AC Drives vs. DC Drives: Which is Better?

Home | Articles | Forum | Glossary | Books

While advancements in technology really have made AC drives competitive, which is "better" depends on your application. The table below lists the advantages of both types.

AC Drive Advantages DC Drive Advantages
Use conventional, low cost, 3-phase AC induction motors for most applications. Less complicated due to single-power conversion (AC to DC only).
AC motors require very little maintenance and are preferred for applications where the motor is mounted in an area not easily reached for servicing or replacement. Often less expensive for most horsepower ratings.
AC motors are smaller, lighter, more commonly available, and less expensive than DC motors. DC motors have traditionally been adjustable-speed machines and a wide range of options have evolved for this purpose
AC motors work better in high-speed operation (over 2500 rpm) since there are no brushes nor commutators. Cooling blowers and inlet air flanges provide cooling air for a wide speed range at constant torque.
Wet, corrosive or explosive operating environments require special motor enclosures. Special AC motor enclosure types are more common and cost less. Availability of accessory-mounting flanges and kits for mounting feedback tachometers and encoders.
Multiple motors in a system can operate simultaneously at a common frequency/speed. DC regenerative drives are available for applications requiring continuous regeneration for overhauling loads. AC drives with this capability would be more complex and expensive.
It is advantageous to use an existing constant-speed AC motor already mounted and wired on a machine. If done properly, brush and commutator maintenance is minimal.
Lack of commutators and brushes are important when the application load varies greatly and light loads are encountered for prolonged periods. DC motor commutators and brushes can wear rapidly under this condition. DC motors can provide starting and accelerating torques in excess of 400% of rated.
Low-cost electronic motor reversing. Some AC drives may be noisy which is undesirable in some applications.
A back up (constant speed) is available if the controller fails.  

Electric Motors + Motor-Control Systems
(Nested-content Series)

Electric Motors

Motor Maintenance and Troubleshooting

Motor Maintenance

In general, motors are very reliable machines that require little maintenance. But while a typical electric motor might be a low-maintenance item, it still requires regular maintenance if it’s to achieve the longest possible service life.


The key to minimizing motor problems is scheduled routine inspection and service. Keep records of all maintenance schedules and procedures performed. The frequency and procedures of routine service vary widely between applications. Motors should be inspected periodically for things such as shaft alignment, motor base tightness, and belt condition and tension.


For DC motors, remove the covers and perform checks on brush wear, spring tension, and commutator wear or scoring. Replace the brushes if there is any chance they won’t last until the next inspection date. The commutator should be clean, smooth, and have a polished brown surface where the brushes ride. Observe the brushes while the motor is running. The brushes must ride on the commutator smoothly with little or no sparking and no brush chatter.


Twice yearly, test winding and winding-to-ground resistance to identify insulation problems. Motors that have been flooded or have low megger readings should be thoroughly cleaned and dried before being energized. The following are typical minimum motor insulation resistance values:

Rated Motor; Voltage; Minimum; Insulation; Resistance;

600 V and below 1.5 M-ohm 2,300 V 3.5 M-ohm

4000 V 5.0 M-ohm


Wipe, brush, vacuum, or blow accumulated dirt from the frame and air passages of the motor. Dirty motors run hot when thick dirt insulates the frame and clogged passages reduce cooling airflow. Heat reduces insulation life and eventually causes motor failure.


Motors that are used continuously are not prone to moisture problems. It’s the intermittent use or standby motor that may have difficulties. Try to run the motor for at least a few hours each week to drive off moisture. Be careful that steam and water are not directed into open drip-proof motors.


Lubricate motors according to manufacturer specifications. Apply high-quality greases or oils carefully to pre vent contamination by dirt or water.


Feel the motor frame and bearings for excessive heat or vibration. Listen for abnormal noise. All indicate a possible system failure. Promptly identify and eliminate the source of the heat, noise, or vibration.


The high current flow during start-up contributes a great amount of heat to the motor. For motors 200 hp and below, the maximum acceleration time a motor connected to a high-inertia load can tolerate is about 20 seconds.

The motor should not exceed more than about 150 "start seconds" per day.

Ill. 73 Instruments used for motor troubleshooting. Megohmmeter Thermometer. Multimeter Clamp-on ammeter.

Troubleshooting Motors

Electric motor failures can be due to mechanical component failure or electrical circuit failure. Any type of electrical testing involves risk, and complacency can lead to injury! When working on any type of motor, to reduce the risk of injury be certain to:

• Disconnect power to the motor and complete lock out and tag-out procedures before performing ser vice or maintenance.

• Discharge all capacitors before servicing the motor.

• Always keep hands and clothing away from moving parts.

