Reforming Capacitors on VFDs

Manufacturers highly recommend reforming capacitors on VFDs that have been stored, without power, for more than 1 year.

Material Required

  • Appropriate single phase or three phase power supply.
    220-230VAC Drives = power supply of 220VAC
    380-480VAC Drives = power supply of 220VAC
    500-600VAC Drives = power supply 300-330VAC
    660-690VAC Drives = power supply 300-330VAC
  • Miscellaneous material for connection of power supply.


All Variable Frequency Drives utilize Electrolytic Capacitors for storage of voltage in the inverter section of the drive. These electrolytic capacitors are made up of basically 3 parts. Two conductive plates (usually one being a metal substance similar to aluminum foil and the other being a porous material impregnated with an acidic substance similar to battery acid) that sandwich a layer of insulator material. The insulator material is generally made up of an oxide that is created when voltage is applied to the capacitor during manufacturing of the capacitor. Subsequent to this initial power up, every time the capacitor receives a charge it “rebuilds” this layer of oxide. As time elapses and there is no voltage applied to “reform” this layer the layer begins to degrade. This layer “thickness” is the determining factor for the voltage rating of the capacitor. If the layer degrades to a certain point one of two failures will occur. 1. The two conducting materials will begin to conduct current at a high rate, and this will cause a boiling of the liquid inside the capacitor. Once this boiling begins the pressure will rise internally and the capacitor will rupture. This failure usually results in a complete destruction of the drive as these capacitors are saturated internally with an acidic mixture. 2. The inrush current will arc across the oxide insulator and create a bridge between the plates. This bridge will short the plates and will cause a direct short in the power circuit that can result in other damage to the electronic components. At a minimum this second failure will require the damaged capacitor to be changed.

VFD Capacitor Reforming


If a VFD has not been powered up for 9 months to 1 year it is REQUIRED that the capacitors be “reformed” prior to the drive being put into service. For 220-230V and 380-480V models apply supply voltage of approximately 220Vac, three-phase or
single-phase input, 50 or 60 Hz, without connecting a motor to the output. This voltage should be applied for a period of 1 hour. After this energizing process, a wait period of 24 hours before installing or utilizing the drive for motor control is vital. Part of the oxide insulation is formed during this “resting” period. For 500-600V, 500-690V and 660-690V models use the same procedure applying a voltage between 300 and 330Vac to the inverter input.


What is a VFD?

You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and-like their DC counterparts-control speed, torque, and horsepower.

Figure 1. Fixed Speed Fan Application

Application As An Example
Let’s take a brief look at a drive application. In Fig. 1, you can see a simple application with a fixed speed fan using a motor starter. You could replace the 3-phase motor starter with Variable Frequency Drive (VFD) to operate the fan at variable speed. Since you can operate the fan at any speed below its maximum, you can vary airflow by controlling the motor speed instead of the air outlet damper.


Figure 2. Basic Induction Motor Construction

A drive can control two main elements of a 3-phase induction motor: speed and torque. To understand how a drive controls these two elements, we will take a short review of AC induction motors. Fig. 2 shows the construction of an induction motor. The two basic parts of the motor, the rotor and stator, work through magnetic interaction. A motor contains pole pairs. These are iron pieces in the stator, wound in a specific pattern to provide a north to south magnetic field.

Figure 3. Operating Principles of Induction Motor

With one pole pair isolated in a motor, the rotor (shaft) rotates at a specific speed: the base speed. The number of poles and the frequency applied determine this speed (Fig. 4). This formula includes an effect called “slip.” Slip is the difference between the rotor speed and the rotating magnetic field in the stator. When a magnetic field passes through the conductors of the rotor, the rotor takes on magnetic fields of its own. These rotor magnetic fields will try to catch up to the rotating fields of the stator. However, it never does – this difference is slip. Think of slip as the distance between the greyhounds and the hare they
are chasing around the track. As long as they do not catch up to the hare, they will continue to revolve around the track. Slip is what allows a motor to turn.

