DC Motors : Constant HP vs Constant Torque
DC Motors : Constant HP vs Constant Torque
The following is offered in response to your comments in the thread...DC Motor brush life....Thread237-77159
- "///Motor load is often considered with a constant shaft load (HP).
Since HP~Watts=ExI, then the only way to preserve the relationship
for E decreasing is by I increasing.\\\"
--- on the surface, you are absolutely correct and theory stands behind you in so far as defining constant watts. But where you fall down is in your assumption that constant HP operation of dc machines abides by that principle. The truth is, they do not, except in a very special case (as will be discussed herein)...
In 99.99% of dc motor applications, they motors operate,
- CONSTANT TORQUE .. below base speed...
- CONSTANT HP .. above base speed.
- base speed = rated rpm at rated voltage, field, & arm. amps
- base speed range is operation at or below base speed
Ergo, at rated armature voltage and below, DC motors are operated Constant Torque.... NOT Constant HP.
In order to operate constant HP in the base speed range the HP must be less than motor rated HP or, the amps must be allowed to increase above rated.
A couple of examples.
Consider a 10 HP, 240v, 38 amps, (81.8% eff), 1150 rpm, dc motor.
That motor develops 10 HP ONLY at 240v & 38 Amps
At 1/2 base speed (575 rpm) 120v armature, and 38 amps, the motor develops 5 HP ...
According to your hypothesis, you would have to allow the current to rise to 200% rated (76 amps) to develop 10 HP at 1/2 armature voltage (120v).
Now Joseph, you and I both know that ain't gonna happen.
There is one exception to this, however; that is in the case of winders and un-winders that operate constant tension (i.e.,constant HP) in the base speed range.
This is a special case.
Using the same motor from the above example....we know that the motor can develop a maximum of 10 HP rated.
This winder is designed as a direct drive, center core, with a 6:1 buildup. Since you cannot obtain more than 10 HP from the motor,
It must be something less.... How much less ???
Consider that at full core, the motor will be operating at
That means that the motor starts at 1.6667 HP at empty core, at rated rpm.... and does not exceed that power rating over 6:1 buildup so as to maintain constant tension while winding ...
The winder starts off at rated speed at rated voltage at 1/6 rated amps and at 1/6 rated HP. As the core builds up, the armature voltage reduces, and the current increases in direct proportion to maintain constant HP.
At full roll, the armature voltage is 40 volts, 191.667 rpm, 38 amps, 1.667 HP ...
That, Joseph is an example of constant HP operation in the base speed range of the motor.
Such winders are typically controlled by dancers or some other type sensor that monitors tension and adjusts the winder torque accordingly to obtain constant hp winding.
In this example... at empty core, the winder is operating at 1150 rpm and as each successive wrap is put on, the speed is reduced as the armature voltage is reduced and the current increases to keep the HP constant.
But, Joseph, in order to obtain constant tension winding, the HP is not allowed to increase (or decrease)..... as the winder speed (armature voltage) is reduced with buildup).
In this example, the winder motor starts at empty core, 1150 rpm, 1/6 HP...... and maintains 1/6 HP over the buildup as the rpm and armature voltage are decreasedwhile armature current increases.
At no time (other than perhaps acceleration) does the armature current exceed rated n/p value.
This Joseph.... is an example of rated hp operation in the "base speed" range of the motor. This is a specialized application of a dc motor... and is not, as you alluded, a typical mode of dc motor operation in the base speed range.
The typical.... and most common.... is Constant Torque in the base speed range.... not Constant HP.
Constant HP is typical for operation ABOVE base speed; i.e., the field weakend range....... which the original post never ascribed to as an application attribute for the question posed.
In the field weakend range above base speed:
HP = (Reduction in Torque x Increase in Speed) / 5250
So, as flux is reduced, torque is reduced, speed is increased, and HP remains constant
But Joseph, don't take my word for it. I've posted some references you can avail yourself of to obtain a better understanding of dc machines and how they operate.
Siemens - Basics of DC Drives
(from which the above graphic is borrowed)
Siemens - Speed/Torque Relationships of Shunt Connected Motors ...page 32 of paper (page 7 of PDF file)
Rockwell Automation (Allen-Bradley)
"A Comparison of the Characteristics of AC and DC Motors
Summing it up Joseph:
Constant HP operation of dc shunt motors occurs in the Field Weakend range above base speed of the motor.
Below base speed, dc shunt motors are operated constant torque.... except in very special cases.
Ergo, your premise that volts decrease and amps increase to obtain constant HP operation of dc machines ... is rejected except in the special case for the winder application. DC machine theory tells us that constant HP operation is obtained while holding arm volts and amps constant while reducing the shunt field flux.
Here's where you started:
- jbartos (Electrical) Nov 1, 2003
Suggestion: If the motor terminal voltage decreases, the motor current through the brushes increases, since HP is a constant. Therefore, the brushes would have to be adjusted to the higher commutation currents through the brushes.
which you posted in response to Clyde38's original post :
- Clyde38 (Electrical) Oct 31, 2003
Why do some dc motor manufacturers say you shouldn't run a motor at a voltage lower that rated? These are small motors, less that 2" in diameter.
12 volt motor at 6 volts.
They state that they need to install different brushes.
Clyde never mentioned operating the motors in the "constant HP" range - standard definition i.e., above base speed.....
You posted in response comments about "motor terminal voltage decreasing, motor current thru the brushes increases since HP is constant."
Your comments Joseph, were both incorrect
- dc machines operate in constant HP with arm volts and current fixed (held constant) ... while field flux is reduced,