AC and DC Drives: Spare Parts

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Stocking spare parts at your facility minimizes downtime and keeps production lines running. Look for a supplier can locate or cross-reference old and discontinued parts for your AC or DC drives. You may save money by looking for reconditioned components, such as circuit boards, complete drives, PLCs, and other equipment.

Spare parts may include:

  • drive spare partsAC/DC Motor Drives
  • Thyristors (SCRs)
  • IGBTs
  • Power Transistors/Darlingtons
  • Bus Capacitors
  • Braking Resistors
  • Circuit Boards
  • Replacement Controls
  • Encoders
  • Semiconductor Fuses
  • Keypads
  • PLCs
  • Varoius Motion Control parts

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Electric Motors + Motor-Control Systems
(Nested-content Series)

Electric Motors

Motor Installation

Knowledge of proper installation techniques is vital to the effective operation of a motor. The following are some of the important motor installation procedures that need to be considered.

Foundation

A rigid foundation is essential for minimum vibration and proper alignment between motor and load. Concrete makes the best foundation, particularly for large motors and driven loads.

Mounting

Unless specified otherwise, motors can be mounted in any position or any angle. Mount motors securely to the mounting base of equipment or to a rigid, flat surface, preferably metallic. An adjustable motor base makes the installation, tensioning, and replacements of belts easier.

Common types of motor mountings are shown and include:

Rigid base, which is bolted, welded, or cast on the mainframe and allows the motor to be rigidly mounted on equipment.

Resilient base, which has isolation or resilient rings between motor mounting hubs and base to absorb vibration and noise. A conductor is imbedded in the ring to complete the circuit for grounding purposes.

NEMA C face mount, which has a machined face with a pilot on the shaft end that allows direct mounting with a pump or other direct-coupled equipment.

Bolts pass through mounted part to threaded hole in the motor face.

Ill. 65 Common types of motor mountings. Leeson, www.leeson.com. Rigid base; Resilient base; NEMA C face mount

Ill. 66 Laser alignment kit. Shaft alignment Sheave alignment

Ill. 67 Motor drive pulley 1,725 rpm 2” diameter Load pulley 1,150 rpm diameter

EXAMPLE:

Problem: You have a motor to drive a load. The motor operates at 1,725 rpm and has a pulley with a 2-inch diameter; the load must operate at 1,150 rpm. What size of pulley is needed for the load?

Solution:

Motor rpm/Equipment rpm = Equipment pulley diameter/ Motor pulley diameter

1,725/ 1,150 = Equipment pulley diameter/ 2

1,725 × 2/ 1,150 = 3-inch pulley

Motor and Load Alignment

Misalignment between the motor shaft and the load shaft causes unnecessary vibration and failure due to mechanical problems. Premature bearing failure in the motor and/or the load can result from misalignment. Different types of alignment devices, such as the laser alignment kit, are used for motor and load alignment. Positioning a motor or placing a shim (thin piece of metal) under the feet of the motor is often part of the alignment process.

Direct-drive motors, as the name implies, supply torque and speed to the load directly. A motor coupling is used to mechanically connect axially located motor shafts with equipment shafts. Direct coupling of the motor shaft to the driven load results in a 1:1 speed ratio. For direct-coupled motors, the motor shaft must be centered with the load shaft to optimize operating efficiency. A flexible coupling permits the motor to operate the driven load while allowing for slight misalignments.

Coupling by means of gears or pulleys/belts may be used in cases where the application requires other than standard available speed. Variable speeds are possible by making available several gear ratios or pulleys with variable diameters. Matching a motor to a load involves transformation of power between shafts, often from a high-speed/low torque drive shaft to a low-speed/high-torque load shaft.

Multiple belts are often used together in order to increase carrying power. If the pulley wheels are different sizes, the smaller one will spin faster than the larger one. Changing pulley ratios does not change horsepower, only torque and speed. The following formula can be used to calculate speed and pulley sizes for belt-drive systems.

Motor rpm / Equipment rpm = Equipment pulley diameter / Motor pulley diameter

Y-belts are common belts used for power transmission.

They have a flat bottom and tapered sides and transmit motion between two sheaves. When servicing a belt-drive system, the belts must be checked for proper tension and alignment. The belt should be tight enough not to slip, but not so tight as to over load motor bearings. Belt deflection should be 1/64 inch per inch of span. A belt tension gauge is used to ensure proper specified belt tension. Misalignment is one of the most common causes of premature belt failure. Angular misalignment is misalignment caused by two shafts that are not parallel. Parallel misalignment is caused by two shafts that are parallel but not on the same axis.

