# Insane amps drawn by motor?

#### strantor

Joined Oct 3, 2010
5,226
Looking at this motor, I see the phase-phase winding resistance is .013Ω. Does that mean that the instant I apply 48V, before there is any BEMF it will draw 3,692A?

How do I get around that? a shunt resistor? I planned on software PWM current limiting, but from what I see here, it looks like my MOSFETs will blow up in the first PWM period.

#### Kermit2

Joined Feb 5, 2010
4,162
It is probably a decimal position error. The closest realistic value would be 10 times that.

For example:
30 ft of 16 gauge magnet wire gives approx. .12 to .13 ohms.
30 ft of 6 gauge is about .012 to .014 ohms.

I seriously doubt they used anything near 6 gauge wire in the motor.

The change of a decimal position also fits much more closely to the spec given of about 200 amps for one minute overload capacity on the motor.

#### shortbus

Joined Sep 30, 2009
7,600
Also you might look at the torque rating of these motors. The data sheet says "Peak Stall Torque if 38 Nm", converting that to Lb/FT = 28 ! Not much torque for the amp input.

The motors you linked to in the other thread had similar ratings. When you look closely at all the data for these motors they don't make much sense, at least not to me. Maybe I'm looking at things wrong, been known to happen.

#### strantor

Joined Oct 3, 2010
5,226
It is probably a decimal position error. The closest realistic value would be 10 times that.

For example:
30 ft of 16 gauge magnet wire gives approx. .12 to .13 ohms.
30 ft of 6 gauge is about .012 to .014 ohms.

I seriously doubt they used anything near 6 gauge wire in the motor.

The change of a decimal position also fits much more closely to the spec given of about 200 amps for one minute overload capacity on the motor.
I think it's right actually, other sellers are saying the same thing:
The phase resistance is very low at 10 milli-ohms
Keep in mind it's supposed to be a 18hp motor that only weights 22kg. It probably comes down to the fact that it's 8 pole, so 4 high gauge wire coils in parallel, with no brush resistance.
here's what the inside looks like:

#### strantor

Joined Oct 3, 2010
5,226
While I am probably axeing my brushless controller, this question is still important to me because I'm back to making a brushed motor controller and the brushed motor I'm considering has a resistance of .025ohms. I've already found this motor on plenty of websites so I am sure about the .025ohm rating.

this document seems to suggest that software PWM current limiting can still work, but I am still skeptical. they are not talking about motors with nearly zero resistance.

#### shortbus

Joined Sep 30, 2009
7,600
These Guys are playing fast and loose with their HP and Torque ratings on all these motors. The one just linked to says -

Peak power: 34.3 hp 7.22KW (HP = KW/746) 7220/746 = 9.68 HP

Torque = 20.5 Nm thats 15.12 lb/ft @ 3480RPM

HP= (rpm * ft/lb) /5252 = (3480 * 15.12) / 5252 = 52617.6 / 5252 = 10.02 HP

So where are they getting their numbers? Formulas here; http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm

Joined Dec 26, 2010
2,148
Perhaps the inductance of the winding has a role in constraining the rate of current rise at start-up. Don't you start with a narrow PWM pulse?

#### steveb

Joined Jul 3, 2008
2,436
Does that mean that the instant I apply 48V, before there is any BEMF it will draw 3,692A?

...

software PWM current limiting can still work, but I am still skeptical. they are not talking about motors with nearly zero resistance.

It's not instantaneous, but the time is dictated by the time constant L/R. The inductance of 0.1 mH and the resistance of 0.013 Ohms gives you about 1 ms for the current to rise to about 10 percent of the 3692 Amps. That's still pretty fast, but it gives you some leeway for electronic current limiting in the brushless design.

#### strantor

Joined Oct 3, 2010
5,226
These Guys are playing fast and loose with their HP and Torque ratings on all these motors. The one just linked to says -

Peak power: 34.3 hp 7.22KW (HP = KW/746) 7220/746 = 9.68 HP

Torque = 20.5 Nm thats 15.12 lb/ft @ 3480RPM

HP= (rpm * ft/lb) /5252 = (3480 * 15.12) / 5252 = 52617.6 / 5252 = 10.02 HP

So where are they getting their numbers? Formulas here; http://www.elec-toolbox.com/Formulas/Motor/mtrform.htm
Considering they would draw way more amps than they're rated for without a current limiting controller, their HP is really only limited by how far you are willing to push them (heat wise) past their rating. it's also common practice in racing community to exceed the voltage rating and the current limit at the same time. that's probaby where he's getting the 'out-to-lunch' figure of 35hp.
here's the specs from a reputable retailer (not ebay):
Motor, Perm-Motor 12 - 72 Volt, 9.5 HP on 72 volts systems, 19.3 HP peak for 10 minutes

Similar and possibly better than the Etek motor. It has a higher efficiency rating at lower RPM's and can be used at higher voltages

This motor runs at an Ideal RPM at 24 volts for Electrathons and at almost 90% efficiency

Applications:
Electrathons, scooters, motorboats, golf cars, electric cars, cleaning vehicles, lawnmowers, industrial trucks / fork lifts, wind generators, go-karts, rehabilitation vehicles

Voltage: 24 - 72 Volts
Inductivity: 0,019 mH
Current: 110 Amps
Resistance: 16 mOhm
Peak Efficiency:
2.2 KW @ 24 volts, 1080 RPM, 2.94 HP
3.5 KW @ 36 volts, 1700 RPM, 4.69 HP
4.74 KW @ 48 volts, 2300 RPM, 6.35 HP
5.97 KW @ 60 Volts, 2870 RPM, 8 HP
7.22 KW @ 72 Volts, 3480 RPM, 9.7 HP
Shaft: 3/4" Diameter, 3/16" Keyway
Protection: IP 20
Inertia of masse: 0.025 kgm²
torque: 20.5 Nm
Temporary service: 10 min = 200 Amps
Max torque: 38.5 Nm
Weight 11 Kg, 24.2 lbs
so 72V X 200A = 14.4KW, or 19.3HP for 10 min. But if you wanted to go for example 300A @96V for 10seconds, you could get 38HP out of it (maybe)

#### shortbus

Joined Sep 30, 2009
7,600
I see that they list your name on Ebay, in the bids page as a***n. Have you been bidding on some Kant-twist clamps lately?