Since power is RPM X Torque it can be readily derived from this curve.  At full speed the motor can supply no torque, this is often call no load speed (nls).  With no torque there can be no power.  At stall the motor provides no RPM but maximum torque.  With no RPM there can be no power.  So we have established that the output power of the motor is zero at both ends of the curve.  With zero at both ends you might assume that power output is maximum in the middle, and that is correct.  The actual curve is a parabola as shown below.

The motor shown in this example can produce from zero to 1,700 Watts (2.3 HP) depending on the load. 

So reviewing what we have discussed so far:

• Maximum power always occurs at ½ the no load speed.
• At no load speed - no power is produced but maximum speed is achieved
• At stall - no power is produced but maximum torque is produced

All of the discussions have assumed that infinite current is available at the rated voltage (in most cases 24 volts).  With a controller there is always a limit to the current, which will limit the output of the motor. 

With the basics of power behind us let's talk about power rating. 

The rating of a DC motor refer to the conditions of voltage, current, speed, and power at which the motor is normally operated.  The rating normally used by ASI is the continuous powerrating.   The continuous power rating is a thermal rating.  At this power, the motor can be operated for long periods of time without a large rise in temperature and beyond the limits of the conductor insulating material, bearings and other components, which are greatly affected by temperature.  It should be noted that batteries can't produce power for an indefinite period. Eventually the batteries will run down and must be recharged.  The continuous power rating is the most conservative power rating of a DC motor.  The continuous rating is most useful for a device that operates (and this may be obvious) continuously!  A fan is a good example of a device that operates continuously. 

Since a transaxle is not often used continuously the continuous rating of a motor does not appear to be very useful.  A vehicle has high power demands when accelerating and climbing hills, and low power demands when driving at a constant speed on level ground.  If we calculate the time averaged power demands of a vehicle we can compare that to the continuous rating of the motor for a fair evaluation.  Unfortunately the average power is very user dependent and usually undefined.  This can make it difficult to size the motor to a new application.  Application experience is often the best judge of which motor to use.

The motor shown in the example above has a continuous rating of 500 Watts.  The peak or maximum output power of this motor is 1,700 Watts (2.3 HP).  Although this may seem significant, the motor would draw 145 Amps to produce this peak power.  Few controllers can output 145 Amps and in fact this motor could only produce this power for less than one minute before overheating. 

Since there is little standardization in the ratings of motors some vendors may engage in "specmanship" and describe a motor by the peak power to show their motor in the best light possible.  This should be taken into account when discussing motor power rating.