DC motors use direct current instead of AC current. This current does not change direction and is determined by the polarity of the source. You may understand direct current in your regular life as the type that is produced by batteries. In industrial applications, the AC current from your power company is supplied to the factory and changed to DC through power supplies or SCRs.
These are more complex and more costly to operate than AC motors, but they can operate at adjustable speed when connected to relatively simple controllers. AC induction motors require much more robust drives in order to adjust speed.
Brush-Type DC Motors
Direct Current motors utilize magnets to operate. Brush-type DC motors can be either permanent magnet or wound field.
In permanent magnet motors, the permanent magnet is the stator and the magnetic field created is a constant. The portion that rotates is called the armature and is made up of steel laminations with slots that hold copper windings. The windings are attached to a segmented copper cylinder called the commutator. Electricity is fed into the armature through carbon brushes that ride on the commutator – hence the name “brush-type.” Permanent magnet motors are typically available from fractional horsepower to 5 HP.
Wound-field motors differ from permanent magnets in that the magnetic field is created by wire-wound poles on the stator. There are three kinds of wound-field motors: series-wound, shunt-wound, and compound (which is a motor that utilizes a combination of shunt and series). Shunt-wound provides high starting torque, but the speed varies widely with a change of load. Shunt-wound DC motors usually have a more constant speed throughout its normal load range, but they may over-speed upon loss of shunt field power. The combination of these two, compound and stabilized shunt-wound DC motors, have a more stable speed yet still have the ability to increase torque output when heavily loaded. Examples of applications that often utilize this are cranes and elevators.
Brushless DC Motors
Brushless DC motors also employ permanent magnets, but they don’t utilize brushes to do so. Instead, the magnets are mounted on the rotor and the windings are in the stator. Also missing from brushless DC motors are the commutators. A solid-state controller electronically provides the commutator function in response to rotor-position signals.
Why would you choose brushless? Brushless DC motors have the capability to delivery rated torque at higher speeds than similar sized brush-type motors.
Permanent Magnet Motors vs. Induction: Why?
Permanent magnet motors are getting more popular. With the added difficulty of having to convert the AC power from the power lines that lead to your plant, why might you choose PM motors instead of AC induction motors? The savings that come in the form of reduced power loss.
Do you have an application that could use a DC motor? Contact us!