Burner Motor Service Facts

We have had many inquiries from you concerning oil burner motors and related service information. Therefore, we will present some basic principles of operation and service.

Theory of Operation

An AC motor consists of four basic components. See Figure 1.828-1

  1. Rotor: Follows the rotating magnetic field and causes the rotation of the motor shaft.
  2. Stator: The start and the run windings are wound around this and cause a magnetic field.
  3. Motor Windings: Create the magnetic field in the stator.
    1. Start Windings: Electrical current flow in this winding is used only to get the rotor turning and is disconnected when motor is up to full speed.
    2. Run Winding: Electrical current flow in this winding causes the rotating magnetic field in the stator. This maintains the shaft rotation after the start winding has been disconnected by the start switch.
  4. Start Switch: A centrifugal switch that connects AC power to the start winding until motor reaches 75 – 80% of full speed. At this point the centrifugal switch opens and disconnects AC power from the start winding while power remains connected to the run winding. The centrifugal throw out speed for 3450 RPM motor is about 2800 RPM and for the 1725 RPM motor it is approximately 1400 RPM.

Service Facts

  • Repair: Generally, the oil burner motor is designed to be a non-serviceable item that is to be replaced rather than repaired.
  • Oiling: There are two types of oil burner motors to consider:
    Permanently lubricated – this type of motor does not have any oiling ports and should not be oiled.
    Motors that require maintenance are oiled according to their duty cycle using SAE 20 oil.
  • Occasional Duty: Less than 2 hours / day. Oil every 5 years.
  • Intermittent Duty: 2 – 12 hours / day. Oil every 2 years.
  • Continuous Duty: 12 or more hours / day. Oil once every year.

THERMAL SWITCH: The thermal protector is a thermally activated switch that is electrically in series with the motor windings. If the motor overheats, the thermal switch will open and turn off the motor. There are two types used in oil burner motors.

Manual reset thermal protectors are the most common and require the serviceman to physically push some form of an indicator button to reset the switch.
Automatic reset will reset itself after the motor cools down.

The temperature at which the switch must open is as follows:
Class A Motor – 140° Celsius / 284° Fahrenheit.
Class B Motor – 165° Celsius / 329° Fahrenheit.


Motor fails to start: If AC voltage is present at motor leads and the thermal overload indicator is not showing, either the motor windings or the start switch have failed.


We will check for a faulty start switch by using our volt/ohm meter set to measure resistance (OHMS). In review, the motor start windings have a much lower resistance than the run windings do. This difference is what we will use to determine if the start switch is defective. When the start switch is functioning properly, both the start and the run windings are connected together and this results in a lower resistance. If the start switch is faulty, the start winding will not be connected or will intermittently be connected and this will result in a higher resistance or a resistance that will fluctuate when the motor shaft is rotated.

Using your meter set to measure resistance or ohms, set the scale on the meter to measure the lowest resistance available on your meter. Note: It would be very helpful to zero out your meter by shorting together the test leads and adjusting the meter to show zero ohms. Turn off power and remove the motor from the burner housing. Place motor in a shaft up position. Disconnect the black and white motor input leads. Connect your meter between the black and white motor input leads. If the start switch is functioning properly, the resistance should be approximately 2-4 ohms. If this resistance is much higher (in the range of 7-10 ohms), then most probably the start switch is bad or your meter is not adjusted properly. To double check if the start switch is bad, pull up on the motor shaft and note if the resistance drops and your meter reads the proper 2-4 ohms. If this happens, the start switch is most definitely bad and motor should be replaced.
The most important function of this test is to make sure that the resistance across the motor leads does not change when the motor shaft is pushed down or is rotated 360°. When rotating the motor shaft, let the meter settle down before noting the resistance. One additional test is to reach through the motor end bell (shaft end) and to push down on the centrifugal start switch. (See Figure 1.) At this point, the resistance should increase to approximately 7-10 ohms from the previously stated resistance of 2-4 ohms. This test cannot be performed on all motors.

In conclusion, if the measured resistance was approx. 2-4 ohms and did not change when you pulled up on the motor shaft and did not change when motor shaft was rotated, then the motor most likely is not at fault. If the motor continues to exhibit starting problems in operation, then it should be replaced.

Thermal Overload: If the Thermal Overload indicator is showing the motor has overheated from either internal failure or because of some external loading condition, check the following:

  1. Bad oil pump – Oil pump has seized and has overloaded the motor.
  2. A partially or completely plugged return line can cause the motor to overload at start due to the extra starting torque it can require.
  3. Misalignment of motor to pump – Check to see if any mounting bolts have loosened and caused improper seating of the motor or pump to the burner housing. Also check and see if the pump coupling is too long causing pressure upon the motor shaft.
  4. Bad start switch – If the centrifugal start switch has failed to disconnect the start winding from the AC power, when the motor has reached approximately full speed, the motor will overheat.
  5. Jammed blower wheel – Remove the obstruction and reset switch.
  6. Undersized motor – If the load requirement exceeds the nameplate rating for horsepower (H.P.), the motor will eventually overheat. Check: Use an Amp probe to measure the motor current and make sure the measured current does not exceed 10% over the motor nameplate current.
    NOTE: When utilizing a Suntec J or H model fuel unit, it will require the use of a 1/5 minimum H.P. motor in place of the 1/7 H.P. motor. You may experience starting problems if you use a motor of a lower horsepower rating.

Excessive motor noise: One cause is shaft end play, or the distance the shaft will move in and out of the motor. A new motor will have no more than .035″ of movement. If shaft end play is more than .060″ and noise is a problem, the motor could be suspect.

Motor sizing and replacement: Use a motor of at least the same HP rating, and of identical rotation.

The typical oil burner motor has provided dependable service over the years. These facts will help you become better acquainted with the motor and service problems you may encounter. For a more detailed service checklist, please refer to Figure 2.

Trouble Cause What To Do
Motor fails to start. Blown fuses. Replace with time-delay fuses. Check for grounded winding.
Low voltage. Inadequate wiring or extension cords. Low system voltage.
Improper line connections. Check connections against diagram supplied with motor.
Overload (thermal protector) tripped. Check and reset overload relay in starter. Check heater rating against motor nameplate current rating. Check motor load. If motor has manual reset thermal protector, check if tripped.
Motor may be overloaded. Reduce load; increase motor size.
If three-phase, one phase may be open. Indicated by humming sound. Check lines for open phase. Check voltage with motor disconnected, one fuse blown.
Defective motor or starter. Repair or replace.
Motor stalls. Overloaded motor. Reduce load or increase motor size.
Low motor voltage. See that nameplate voltage is maintained.
Motor does not come up to speed. Not applied properly. Consult motor service firm for proper type. Use larger motor.
Voltage too low at motor terminals due to line drop. Use higher voltage tap on transformer terminals, increase wire size. Check for poor connections. Voltage unbalance.
Starting load too high. Check load motor is carrying at start.
Motor takes too long to accelerate. Excess loading; tight belts. High inertia load. Reduce load; increase motor size. Adjust belts.
Inadequate wiring. Increase wire size. Check for poor connections.
Applied voltage too low. Reconnect to a higher tap. Increase wire size. Check for poor connections.
Defective motor. Repair or replace.
Inadequate starting torque. Replace with larger motor.
Motor vibrates or is excessively noisy. Motor misaligned. Re-align.
Three-phase motor running single phase. Check for open circuit, blown fuses or unbalanced voltages.
High or unbalanced voltages. Check wiring connections, transformer.
Worn, damaged, dirty or overloaded bearings. Replace; check loading and alignment.
Loose sheave or coupling. Tighten set screw(s); replace.

Figure 2