# What does a commutator do in dc machines?

It's my exam and in need help I am sure it's reversers current and I don't know my english lecturer say it's no it's provides current I need help please

• Your lecturer is wrong, if it helps. – Eugene Sh. May 9 '17 at 16:16
• Actually you are both right.. – Trevor_G May 9 '17 at 16:20
• commutators reverse the voltage at the end of the pole phase where torque drops null thus pulling current and torque for the next angular pole. The same happens with BLDC commutators and Full wave bridges by complementing the differential voltage with 2 switches half-bridge or SPDT. In the motor it just reverse the DC voltage in the direction where torque continues in the same direction. so you can say applied voltage to switch load current or "reverses current" – Sunnyskyguy EE75 May 9 '17 at 16:20
• But I don't know how to make him change his view and he have 70 years old and I afraid make me fail on final exam – Mohamed aamer May 9 '17 at 16:30
• Well, this is not an engineering problem. – Eugene Sh. May 9 '17 at 16:32

Actually you are both right.

A motor contains a coil that is free to rotate around a pivot mounted across a magnetic field. When current is applied to that coil the coil generates it's own magnetic field. The difference in angles of those fields causes a force that turns the coil.

If there were no commutator, the coil would oscillate a bit due to it's inertia and finally end up lined up vertically with the magnets.

The commutator is a mechanical switch that, at an appropriate point in the rotation of the coil, sends current through the coil in the appropriate direction.

So, for a simple single coil DC motor, your original answer is correct.

The torque, the force driving the coil around, follows a sinusoidal function as shown below. The maximum torque occurs when the coil is at 90 degrees to the magnetic field and goes to zero when the coil lines up with the field. The left graph below is for no commutator. The centre one is for a single coil commutated motor.

In real DC motors there are multiple coils. The coils are arranged such that the torque curves look like the right hand graph above, a three coil motor. The commutator then supplies current to the appropriate coil at the appropriate angle to keep the torque close to the peak of those curves. One or more coils will be off entirely.

So, yes, the commutator switches current to the coils. It just so happens that the coils are also, eventually, fed current in the other direction after the motor has turned 180 degrees.