• Be sure required safety guards are in place before starting equipment.

Electrical contact accounts for one-fifth of all construction deaths. Never work on energized equipment unless this is absolutely necessary for examination, adjustment, servicing, or maintenance. When you find you must work on energized equipment, always wear the appropriate personal protective equipment and use appropriate tools and equipment. Use the "buddy rule" and never work on energized equipment alone. Always have a partner working with you, in case of emergency.

Typical instruments used for troubleshooting motor operation problems include a multimeter, clamp-on ammeter, megohmmeter, and infrared thermometer. These instruments, are used to measure voltage, current, resistance, insulation resistance, and temperature.

The basic motor system consists of the power supply, controller, motor, and driven load. When a motor problem occurs, it’s first necessary to find which of the parts of the system is at fault. Power supplies and controllers can fail as well as the motor itself. Mechanical loads can increase because of an increased size of the load the motor is driving, or failure of bearings or coupling mechanisms.

Mechanical overloading is a prime cause of motor failure.


Once it has determined that the motor is at fault, you can proceed to locate the problem with the motor. A troubleshooting guide outlines a comprehensive variety of motor problems.

Generally the categories are arranged according to symptoms, offering brief suggestions concerning what to look for when investigating motor failures and often providing advice on how to correct the problem once it has been identified.

The following is an example of a troubleshooting guide that outlines fault symptoms common to most types of motors.

1. Symptom: The motor fails to start. Possible causes:

  • Blown fuse or open circuit breaker. Check the volt age at the input and output of the overcurrent protection device. If voltage is measured at the input but not at the output, the fuse is blown or the circuit breaker is open.
  • Check the rating of the fuse or circuit breaker. It should be at least 125 % of the motor's full-load current.
  • Motor overload relay on starter tripped. Allow overload relay to cool and reset it. If the motor causes the overload relay to open after a short period, check for motor short circuits and grounds. Check the full load current of the motor and compare it to the setting of the overload relay.
  • Low voltage or no voltage applied to the motor.
  • Check the voltage at the motor terminals. The volt age must be within 10 % of the motor nameplate voltage. Determine the cause of the low voltage.
  • Loose fuse clips and connections at the terminals of the disconnect switch or circuit breaker can result in low voltage at the motor.
  • Mechanical overload. Rotate the motor shaft to see if a binding load is the problem. Check for frozen bearings.
  • Check the air gap between the stator and rotor. Reduce the load or try operating the motor with no load applied.
  • Defective motor windings. Make resistance checks of the motor windings for opens and shorts in coil windings and coils shorted to ground faults. An ohmmeter reading of infinity across a set of coil windings means that there is an open somewhere-sometimes it’s at one end of the coil and accessible for repair. A short circuit in only a few turns of a coil, while difficult to detect, will still result in a motor overheating. One way to test for a short circuited coil winding is to compare its resistance reading with that of a known good identical coil.
  • Burnt-out motor. If one or more of the motor windings looks blackened and smells burnt, it’s most likely burnt out and needs to be replaced.

2. Symptom: The motor overheats. Possible causes:

  • Load. A basic rule is that your motor should not get too hot to touch. Check ammeter reading against full load current rating of motor. For a higher-than-normal current reading, reduce the load or replace motor with a larger sized one.
  • Insufficient cooling. Remove any buildup of debris in or around the motor.
  • Ambient temperature. Higher-than-normal ambient temperatures. Take steps to improve the motor's ventilation and/or lower the ambient temperature.
  • Bearings and alignment. Bad bearings or poor coupling alignment can increase friction and heat.
  • Source voltage. If the operating voltage is too high or too low, the motor will operate at a higher temperature. Correct voltage to within 10 % of the motor's rating.

3. Symptom: Excessive motor noise and vibration. Possible causes:

  • Bearings. With the motor stopped, try gently moving the shaft up and down to detect bearing wear. Use a stethoscope to check the bearings for noise. When the handle of a screwdriver is placed to the ear and the blade to the bearing housing, the screwdriver will amplify the noise, like a stethoscope. Replace worn or loose bearings. Replace dirty or worn-out oil or grease.
  • Coupling mechanism. Check for bent shaft on motor or load. Straighten if necessary. Measure the alignment of the couplings. Realign if necessary.
  • Loose hardware. Tighten all loose components on the motor and load. Check fasteners on the motor and load mounts. Motors with centrifugal mechanisms, brushes, slip rings, and commutators can cause noise due to wear and looseness of the mechanisms.

4. Symptom: Motor produces an electric shock when touched. Possible cause:

  • Grounding. Broken or disconnected equipment grounding conductor. Motor winding short-circuited to frame. Check motor junction box for poor connections, damaged insulation, or leads making electrical connection with the frame.

