230V DC Motor in rush current issue

Is the 2.8A the "do not exceed" current, or does it mean that the armature resistance is 82 ohms? In the former case, then something weird is happening (or you've got more stuff hanging off of that relay than one motor). In the latter case, you're abusing your motor as well as your circuit breaker.

You could switch the motor on with two relays: one that supplies the motor through enough resistance to keep the circuit breaker from tripping (and really should be enough to limit the motor current to something that'll make it live a long time), then after enough time has passed for the motor to come up to speed, switch in the other relay to bypass the resistor and give the motor full torque.

In this day and age, you really should have a real controller and not just relays.

NTC works on temperature/heat, it is obviously not cooling down enough between stops to provide inrush limiting.

Tim's suggestion with relays works but it is prone to contact wear if switched a lot. It also comes with a speed-up penalty.

If you are feeding it DC the clean way would be a PWM-controlled ramp-up. Then you can train the slope to be close to the limit for maximum acceleration but far enough from the limit in order not to trip the breaker.

If you are feeding is AC there's other ways. For AC it works like this but I developed it about 30 years ago at the university and the IC is unobtanium by now, just to show the principle of operation:

In a nutshell it's a glorified AC dimmer with reset and automatic ramp-up. Make sure it can handle inductive loads if you design one.

Hmm. DC motors don't experience the same sort of magnetizing inrush that AC devices do. In fact, due to their winding inductance, the current draw upon energizing a 'stalled' motor tends to be less than the locked rotor current, ramping up to that value as controlled by the R/L time constant. What kind of DC motor is this (PM, shunt, series, compound)?

Are you reversing the motor between starts? Is it possible that the motor hasn't come to a stop in one direction before energizing it in the other?

Also, I'd look at the motor's duty rating. Its possible that it is being restarted too frequently and its operating at too high a temperature, reducing its winding resistance somewhat. But from 2.8A locked rotor to blowing a 5A breaker? Perhaps your breaker is bad.

if it sorta works with a relay, something simple like a switching current limiter at around the stall limit of 2.8A should work and give maximum acceleration and never trip the breaker

something simple like an npn and a current sense resistor controlling a fetdriver and a fet

shouldn't need to be super accurate

-Lasse

That's why I questioned the use of relays in my post. But if all the OP understands is relays...

(Note, too, that if the OP is using SSRs but is saying "relay", that the problem of contact wear goes away).

Train? As in control it open-loop? Why not just monitor the current and hold that to the maximum rated motor current? That's kinda what I was thinking when I said "real controller"

I tried reading the schematic, but I don't understand German so I couldn't make heads or tails of it. (It's how you Euro-guys spell "resistor" that gets me).

Presumably an SCR switching dingus could also servo the motor current, but the pain might not be worth the gain.

Yes, SSR would be the ticket if he wants relays but the motor starts and stops a lot. Can get expensive.

What I mean is ride a current spike that is acceptable to the breaker. They can take a certain inrush, just not a whole lot and not for too long. It's a matter of current over time, not just peak current. So if ok with the manufacturer you can exceed the rated motor current for a brief moment. Just like many jet engines where you can push them a bit harder for take-off but must back off a bit after x minutes.

Yeah, resistors are squares instead of squigglies. The only German words in there are start-up current limiter (Anlaufstrombegrenzer) and Input/Output (Eingang/Ausgang). And, mea culpa, I hate it when people don't used reference designators. Here it was me who didn't ... whoops.

The date is also reversed over there. 13/4 is not the 4th of Fruptember but April-13 :-)

Well, the circuit in the link worked really well. At the university they used it for big hand-held machines. Some of the angle grinders were not all that healthy on your wrist when you turned them on. This circuit made them come on with a greatly reduced roll and yaw. The electrical effect is the same as the OP wants, just for a different purpose.

