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comprised of companies that distribute and manufacture the components that keep your world running smoothly.

Tech Tip: Motors

Enhance your employees’ product and industry knowledge with PTDA’s Tech Tips. This library of online tips, hints and techniques may be used to educate new and current employees on power transmission/motion control (PT/MC) products, technologies and concepts and serve as reference and reminder for more experienced staff. Tech Tips are based on the expertise of PTDA member companies, content from Motion Control Made Simple or PTDA’s Power Transmission Handbook®, the definitive resource and training tool on PT/MC products.

Visit the full library of Tech Tips for additional products.

How can the drivesystem produce energy savings in typical warehouse conveying applications?

Often conveyor applications in warehouse fulfillment centers run at the full speed even when the output volume demand fluctuates regularly.  Matching the conveyor speed to the load volume will result in a direct energy savings. Simply put, when the load volume demand is at 50% capacity, operating the drive system at 50% of the speed will result in 50% less energy usage. A cost effective solution is to consider efficient helical or helical-bevel gearmotors with premium efficient motors and AC variable speed drives.  Eliminating less efficient worm gearing and external belt or chain drives in favor of directly mounted or shaft mounted gearmotors will also increase the system efficiency.

What is the significance of the NEMA Code Letter found on a motor's nameplate?

The code letter is a simple visual indication of the magnitude of the locked rotor (starting or in-rush) current a motor will draw at the moment full motor voltage is applied during a direct-across-the-line start. The higher the code letter is, the higher the locked rotor current will be per motor HP. 

See NEMA table MG1 Part 10.37 for the range represented by each letter. A motor's locked rotor current can be calculated using this formula: [1000 x HP x (Code Letter mid-range value)] / (√3 x motor voltage) Example: 7.5 HP 230V motor, Code Letter G ≈ (1000 x 7.5 x 5.9) / (√3 x 230) ≈ 111 amps.

Can a VFD power more than one motor?

Yes, as long as the following application factors are considered:

  • Add up the full load ampere (FLA) values of all of the laod motors and use a VFD whose output ampere rating is equal or greater than the resultant sum.
  • Some VFD manufacturers recommend adding 20 percent to the sum of the motor FLAs as a safety factor.
  • If any motor will be connected to a VFD already operating at full speed, the locked rotor current of that motor needs to be included in the above motor current summation, not its FLA rating.
  • Separate overload, ground fault and short circuit protection need to be provided for each of the individual motors.
  • Flux vector motor control will not be possible. Motor performance will suffer.
  • Each motor will operate at the same speed. A tripped VFD will stop all of the motors.
  • A load reactor or a common mode core should be installed between the VFD and all motor loads.
  • Some VFD manufacturers recommend the use of more expensive XLPE motor cable.

Can a 230V nameplated motor be used on a 208 volt power network?

The answer is yes, as long as the 208 volt supply line does not drop below 207 volts. While NEMA MG1-2011 requires that a motor operate successfully over a +/- 10% range from nameplate voltage, it cautions that motor performance will suffer. A 230V motor operating at 207 volts (230 volts x 90%) will experience the following effects on its performance:   
  • Increased motor heating at rated HP load resulting in accelerated insulation deterioration and shortened motor life
  • A 10% drop in supply voltage reduces available motor torque by 19%. Slip will increase by about 21% 
  • While power factor will improve, efficiency will drop.
  • The motor’s service factor will drop to 1.0. 
Often a published power deration factor needs to be applied. Most 208 volt power systems are found in buildings with short distances between the incoming power service and the operating equipment. This results in stable power systems that do not often deviate far from the rated 208V value. For power requirements exceeding 10 HP, a 200V rated motor is the best choice.

What Can Be Done to Help Increase the Life of a Motor?

In order to help increase the life of a motor, inspect them at regular intervals.  Keep units clean and ventilation openings clear of dust, dirt or debris.  If re-lubrication is necessary, do not over grease as excessive lubrication may damage the motor.  Follow the re-greasing instructions in accordance with the supplier recommendations.

Why is Maximum Ambient Temperature Important?

It’s important that the ambient operating temperature of a motor does not exceed its maximum design ambient rated temperature, so that it does not overheat. The maximum ambient temperature in degrees Celsius determines the limit at which the motor can operate continuously at the rated horsepower and service factor without overheating. The NEMA standard is 40˚C (approximately 110˚F), but check the motor name plate for any exceptions.

What to Consider When Fitting a NEMA Premium® Electric Motor to Your Application

When fitting your NEMA Premium motor to your application, there are a number of items to consider.  For example, it should be noted that any increases in efficiency reduce the slip, which will increase the speed of the motor shaft.  If you want to maintain the same output with a more efficient motor, you may have to adjust your driven equipment.  The KVA code letter should be reviewed to determine motor starter size compatibility.  To obtain the best efficiencies, you should run the motor between 75 to 100 percent load.
 

NEMA & NEMA Premium marks are owned by National Electrical Manufacturers Association.

How Can Excessive Corrosion Be Avoided?

Some corrosion of metal components is unavoidable in applications where humidity can reach 100 percent. Normal levels of corrosion may look ugly, but it won't prevent a motor from operating.  However, excessive corrosion of the bearing/shaft journal or between the rotor and stator can lead to premature motor failure. To avoid excessive corrosion:
  • Reapply a rust preventive to motor surfaces during routine maintenance.
  • Turn the motor shaft monthly when the motor is not in operation.
  • Use space heaters or trickle voltage to eliminate condensation.

The information provided in Tech Tips is not meant to be all-encompassing, but rather to draw attention to and provide information about the particular subjects covered. All suggestions and recommendations contained in Tech Tips are based upon information that is believed to be accurate to the best of the experience and knowledge of PTDA’s contributing members, but are made without guarantee or representation as to results. PTDA and Tech Tip contributors expressly disclaim any warranties or guaranties, express or implied, as to the accuracy or completeness of any information published in Tech Tips, and disclaims and makes no warranty that the information in Tech Tips will fulfill any of your particular purposes or needs. PTDA and Tech Tip contributors disclaim liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on Tech Tips.