## What determains the steaty-state velocity of an open loop DC motor?

## What determains the steaty-state velocity of an open loop DC motor?

(OP)

I have a Simulink model of an open-loop DC motor.

I am trying to understand what parameters effect the final steady state angular velocity of the motor.

From playing around with different values, it seems that:

A. The input voltage, electric resistance, viscous resistance (damper) and torque constant have an effect on the steady state velocity

B. The Inertia and electric inductance do not affect the final speed, but rather only effect the time it takes to reach this speed.

Am I correct? Why is this? my intuition says that also the inertia should influence the steady state speed, but it seems my intuition is wrong.

Thanks!

I am trying to understand what parameters effect the final steady state angular velocity of the motor.

From playing around with different values, it seems that:

A. The input voltage, electric resistance, viscous resistance (damper) and torque constant have an effect on the steady state velocity

B. The Inertia and electric inductance do not affect the final speed, but rather only effect the time it takes to reach this speed.

Am I correct? Why is this? my intuition says that also the inertia should influence the steady state speed, but it seems my intuition is wrong.

Thanks!

## RE: What determains the steaty-state velocity of an open loop DC motor?

1. Why does your intuition say that inertia should influence steady state speed? (Don't try to answer that - who can explain intuition). If the problem was linear and there was no electricity involved (say you were seeing how fast you and a friend could push a wagon along a level rail track), would you still expect inertia to govern ultimate speed?

2. How confident are you that your model is complete? Have you come across the concept of "Back EMF" yet? This is a really significant factor in the steady state speed of dc motors.

A.

## RE: What determains the steaty-state velocity of an open loop DC motor?

Imagining the wagon setup you described, yes, i would have that same expectation. You say that there as well, the ultimate speed would not be dependent on the inertia of the wagon?

The model was taken from a tutorial I saw a while ago, I couldn't find it right now. I am pretty sure it is correct, yet I am sure it is not 'complete', since there are probably many more effects in real life...

## RE: What determains the steaty-state velocity of an open loop DC motor?

The inertia affects the transient state, i.e. how long does it take to get to full speed.

It has no impact on the final steady state, which could arrive in 0.1 sec, 1 second, 1 minute, 1 hour depending on the amount of inertia. But the end result will be the same once it stops accelerating.

Remember - More details = better answers

Also: If you get a response it's polite to respond to it.

## RE: What determains the steaty-state velocity of an open loop DC motor?

The no-load speed of a dc motor is a similar story. As speed increases, electromagnetic effects (Back EMF again) reduce the torque generated by the windings until you reach a speed where the motor stops generating torque altogether.

I think that behaviour is already baked into the Simulink dc motor block so, when you specify the torque constant, you are also specifying the no-load speed (per volt). There are real-world strategies for trading torque for speed (look up field weakening for example), so it's a legitimate parameter to alter as part of your investigation.

A.

## RE: What determains the steaty-state velocity of an open loop DC motor?

The same machine may be a generator or motor depending on the application.

For this discussion and for understanding assume that this is a test machine connected to a dyno and that the field and armature voltages may be controlled or varied.

The dyno may control the speed and is capable of overcoming the motor torque or supplying the generator torque.

To start, apply rated field voltage and rated current and no load from the dyno.

The motor will stabilize at no-load speed.

Now over-drive the machine with the dyno until the armature current is zero.

The back EMF now matches the applied voltage, no armature current flows, no torque is developed and the machine is neither a motor nor a generator, but it is still a DC machine.

Now drop the speed a little below the neutral speed;

In a well designed, efficient DC machine, held at constant speed, there is only a few volts between motoring and generating.

In a well designed, efficient DC machine, with constant voltage applied, there is only a few RPM between motoring and generating.

If you understand DC motor basics, you should be able to discern what will happen when the voltage and speed of a DC machine are both held constant but the field strength is varied.

I leave it to you to work that out.

ps; I'm old and tired so beware of typos.

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Ohm's lawNot just a good idea;

It's the LAW!## RE: What determains the steaty-state velocity of an open loop DC motor?

## RE: What determains the steaty-state velocity of an open loop DC motor?

--------------------

Ohm's lawNot just a good idea;

It's the LAW!