5. Symptom: Motor overload protector continually trips. Possible cause:

  • Load. Load too high. Verify that the load is not jammed. Remove the load from the motor and measure the no-load current. It notably should be less than the full-load rating stamped on the nameplate.
  • Ambient temperature too high. Verify that the motor is getting air for proper cooling.
  • Overload protector may be defective. Replace the motor's protector with one of the correct rating.
  • Winding short-circuited or grounded. Inspect windings for defects and loose or cut wires that may cause a path to ground.


Troubleshooting Q&A may be used to quickly identify common problems and possible corrective courses of action. The following are examples that pertain to specific motor types.


Single-Phase Motors


  • Split-phase motor hums, and it will run normally if started by hand.
  • Capacitor-start motor hums, and it will run normally if started by hand.
  • Start capacitors continuously fail.
  • Run capacitor fail.
  • Universal motor sparks.

Probable Cause and Course of Action:

  • Centrifugal switch is not operating properly. Disassemble the mechanism. Clean the contacts. Adjust spring tension. Replace switch.
  • Centrifugal switch (same as for split-phase motor). Defective capacitor. Test capacitor. If defective replace.
  • The motor is not coming up to speed quickly enough as a result of not being sized properly.
  • The motor is being cycled too frequently. Capacitor manufacturers recommend no more than twenty 3-second starts per hour.
  • Starting switch may be defective, preventing the motor from opening the start winding circuit.
  • Ambient temperature too high.
  • Possible power surge to motor caused by high transient voltage. If a common problem install a surge protector.
  • New brushes not properly seated. Seat brushes with fine sandpaper to fit contour of commutator.
  • Worn or sticky brushes. Replace brushes or clean brush holder.
  • Open- or short-circuited armature coils. Replace armature.


Three-Phase Motors


  • Single-phasing-one phase of the three phase system is lost. Motor won’t start, but if in operation may continue to operate at increased current and diminished capacity.
  • Unique high-pitched sound from motor.
  • Unbalanced three-phase voltage-the voltages of all phases of a three-phase power supply are not equal. A voltage imbalance of 3.5% between phases will cause a temperature rise of 25°C in the motor. Motor operates at a higher-than normal temperature and reduced efficiency.
  • Wound-rotor induction motor fails to start or starts and runs erratically.
  • Synchronous motor experiences increased start-up times or erratic acceleration.

Probable Cause and Course of Action:

  • A fuse is blown or one leg of a circuit breaker is open. Check each of the three-phase power lines for correct voltage.
  • Blown fuse on power factor correction capacitor bank-find and replace fuse.
  • Uneven single-phase loading-distribute single-phase loads more evenly on the three-phase circuit.
  • Utility unbalanced voltages-if the incoming voltages are substantially unbalanced, contact the utility and ask them to correct the problem.
  • Harmonic distortion-The presence of harmonic distortion in the applied voltage to a motor will increase motor temperature, which could result in insulation damage and possible failure.
  • Locate the sources of the harmonics and use harmonic filters to control or reduce harmonics.
  • External rotor resistors-Look for failed components in the resistor bank when troubleshooting. Clean slip rings and check brushes for wear and proper pressure.
  • Damaged or defective amortisseur windings-Historical inrush testing that records the stator's current during start-up can greatly assist in determining if these windings have degraded over the life of the motor.


Direct-Current Motors


  • Excessive arcing at brushes.
  • Rapid brush wear.

Probable Cause and Course of Action:

  • Worn or sticky brushes. Replace brushes or clean brush holder.
  • Incorrect brush position with respect to neutral plane. Rotate brush rigging to the correct position to aid in commutation.
  • Overload. Measure current to the motor and compare to full-load current rating. If necessary, reduce motor load.
  • Dirty commutator. The commutator surface should be clean and bright; slight scratches and discoloring can be removed with emery paper. Deep scratches/ridges require the commutator to be machined and mica-undercut.
  • Armature faults. Test for open- and short-circuited windings in the armature and correct or replace motor.
  • Field-winding faults. Test for short circuits, open circuits, and ground faults and correct or replace motor.
  • Wrong brush material, type, or grade. Replace with brushes recommended by manufacturer.
  • Incorrect brush tension. Adjust brush tension so that the brush rides freely on the commutator. Replace brush springs if tension measured by a scale is insufficient.



A troubleshooting ladder or tree may be used to guide you through the steps of the troubleshooting process. A trouble shooting ladder is sequential in nature, and its simplicity can often save time in arriving at the source of a motor problem. The following is a typical example of a trouble shooting ladder used to determine the cause of overheating of a three-phase squirrel-cage induction motor.

