Tech Tip: Motors

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Why can an SPM or IPM type motor not be started directly from utility power? 

To generate motor torque, the position of the rotating magnetic field generated by electrical power fed into the PM motor must be in synchronization with the actual position of the magnets on the motor’s rotor.

VFDs designed for PM motor operation carry out a rotor position detection routine before applying power to start the motor. This is not possible with standard utility power.   If an attempt was made to start an SPM or IPM type motor with a full voltage motor starter, the motor would generate no starting torque and would be damaged.

Most current generation VFDS include a control algorithm for Permanent Magnet motor control and at least one brand can also control Brushless DC motors and Synchronous Reluctance motors.

Can a Permanent Magnet AC Motor be started with a full voltage motor starter?

In most cases, no. The most common and cost-effective Permanent Magnet motor designs require the use of a VFD equipped with a motor control algorithm designed for PM motors. Line-start Permanent Magnet motors can be used with a full voltage motor starter but these type of motors are considerably more expensive than the more common SPM (surface permanent magnet) or IPM (interior permanent magnet) motors.

What is the KVA Code on a motor template?

The KVA code defines the low and high voltage inrush values on dual voltage motors. AC Motors, at startup, create an inrush current much higher than the motor rated full-load current. This large current draw can be so great that it could negatively affect other equipment inside the facility. 

The inrush current has been standardized and defined by a series of code letters which classify motors based on the amount of current represented as kilovolt amps (KVA). 

The KVA code is supplied on a motor nameplate as a standardized qualifier when sizing a motor with a starter. 

NEMA Standard MG 1-10.37.2

Do three phase motors have a default shaft rotation direction?

Three phase motors that have been designed per the IEC standard 60034 have a default motor shaft rotation. When the voltage supply cables are connected to the motor terminal board, in phase order, then the motor shaft rotation will be indicated as shown in the the table below. 

What is the purpose of a NEMA 4 and a NEMA 4X motor enclosure?

NEMA 4  provides a level of protection against dust and rain and washdown low pressure hose-directed water. NEMA 4X is a level of protection that meets the same standards as a NEMA 4 motor but the motors are also subjected to corrosive environmental testing. It meets NEMA-approved standards after 200 hours of testing. Both are approved for indoor as well as outdoor usage

Define the four critical torque levels, on the speed/torque curve, of an AC Induction motor.

1. Rated or Full Load Torque – This is the torque required to produce the motors rated horsepower at full load speed.
2. Locked Rotor Torque/Starting torque – This is the torque the motor has available at zero speed for accelerating the load. 
3. Pull up torque – This is the minimum torque available during acceleration from rest to the speed at which the
    breakdown torque occurs.
4. Breakdown or Stall torque – This is the torque produced by the motor just before it stops operating. This torque is three
    to four times the full rated torque. It occurs at a speed range of 50% - 80% of the motors synchronous speed.

What are three types of motor load classifications?

1. Constant torque - This load produces a demand for horsepower that is in direct proportion to the speed of the load.
    Conveyors, hoists and positive displacement pumps are all application examples that demand constant torque.
2. Variable torque - This load requires an increase in horsepower in greater proportion that the increase in speed.
    Centrifugal fans, centrifugal pumps, blowers and compressors are applications that demand variable torque.
3. Constant horsepower - These loads will have torque requirements that decrease as their speed increases. Winding
    application and metal working applications are examples of this motor load. 

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.