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ABB - General about motors

ABB - General about motors

Modern electrical motors are available in many different forms, such as single phase motors, three-phase motors, brake motors, synchronous motors, asynchronous motors, special customised motors, two speed motors, three speed motors, and so on, all with their own performance and characteristics.
For each type of motor there are many different mounting arrangements, for example foot mounting, flange mounting or combined foot and flange mounting. The cooling method can also differ very much, from the simplest motor with free self-circulation of air to a more complex motor with totally enclosed air-water cooling with an interchangeable cassette type of cooler.

To ensure a long lifetime for the motor it is important to keep it with the correct degree of protection when under heavy-duty conditions in a servere environment. The two letters IP (International Protection) state the degree of protection followed by two digits, the first of which indicates the degree of protection against contact and penetration of solid objects, whereas the second states the motor’s degree of protection against water.
The end of the motor is defined in the IEC-standard as follows:

  • The D-end is normally the drive end of the motor.
  • The N-end is normally the non-drive end of the motor.

Note that in this handbook we will focus on asynchronous motors only.

Squirrel cage motors

In this chapter the focus has been placed on the squirrel cage motor, the most common type of motor on the market. It is relatively cheap and the maintenance cost is normally low.

There are many different manufacturers represented on the market, selling at various prices. Not all motors have the same performance and quality as for example motors from ABB. High efficiency enables significant savings in energy costs during the motor’s normal endurance. The low level of noise is something else that is of interest today, as is the ability to withstand severe environments.

Current diagram for typical squirell cageThere are also other parameters that differ. The design of the rotor affects the starting current and torque and the variation can be really large between different manufacturers for the same power rating. When using a softstarter it is good if the motor has a high starting torque at Direct-on-line (D.O.L) start. When these motors are used together with a softstarter it is possible to reduce the starting current further when compared to motors with low starting torque. The number of poles also affects the technical data. A motor with two poles often has a lower starting torque than motors with four or more poles.


Three-phase single speed motors can normally be connected for two different voltage levels. The three stator windings are connected in star (Y) or delta (D). The windings can also be connected in series or parallel, Y or YY for instance. If the rating plate on a squirrel cage motor indicates voltages for both the star and delta connection, it is possible to use the motor for both 230 V, and 400 V as an example.

The winding is delta connected at 230 V and if the main voltage is 400 V, the Y-connection is used. When changing the main voltage it is important to remember that for the same power rating the rated motor current will change depending on the voltage level. The method for connecting the motor to the terminal blocks for star or delta connection is shown in the picture below.

Wiring diagram for Y- and Delta connection

Power factor

A motor always consumes active power, which it converts into mechanical action. Reactive power is also required for the magnetisation of the motor but it doesn’t perform any action. In the diagram below the active and reactive power is represented by P and Q, which together give the power S.

Diagram indicating P, Q, S and Cos φThe ratio between the active power (kW) and the reactive power (kVA) is known as the power factor, and is often designated as the cos φ. A normal value is between 0.7 and 0.9, when running where the lower value is for small motors and the higher for large ones.


The speed of an AC motor depends on two things: the number of poles of the stator winding and the main frequency. At 50 Hz, a motor will run at a speed related to a constant of 6000 divided by the number of poles and for a 60 Hz motor the constant is 7200 rpm.

To calculate the speed of a motor, the following formula can be used:

n = speed
f = net frequency
p = number of poles

4-pole motor running at 50 Hz

This speed is the synchronous speed and a squirrel-cage or a slip-ring motor can never reach it. At unloaded condition the speed will be very close to synchronous speed and will then drop when the motor is loaded.

Diagram showing syncronous speed vs.rated speedThe difference between the synchronous and asynchronous speed also named rated speed is ”the slip” and it is possible to calculate this by using the following formula:

s = slip (a normal value is between 1 and 3 %)
n1 = synchronous speed
n = asynchronous speed (rated speed)

Table for synchronous speed at different number of poles and frequency:

Table for synchronous speed at different number of poles and frequency


The starting torque for a motor differs significantly depending on the size of the motor. A small motor, e.g. ≤ 30 kW, normally has a value of between 2.5 and 3 times the rated torque, and for a medium size motor, say up to 250 kW, a typical value is between 2 to 2.5 times the rated torque. Really big motors have a tendency to have a very low starting torque, sometimes even lower than the rated torque. It is not possible to start such a motor fully loaded not even at D.O.L start.

