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Single-Phase Electric Motor Performance Characteristics

By M Jessup  –  Published 9/16/2008

Summary

Pull-up torque during starting is critical when specifying electric motors

Note: For the following discussion, assume the voltage and signal frequency across the motor remains constant.  Power factor, variable-speed controls, and other arcane AC phenomena will be ignored.

Motor Rating

Continuous-duty electric motors are rated by horsepower, RPM, and service factor.  Maximum motor power is limited by the highest temperature at which components in the motor can continue to function.  During operation, any electrical energy which does not do mechanical work gets converted into heat in the motor; that heat is removed by cooling air forced through the motor case by the integral motor fan.  A service factor is used to define the load at which the motor will stabilize at its maximum operating temperature.  When the (load) = (rated power x service factor), the net heat gain/loss is zero.  Any additional load placed on the motor at that point will add heat to the motor and cause damage.

Service factors (typ 1.15-1.25) are published at a given ambient condition (typ 40º C).  Installation affects a motor’s cooling ability.  Low ambient temperature helps cool the motor, effectively increasing its service factor.  High altitude reduces the cooling air’s effectiveness, decreasing the service factor.

Normal Operation

When a motor is operating at its rated horsepower and RPM, it is applying full-load torque, and the current is full-load amps.  It is the nature of electric motors that, as long as the applied load is less than full-load torque, the motor will operate a near-synchronous speed (i.e. above 1725 RPM for an 1800 RPM nominal motor) draw less than full-load amps.  When operating at near0synchronous speeds, motors exhibit very good performance because, up to the breakdown torque (also called pull-out torque), they will respond to changing loads by drawing more or less current as required to maintain a near constant (typ within 100 RPM) operating speed.  However, if the applied load exceeds breakdown torque, the motor speed will fall off dramatically and, if the load is not immediately reduced, the motor will stall.

Operation at loads above (full-load x service factor) is allowable for short periods.  Extended operation at these levels will raise the temperature of the motor to damaging levels.  In practice, motors can deliver more than twice their rated power for short periods without damage.

Starting

All motor operation at speeds below near-synchronous is potentially damaging.  At these speeds motor efficiency is very low, fan-cooling is less effective, and the resultant heat build-up is rapid.  Ideally loads are accelerated to near-synchronous as quickly as possible.

At start-up, the motor rotor is normally not turning.  The motor will apply its highest torque, locked-rotor torque (also called breakaway torque, or starting torque), and draw its highest current, locked rotor amps, when starting.  The motor can draw 600% of its nominal current at this point, and will quickly overheat if operated for more than a few seconds if the load does not turn.

As the rotor begins to turn, torque output decreases.  Torque will decrease to the pull-up torque limit.  This is the lowest torque the motor will apply while below near-synchronous speed, and is critical in determining if a motor will operate satisfactorily.
  • If the motor is driving a high inertial load, the motor may overheat before the load accelerates up to speed.  The motor pull-up torque rating may well be sufficient to eventually accelerate the load to full speed, but the time necessary to do so may allow enough heat to build in the motor to cause damage.
  • If the motor is starting a hydraulic pump against system pressure, is too high.  This can be particularly troublesome because a motor can satisfactorily operate the pump to position a load while operating at near-synchronous speed, but then not be able to start against the same load after a shut-down.
As the motor accelerates out of the pull-up torque trough (ref attached graph), torque output increases with increasing speed up to another maxima of the aforementioned breakdown limit, and then decreases again up to full speed.

Single Phase Electrice Motor Performance Characteristics - Graph

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