Frequently Asked Questions on AC Motors
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Frequently asked questions on AC motors Selection guide to ac motors 1. What are the power supply requirements? 2. What speeds are available? 3. What speed range can motors cover? 4. What duty ratings are possible? 5. What are the limits for the operating environ ment? 6. How well are motors protected? 7. What are the mounting options? 8. What is the situation with motor efficiency? 9. What are brake motors? 10. What about other motor options? 1. What are the power supply requirements? Motors can be wound for 1 phase or 3 phase supply. Single phase motors tend to have lower performance, for example low starting torques, or be more complex and expensive. Their supply voltage tolerance is fairly tight, for example 230V + 10%. Three phase motors are the most common. They are low in cost and robust . Voltage tolerance may be wider, for example 400 - 480 V + 10%, and lower power motors can be connected to run on 230V 3 phase supplies. Other supply voltages such as 600V require motors with speci fic windings Our UK motors are designed for mains 50Hz frequency operation which gives standard synchronous speeds of 3000 r/min (2 pole), 1500 r/min (4 pole) and 900 r/min(6 pole). In practice motors exhibit slip and actual running speeds tend to be 3 to 12% lower. Our motors can also be run at the North American frequency of 60Hz. If the voltage is the same, speeds increase by 20% but output torques fall by 17% 2. What speeds are available? As described above in 1, the synchronous speed depends on the number of pole pairs in the motor construction. Most common are 2 pole pairs for 3000r/min, 4 pole pairs for 1500 r/min and 6 pole pairs for 900 r/min. However 8 or more pole pairs are possible. All motors run with a sl ip speed 3 t o 12% below the synchronous speeds, the difference becoming smaller as frame size increases. Pole changing motors that run at two speeds according to connection are possible. However we recommend that inverter operation be used instead for reasons of cost and availabilit y. Our variable speed motors for inverter operations can run in a range 60 to 2500r/min 3. What speed range can motors cover? Ac asynchronous motors are essentially fixed speed devices. To vary speed you need afrequency inverter. Simple inverters can control speed down to 2Hz and it may be possible to overspeed the motor, for example to 70Hz or 2000 r/min on a 4 pole motor. This gives a speed range of 2 to 70, that is 35/1 or 60 to 2000 r/min. More sophisticated vector controlled inverte rs can control ac motors down to zero speed. Some motors are wound to suit inverter control with a nominal frequency up to 120Hz equivalent to 3500r/min. This results in more power or a smaller frame size at the same power, also a wider speed range with inverter operation. Normal ac motor ventilation is by a bi-direction fan mounted under a cowl at the non-drive-end (NDE) of the motor. As speeds fall, ventila tion becomes ineffectiv e and overheating may occur. We recommend that below 20Hz operation the motor be derated or a forced cooling blower be fitted.
Transcript of Frequently Asked Questions on AC Motors
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Frequently asked questions on AC motorsSelection guide to ac motors
1. What are the power supply requirements?2. What speeds are available?3. What speed range can motors cover?
4. What duty ratings are possible?5. What are the limits for the operating environment?6. How well are motors protected?7. What are the mounting options?8. What is the situation with motor efficiency?9. What are brake motors?10. What about other motor options?
1. What are the power supply requirements?
Motors can be wound for 1 phase or 3 phase supply. Single phase motors tend to have lower performance,for example low starting torques, or be more complex and expensive. Their supply voltage tolerance is fairlytight, for example 230V + 10%.
Three phase motors are the most common. They are low in cost and robust. Voltage tolerance may bewider, for example 400 - 480 V + 10%, and lower power motors can be connected to run on 230V 3 phasesupplies. Other supply voltages such as 600V require motors with specific windings
Our UK motors are designed for mains 50Hz frequency operation which gives standard synchronous speedsof 3000 r/min (2 pole), 1500 r/min (4 pole) and 900 r/min(6 pole). In practice motors exhibit slip and actualrunning speeds tend to be 3 to 12% lower. Our motors can also be run at the North American frequency of 60Hz. If the voltage is the same, speeds increase by 20% but output torques fall by 17%
2. What speeds are available?
As described above in 1, the synchronous speed depends on the number of pole pairs in the motor
construction. Most common are 2 pole pairs for 3000r/min, 4 pole pairs for 1500 r/min and 6 pole pairs for 900 r/min. However 8 or more pole pairs are possible. All motors run with a slip speed 3 to 12% below thesynchronous speeds, the difference becoming smaller as frame size increases.
