General Motion Control

Finding the motor technology to suit your needs

Author: Bob Parente | March 1, 2011

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

AC Motors with no control. (Line driven) continued

  • Split phase and Capacitor start / Induction run
    • Advantages
      • High starting and running torque
      • Relatively constant speed
      • Fast acceleration
      • Brushless, more reliable
      • Economical
      • Size ranges from about 1/10 to about 10 HP
    • Disadvantages
      • Can’t be used with a speed control
      • Low start and stop rate
      • High starting current
      • Prolonged starting time can cause over heating
      • Has a mechanical centrifugal switch

My daughter was just finishing putting her clothes in the washer that was just off the kitchen as I plodded in early one morning, in search of my first cup of coffee. My wife and I sat down for breakfast and a couple of minutes later the washer had filled with water and started. Or should I say tried to start.

I heard the motor start and the centrifugal switch click out as it reached its speed, but the load inertia was too great and the running torque wasn’t sufficient to sustain the speed. The motor slowed down and the centrifugal switch closed again. The motor speed increased causing the switch to reopen. It went through about four close-open cycles before the motor was able to sustain its high speed.

“Did you hear that” I said to my wife. “Hear what?” she replied. We’re going to kill the washer’s motor if we don’t take some things out of it. She looked at me as if I had two heads. “You weren’t even here when it was loaded. How do you know it’s over loaded?”

Knowing that the centrifugal switch kicks out around 75% of its rated speed and that the starting torque is around 175% of the running torque were the clues that made me think that. Also, with the centrifugal switch that is speed sensitive, you can see why this isn’t a good motor to use with a speed control.

We went over to the washer and removed numerous pairs of jeans and sweat shirts (enough to start a small boutique) and set them aside for a second load. The next start the motor ran up to speed on one try.

Don’t try to explain centrifugal switches early in the morning.

I had a second cup of coffee and a conversation with our daughter.

  • Synchronous Motors
    • Advantages
      • Operates at an exact, constant speed
      • Brushless, more reliable
    • Disadvantages
      • Low starting torque
      • Starting the load inertia must be within the motor’s capability.
  • Poly phase
    • Advantages
      • High starting torque
      • High efficiency
      • No capacitor, more reliable
      • Brushless, more reliable
      • No centrifugal switch, more reliable
      • Low current
      • From about ¼ to 300 HP.
    • Disadvantages
      • Requires polyphase power or an inverter
      • Speed controlled by a Variable Frequency Drive (VFD)
  • Universal Motor / DC series wound
    • Advantages
      • Small size, operates at 7,500 to 10,000 rpm
      • Lighter weight per HP rating
      • Can operate on AC or DC
      • Ideal for hand held tools
      • Ideal for home appliances like vacuum cleaners
      • Low cost
      • High starting torque
    • Disadvantages
      • Operates at 7,500 to 10,000 rpm
      • Requires gearing to reduce the output speed but gains output torque (adds to the audible noise)
      • Brushes, less reliable
      • Noisy
      • Unidirectional
      • Poor speed regulation

Did you ever wonder why a vacuum cleaner motor sounds like it’s going faster when you cover the intake hose with you hand or it gets clog with debris?

Well that’s because it is going faster.

When the hose is clean and unobstructed the motor is working and under a load by moving the air. Block the air flow and you have removed the load (no air flow) and the motor speeds up to its no-load speed.

For DC Motors and controls we have:

  • Brushed DC motors
    • Permanent magnet (PM)
      • Advantages
        • Continuous duty
        • Reversible
        • Reasonable constant speed
        • Adjustable speed with inexpensive controls.
        • Speed is proportional to the applied voltage
        • Torque is proportional to the current
        • From 1/10 to 3 HP
      • Disadvantages
        • Brushes wear and create an eclectically conductive dust
        • Electrical noise (EMI & RFI Interference) from the brush commutator interface
        • Commutator wear
        • Cogs at low speed
    • Shunt wound
      • Advantages
        • Same as PM
      • Disadvantages
        • Same as PM
        • Physically larger than a PM DC motor in order to accommodate the shunt winding
        • Need a power supply to energize the shunt field.

For AC Motors and controls we have:

  • See Poly Phase motors
    • Controls can vary from the moderately priced VFD to the expensive vector drives. Vector drives require a feedback device and can mimic servo like performance.

For Brushless motors and controls we have:

  • Brushless DC motors
      • Advantages
        • Continuous duty
        • Reversible
        • Constant speed
        • No brushes, more reliable
        • DC brushless controls can be less expensive than VFD.
      • Disadvantages
        • Electrical noise (EMI & RFI Interference) from the controller and the motor
        • Cogs at low speed
        • Needs a more complex drive.
        • The control requires feedback in order to electronically commutate the motor

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

We’ll continue with even more motor technologies next time

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