# Questions Regarding DC Machines

Discussion in 'Homework Help' started by jegues, Nov 29, 2011.

1. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
Hello all,

We've been given some questions related to DC Machines and I'm not entirely if what I've come up with so far makes any sense.

Here are the questions,

1) Why is the armature current non-zero even when there is no load on the shaft of the machine?

2) Why does the armature current slightly increase although the shaft load remains constant?

3) Explain a practical strategy for speed control of a DC machine when both terminal voltage and field current are available for control.

Here are my thoughts on the answers,

1) It is because Ea = Vt + IaRa, Since Ea is non-zero and Vt=0, Ia must be nonzero?

2) I'm not entirely sure, can someone explain?

3) The speed relates to Ea, which can be controlled by means of Vt and If through the following equations,

Ea = Vt + IaRa

If we ignore magnetic non linearity the flux will be proprtional to the field current. Thus,

Ea = k'Ifω

Is there a better explanation/way to the answer the following questions?

I look forward to your input.

2. ### jimkeith Active Member

Oct 26, 2011
539
99
2) This is a thermal issue--as the motor increases in temperature, the field flux decreases so armature speed increases causing more windage and mechanical losses in both the motor and load--the reduction in flux also causes a reduction in the torque constant, so additional current is required to maintain load torque.

3. ### jegues Thread Starter Well-Known Member

Sep 13, 2010
735
43
Are my answers for 1) and 3) correct as well?

4. ### jimkeith Active Member

Oct 26, 2011
539
99
1) It is because Ea = Vt + IaRa, Since Ea is non-zero and Vt=0, Ia must be nonzero?

I question your logic here because Vt (assuming Vt is CEMF) cannot = 0 when it is rotating.

My take is that a rotating motor has both windage and bearing (mechanical) losses that are reflected as a load torque. Since Ia is a function of torque, it cannot be 0.

Haven't figured out how do do the quote function yet...

5. ### jimkeith Active Member

Oct 26, 2011
539
99
I am assuming that this is a shunt field motor. The question does not indicate the required accuracy of speed regulation and /or speed stability.

The field current may generally be considered fixed, but to provide high speed stability, it may be driven via a constant current source.

Practical applications of a DC shunt motor simply regulate terminal voltage as this is the major component of the speed (90 to 95%)(or 10 to 5% speed regulation respectively). For better regulation, a technique called IR Compensation measures Ia and subtracts this signal from Ea feedback thus calculating Vt. Via this technique, speed regulation may be improved to about 1%--attempting to obtain 0% puts the system on the edge of instability--positive feedback.

Note the distinction between speed stability and speed regulation. Regarding specsmanship, motor drive controls always boast high speed regulation, while ignoring inherent relatively poor speed stability. What good is 1% speed regulation, if the speed drifts some 10% as the motor heats up?

Hope this is helpful--figured out how to do the quote function.

Last edited: Nov 30, 2011