This doesnt make sence, if the stall current is 2.8A then you cant trip a 5A breaker unless it or the motor are faulty. Where did you get the stall current value from? Have you tested your motor for armature shorts and shorts to earth?

ok, let me ask you this.. when you switch off the motor does the load force the motor to retreat in the reverse direction and if so, are you switching on the motor while it may still be coming back?

If you don't wait for the DC motor to come to a rest before turning it back on, you'll get twice the current because the motor is acting as a generator but producing reverse voltage. That would be like twice the current until it can recover.

What you may want to try is use a voltage detector on the armature before you are allowed to switch on. If voltage is there, your on circuit will simply not be allowed and when it does come on, it'll simply latch from there on so that it won't get switched off from the voltage sensor or the sensor gets switched out.

What some do is put a dynamic braking resistor that connects to the armature during the off state that will slow the reverse spin and acts like a soft stall for your machine at the same time. Of course, you'll need to contact that can switch the resistor in.

The Peak average of watts on your motor is 230*2.8Amps = 644 watts. Then if you calculate the actual speed verses the time it takes to slow down, we can derate the wattage required for the R.

If you take the Average Watts/2 which will be a good start of a R of around 75 ohms or so. Maybe even a standard 100 ohm 500 watt in your case will cut it.

Of course, this is a soft R compared to what some use, many like to really clamp the motor hard and use just enough R to lower Q and save some contact surface. a R in the range of 10 ohms is used in this case.

For proper calculations. You need the Average Power, the rotation speed of initial braking minus the target speed and time it takes to reach that speed. This Speed is normally done in RADIANS (w) but I've done in RPMs. Its the same as long as you use the same units through out.. With that you can calculate a derated R.

Something to think about.

Jamie

g it

This has been suggested a few times, but I dont think it can be the problem in this case. 2x2.8A is only upto 5.6A, no 5A breaker will trip on that for under a second. Something else must be going on.

I presume its tripping at switch on. One simple solution using similar technology to whats already in use is just to use a 3 position switch instead of 2, so it goes fom off to on via a resistance then quickly to full power on. Thermistors wont do because they take too long to cool down. Beware of downrating the resistor if the switch could possibly be left in the series R position. A heating element often makes a practical high power R, but check its cold resistance.

Really the OP has the choice to troubleshoot properly, and implement a costly repair (in downtime if nothing else), or to do something like the above to just get it working in practice.

It might just be simpler to replace the beaker with a slower acting type. In UK parlance one would replace the B type mcb with a C or D type. 10A spikes on a 5A rated circuit running at 2.8A intermittent wont cause any harm.

NT

If you say so We use thermo breakers that are designed for the final current which is usually the max current over all and it then has a magnetic current of 2 to 3 times that for inrush devices like transformers and motors.

A breaker designed for a motor is not the same as one for conventional circuits found in basic places like your home.

And for the record, if that motor will spinning backwards at full rate when the forward voltage was applied, it will generate twice the current and if the breaker is already hot from repeating sessions of this, it will trip..

On top of all that, breakers are designed, depending on the usage code, to get weaker over time if the trip keeps happening. This forces you to have some one look into the problem.

Jamie

g it

The thermal element is designed to never trip at rated current. That means 5A continuous on a hot summer day, with rather warm cables entering the CU. Then there's tolerances to take into account, so tripping will never happen at 5.6A, ever.

Magnetic component having 2-3 times trip threshold is the most senitive type of mcb available, not the best choice for a motor load. But even so 5.6A will never trip it.

onal

No, type D mcbs use a higher current threshold for the magnetic element, of 10-20 x rated current.

Only if its defective. Mean current is far below the 5A breaker rating.

I dont think I've ever encountered that happening.

NT

The real question here is Did this setup ever work properly?

If YES, then you have a problem that should be fixed. not band aided.

If NO, then you need to go back to the designer/supplier and get it designed by a competant engineer.

It somewhat depends what country the OP is in. A press run on a 5A panel breaker sounds a bit out of the ordinary for the first world.

NT

It is time to rebuild the motor.

?-/