The rated torque of a motor can be calculated using the following formula:

Mr = Rated torque (Nm)
Pr = Rated motor power (kW)
nr = Rated motor speed (rpm)

Torque diagram for a typical squirrel cage motorSlip-ring motors

In some cases when a D.O.L start is not permitted due to the high starting current, or when starting with a star-delta starter will give too low starting torque, a slip-ring motor is used. The motor is started by changing the rotor resistance and when speeding up the resistance is gradually removed until the rated speed is achieved and the motor is working at the equivalent rate of a standard squirrel-cage motor.

Torque diagram for a slip-ring motor | Current diagram for a slip-ring motor

In general, if a softstarter is going to be used for this application you also need to replace the motor.

The advantage of a slip-ring motor is that the starting current will be lower and it is possible to adjust the starting torque up to the maximum torque.



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Cost benefits of AC drives

Cost benefits of AC drives

In addition to their technical advantages, AC drives also provide many cost benefits. In this chapter, these benefits are reviewed, with the costs divided into investment, installation and opera- tional costs.

At the moment there are still plenty of motors sold without variable speed AC drives. This pie chart shows how many motors below 2.2 kW are sold with frequency converters, and how many without. Only 3% of motors in this power range are sold each year with a frequency converter; 97% are sold without an AC drive.
This is astonishing considering what we have seen so far in this guide. Even more so after closer study of the costs of an AC drive compared to conventional control methods. But first let’s review AC drive technology compared to other control methods.

How many motors below 2.2 kW are sold with frequency converters, and how many without

How many motors below 2.2 kW are sold with and without frequency converters


Technical differences between other systems and AC drives

AC drive technology is completely different from other, simpler control methods. It can be compared, for example, to the dif- ference between a zeppelin and a modern airplane.

We could also compare AC drive technology to the develop- ment from a floppy disk to a CD-ROM. Although it is a simpler information storage method, a floppy disk can only handle a small fraction of the information that a CD-ROM can.

The benefits of both these innovations are generally well known. Similarly, AC drive technology is based on a totally different technology to earlier control methods. In this guide, we have presented the benefits of the AC drive compared to simpler control methods.

Technical differences between other systems and AC drives

Technical differences between other systems and AC drives


No mechanical control parts needed

To make a proper cost comparison, we need to study the configurations of different control methods. Here we have used pumping as an example. In traditional methods, there is always a mechanical part and an electrical part.

In throttling you need fuses, contactors and reactors on the electrical side and valves on the mechanical side. In On/Off control, the same electrical components are needed, as well as a pressure tank on the mechanical side. The AC drive provides a new solution. No mechanics are needed, because all control is already on the electrical side.

Another benefit, when thinking about cost, is that with an AC drive we can use a regular 3-phase motor, which is much cheaper than the single phase motors used in other control methods. We can still use 220 V single phase supply, when speaking of power below 2.2 kW.

Conventional methods:AC drive:
• Both electrical and mechanical parts• All in one
• Many electrical parts• Only one electrical component
• Mechanical parts need regular maintenance• No mechanical parts, no wear and tear
• Mechanical control is energy consuming• Saves energy


Factors affecting cost

This list compares the features of conventional control methods with those of the AC drive, as well as their effect on costs. In conventional methods there are both electrical and mechanical components, which usually have to be purchased separately. The costs are usually higher than if everything could be pur- chased at once. Furthermore, mechanical parts wear out quickly. This directly affects maintenance costs and in the long run, maintenance is a very important cost item. In conventional methods there are also many electrical components. The installation cost is at least doubled when there are several different types of components rather than only one.

And last but not least, mechanical control is very energy con- suming, while AC drives practically save energy. This not only helps reduce costs, but also helps minimise environmental impact by reducing emissions from power plants.

