not positive of what it is doing. Our Ac models are great but our DC have two big problems, 1- Its battery powered, 2-Brushed motor.

author: james_s
author_rank: Super Contributor
timestamp: October 28, 2019, 08:54:01 pm
content: If you're stuck with that motor, what you probably want to do is monitor the current with a current sense IC and PWM the drive to the motor, using the current sensing to limit the pulse width cycle by cycle.This is not trivial to do, but with the tech available these days it's not too hard either.Your failures could be caused by either inrush current (likely MUCH higher than FLA) or by excessive voltage, it's not unusual to see spikes of 3x or more the supply voltage when driving inductive loads like that.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 29, 2019, 06:57:17 am
content: Brushed motor is likely OK (unless of crappy quality), the problem is that you don't have acontroller. When suggested a brushless motor, the key is that a brushless motor can't run without a controllerat all, so the controller is a no-brainer. A brushed motorcanrun without one, but it's very crappy.A motor controller is a device which actively controls the motor current. By limiting the current to something the brushes and windings can handle, you can run against the current limit and even against physical stall for indefinite time, no problem whatsoever: it will run in constant torque mode. It's not an inrush limiter only; needing to regulate the current can happen, and often happens, in other situations as well, not only at initial startup.A brushed DC motor controller is simpler than a brushless controller, since the only thing it needs to do is to PWM a bridge, using current measurement (and possibly a speed signal) as a feedback. A brushless controller does the same, but in addition, needs to handle commutation.So look at DC motor controllers, with adjustable current limit. Off-the-shelf, they are pretty expensive, more than a simple series resistor, but with a proper controller, you reduce the system losses significantly, and it will last the lifetime.

author: max_torque
author_rank: Super Contributor
timestamp: October 29, 2019, 01:17:25 pm
content: Wait.the OP says "40 to 60 times per minute"then a later post says "40 to 60 times an HOUR"Which is correct? 40 to 60 times an hour is nothing. 40 to 60 times a minute a whole different kettle of fish!(once a minute vs once a second!!!)A lift access gate, that operates once a minute is fine with a dc motor when properly setup and controlled. Frankly, for something like that, you'll want a soft start for it anyway, so it glides open and shut and not slams open and shut! Use a PWM driver and current control to control motor torque.

author: max_torque
author_rank: Super Contributor
timestamp: October 29, 2019, 01:25:41 pm
content: Also, rather than let the tail wag the dog, lets start with some actual objective requirements.1) what is the mass of the load2) what is the friction of the load3) how quickly does the gate need to open? ie 1 second, 3 seconds, 5 seconds 10 seconds??4) can you determine the end stops BEFORE they are hit (ie limit sensors or a hard crash landing for the gate) what are the "end stops" of the load like, ie (hard, or soft)5) How often does the gate need to be opened in any given time period (worst case)Then,calculate the starting force required (which is effectively your stall torque of your driving system) allowing perhaps 50 to 100% over range to cope with "sticky" gates due to wear or a lack of maintenance etccalculate the peak power for the driver (force x velocity) and the average power during the opening, and then calculate the average power during worst case useage (ie opening as often as is likely)Once you've done all that, none of which is difficult to do, THEN look at what the best way of driving the gate actually is.......