Motor Slip:
Shaft Speed = 120 X F
– Slip
Slip for NEMA B Motor = 3 to 5% of Base Speed which is 1800 RPM at Full Load
F = Frequency applied to the motor
P = Number of motor poles
Shaft Speed = 120 X 60 Hz
– Slip

Figure 4, Induction Motor Slip Calculation

We can conveniently adjust the speed of a motor by changing the frequency applied to the motor. You could adjust motor speed by adjusting the number of poles, but this is a physical change to the motor. It would require rewinding, and result in a step change to the speed. So, for convenience, cost-efficiency, and precision, we change the frequency. Fig. 5 shows the torque-developing characteristic of every motor: the Volts per Hertz ratio (V/Hz). We change this ratio to change motor torque. An induction motor connected to a 460V, 60 Hz source has a ratio of 7.67. As long as this ratio stays in proportion, the motor will develop rated torque. A drive provides many different frequency outputs. At any given frequency output of the drive, you get a new torque curve.

Figure 5. Volts/Hertz Ratio

 How Drive Changes Motor Speed
Just how does a drive provide the frequency and voltage output necessary to change the speed of a motor? That’s what we’ll look at next. Fig. 6 shows a basic PWM drive. All PWM drives contain these main parts, with subtle differences in hardware and software components.

Figure 6. Basic PWM Drive Components

Although some drives accept single-phase input power, we’ll focus on the 3-phase drive. But to simplify illustrations, the wave forms in the following drive figures show only one phase of input and output.

The input section of the drive is the converter. It contains six diodes, arranged in an electrical bridge. These diodes convert AC power to DC power. The next section-the DC bus section-sees a fixed DC voltage.

The DC Bus section filters and smooths out the waveform. The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you’d measure an average DC bus voltage of about 650V to 680V. You can calculate this as line voltage times 1.414. The inductor (L) and the capacitor (C) work together to filter out any AC component of the DC waveform. The smoother the DC waveform, the cleaner the output waveform from the drive.

The DC bus feeds the final section of the drive: the inverter. As the name implies, this section inverts the DC voltage back to AC. But, it does so in a variable voltage and frequency output. How does it do this? That depends on what kind of power devices your drive uses. If you have many SCR-based drives in your facility, see the Sidebar. Bipolar Transistor technology began superseding SCRs in drives in the mid-1970s. In the early 1990s, those gave way to using Insulated Gate Bipolar Transistor (IGBT) technology, which will form the basis for our discussion.

Switching Bus With IGBTs Today’s inverters use Insulated Gate Bipolar Transistors
(IGBTs) to switch the DC bus on and off at specific intervals. In doing so, the inverter actually creates a variable AC voltage and frequency output. As shown in Fig. 7, the output of the drive doesn’t provide an exact replica of the AC input sine waveform. Instead, it provides voltage pulses that are at a constant magnitude.

Figure 7. Drive Output Waveform

The drive’s control board signals the power device’s control circuits to turn “on” the waveform positive half or negative half of the power device. This alternating of positive and negative switches recreates the 3 phase output. The longer the power device remains on, the higher the output voltage. The less time the power device is on, the lower the output voltage (shown in Fig.8). Conversely, the longer the power device is off, the lower the output frequency.

Figure 8. Drive Output Waveform Components

The speed at which power devices switch on and off is the carrier frequency, also known as the switch frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3KHz to 4KHz). (With an older, SCR-based drive, switch frequencies are 250 to 500 times per second). As you can imagine, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies decrease the efficiency of the drive because of increased heat in the power devices.

Shrinking cost and size

Drives vary in the complexity of their designs, but the designs continue to improve. Drives come in smaller packages with each generation. The trend is similar to that of the personal computer. More features, better performance, and lower cost with successive generations. Unlike computers, however, drives have dramatically improved in their reliability and ease of use. And also unlike computers, the typical drive of today doesn’t spew gratuitous harmonics into your distribution system-nor does it affect your power factor. Drives are increasingly becoming “plug and play.” As electronic power components improve in reliability and decrease in size, the cost and size of VFDs will continue to decrease. While all that is going on, their performance and ease of use will only get better.


Sidebar: What if you have SCRs?

With the large installed base of SCRs, you might want to know how these operate. An SCR (originally referred to as a thyristor) contains a control element called a gate. The gate acts as the “turn-on” switch that allows the device to fully conduct voltage. The device conducts voltage until the polarity of the device reverses-and then it automatically ‘turns off.” Special circuitry, usually requiring another circuit board and associated wiring, controls this switching. The SCR’s output depends on how soon in the control cycle that gate turns on. The IGBT output also depends the length of time the gate is on. However, it can turn off anytime in the control cycle, providing a more precise output waveform. IGBTs also require a control circuit connected to the gate, but this circuitry is less complex and doesn’t require a reversal of polarity. Thus, you would approach troubleshooting differently if you have an SCR-based drive.

