3v dc motor problem and strange R-C attached

[Fred]

Hi,

In the spirit of ukdiy, my son was given (not by me) a toy circular saw! It is powered by two AA batteries but after a couple of months of "use", it has stopped working. Taking it apart it seems to use a motor that looks like this:

I have checked with a DMM and there is a voltage at the terminals but it won't turn. How do these fail? Since a new motor is only 70 pence, I thought I would swap it for a new one.

When I have seen these motors in toys before, they re usually connected directly to the battery. The data sheet on the page linked above does not say anything about special requirements, so I was puzzled that the existing motor has some resistors and capacitors soldered on and around it.

Each of the motor's terminals has a resistor in series. Brown, brown, gold, silver as far as I can tell. I tried to measure with the DMM and the numbers kept dancing; perhaps it was too low for the meter to measure reliably? By my calculation, each one is 1.1 Ohm, which seems a strange value. Why put one on each terminal, rather than use one 2.2 Ohm resistor on one terminal? I presume they are to lower the current and thus the speed of the motor?

And then there are three ceramic disc capacitors marked 104, which if my maths is right means 100nF? One capacitor is connected parallel to the motor's terminals.

One side of the second capacitor is connected to the +ve motor terminal and one side of the third capacitor is connected to the -ve terminal of the motor.

The other sides of C2 and C3 are soldered together on the case of the motor. As far as I could tell, the case is electrically independent of the terminals, so must be just a cheap way of soldering the two capacitors together.

But what is the point of these? Effectively C2 and C3 are in series and the C2 and C3 combination is in parallel with C1.

C2 and C3 in parallel make 50nF but that in parallel with C1 makes a total of 150nF. I can't believe the value is that critical so why not just use one capacitor of roughly that value, to save on production costs? And what is the point of them all anyway?

I realise that because the capacitors charge/discharge through the resistors but the values are so small, will they have any real effect? Is it to be kinder to the motor on starting and stopping the current?

TIA

[meow2222]

The RC network is to filter out interference. The thing should run perfectly well without it.

NT

[The Natural Philosopher]

shouldnt fail but can burn brushes out.

Not sure I haven't a few of these lying about..ex model aircraft. we ran em on 9V to get decent power out. Ok with a gearbox.

that is a interference suppression arrangement.

No, to suprress RF.

[The Natural Philosopher]

well without it.

the C is, but the R is to probably stop the motor burning out when stalled.

[Chris Holford]

In article , Fred writes snip

Snip The capacitors are for suppressing the interference from the commutator;

- it's quite common to connect them from each terminal to the case as you have seen so that radio frequency interference is shorted to the case. I wonder if the 'resistors' are actually inductors? This would tie in with the suppression of interference.

[harry]

The resistor capacitor network is to reduce sparking at the brushes so prolonging their life. The problem with this sort of motor is that for different loads/ currents drawn, ideally the brushes need to be in a slightly different position . As this is not possible, they try to damp out the resulting sparking with a resistor/capacitor network. Also there is a slight "soft start" effect. (ie lowers inrush current)

[Andrew Gabriel]

Brush wears out. It's probably just the end of a piece of sprung metal. I have opened these up and got them working again. Getting them closed with the brushes on the commutator can be a bit of a challenge - sometimes there are holes in the base to hold the brushes back during assembly.

They are probably minature inductors (which look like resistors) for RFI suppression.

If they're inductors, it's a standard LC suppressor network.

[Fred]

Thanks. As the new motor is only 70p, it sounds easier to replace it than try to open it and see.

If it is for interference suppression that would explain why they also connect to the body of the motor. It's very considerate of the manufacturer to do this; I've never seen it on a toy before. My education only went as far as resistors and capacitors, so I know nothing about inductors; it could well be that this is what they are. Would the brown brown gold value make sense in this application?

[Ian Jackson]

In message , Fred writes

Unless I've missed it, although you've measured the resistance of what appear to be resistors (but could be RF chokes), you don't say whether there is any sign of continuity through the motor (between the 3V supply input terminals).

Assuming your motor is very similar to the Farnell MM10, from the data sheet I can't immediately see any value for the resistance of the armature (plus brushes). However, on 3V, the input current is 1.1A for a torque of 10g-cm. This means that the resistance cannot possibly exceed

3 ohms - and is almost certainly a lot less. So I reckon that any resistance you measure will be essentially the two chokes in series (ie around 2.2 ohms). The most the motor could read is 5 ohms - but it's probably not much more than 2.2 ohms. If it reads more, something has come adrift. The armature may be open circuit, or the brushes may not be making contact with the commutator - or whatever.

BTW, if the motor WAS working OK, if you connect a DC millivoltmeter to the supply terminals, and spin the spindle with your fingers, the motor will behave as generator, and the meter should momentarily 'kick'.