In robotic terms motors are the actuators that drive the joints. You could say that the motors are the muscles that manipulate the joints. The motors must be powerful enough to drive the load through acceleration and deceleration but must also be responsive, light, accurate and reliable. Electric motors are the most common type with servo, stepper and direct drive motors being found. With all electric motor the torque (power output) is a direct function of the magnetic fields and the current in the windings of the motor. A consideration when designing and operating electric motors is the heat that generated, this must be dissipated to allow safe operation and prevent failure.
A D.C motor has two main parts, the ‘stator’ and the ‘rotor’. Normally many conductors are wound lengthwise in slots on the rotor. The magnets or poles are replaced in the stator. When current is supplied to the loops, the magnetic field produced interacts with the field from the poles producing torque and rotation. In order to keep the process going, the current must be switched to the loops moving into position under the poles. This may be done with a ‘commutator’ and ‘brushes’ but it can be done electronically.
The magnetic poles may be permanent magnets but in more powerful motors they are electro-magnets with coils wounds on the poles called ‘field windings’ The stator and rotor form a magnetic circuit and flux crosses the loops on the armature. The strength of the magnetic flux can be changed by changing the current. The rotor would be called the ‘armature’
A simple generator may be produced by rotating a simple loop of conducting wire in magnetic field. In the position shown one side of the loop is moving up and the other down cutting the flux at a right angle. A voltage is generated across the end of the loop. The current flowing from the terminals is governed by the resistance connected between them. Two slip rings and brushes are needed to connect the loop to the external circuit. At any other position of the loop the conductors cut the flux at an angle. The voltage generated is directly proportional to the angle of the loop to the flux ‘θ’
A pulse width modulator is used to control the average DC voltage applied to the armature. A PLC can control the speed of rotation of a motor by controlling the electronic circuit used to control the width of the voltage pulses.
In many cases it is only required for the PLC to switch a DC motor on or off. This can be done by using a relay. Although sometimes a PLC is required to reverse the direction of rotation of the motor. This can be done using relays to reverse the direction of the current applied to the armature coil.
A Three phase AC motor has two main parts: a stationary stator and a revolving rotor. The rotor is separated from the stator by a small air gap that ranges from 0.4mm to 4mm, depending on the power of the motor.
The stator consists of a steel frame that supports a hollow, cylindrical core made up of stacked laminations. A number of evenly spaced slots, punched out of the internal circumference of the laminations, provide the space for the stator winding.
The rotor is also composed of the punched laminations. These are carefully stacked to create a series of rotor slots to provide space for the rotor winding. There are two types of rotor windings: (1) conventional 3 phase windings made of insulated wire and (2) squirrel-cage windings. The type of winding gives us two main classes of motor: squirrel cage induction motors and wound-rotor induction motors.
A squirrel-cage rotor is composed of bare copper bars, slightly longer than the rotor, which are pushed into the slots. The opposite ends are welded to two copper end-rings, so that the bars are short-circuited together. The entire construction resembles a squirrel cage, hence the name. In small and medium size motors, the bars