Consider the following when designing or servicing your gear units

  • Mounting Position: The orientation of the gear unit determines the amount of oil. M1 contains the least; M2 and M4 contain the most. Whenever possible, design your system for M1.
  • Input Speed: The faster the oil churns, the higher the churning losses. Therefore, a gear unit with 4-pole motor (~1475 rpm) will run cooler than with a 2-pole motor (~2800 rpm).
  • Synthetic Oil: Synthetic oil is known to reduce friction by 25%, which can be significantly on worm gear units or large gear units. Synthetic oil allows the gear unit to run cooler, doubling the oil service life, which reduces your maintenance interval and costs.
  • Viscosity: The “thicker” the oil, the more resistance it has to flow and the more energy it requires moving. Always use the correct viscosity, considering the type of application and ambient temperature.


The Truth about Premium-Efficient Motors

1. The motor is only part of the efficiency equation.

As you have seen, a motor is at best one-sixth of the total energy loss potential for an electro-mechanical drivetrain. And, what’s more, it typically isn’t even the most inefficient part. Mechanical devices, such as external transmission elements, have far more inefficiencies than electrical devices do. So, look there first to find your largest energy savings.

2. By revamping your entire drivetrain, you may actually be able to use a
smaller motor and save even more.

Right now, you are probably using a motor of a particular power rating to produce a certain output from your drivetrain. You may be pleasantly surprised to find that, by upgrading your gearbox, drive, and external transmission components, you will have gained enough efficiency that your motor power is now higher than you actually need. Therefore, you may be able to save additional costs by purchasing a lower horsepower motor. For example:

Before: 45 kW load ÷ 53.5% efficiency = 84.1 kW (use 90kW motor)

After: 45 kW load ÷ 72.5% efficiency = 62.1 kW (use 75kW motor)

SPIROPLAN Right-Angle Gear Units

3. Motors are most efficient when integrated with other drivetrain components
from the same manufacturer.

Systems where the VFD, motor, and gearbox are all engineered by the same company are by nature designed to work well together, eliminating unnecessary inefficiencies and allowing additional energy savings. For example, integrating an SEW-EURODRIVE DRP motor, helical-bevel gear unit, and VFD will provide dramatically higher energy savings than simply replacing the motor.

4. The motor must be well-suited to your application.

Just placing a premium-efficient motor on the line may not automatically solve all your energy problems, even if all the other components are as efficient as possible. Ensure that the specifications of the motor fit your application, especially if you have a high-cycling application that is greater than 10 to 30 cycles/hour. If so, use a premium efficient motor designed for such an application with an appropriately sized integral brake.

Also, where possible, use the smallest motor for the application so that it is loaded near 80% and operates as close to it nameplate efficiency as possible.

5. Mechanical efficiencies matter, too.

Worm gear units, which are common in the industry, have an efficiency range of 50 to 88 percent, depending on the number of starts (teeth) on the worm gear or gear ratio, as shown below.

number of starts —– typical efficiency range
1                                        50-69%
2                                        70-79%
5                                        80-88%

Their poor efficiency is due to sliding gear contact. Since sliding produces friction, much of the energy is wasted through heat. Conversely, helical bevel gear units use rolling friction, so they lose only 1.5% of efficiency for each stage. Thus, a three-stage helical bevel gearbox is 95.5% efficient.

Although helical-bevel gear units are higher in initial cost, they will save money in energy over the lifetime of the system. If you are an end-user, consider specifying helical-bevel gear units the next time you purchase equipment for your plant. It is in your best interest.

6. Gearmotors eliminate even more efficiency losses.

Gearmotors inherently yield tremendous increases in efficiency compared to the average flexible transmission system. Since a gearmotor contains a motor that is rigidly coupled and precisely aligned with the gear unit, the connection is nearly 100% efficient. By eliminating the friction and slippage associated with v-belts, pulleys or chains, you can quickly yield a potential 12-15% increase in efficiency. You will save even more on the replacement and maintenance of belts. And, don’t forget about safety.

7. Oil may be costing you.

Oil plays a role in energy savings because it creates heat as it churns inside a gear unit. And, the amount of heat increases as the oil volume increases. Not only does heat increase your energy bill, it also damages gears and seals. Excessive heat is especially problematic for larger gear units – typically with an output shaft diameter greater than 60mm.


