Low Voltage Effects on Ac Motors

Only available on StudyMode
  • Download(s) : 890
  • Published : November 4, 2012
Open Document
Text Preview
Low Voltage Effects on AC Motors
A drop in voltage will result in a proportionate increase in current. If the current exceeds the nameplate rating and is not corrected, this can result in damage to the motor from overheating

Nameplate Rating

An electric motor will have a nameplate rating for both voltage and amperage. When a load is on a motor, the motor must draw a fixed amount of power. The required power is about equal to volts times amps. Should the voltage fall below the nameplate rating, the amperage will increase. This can result in increased heat that will shorten the motor's life, according to Motorsanddrives.com. Torque

The motor is subject to several kinds of torque. The starting or start-up torque is the amount present when the motor is stationary and power is applied. The pull-up torque is the minimum amount necessary for the motor during the starting sequence.

Effects of Low Voltage on Torque

A reduced amount of voltage will reduce the amount of torque. This can result in difficulty for start loads. A reduction of voltage to 80 percent would result in a torque value of only 64 percent. In light loads, a reduction in voltage may actually be preferable because it will result in increased efficiency.

Even a small voltage unbalance will result in large current unbalance during the running of motor by a factor of 6 times.
• Negative phase sequence components will lead to heating of motor • Negative phase sequence currents leads to reduction in motor output torque. • Motor is forced to run at higher slip leading to increased rotor loss and reduced efficiency.

• Electricity boards should look in to this phenomenon seriously where irrigation pump sets suffer the voltage imbalance, even 1% loss of efficiency for the country like India would mean a great loss. 5 HP irrigation pump sets with a quantity of 6 Lakh approximate, the loss of power would be 22MW and annual wastage of Rs.47.5 millions (At 3 Rs/unit, 4 Hrs of Pump working and 6 month’s season

Keys to maximize the service life of industrial motors

Why do motors fail?
Certain components of motors degrade with time and operating stress. Electrical insulation weakens over time with exposure to voltage unbalance, over and under-voltage, voltage disturbances, and temperature. Contact between moving surfaces causes wear. Wear is affected by dirt, moisture, and corrosive fumes and is greatly accelerated when lubricant is misapplied, becomes overheated or contaminated, or is not replaced at regular intervals. When any components are degraded beyond the point of economical repair, the motor’s economic life is ended. For the smallest and least expensive motors, the motor is put out of service when a component such as a bearing fails. Depending upon type and replacement cost, larger motors—up to 20 or 50 horsepower (hp)—may be refurbished and get new bearings, but are usually scrapped after a winding burnout. Still larger and more expensive motors may be refurbished and rewound to extend life indefinitely. An economic analysis should always be completed prior to a motor’s failure to ensure that the appropriate repair/replace decision is made.

Extend Motor Life with Improved Bearing Care

Bearing failures are the root cause for the great majority of electric motor downtime, repair and replacement costs. Bearing and motor manufacturers are aware of the situation. Motor repair shops can attribute much of their business to bearing failures. And motor users see bearing failure as the fundamental cause of virtually every electric motor repair expense. Studies conducted by the Electrical Apparatus Service Association also demonstrate that bearing failures are by far the most common cause of motor failures.

Knowing that shaft bearings are the Achilles’ heel of industrial electric motors is not a new idea in maintenance departments, but what is new is recognizing that something can be done to prevent most...
tracking img