Field of D.C motor needs D.C field control(variation of current by D.C voltage control, potential drop by rheostat  used in potentiometer arrangement). Transformer used to step down A.C voltage, done by auto transformer(working on induction principle), which gives reduced variable A.C voltage output . Can be used only after rectification, not directly, without rectifier, yet method would be energy saving, compare to rheostat control..

The transformer is used only for alternatives systems, it works with induction principals which are not applicable on DC. Cordialy

It is impossible to use an autotranformer.

The DC machine uses a Direct Current, but the auto transfomer (like the transformer) provides AC power.

To control DC motors we must use converters.

If the control is manually we can use rheostats with DC motors, and transformer for AC motor, but if we need automatic control the power-electronics converters are Irreplaceable for all types of motors. cordially

As M.S says the answer is NO.

Under DC a transformer has no reactance (0 Hz) and the low resistance of the transformer winding will cause large currents to flow and severe damage. That is why a rheostat is used as it has a very high resistance and very low reactance and why it is used on DC systems.

Inductance or reactance is only present in DC systems during dynamic or transient changes in the magnetic field after which they die away.

Of course an auto-transformer is  much better than a reostat from the pint of view of efficiency. Just put a rectifier bridge in the secondary. The high inductance of the field winding will filter the current. But a thyristor or triac regulator  will give a smaller and cheaper solution.

Obviously you can't use an autotransformer for directly regulating the dc current, but you could use a variable autotransformer (VARIAC) in order to regulate the ac input voltage of a diode rectifier, which in turn feeds the field winding of the dc motor. By changing the transforming ration you could thus control the field current.  

to use an autotransformer, one must use a diode rectifier (a three-phase).

The above comments only apply if it is a separately excited DC machine. All others such as series (diverter), short and long shunt one cannot do this.

That is why when questions are posed they need to be clear and not be lazy. I say this as to date Mahboobeh has not yet responded.

Hi dear.

I think that:

Such as multi-coil transformers, auto transformers are the property of the magnetic field varies with time to convert the direct current voltage and thus not applicable in the auto transformer.