Common Premium-Efficient Motor Misconceptions

1. Efficiency automatically equals savings.

Not necessarily; it depends on the application. Current Australian MEPS requirements mandate that new motor efficiencies be as a minimum IE2 High Efficiency motor or IE3 Premium Efficiency with efficiencies based on AS1359.5:2004, which “roughly” aligns with the international IEC60034:30 standard on induction motor energy efficiency. The efficiency levels are typically based upon 80-100% loading with the motor connected to constant power source (i.e. across the line). So purchasing a High or Premium Efficiency motor from one manufacturer will very likely give you a motor with basically the same efficiency as a High or Premium Efficiency motor purchased from another manufacturer.

If you only install a premium-efficient motor, you are not automatically saving all the money you could be saving. There are multiple reasons why this might be possible, as discussed below:

  • Your new motor may only be a few percent more efficient than your previous motor. Therefore, in cycling or intermittent duty applications, the savings you recognize are so small that they are outweighed by the higher cost of the new motor.
  • Your new motor may not be well-suited to saving energy in your type of application, e.g. high-cycling applications.
  • Your new motor may be over-sized for the application, yielding much less efficiency than what the nameplate says.
  • Other parts of your drive-train may be much less efficient, causing higher-than necessary energy consumption from your efficient motor.

While a premium-efficient motor is important, it’s critical to evaluate your entire drive-train for efficiency and to realize that the motor is just a single part of the overall equation.

2. Replacing my motor will give me the best bang for my buck.

It depends. A motor is only one component in the drive-train (and, truth be told, motors for some time have been comparatively efficient). Each component in a system will inherently have some inefficiency and these energy losses multiply together to provide an overall system efficiency. Just one component with poor efficiency will quickly drag down the rest of the system. Consider the following theoretical example where every component has an almost-impossible efficiency of 99%:


You’ll see that even in this example with 6 components of ideal efficiency, you are still
losing almost 6% of the energy.

Now, to consider two more realistic examples:



You can see in both examples that you are losing over 42% of the energy going into the system. You can also see that replacing your motor with a premium-efficient model will save you just over 4% efficiency, even though the new motor is 7% more efficient that the old motor. That is because the other, less efficient components in your drivetrain are still wasting energy. Therefore, the investment you have made in a premium-efficient motor will take longer to recoup than you had planned.

3. Replacing my motor will automatically make my line more efficient.

Well, yes- but by less than you can expect. However, replacing some of the other components along with your motor can provide some very substantial efficiency gains. Consider, for instance, that you replace the gear unit as well as the motor. Worm gear units, which are installed in many manufacturing environments, are inherently inefficient, as the gears are essentially sliding against one another causing heat (energy loss). Sure, there are instances on which worm drives are necessary for the application (e.g. withstanding heavy shock loads, or providing back-driving resistance). But, in many applications a helical-bevel gear units, which operates with rolling contact, will be much more efficient.

Take the previous “real world” example and replace both the motor AND the gear unit.


Now you are quickly recognizing substantial, double-digit efficiency grains- nearly 20%- and your line begins to become much more efficient.

To gain even more efficiency, consider changing or eliminating your transmission elements. Replacing a v-belt with a direct drive or use a shaft-mounted gear unit. Shaft-mounted units, such as the TorqLOC® from SEW-EURODRIVE, offers a keyless, taper hollow shaft with a shrink disc that has a liberal tolerance so it installs easily. It can even retrofit onto an existing keyed shaft.

4. A premium-efficient motor is an appropriate energy-saving choice for all applications.

Again, it depends. Most premium-efficient motors used in continuously-running applications will begin to show at least modest energy savings (depending, as previously shown, on the other elements in the drive-train).

But, motors used in high-cycling applications may never recognize the entire efficiency gain of a premium-efficient motor, due to the start-and-stop nature of the application fighting against the higher rotor inertia of many premium-efficient motors. Hence, the extra investment in a high-efficiency motor may not ever be completely recouped.

However, some Premium Efficiency motors, such as the DRP motor from SEW-EURODRIVE are engineered to make them more efficient in high-cycling applications. These motors are designed with low losses, and less heat accumulation in the windings, which increases efficiency and provides a very high number of starts and stops per hour.

Be sure that you consider all the options available to you, and be careful to choose the premium-efficient motor that is best suited to your need.