Pole changing motors that run at two speeds according to connection are possible. However werecommend that inverter operation be used instead for reasons of cost and availability. Our variable speedmotors for inverter operations can run in a range 60 to 2500r/min
3. What speed range can motors cover?
Ac asynchronous motors are essentially fixed speed devices. To vary speed you need a frequencyinverter. Simple inverters can control speed down to 2Hz and it may be possible to overspeed the motor, for example to 70Hz or 2000 r/min on a 4 pole motor. This gives a speed range of 2 to 70, that is 35/1 or 60 to
2000 r/min. More sophisticated vector controlled inverters can control ac motors down to zero speed.
Some motors are wound to suit inverter control with a nominal frequency up to 120Hz equivalent to3500r/min. This results in more power or a smaller frame size at the same power, also a wider speed rangewith inverter operation.
Normal ac motor ventilation is by a bi-direction fan mounted under a cowl at the non-drive-end (NDE) of themotor. As speeds fall, ventilation becomes ineffective and overheating may occur. We recommend thatbelow 20Hz operation the motor be derated or a forced cooling blower be fitted.
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For precise speed control it is necessary to go to closed loop operation. This can be done by fitting anincremental encoder on the back of the motor inside the fan. A two channel encoder with a vector controlledinverter can give very precise speed accuracy. The Lenze 8400 Highline and StateLine inverters are idealfor control.
4. What duty ratings are possible?
The majority of motors are general purpose and designed for S1 duty, that is continuous 100%. If your application is intermittent, there can be advantages to either uprate the motor power, or have a motor specially wound for your duty, also resulting in increased power. There are standard definitions of dutycycles S1 to S6, information available on request. Also motors can be supplied to duty S9 which suitsinverter operation.
5. What are the limits for the operations environment?
The main factors to consider are temperature, humidity and height above sea level. Generally standard ac
motors can operate in ambient temperatures from -15 to +40°C, although there may be problems with
condensation and corrosion. See point 6 below. Special motors can be provided for higher temperatures.Similarly humidity should be in a range 30 to 95% or else special protective measures are needed.Operating heights up to 1000m above sea level are possible with derating from there up to 4000m.
6. How well are motors protected?
The general standard here is IP54/55 meaning protection against dust and splashedwater. Open framed motors with lower protection to IP20 can be cost effective for volume applications.Motors with higher protection including stainless steel construction can raise the protection level to IP65,IP66, IP67 or even higher. Costs tend to increase sharply. Other measures for protection against theenvironment may be needed. Special paint finishes resist corrosion. Tropicalisation suits operation with highhumidities.
Anti condensation heaters suit where temperatures cycle down to low values. Rain covers and sun shadescan be provided.
7. What are the mounting options?
Apart from special designs where the mounting options are unlimited, there are three standard mountings
B3 - foot mountingB5 - flange mounting, sometimes different flange diameters are availableB14 - face mounting, usually available up to 90 frame and offering a more compact arrangement than B5.
(sometimes combinations are offered such as B34 and B35)
8. What is the situation with motor efficiency?
New legislation has put the focus on motor efficiency from the point of view of energy savings. Electricmotors are one of the main consumers of energy. There are three levels of efficiency relevant:IE1 - effectively low efficiency, will continue as the standard for small motors below 0.75kW.IE2 - higher efficiency, previously known as Eff1, this level is compulsory for all new motors from 0.75 to375kW in Europe from June 2011.IE3 - a higher efficiency level to be introduced in 2015. As an alternative IE2 motors with frequencyinverters can be used.
