Motors and Drives

Motors and Drives Testing Services

Motor efficiency is the ratio of output power at the shaft to electrical input power. Output power, the effective work or motor horsepower, is a product of the motor’s shaft speed and its output torque. Input power is the amount of electric power the motor draws from the line to create the output power and to cover its own internal losses. These internal motor losses include stator copper losses, rotor conductor losses, iron core losses, friction and windage losses and stray load losses. Efficiency gains are made by reducing some or all of these component losses.

There are several reasons to be concerned about the efficiency of your motor:

The operating cost of a motor is much higher than its initial cost. A higher efficiency motor requires less input power and therefore costs less to operate over the life of the motor than a lower efficiency motor. Typically 95 to 97 percent of the total lifecycle cost is electricity converted by the motor.
Motor efficiency is a reliability issue. Since motor losses dissipate as heat, higher efficiency motors usually run cooler than less efficient motors of the same rating. The electrical wiring in motors, called “windings,” is insulated to prevent electrical shorts. A 10º C rise in winding’s temperature shortens the life of this insulation and consequently the life of the motor by half.

At Advanced Energy, we conduct efficiency tests for three phase motors according to the Institute of Electrical and Electronics Engineers (IEEE) Standard 112, Method B, the most recognized method of determining induction motor efficiency. Other equivalent test methods such as the CSA 390 standard are available upon request. Single phase motors are tested according to IEEE Standard 114. Custom efficiency test protocols are also available.

Temperature Rise

Excessive heat shortens the life of a motor. Motor losses dissipate as heat and are removed from the motor by the motor cooling system. The motor cooling system includes internal or external fans, fins, and even coolant-based systems for special application motors. While losses increase with the load, the cooling ability of the motor does not; therefore, the temperature of the motor increases with load. When a motor application overloads the motor or restricts air flow over the motor, inhibiting the fans and fins most commonly used to cool the motor, the motor temperature can rise over the designed limit shortening the life of the motor.

Overloading the motor, improper supply voltage (high/low), frequent start/stops, poor input power quality, restricting the cooling system and using variable frequency drives increase temperature rise. This heat destroys motors in two main ways:

Heat reduces the expected life of lubricants and seals. At higher temperatures, the soap component of lubricants that time-releases oil becomes sticky increasing friction losses, reducing the life of the bearing and leaking the released oil onto the motor windings. Lubricating oils deteriorate winding insulation causing premature winding insulation failure.
Winding insulation in motors is classified by its life expectancy at various temperatures. When the rated temperature is exceeded, the insulation breaks down at a faster rate resulting in premature failure.

When selecting a motor for a particular application it is important to consider the operating temperature of the motor. At Advanced Energy, we conduct heat run tests to establish thermal stabilization at a specific load. Tests to determine temperature rise at rated conditions and at application load and voltage conditions are available. We measure the temperature rise of motors using the stator winding resistance method as specified in IEEE Standard 112. This measurement provides an average temperature through the entire stator windings and is a very reliable and repeatable measurement. Additionally, we install thermocouples in the windings to determine when the motor has reached its thermal stabilization.

Locked-Rotor Tests

Locked rotor tests determine the starting characteristics of the motor, quantities often referred to as starting current and starting torque. The locked rotor torque and current are, respectively, the minimum torque developed and the steady-state current drawn by the motor at standstill (RPM=0) for all rotor angular positions at rated voltage and frequency.

At Advanced Energy, we measure these quantities to determine whether the motor meets the locked-rotor torque and current requirements set by the National Electrical Manufacturers Association (NEMA). A motor that draws excessive current under locked rotor conditions is more likely to cause nuisance tripping of protection devices during motor start-ups. On the other hand, the minimum torque requirements ensure that the motor produces adequate torque to start the load.

Torque-Speed Curves

To drive a load effectively a motor must produce torque that is greater than the torque required by the load at all speeds from start to full speed. A motor that is unable to quickly accelerate the connected load to operating speed will generate damaging excessive heat and either fail immediately, cause the breaker or starter to trip or at the very least experience a shortened life. A torque-speed test ensures a motor’s characteristics are adequate for a specific application.

NEMA MG1 defines various parts of the torque-speed curve for various types of motors such as the locked rotor, pull-up, breakdown and full-load (rated) torque. All applications must consider how a motor performs at these points:

The full-load (rated) torque of a motor is the torque necessary to produce its rated horsepower at full-load speed.
The locked-rotor torque is the minimum torque developed at rest (RPM=0) for all angular positions of the rotor with rated voltage applied at rated frequency.
The breakdown torque is the maximum torque the motor will develop with rated voltage applied at rated frequency without an abrupt drop in speed.
The pull-up torque is the minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. This characteristic is only available for alternating-current (AC) motors. For motors that do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed.

At Advanced Energy, we use high-speed data acquisition systems and a variety of dynamometers and methodologies to create torque-speed curves. These curves show the intricate details of the various torque-speed characteristics described above for the motor.

Current Unbalance (Three-phase motors)

The most common cause of current unbalance in three-phase motors is an unbalance in the source voltage applied to the motor. A one percent voltage unbalance in a motor results in a seven to 10 percent current unbalance. Current unbalance leads to higher losses and increased heat in the motor. Given this fact, NEMA recommends that motor horsepower be de-rated if operating at voltage unbalances above one percent to mitigate the negative effects of the current unbalance on the motor.

A high current unbalance can also lead to circulating currents in windings that eventually return as shaft currents. These currents can cause pitting and overheating in the bearings leading to premature bearing failure. Also, current unbalance causes nuisance tripping of motor protective equipment and even fuse failures.

In the Advanced Energy lab, our phase-independent variable voltage (variacs) source supplies all three-phase motors under test with a voltage unbalance of less than 0.1 percent. Any significant current unbalance is attributable to impedance and/or magnetic asymmetries in the design of the motor. We are also able to create specific unbalanced voltages that enables us to quantify the effects on motor design and construction.

For More Information

Please contact Kitt Butler or David Berkowitz at 800 869-8001 or to learn more about how Advanced Energy can support your needs.