# What is the actual stall current of a Mabuchi RS550VC-8518 12v DC motor (out of a Dewalt 12v battery-drill)?

This question is part of a long-winded way of working out the maximum working-under-duress current-load on a 3S Li-Ion battery from a 10.8v/12vMax Dewalt Drill.

The replacement motor for the drill is a Mabuchi RS-550VC-8518. I've hunted high and low for a data sheet but the motor seems too old to have one available. Various listings I've found for these specific motors on E-Bay and the like list the stall current at any where from 3.0 Amps to 10 Amps (with no torque figures given...), while the closest wind I can find on Mabuchi's site (RS-550VC-7525) is rated at a wooping 85A/549nM.m stall and a similar Kysan motor (RS-550SH-7524) is rated at 32.4A/117.6nM.m stall.

Firstly, can both the figures for the RS-550VC-8518 and RS-550SH-7524/7525 be correct? ie: is there something about DC motors at this scale that could account for such a huge difference in stall current with that amount of difference in wind spec? (I'm more used to 130/180 sized motors...)

If not, which figure is more likely to be correct? 3-10A or 32A or 85A?

Best regards, Nscrup

• Finding specs is not the purpose of this site. But consider that at stall there is no back EMF, so if no other losses get in the way, the motor is basically a length of wire across the supply. Brush or electronic commutation losses, over current protection, or or battery internal losses may dominate. – Chris Stratton Feb 25 at 5:42
• When motor is stalled, then N=0, further back EMF=0V, so we have a siplification V=R*I. The stall current I=V/R is proportional to applied voltage and inverse to the winding resistance. If you know the winding resistance you can calculate the stall current. – Marko Buršič Feb 25 at 7:56
• Keep in mind that DC motors also differ in number of poles in armature, 3, 5, etc, which makes big difference in resulting torque. – Ale..chenski May 1 at 1:49

## 2 Answers

If you are searching a replacement motor, then you should firstly check if the dimensions match.

The second importan thing is that you may have already discovered is that the same size motor can have different specification, how come? It's because of the different windings, that motor is built for.

Same size means almost same nominal power, but different nominal working points are possible.

• Tiny wire, lots of turns: high voltage, low torque (amps), high revs
• Thick wire, small number of turns: low voltage, high torque (amps), low revs

How you'll find almost equal motor? By choosing one with same motor constants kt- torque constant or kv-voltage constant. Note: these two constants are the same, just fliped arround, so if you identify one of them you are done.

kt - torque constant A/Nm.

Mabuchi:

• stall current/stall torque = 85A/550mNm = 0.1545 A/mNm
• max eff current/max eff torque = 10.1A/58.3mNm = 0.1732 A/mNm

At the stall point the kt tends to be lower due to core saturation, so compare whatever you have for equivalent.

• Hey, thanks for the reply. What we're chasing isn't so much a replacement motor as trying to ascertain what sort of "rated" stall the Li-Ion battery that came with the drill actually has to deal with in real life, with an eye to building an analoge picture of how they'd perform in applications currently powered by small high-current LiPo's. Knowing the rated stall current of this motor would help, but also understanding more of how winding spec's affect performance (ie: there's only 1 wind difference between the RS-550SH-7524 and the 7525, but a gap of 20A in the stall current, so ??) – Nscrup Feb 25 at 10:56
• @Nscrup You ave linked the datasheet only for RS-550SH-7524. – Marko Buršič Feb 25 at 11:12
• Sorry, both were linked above but linking here too :) Mabuchi RS-555VC-7525, Kysan RS-555VC-7524. Again, really just trying to use these examples to get a better guesstimate of what stall current would likely be for Mabuchi RS-550VC-8518 on 12v. I'd imagine much of it though might come down to the other components used in the motor(?) Links to RS-550VC-8518's were too long (EBay) to fit in character-limit here, and all were lacking good data. – Nscrup Feb 25 at 12:06

The datasheet is correct but the Drill will add battery ESR and driver resistance to the motor DCR* of the motor coils to limit stall current in the drill. So it won't be that big.

My Rule of Thumb for DC Motors is close to your 1st link specs.

• No Load amps = NLA depends on gear friction.
• Stall Current = SC
• Full Load Amps depends on duty cycle and temp rise
SC/NLA=100:1 or NLA/SC=1%
SA/FLA = 10x no load current +/-20-%
• Stall current = 5x to 12x full load current which is still proportional to torque.
• Torque is proportional to stall current and operational current minus no load current which is only 1% + friction

• Motor DCR = DC coil resistance = Vnom/SCA for all windings ( stall current factor )

# RS-550VC-7525 specs

                    VOLTAGE             NO    AT
OPERATING          LOAD   MAX. EFFIC.   STALL
MODEL        RANGE   NOMINAL SPEED CURRENT SPEED CURRENT CURRENT
units=>       V     V   RPM    A    RPM     A        A
RS-550VC- 7525  6.0-14.4  12V   17600 1.2   15730 10.1      85.0

• Stall Amps 85A
SA/FLA ratio= 14 to 5.9 Avg = 10 ok
SA/NLA ratio= 70 ok
• Full Load Amps (FLA) 6~14.4A = 10.2A avg
FLA/NLA= 8.5 ok
• No Load Amps (NLA) 1.2 A ( no gears either)
NLA/SA=1.4% ok

# Recommendation

• Choose the motor with the highest efficiency and/or stall current rating for same V nom.
• rely on drill trigger control for user acceleration and clutch for torque for curent limiting.

As I stated in another recent question Li-Ion cells have an ESR about 50x Ah per cell but this improved with C charge ratings.

• Hey, thanks for the reply. What we're chasing isn't so much a replacement motor as trying to ascertain what sort of "rated" stall the Li-Ion battery that came with the drill actually has to deal with in real life, with an eye to building an analogue picture of how they'd perform in applications currently powered by small high-current LiPo's. Knowing the rated stall current of this motor would help, but also understanding more of how winding spec's affect performance (eg: there's only 1 wind difference between the RS-550SH-7524 and the 7525, but a gap of 20A in the stall current, so ??) – Nscrup Feb 25 at 10:58