Comparing Stepper, Servo and Direct Drive Technologies

If we have a good understanding of the advantages and disadvantages of each of the motor technologies and armed with the application’s constraints, then we can begin to zero in on the motor that best suits our needs.

Our motor technology list is:

• AC Motors with no control. (Line driven.)
• DC Motors and controls
• AC Motors and controls
• Brushless motors and controls
• Stepper motors and controls
• Servo motors and controls
• Direct drives and controls

We’ll review the bold listings in this blog posting

The following motor types are best suited for positioning, torque and other types of applications other than controlling the shaft speed. (hint: Our fan motor technology choice is not in this group.)

For Stepper motors and controls we have:

• Two phase steppers
  • Advantages
    • Least expensive positioning system
    • Brushless, more reliable
    • Very accurate rotor positioning
    • 200 full steps per revolution
    • Up to 51200 steps per revolution with a micro stepping control
    • Reversible
    • Very high holding torque at stand still
    • Inexpensive open loop or closed loop position control
    • Highest torque per cubic inch of motor (Torque density.)
    • Newer control designs prevent stalling and allow torque control mode and can provide higher efficiency
    • Good up to 3,000 RPM, but torque falls off.
  • Disadvantages
    • Low efficiency
    • Load inertia must be within the motor’s capability
    • Open loop operation can stall and lose position
    • Full step resonance
    • Most closed loop controls will quickly detect a stall and correct after the fact.
    • Electrical noise (EMI & RFI Interference) from the controller and the motor
    • Requires eight power devices to control the phase current (four per phase) less reliable
    • Noisy at high speeds
• Three phase steppers
  • Advantages
    • Similar to the two phase
    • 200 full steps per revolution
    • Requires six power devices to control the phase current (two per phase)
  • Disadvantages
    • Similar to the two phase
• Five phase steppers
  • Advantages
    • Similar to the two phase
    • 500 or1000 full steps per revolution
    • Smoother full step operation because the steps are smaller.
  • Disadvantages
    • Similar to the two phase
    • Requires 10 power devices to control the phase current (two per phase) less reliable.

For Servo motors and controls we have:

• Brushed
  • Advantages
    • Intermediate motor and control costs
    • Continuous duty
    • Reversible.
    • Speed is proportional to the applied voltage
    • Torque is proportional to the current
    • Very efficient
    • No power required to hold a static load in position
    • Flat speed-torque curve
    • Peak torque available for short periods of time
    • Smooth rotation at low speeds
    • Good up to 3,000 RPM
  • Disadvantages

• • • Brushes wear and create an eclectically conductive dust
    • Electrical noise (EMI & RFI Interference) from the brush commutator interface

• • • Commutator wear
    • Position is limited to the feedback resolution (2000 line encoder produces 8000 steps per revolution
    • Poor thermal performance. Current/heat is in the rotor.
    • Requires tuning
• Brushless
  • Advantages
    • Brushless, more reliable
    • Continuous duty
    • Reversible.
    • Very efficient
    • No power required to hold a static load in position
    • Flat speed-torque curve
    • Peak torque available for short periods of time
    • Smooth rotation at low speeds
    • Good up to 5,000 RPM
  • Disadvantages
    • Higher motor and control costs
    • Electrical noise (EMI & RFI Interference) from the controller and the motor
    • More complex control. Less reliable
    • Requires tuning

For Direct drives and controls we have:

• • Advantages
    • Similar to a stepper motor
    • High torque at low speeds
    • Up to 4,096,000 steps per revolution
    • Capable of handling high inertias
    • Generates high toque without gearing
  • Disadvantages
    • Highest motor and control costs
    • More complex control. Less reliable
    • Electrical noise (EMI & RFI Interference) from the controller and the motor
    • Requires tuning

Are there other advantages or disadvantages that you would like to add?

Are we ready to make the choice for our fan application?