View Single Post
What makes a "good" motor?
RC-Monster Admin
BrianG's Avatar
Posts: 14,606
Join Date: Nov 2005
Location: Des Moines, IA
What makes a "good" motor? - 01.13.2011, 05:38 PM

More great info from Pdelcast where he explains what makes a good motor.

The no-load current tells you the approximate magnetic efficiency of the motor. Lower is better.

The steel for laminations comes in many varieties. Most of the cheap motors use either .35mm or .5mm laminations. The thinner laminations are more efficient. The cost for the steel goes up significantly with thinner steel (because it needs a LOT more processing to make thinner steels.)

The other variable in steel is the amount of silicon (sand) that they add to the steel. The silicon increases the electrical resistance of the steel, increasing the magnetic efficiency by lowering electrical losses (you want the steel to be good at conducting magnetism, but poor at conducting electricity.) The higher the silicon content, the more brittle the steel and the longer it takes to process (and therefore, the more expensive it is.)

So, the cheap steels are thick with a low silicon content, and the expensive steels are thin with a high silicon content.

We use .2mm thick, high silicon content steel. Most of our competitor use .35mm or .5mm low silicon content steel. The .2mm high silicon steel is about four to five times more expensive than .35mm high silicon steel, and about ten times more expensive than .5mm low silicon steel.

But the difference is large: A 1415-2400Kv motor from Castle has a no-load current of about 2.4A.

To figure the magnetic losses, you multiply the no-load current by the battery voltage.

So, for example:

Hobbyking motor: 5A * 24V = 120 watts of magnetic loss
Castle 1415-2400Kv motor: 2.4A * 24V = 57.6 watts of magnetic loss


The second type of loss is resistive loss. This is the loss caused by the current flowing through the copper.

The Hobbyking motor lists a 5.8 milliohm resistance. The Castle 1415-2400kV motor has a resistance of 4.2 milliohms.

The formula for resistive losses is (Current in amps) ^2 * resistance (amps squared times resistance)

So, at 125 Amps (the "rating" from the hobbyking motor) the losses would be:

Hobbyking motor: (125)^2 * .0058 (ohms) = ~90 watts
Castle 1415-2400: (125)^2 * .0042 (ohms) = ~65 watts


These two types of loss (magnetic and resistive) add up in the motor, and get turned into heat.

So to make a good motor, you need both low resistance (for low copper losses) and low no-load current (for low magnetic losses.)

To compare the two motors:

Losses at 24V battery voltage, 120A current:

HobbyKing motor: 90 watts (resistive) + 120 watts (magnetic) = 210 watts of loss
Castle 1415-2400Kv: 65 watts (resistive) + 58 watts (magnetic) = 123 watts of loss

Because the "loss" watts ALL TURN INTO HEAT, the Castle motor will run much cooler in the same setup -- it's efficiency is much higher.

It all comes down to this: It's expensive to make a good quality motor. And, you get what you pay for.

Last edited by BrianG; 01.13.2011 at 05:40 PM.
  Send a message via Yahoo to BrianG Send a message via MSN to BrianG  
Reply With Quote