For applications requiring a motor larger than our 2.4-inch or 3-inch motors, we offer the A40-300 and the G43-500.

We stock these motors but if they are not quite right for your project, we can design custom brushed or brushless motors.



Type High Performance Standard Custom
Model A40-300 G43-500  
Diameter (inches) 4.0 4.3 1.4 to 5
Length (inches) 6.9 5.3 2 to 9
Peak HP 3.8 Detail Page Varies
Stall Torque (oz-in) 3840 Detail Page Varies
Efficiency 84% 80% Varies
Voltage* 24V* 24V* 6V to 90V
RPM @ 24V 4000 3300 Custom
Shaft Dia. (inches) 5/8 Sprocket Custom
Shaft Length (inches) 1.75 Detail Page Custom
Keyway (inches) 3/16 N/A Custom
Capacitors Yes No Available
Magnet Type Ferrite Ferrite Custom
No Load Amps 3.5 1.7 Varies
Resistance (Ohms) 0.050 0.144 Varies
Kt (oz-in/Amp) 8.05 9.74 Varies
Kv (RPM/Volt) 168 137 Varies
Weight (pounds) 11.9 8.6 Up to 20
Price $350 $69 Varies
Pictures A40-300 G43-500 N/A
Drawings A40-300 G43-500 N/A
Performance Charts A40-300 G43-500 N/A

* The above specifications are for 24V. The motors can also be used at 12V or 18V. The RPM is proportional to the voltage so running at 12V will result in half the no-load RPM that is shown above for 24V. The lower voltage also reduces the maximum achievable torque by 50%. The motors have also been tested and used at higher voltages resulting in higher RPM, torque, and power. Shorter duty-cycles are recommended for higher voltages to allow the motors time to cool. Please note that the above torque figure is the theoretical peak torques when stalled. Operating any high-performance motor while stalled will damage it.




Getting high efficiency from a low-voltage motor is not easy. There are many factors that effect efficiency. We have optimized each of these factors to achieve very high efficiency in these motors.

Model A40-300 G43-500
Peak Efficiency (PE) 84% 80%
RPM at PE 3700 3000
Torque at PE (oz-in) 300 120
Horsepower at PE 1.1 HP 0.44 HP
Current at PE 41 Amps 17 Amps
Current at 500 Watts output 25.5 Amps 27 Amps
HP Range for 75%+ Efficiency 0.4 to 2.6 0.16 to 0.85

As you can see from the above chart, the efficiency of the motors is very high over a broad range of power. (The RPM is the same in both directions to within about 3%).

The A40-300 motor uses a 42-bar commutator. The huge 1.75" diameter of the commutator combined with the four massive brushes and the 42 bars enabled us to get high power and high efficiency from this smooth-running motor. As far as we know, our high-performance and mid-range motors have the highest efficiency of any 24-volt reversible permanent magnet DC brush motors in their size ranges.

Another benefit of the four-brush, 42-bar designs is the absence of excessive electrical noise. While all motors produce some electrical noise, cheaper motors with fewer commutator bars are more likely to send noisy voltage spikes back to your motor controller.

Radio-frequency noise is further reduced by the use of internal capacitors. Each of the four brushes has a capacitor wired to its nearest neighbor on each side. This makes the motors ideal for radio-controlled applications.

Using rare earth magnets is normally the only way to get high efficiency from a low-voltage motor. While these magnets are very strong, they are also extremely expensive. To reduce the cost of the A40-300, we use a high-energy ferrite magnet known as "T9". The T9 magnet material along with the other design features listed above have enabled us to achieve the same high efficiency of rare-earth in a much less expensive motor.

Here are our recommendations regarding the selection of a proper reduction ratio.


The motors are warranted to be free from manufacturing defects, and fully operational when you receive them. AmpFlow gives no other warranty, either expressed or implied. Warranties are not transferable. In no event shall AmpFlow's liability exceed the buyer's purchase price, nor shall AmpFlow be liable for any indirect or consequential damages.

Shipments must be to a street address; we can not ship to a PO Box. Please allow 7-10 days for orders shipped by UPS ground, 3-4 days for orders shipped by 2nd day air, and up to 14 days for international orders.


  We accept Visa, MasterCard, and PayPal.

A40-300 Motor - $350      Quantity

G43-500 Motor - $69      Quantity

Replacement brushes are available here

Contact us for custom motors and volume pricing.



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