5. Adding a variable frequency drive (VFD) will automatically make my line more efficient.

Maybe. As the efficiency equation shows, a VFD is a load. It produces heat (losses) from electricity conversion, switching frequency, and harmonics. So by itself, it will decrease your system efficiency. What’s more, many VFD’s have an adjustable carrier frequency that reduces audible noise during operation. Unfortunately, when the carrier frequency increases, so does the heat. In fact, the heat produced as a high carrier frequency can be so significant that a room full of VFD’s may require a substantial increase in air conditioning.

Thus, the key to energy savings is to use a VFD to reduce other losses in the system (i.e. smart control), as in the following applications.

  • Regenerative Energy: When a motor is trying to stop a high inertia load or lower a load, it acts as a generator. All of the kinetic or potential energy stored in the machine has to be removed. All of the kinetic or potential energy stored in the machine has to be removed. Typically, it is wasted as heat through a braking resistor. But, a regenerative VFD can put the energy back onto the grid. Some even allow the energy to be directly given to another VFD as it accelerates, such as in a storage retrieval system.
  • HVAC: Typical systems used in HVAC contain mechanical dampers with motors that run continuously. Using a VFD to turn off the motor or to optimize the motor speed is much more efficient, especially since the load decreases more than four times at half the speed.
  • Soft Start: Using a VFD to control the acceleration on a cycling application lowers the motor starting current. So, the motor runs cooler since less energy is converted to heat.
  • Motor Efficiency Correction: Motor nameplate is usually rated at 80% loading. Therefore, when a large motor is applied to a small load (e.g. 1 kW used instead of 0.25 kW), its actual efficiency decreases considerably. Using a VFD with vector control (or VFC technology) optimizes the motor efficiency, regardless of the loading conditions.

The bottom line? If properly used, VFDs can have some big efficiency benefits when added as part of a complete drive-train efficiency solution.

Energy Efficiency Legislation Regulations

DOE & EISA Ruling Explained

In March of 2010, the DOE (Department of Energy) instituted a ruling that incorporates small motors into their Energy Efficiency Programs. The DOE, as required by law, is mandated to review the readiness of the Energy regulations based on current materials and manufacturing technologies. This SMR (Small Motor Efficiency Rule) includes motors that are described in the information below. This rule goes into effect on March 9th, 2015. Motors requiring outside agency approval, such as UL or CSA, have a 2-year extension
and need to comply by March 9, 2017.

W22 Severe Duty Super Premium Electric Motor

In addition, in May of 2014, the DOE released communications with required expansions of 3 phase, single-speed, low voltage, integral HP motors, 1-500 HP to meet NEMA Premium efficiency levels, excluding some exceptions. This ruling expands current motor
regulation for motors that were not previously covered in the EISA 2010 regulations. These families of motors are also listed in the information below. This rule goes
into effect on June 1, 2016.

This new rule is predicted to save approximately 7 quads of energy and result in approximately $41.4 billion in energy bill savings for products shipped from 2016 – 2045. This rule is also predicted to reduce 395 million metric tons of carbon dioxide emissions.


  • 2-digit frame numbers – 42, 48 and 56 frame motors and their IEC equivalent frame size motors
  • The Speed or Poles of the motors would include 2, 4 and 6 pole designs from 1/4 to 3 HP
  • Open construction motors that are either 3 phase (Polyphase), Cap Start – Induction Run or Cap Start/Cap Run designs
  • Continuous duty rated and also meet NEMA Service Factor


  • Definite or Special Purpose OPEN construction design motors
  • Motor speeds that are outside of the 2, 4 and 6-pole speeds
  • Enclosed motors
  • Motor types that are not classified as being 3 phase, Cap Start – Induction Run or Cap Start/Cap Run
  • Intermittent duty motors as well as designs outside the HP and frame size listing as described above
  • Motors that are already covered by other efficiency legislation are also not covered by this rule

EISA Expansion Rule

The EISA (Energy Independence and Security Act) expansion and compliance rule, or sometimes referred to as the EISA Expansion Rule, expands the following list of motor designs to meet NEMA Premium efficiency: 1 – 500 HP, NEMA Design A, B & C (1 – 200
HP only today for Design A & B motors); IEC Design N, H, 8 Pole designs, enclosed 56 Frame IHP (1 HP and larger) that are either of General Purpose, Special or Definite Purpose design electric motors.

The Efficiency levels must meet NEMA Premium levels as listed in Table 12-12 (IE3 – 60Hz).


