Electric motors convert electrical energy into mechanical rotation, making them essential in tools, vehicles, fans, pumps, robots, and industrial machines. Different motor types use different methods to create magnetic forces between a stationary stator and a rotating rotor. Comparing DC, induction, synchronous, and stepper motors helps engineers choose the right motor for speed, torque, efficiency, cost, and control needs.
A good motor choice can improve performance, reduce wasted energy, and simplify the control system.
Key Facts
- Motor torque comes from magnetic force: a current-carrying conductor in a magnetic field experiences a force.
- Mechanical power output is P = τω, where τ is torque in newton meters and ω is angular speed in radians per second.
- DC motor speed is approximately proportional to applied voltage when load and field strength are constant.
- Induction motor synchronous speed is ns = 120f / p, where f is supply frequency in hertz and p is the number of poles.
- Induction motors require slip: slip = (ns - n) / ns, where n is rotor speed.
- Stepper motor step angle is θstep = 360° / N, where N is the number of full steps per revolution.
Vocabulary
- Stator
- The stator is the stationary part of a motor that produces or guides the magnetic field.
- Rotor
- The rotor is the rotating part of a motor that turns the shaft and delivers mechanical output.
- Torque
- Torque is the turning effect of a force and is measured in newton meters.
- Slip
- Slip is the difference between the rotating magnetic field speed and the rotor speed in an induction motor.
- Commutation
- Commutation is the process of switching current direction in motor windings to keep torque acting in the desired direction.
Common Mistakes to Avoid
- Confusing induction motors with synchronous motors is wrong because an induction motor normally runs slightly below synchronous speed, while a synchronous motor locks to the rotating magnetic field speed.
- Assuming higher voltage always means proportionally higher speed is wrong because load torque, back emf, current limits, and controller behavior also affect motor speed.
- Ignoring starting torque is wrong because some loads, such as conveyors and compressors, need high torque at low speed before normal running begins.
- Treating stepper motors as perfectly accurate under any load is wrong because they can miss steps if the demanded torque exceeds the available torque.
Practice Questions
- 1 An induction motor is connected to a 60 Hz supply and has 4 poles. Find its synchronous speed in revolutions per minute using ns = 120f / p.
- 2 A motor delivers 8.0 N m of torque at an angular speed of 150 rad/s. Calculate its mechanical output power using P = τω.
- 3 A robot joint must move to repeatable positions without a feedback sensor, but it does not need very high speed. Explain why a stepper motor may be a better first choice than an induction motor.