Investment costs: Mechanical and electrical components

Price Comparison For Pumps

Price Comparison For Pumps

In this graph, the investment structure as well as the total price of each pump control method is presented. Only the pump itself is not added to the costs because its price is the same regardless of whether it’s used with an AC drive or valves. In throttling, there are two possibilities depending on whether the pump is used in industrial or domestic use. In an industrial environment there are stricter requirements for valves and this increases costs.

The motor
As can be seen, the motor is much more expensive for traditional control methods than for the AC drive. This is due to the 3-phase motor used with the AC drive and the single phase motor used in other control methods.
The AC drive
The AC drive does not need any mechanical parts, which reduc- es costs dramatically. Mechanical parts themselves are almost always less costly than a frequency converter, but electrical parts also need to be added to the total investment cost.
After taking all costs into account, an AC drive is almost always the most economical investment, when compared to differ- ent control methods. Only throttling in domestic use is as low cost as the AC drive. These are not the total costs, however. Together with investment costs we need to look at installation and operational costs.

ThrottlingAC drive
Installation material20 USD10 USD
Installation work5h x 65 USD = 325 USD1h x 65 USD = 65 USD
Commissioning work1h x 65 USD = 65 USD1h x 65 USD = 65 USD
Savings in installation: 270 USD!


Installation costs: Throttling compared to AC drive

Because throttling is the second lowest investment after the AC drive, we will compare its installation and operating costs to the cost of the AC drive. As mentioned earlier, in throttling there are both electrical and mechanical components. This means twice the amount of installation material is needed.

Installation work is also at least doubled in throttling compared to the AC drive. To install a mechanical valve into a pipe is not that simple and this increases installation time. To have a mechanical valve ready for use usually requires five hours compared to one hour for the AC drive. Multiply this by the hourly rate charged by a skilled installer to get the total installation cost.

The commissioning of a throttling-based system does not usu- ally require more time than commissioning an AC drive based system. One hour is usually the time required in both cases. So now we can summarise the total installation costs. As you can see, the AC drive saves up to USD 270 per installation. So even if the throttling investment costs were lower than the price of a single phase motor (approximately USD 200), the AC drive would pay for itself before it has even worked a second.

ThrottlingAC drive
Power required0.75 kW0.37 kW
Annual energy 4000 hours/year 3000 kWh1500 kWh
Annual energy cost with 0.1 USD/kWh300 USD150 USD
Maintenance/year40 USD5 USD
Savings in installation: 185 USD!


Operational costs: Maintenance and drive energy

In many surveys and experiments it has been proved that a 50% energy saving is easily achieved with an AC drive. This means that where power requirements with throttling would be 0.75 kW, with the AC drive it would be 0.37 kW. If a pump is used 4000 hours per year, throttling would need 3000 kWh and the AC drive 1500 kWh of energy per year.
To calculate the savings, we need to multiply the energy con- sumption by the energy price, which varies depending on the country. Here USD 0.1 per kWh has been used.

As mentioned earlier, mechanical parts wear a lot and this is why they need regular maintenance. It has been estimated that whereas throttling requires USD 40 per year for service, maintenance costs for an AC drive would be USD 5. In many cases however, there is no maintenance required for a frequency converter.

Therefore, the total savings in operating costs would be USD 185, which is approximately half of the frequency convert- er’s price for this power range. This means that the payback time of the frequency converter is two years. So it is worth considering that instead of yearly service for an old valve it might be more profitable to change the whole system to an AC drive based control. To retrofit an existing throttling system the pay-back time is two years.

Total cost comparison

Total Savings Over 10 Year - USD 1562

Total Savings Over 10 Year - USD 1562

In the above figure, all the costs have been summarised. The usual time for an operational cost calculation for this kind of investment is 10 years. Here the operational costs are rated to the present value with a 10% interest rate.

In the long run, the conventional method will be more than twice as expensive as a frequency converter. Most of the savings with the AC drive come from the operational costs, and especially from the energy savings. It is in the installation that the high- est individual savings can be achieved, and these savings are realised as soon as the drive is installed.

Taking the total cost figure into account, it is very difficult to understand why only 3% of motors sold have a frequency con- verter. In this guide we have tried to present the benefits of the AC drive and why we at ABB think that it is absolutely the best possible way to control your process.



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