Ill. 68 Servicing a belt-drive system. Span length Deflection; Force; Belt deflection should be 1/64 inch per inch of span; Angular misalignment; Parallel misalignment; Parallel misalignment; Correct alignment

Ill. 69 Motor bearings. Canadian Babbitt Bearings, www.cbb.ca. The Timken Company. (a) Split-sleeve bearing (b) Ball bearing (c) Roller bearing (d) Thrust bearing

Motor Bearings

The rotating shaft of a motor is suspended in the end bells by bearings that provide a relatively rigid support for the output shaft. Motors come equipped with different types of bearings properly lubricated to prevent metal-to-metal contact of the motor shaft (Ill. 69). The lubricant used is usually either grease or oil. Most motors built today have sealed-bearing lubrication. This should be checked periodically to ensure the sealing has not been compromised and the bearing lubricant lost. For installations using older motors that require regular lubrication, this should be done on a scheduled basis, in conformance with the manufacturer's recommendations.

Sleeve bearings used on smaller light-duty motors consist of a bronze or brass cylinder, a wick, and a reservoir. The shaft of the motor rotates in the bronze or brass sleeve and is lubricated with oil from the reservoir by the wick, which transfers oil from the reservoir to the sleeve. Large motors (200 hp and over) are often equipped with large split-sleeve bearings that mount on the top and bottom half of the motor end shield. These bearings are usually poured with a material called babbitt and then bored to size. Sleeve bearings are furnished with oil reservoirs, sight gauges, level gauges, and drain provisions.

Ball bearings are the most common type of bearing.

They carry heavier loads and can withstand severe applications. In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to the inner race. Ball bearings come in three different styles: permanently lubricated, hand packed, and bearings that require lubrication through fitting. Not lubricating the bearings can damage a motor for obvious reasons; too much grease can overpack bearings and cause them to run hot, shortening their life. Excessive lubricant can find its way inside the motor where it collects dirt and causes insulation deterioration and overheating.

Roller bearings are used in large motors for belted loads. In these bearings, the roller is a cylinder, so this spreads the load out over a larger area, allowing the bearing to handle much greater loads than a ball bearing.

Thrust bearings consist of two thrust races and a set of rollers that are designed to handle higher than normal axial forces exerted on the shaft of the motors, as is the case with some fan and pump blade applications. Motors for vertically mounted motors typically use thrust bearings.

Electrical Connections

NEMA standards and Article 430 of the NEC, as well as state and local codes, provide specific electrical and mechanical installation requirements and recommendations covering motors and motor controls. The motor must be connected to a power source corresponding to the volt age and frequency rating shown on the motor nameplate.

After you've verified that the supply voltage requirements are correct, you then can make the motor terminal connections. Stator winding connections should be made as shown on the nameplate connection diagram or in accordance with the wiring diagram attached to the inside of the conduit box cover.

Grounding

Both your motor and the equipment or apparatus to which it’s connected must be grounded as a precaution against the hazards of electrical shock and electrostatic discharge.

This is done by using an equipment-grounding conductor that establishes a path or circuit for ground-fault current to facilitate overcurrent device operation. The equipment grounding conductor may be a conductor (insulated or bare) run with the circuit conductors, or where metal raceways are used, the raceway may be the equipment- grounding conductor. The color green is reserved for an insulated grounding conductor. In addition to helping prevent electrical shock, grounding of an electronic motor drive also helps to reduce unwanted electrical noise that can interfere with the proper operation of the electronic motor drive circuits.

Electrical currents are induced onto the motor's rotor shaft and seek the least resistant path to ground-usually the motor bearings. Shaft voltages accumulate on the rotor until they exceed the dielectric capacity of the motor bearing lubricant; then the voltage discharges in a short pulse to ground through the bearing. The random and frequent discharging has an electric discharge machining (EDM) effect, causing pitting of the bearing's rolling elements and raceways that eventually can lead to bearing failure. This occurs more often in AC motors controlled by variable-frequency drives. For this reason proper grounding is especially critical on the motor frame, between the motor and drive, and from the drive to earth. Grounding the motor shaft by installing a grounding device, prevents bearing damage by dissipating shaft currents to ground.

===

EXAMPLE

Problem: What size THW CU conductors are required for a single 15-hp, three-phase, 230-V squirrel cage motor?

