Dc motor speed and torque

  • Thread starter david90
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  • #1
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Main Question or Discussion Point

Torque has an inverse relationship with motor speed and I can understand why it's like that (more speed = more cemf = less current = less torque).

On the contrary, my SCR controlled DC drive manual said that a DC drive could control a DC motor in a way that give constant torque from 0 rpm to base speed.

How is it possible to have constant torque when you increase the motor rpm? It's also not consistent with the torque curve shown above.
 

Answers and Replies

  • #2
176
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Good question..
In a dc motor the speed of the the motor is controlled by varying the stator voltage, and the torque is controlled by varying the armature current, so if you vary the motor speed you should vary the armature current in oder to have a constant torque for different speeds, you can build a control system with PI regulator which controls the armature current in order to get a constant torque while varying the speed.
 
  • #3
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Thanks for the help. The DC drive that I work on have a constant field output (stator). Increasing the field voltage will able the motor to spin above it's base speed.

Let say that a motor spins at 1750rpm with no load, 500VDC arm input and a constant 300VDC field input.

Now if I put a 50% load on the motor and want it to spin at 1750rpm, should the arm voltage be higher than 500VDC in order to increase the arm current?

Not sure if I make any sense
 
  • #4
702
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Most DC brushless motors have an almost square torque speed curve (except at top speeds).
As a motor's back EMF approaches that of the source, the leakage inductance and series resistance begins to limit your current because there's little voltage drop. However, many brushless designs have little resistance and series inductance. Also, the controller can simply increase the applied voltage in an effort to compensate for any speed reduction (associated with torque load).

Of course, as you reach higher speeds, you eventially approach the power supply's EMF and this all falls apart. You simply don't have enough voltage difference to maintain the current required for a constant torque.
 
  • #5
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DC motors are really very simple to first approximation, speed controlled or otherwise, and can be modeled by a series resistor in series with a rheostat in parallel with another resistor. The rheostat represents the variable load.

At any given field strength the motor K factor is the proportionality constant of torque/current and voltage/angular velocity.
 
  • #6
305
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I'm still a bit confused.

Let's use this motor with the follow specs as an example:

Varm_max= 500VDC, Iarm_max=100A, Vfield=300V, Ifield=2A, Base Speed=1800RPM.

Assuming I use an SCR controlled DC drive, and I can spin the motor to 1800RPM by applying 500VDC to the armature with no load.

Will the motor still run at 1800RPM if I load it down until Iarm=100A? If yes, will the amature voltage still reads 500VDC or will it be higher in order to increase current (to overcome cemf)?
 
  • #7
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The only way you can maintain 1800 rpm at a field current of 2A is to exceed the max armature voltage, or increase the the field strength--that is, increase the field current over your given 2A value.

By the way, you cannot switch DC with an SCR, but with GTOs (gate turn-off SCR) or something else--typically power MOS FETs or IGBTs (insulated gate bipolar transistor)
 
  • #8
176
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What is the type of your motor?(shunt, separate excitation or what?)
 

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