A DC motor has a couple of advantages, Torque and Speed Variation.
When you think about what an electric motor is, the answer is fairly simply, it’s an object that changes electrical energy in to mechanical energy. The mathematics behind it are more complex, but for practical purposes all it does is change one form of energy into another form. To be more accurate we could say that a motor is really a device that generates torque.
In the case of our motors, torque is a turning force that produces rotation of the output shaft. DC motors can develop a constant torque over a wide speed range.
For a DC motor : Torque is proportional to armature current - this means controlling torque requires simply controlling the motor DC current - easily achieved with a simple DC drive.
You can vary the speed by changing the field voltage, the armature voltage or a mix of the two. To put this simply if you run your motor at a base speed of 2000 rpm and an armature voltage of 500v, if you reduce the armature voltage to 250v your motor will run at 1000 rpm.
If we look at controlling the armature voltage, then we need to maintain a constant shunt field excitation. The torque of the motor is proportional to the product of the armature current, the main pole flux(field) and a machine constant which is a function of the armature windings. This means that if the shunt field excitation is kept constant, the torque is dependent solely upon the armature current.
If we use armature voltage control with a fixed field excitation, our DC motor will develop the rated torque at the rated armature current independent of speed, this is usually referred to as constant torque operation. The speed will be proportional to armature voltage.
However, if we control the voltage applied to the shunt field using a field controller, we will also control the motor speed. Reducing the voltage to the shunt field decreases the field current, as a result this reduces the field flux and allows the motor speed to increase.
You cannot increase the field voltage to obtain a speed below base speed, this would cause the field to overheat at higher than rated current. DC motors running with a constant armature voltage and field weakening have constant horsepower capacity over their speed range. Field control speed values vary from 1:1 to 6:1
As can be seen using both shunt field control and armature voltage control will give a wide range of speed. With shunt field control a DC motor can achieve speeds above base speed and produce a constant horsepower capacity. Armature voltage control is used for speeds below base speed, giving us a constant torque capacity.
One advantage of using a DC motor with a DC drive is that regenerative energy can be fed back into the mains. The regenerative DC drive (4 quadrant drive or APSH) is significantly cheaper than a similar regenerative AC drive. It also takes up less space and has a simple construction.
Drives and Automation are able to supply a range of DC motors, and are the UK agent for Sicme Motori. Sicme Motori was founded in 1967 in Turin, Italy and with 50 years experience in electric motor design and manufacture it is one of the leading companies in this sector. With two modern facilities in Turin, Italy, Sicme Motori supplies supply square frame AC and DC motors from 1 to 3600kW in 42 countries all over the world.
People often over estimate the maintenance requirement of a DC motor, if you experience a DC motor breakdown however it is often more cost effective to replace it with another DC motor. We are one of the few integrators in the UK with an in depth knowledge and experience of working with DC motors and Drives and are able to advise on motor and drive selection and integration into your lines.