Solution: Step 1 Determine the full-load current (FLC) rating of the motor to be used in determining the conductor size.

NEC 430.6 requires that tables 430.247 through 430.250 be used to determine the FLC and not the nameplate rating. Table 430.250 deals with three-phase alternating cur rent motors, and using this table, we find that for a 10-hp, 208-V, three-phase motor the FLC is 42 amperes.

Step 2 NEC 430.22 requires branch circuit conductors supplying a single motor to have an ampacity not less than 125 % of the motor FLC. Therefore, Rated ampacity = 42A × 125% = 52.5A

Step 3 According to Table 310.16, the conductor size required would be:

6 AWG THW CU

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Problem: What is the % voltage unbalance for a three-phase supply voltage of 480 V, 435 V, and 455 V?

Solution: Average voltage deviation = 480 + 435 + 445 /3 = 1,360/3 = 453 V

===

Conductor Size

The size of the motor branch circuit conductors is deter mined in accordance with Article 430 of the NEC, based on the motor full-load current, and increased where required to limit voltage drop. Undersized wire between the motor and power source will limit stating abilities and cause overheating of the motor.

Ill. 70 Motor shaft grounding ring. Electro Static Technology-an ITW Co., www.est-aegis.com.

Voltage Levels and Balance

Motor voltages should be kept as close to the nameplate value as possible, with a maximum deviation of 5 %.

Although motors are designed to operate within 10 per cent of nameplate voltage, large voltage variations can have negative effects on torque, slip, current, efficiency, power factor, temperature, and service life.

Unbalanced motor voltages applied to a polyphase induction motor may cause unbalanced currents, resulting in overheating of the motor's stator windings and rotor bars, shorter insulation life, and wasted energy in the form of heat. When three-phase line voltages are not equal in magnitude, they are said to be unbalanced. A voltage unbalance can magnify the % current unbalance in the stator windings of a motor by as much as 6 to 10 times the percent voltage unbalance. Acceptable volt age unbalance is typically not more than 1 %. When there is a 2 % or greater voltage unbalance, steps must be taken to determine and rectify the source of the unbalance. In cases where the voltage unbalance exceeds 5 %, it’s not advisable to operate the motor at all.

The voltage unbalance is calculated as follows:

Percent voltage unbalance = Maximum voltage deviation from the voltage average / Average voltage × 100

Maximum deviation from the average voltage = 480 - 453 = 27V.

Percent voltage unbalance = Maximum voltage deviation from the voltage average/Average voltage × 100 = 27 /453 × 100 = 5.96%

Ill. 72 Built-in thermal motor protection. Wired in series Integrated into the control circuit

Built-in Thermal Protection

Overload relays mounted on the motor starter enclosure protect the motor by monitoring the motor current and resultant heat it creates inside the motor. They do not, however, monitor the actual amount of heat generated within the winding. Motors subject to such conditions as excessive starting cycles, high ambient motor temperatures, or inadequate ventilation conditions may experience rapid heat buildup that is not sensed by the overload relay. To minimize such risks, the use of motors with thermal protectors inside that sense motor winding temperature is advisable for most applications. These devices may be integrated into the control circuit to offer additional overload protection to the motor or connected in series with the motor windings on smaller single-phase motors. Basic types include:

Automatic reset: After the motor cools, this line interrupting protector automatically restores power. It should not be used where unexpected restarting would be hazardous.

Manual reset: This line-interrupting protector has an external button that must be pushed to restore power to the motor. Use where unexpected restarting would be hazardous, as on saws, conveyors, compressors, and other machinery.

Resistance temperature detectors: Precision calibrated resistors are mounted in the motor and are used in conjunction with an instrument to detect high temperatures.

QUIZ:

1. List three popular types of motor mountings.

2. A motor with a 3-inch drive pulley operating at a speed of 3,600 rpm is belt-coupled to an equipment pulley 8 inches in diameter. Calculate the speed of the driven load.

3. List four basic types of bearing and give a typical application for each.

4. How can a motor be damaged by over-lubricating a ball bearing?

5. Which NEC article deals specifically with requirements for electric motors?

6. Why is it desirable to ground the motor shaft in addition to the frame?

7. In what ways can undersized wiring between the motor and power source affect the operation of a motor?

8. What negative effects can unbalanced three phase line voltages have on the operation of a motor?

9. In what type of application is it advisable to use manual-reset built-in thermal protectors?

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