Motor efficiency depends on construction, on the frame size, but also on the loading. A motor running onpartial load exhibits a big loss in efficiency, which can be counteracted by using a frequency inverter.
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It should be noted that stepping up from an IE1 to an IE2 motor may result in an efficiency gain of 2-4%.This may be worthwhile, but changes to other parts of the drive system can produce far larger savings,sometimes 50% or more.
9. What are brake motors?
A brake motor is effectively a standard motor with a spring applied brake mounted at the non-drive endunder the fan. These exist in a standard range at lower powers up to 7.5kW and are commonly available onrequest for higher powers.
The brake is usually fitted with a dc coil and energised from the motor terminals through a rectifier.Therefore the brake automatically releases when the motor is switched on, and automatically engages whenpower to the motor is removed. Brakes are sized appropriate to the motor torque and brake torque may bemechanically adjustable. Sometimes more than one size of brake is offered on the frame of motor.
Where gravity acts on falling loads, the standard rectified brake control may not be suitable and adviceshould be sought for a separate control circuit.
10. What about other motor options?
There are many other options and a motor specialist can give advice. For example:
Thermistors to give a warning signal on overheating. Operation at 87Hz to achieve increased power from agiven motor frame size. Operation at 120Hz to reduce size, increase efficiency and lower costs (see LenzeMF motors) Special shafts. Special frame dimensions. Windings to suit intermittent operation.
Our MF motors are inverter optimised and are designed for a nominal speed of 120Hz/3500 r/min. Thisgives several advantages. For a given power the physical size will be 1 or 2 frames smaller saving spaceand purchase cost. They are also more dynamic and run with increased efficiency.
Modular spring applied brakes INTORQ BFK458
Brakes with electromagnetic release 2 to 600NmThe BFK458 modular system of spring applied brakes is world class and proven in thousands of differentmachines with torques from 2 to 600Nm. High quality construction with long lasting materials suits use in alltypes of safety sensitive applications. A cold-climate version is available for extreme ambients and this isused in wind turbines. For mechanical handling there is a long-life variant that can achieve more than 10million operations. Nine sizes with CSA and UL approval are available for operation with dc voltages from24 to 205V.
All the options: mounting options include friction plate, mounting flange and terminal box. Functionaloptions are manual release, limit switches, long life, double safety brake and low noise.Enclosure options are IP54, external seal or cover.Brake rectifiers are available.
Brakes are designed for dry running and brake torque can be adjusted (type E).
Where the duty is mainly static holding torques, rated torques can be increased to a maximum by factorymodification of the springs.
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Brake sizeBFK 458
Min torqueNm
Rated torqueNm
Maxtorque Nm
Body diamm
Max boremm
06 1.5 4 6 87 15
08 3.5 8 12 105 20
10 7 16 23 130 2012 14 32 46 150 25
14 25 60 80 165 30
16 35 80 125 190 38
18 65 150 235 217 45
20 80 260 400 254 50
25 175 400 600 302 70
Data for response times, maximum permissible energy per stop, stoppingcalculations and electrical connections are given in the catalogue.
Standard spring applied brakes INTORQ BFK457
Brakes with electromagnetic release 0.12 to 125NmThese simple energise-to-release multi-spring brakes suit both dynamic and static duties in a torque range0.12 to 125Nm. They are available pre-assembled with integral fixing screws and a preset air gap reducesmachine build times to a minimum. Application of a dc voltage of either 24 or 205V releases the brake.
Other features: available in 11 sizes, optional manual release lever, double brake option for highest safetyrequirements, low noise option, thermal class F, high torque spring option..
Brake sizeBFK 457
Rated torqueNm
Max torqueNm
Body diamm
Max boremm
01 0.12 0.24 37 6
02 0.25 0.5 47 7
03 0.5 1.0 56 9
04 1 2.0 65 10
05 2 4.0 75 12
06 4 6 84 15
08 8 12 102 20
10 16 23 130 20
12 32 46 150 25
14 60 95 165 30
16 80 125 190 38
Maximum torque is achieved by factory modification to the springs and is
best suited to emergency stop and holding duties.