  • NEMA Design A & B motors from 201 to 500 HP
  • NEMA Design C motors from 1 to 500 HP
  • All voltages ≤ 600 volts
  • 8-pole motors
  • Electric motors with non-standard end-shields, flanges or shafts
  • Motors with moisture resistant windings, like encapsulated or sealed windings
  • Motors that use any non-standard mounting like a base or cradle
  • Motors that do not have a base or cradle footless designs
  • Partial designed electric motors but not rotor and stator sets
  • Vertical hollow shaft motors
  • TENV designed motors
  • JM and JP Pump motors
  • Electric motors having thrust or roller bearings
  • Integral brake motors
  • Motors with separately cooled blowers on them
  • Enclosed 56 frame 1 HP and larger – 56 Open motors are covered by the SMR
  • Gearmotors if the motor can be removed from the reducer and work as independent motor


  • Fire pump motors
  • Liquid cooled motors
  • Submersible motors
  • Air over design motors
  • Component sets (stator, rotor sets)
  • Small electric motors below 56 frame – see SMR rules
  • Advanced Motor Technology motors which include PMAC, ECM, Brushless DC, etc.
  • Inverter duty only motors

What’s the Difference Between Spur, Helical, Bevel, and Worm Gears?

by Carlos Gonzalez

Gears are a crucial part of many motors and machines. Gears help increase torque output by providing gear reduction and they adjust the direction of rotation like the shaft to the rear wheels of automotive vehicles. Here are some basic types of gears and how they are different from each other.

Spur Gears

Spur Gear

Spur gears are mounted in series on parallel shafts to achieve large gear reductions.

The most common gears are spur gears and are used in series for large gear reductions. The teeth on spur gears are straight and are mounted in parallel on different shafts. Spur gears are used in washing machines, screwdrivers, windup alarm clocks, and other devices. These are particularly loud, due to the gear tooth engaging and colliding. Each impact makes loud noises and causes vibration, which is why spur gears are not used in machinery like cars. A normal gear ratio range is 1:1 to 6:1.

Helical Gears

Helical Gear

Helical gears have a smoother operation due to the angle twist creating instant contact with the gear teeth.

Helical gears operate more smoothly and quietly compared to spur gears due to the way the teeth interact. The teeth on a helical gear cut at an angle to the face of the gear. When two of the teeth start to engage, the contact is gradual–starting at one end of the tooth and maintaining contact as the gear rotates into full engagement. The typical range of the helix angle is about 15 to 30 deg. The thrust load varies directly with the magnitude of tangent of helix angle. Helical is the most commonly used gear in transmissions. They also generate large amounts of thrust and use bearings to help support the thrust load. Helical gears can be used to adjust the rotation angle by 90 deg. when mounted on perpendicular shafts. Its normal gear ratio range is 3:2 to 10:1.

Bevel Gears

Bevel Gears

The image above shows two different configurations for bevel gears: straight and spiral teeth.

Bevel gears are used to change the direction of a shaft’s rotation. Bevel gears have teeth that are available in straight, spiral, or hypoid shape. Straight teeth have similar characteristics to spur gears and also have a large impact when engaged. Like spur gears, the normal gear ratio range for straight bevel gears is 3:2 to 5:1.

Spiral teeth operate the same as helical gears. They produce less vibration and noise when compared to straight teeth. The right hand of the spiral bevel is the outer half of the tooth, inclined to travel in the clockwise direction from the axial plane. The left hand of the spiral bevel travels in the counterclockwise direction. The normal gear ratio range is 3:2 to 4:1.

Hypoid gears are a type of spiral gear in which the shape is a revolved hyperboloid instead of conical shape. The hypoid gear places the pinion off-axis to the ring gear or crown wheel. This allows the pinion to be larger in diameter and provide more contact area.

Pinion & Gear

The pinion and gear are often always opposite hand and the spiral angle of the pinion is usually larger then the angle of the gear. Hypoid gears are used in power transmissions due to their large gear ratios. The normal gear ratio range is 10:1 to 200:1.

Worm gears are used in large gear reductions. Gear ratio ranges of 5:1 to 300:1 are typical. The setup is designed so that the worm can turn the gear, but the gear cannot turn the worm. The angle of the worm is shallow and as a result the gear is held in place due to the friction between the two. The gear is found in applications such as conveyor systems in which the locking feature can act as a brake or an emergency stop.