Quote:
Originally Posted by chala
... If the voltage is lower that the spec'd voltage then the current will be higher.
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Good point!
Operating a motor at the "outer limits",
either high or low, of its Voltage requirements (Voltage tolerance band)
reduces its efficiency and can cause premature failure. Don't think that you're okay, just because your supply voltage falls within these tolerance bands. The purpose of these bands is to accommodate the normal hour-to-hour swings in utility supply voltage. Operation on a continuous basis, at either the high or low extreme, will shorten the life of the motor. The best performance will always occur at the rated voltage*.
* Motors meeting the criteria contained in the NEMA standard will operate satisfactorily within plus-or- minus 10% of the rated nameplate voltage.
Low Voltage:
When you subject a motor to low voltage (below the nameplate rating), some of the motor's characteristics will change slightly, and others will change dramatically. Low voltage can lead to
overheating, shortened life, reduced starting ability, and reduced pull-up and pullout torque.
To drive a fixed mechanical load connected to the shaft, a motor must draw a fixed amount of
power from the line. In a single phase motor, Power in Watts* = Volts x Amps (conversely, Amps = Watts ÷ Volts). Thus, when voltage gets low, the current must increase to provide the same amount of power.
An increase in current is a
danger to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat may damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.
High Voltage:
An assumption people often make is that since low voltage increases the amperage draw on motors, then high voltage must reduce the amperage draw and
heating of the motor. This is not the case. High voltage on a motor tends to push the magnetic portion of the motor into saturation. This causes the motor to draw excessive current in an effort to magnetize the iron beyond the point where magnetizing is practical.
Motors will tolerate a certain change in voltage above the design voltage. However, extremes above the design voltage will cause the amperage to go up with a corresponding increase in
heating and a shortening of motor life.
Such sensitivity to voltage is not unique to motors. In fact, voltage variations affect other magnetic devices in similar ways. The solenoids and coils you find in relays and starters tolerate low voltage better than they do high voltage. This is also true of ballasts in fluorescent,
mercury, and high-pressure sodium light fixtures. And it's true of transformers of all types. Incandescent lights are especially susceptible to high voltage. A 5% increase in voltage results in a 50% reduction in the life of the lamp. A 10% increase in voltage above the rating reduces incandescent lamp life by 70%.
Rules of Thumb for High and Low Voltage:
• Small motors tend to be more sensitive to overvoltage and saturation than do large motors.
• Single-phase motors tend to be more sensitive to overvoltage than do 3-phase motors.
• U-frame motors are less sensitive to overvoltage than are T-frames.
• Premium efficiency Super-E motors are less sensitive to overvoltage than are standard efficiency motors.
• Two- and 4-pole motors tend to be less sensitive to high voltage than are 6- and 8-pole designs.
• Overvoltage can drive up amperage and temperature even on lightly loaded motors. Thus, high voltage can shorten motor life even on lightly loaded motors.
• Efficiency drops with either high or low voltage.
• Power factor improves with lower voltage and drops sharply with higher voltage.
• Inrush current goes up with higher voltage.
* Power in H.P. = (Volts x Amps x Motor Efficiency) ÷ 746
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