Problem--Motor Overheats:

Step 1 Is ambient temperature too high? NO/YES Reduce ambient, increase ventilation or install larger motor.

Step 2 Is motor too small for present operating conditions? NO/YES Install larger motor.

Step 3 Is motor started too frequently? NO/YES Reduce starting cycle or use larger motor.

Step 4 Check external frame. Is it covered with dirt, which acts as insulation and prevents proper cooling? NO/YES Wipe, scrape, or vacuum accumulated dirt from frame.

Step 5 Feel output from air exhaust openings. Is flow light or inconsistent, indicating poor ventilation? NO YES Remove obstructions or dirt preventing free circulation of airflow. If needed, clean internal air passages.

Step 6 Check input current while motor drives load. Is it excessive, indicating an overload? YES NO Go to Step 11 Step 7 Is the driven equipment overloaded? NO YES Reduce load or install larger motor.

Step 8 Are misalignments, bad bearings, or damaged components causing excessive friction in driven machine or power transmission system? NO YES Repair or replace bad components.

Step 9 Are motor bearings dry? YES Lubricate. Does motor still draw excessive current? Step 10 Are damaged end bells, rubbing fan, bent shaft, or rubbing rotor causing excessive internal friction? NO YES Repair or replace motor.

Step 11 Are bad bearings causing excessive friction? NO YES Determine cause of bad bearings.

Step 12 Check phase voltage. Does it vary between phases? NO YES Restore equal voltage on all phases.

Step 13 Is voltage more than 10% above or 10% below nameplate? NO YES Restore proper voltage or install motor built for the voltage.

Step 14 Check stator. Are any coils grounded or short-circuited? YES Repair coils or replace motor.


1. From a safety perspective, what is the first step to be taken before performing any type of motor maintenance?

2. Outline five common motor maintenance tasks that should be performed as part of a motor preventive maintenance program.

3. Outline how to test for each of the following suspected motor problems.

a. Blown fuse or open circuit breaker.

b. Low voltage applied to the motor.

c. Defective motor windings.

4. List five possible causes of motor overheating.

5. The centrifugal switch of a spilt-phase motor fails and remains open at all times. How would this affect the operation of the motor?

6. The centrifugal switch of a capacitor start motor fails and remains closed at all times. How would this affect the operation of the motor?

7. List four possible causes of unbalanced voltages on the supply voltage of a three-phase motor circuit.

8. List five possible causes of excessive arcing at the brushes of a DC motor.


1 . Assume the tags used to identify the six motor leads of a compound-wound DC motor are suspected of being incorrectly marked or missing.

a . Outline how an ohmmeter would be used to identify the armature, shunt field, and series field leads.

b . What operating test could be made to ensure cumulative connection of the shunt and series field?

2. One of the three-phase line fuses to a squirrel-cage induction motor burns open while the motor is operating.

a . Will the motor continue to rotate? Why?

b . In what way might this operating condition damage the motor?

c. Should the motor be able to restart on its own? Why?

3. A defective motor start capacitor rated for 130 µF and 125 V AC is replaced with one rated for 64 µF and 125 V AC. What would happen?

4. The speed of a motor is to be reduced by one-half by using two different size pulleys. What must the relative diameters of the motor drive and load pulley be?

5. A motor feels hot to the touch. Does this always indicate it’s operating at too high a temperature? Explain.


1 . Explain how a squirrel-cage rotor produces a magnetic field.

2. List the different types of motor measurements that are used for troubleshooting motors.

3. Why does a single-phase motor have no starting torque if only a single winding is used?

4. How would you determine the running and starting winding of a single-phase motor from a visual inspection of the stator?

5. Arrange the following single-phase motors in the order of decreasing torque, with the highest torque first: split-phase, universal, shaded pole, capacitor.

6. How does slip affect motor speed?

7. Describe the major physical and electrical differences between the three major three-phase motor types.

8. Can a single-phase motor be operated from a three phase power supply? Explain.

9. Assume you have to purchase a motor and load laser alignment kit. Search the Internet for suppliers and prepare a report on the features and operation of the one you would consider purchasing.

10. An energy-efficient motor produces the same shaft output power (hp), but uses less input power (kW) than a standard-efficiency motor. Visit the website of a motor manufacturer and compare the price and features of a standard-efficiency motor with that of an equivalent energy-efficient motor.

11. Explain why motors are more efficient at full load.

<<Prev. | Index | Next>>

HOME | PREVIOUS: AC and DC Drives: Spare Parts | NEXT:
Home  Forum  Books  Glossary  Sitemap