Frequently asked questions on AC motors
Selection guide to ac motors
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.
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 and 4 pole pairs for 1500 r/min. However 8 or more pole pairs are possible. All motors run with a slip speed 3 to 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 availability.
3. What speed range can motors cover?
Ac asynchronous motors are essentially fixed speed devices. To vary speed you need a frequency 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 inverters can control ac motors down to zero speed.
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, ventilation becomes ineffective and overheating may occur. We recommend that below 20Hz operation the motor be derated or a forced cooling blower be fitted.
For precise speed control it is necessary to go to closed loop operation. This can be done by fitting an incremental encoder on the back of the motor inside the fan. A two channel encoder with a vector controlled inverter can give very precise speed accuracy.
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 duty cycles S1 to S6, information available on request.
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 splashed
water. 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 the environment may be needed. Special paint finishes resist corrosion. Tropicalisation suits operation with high humidities.
Anti condensation heaters suit where temperatures cycle down to low values. Rain covers and sun shades can 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 mounting
B5 - flange mounting, sometimes different flange diameters are available
B14 - 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. Electric motors are one of the main consumers of energy. There are three levels of efficiency relevant:
IE1 - effectively low efficiency, but the vast majority of motors in use today meet this standard.
IE2 - higher efficiency, previously known as Eff1, this level will be compulsory for all new motors in Europe from 2011.
IE3 - a higher efficiency level to be introduced in 2015. As an alternative IE2 motors with frequency inverters can be used.
Motor efficiency depends on construction, on the frame size, but also on the loading. A motor running on partial load exhibits a big loss in efficiency, which can be counteracted by using a frequency inverter.
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 end under the fan. These exist in a standard range at lower powers up to 7.5kw and are commonly available on request 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 when power to the motor is removed. Brakes are sized appropriate to the motor torque and brake torque may be mechanically 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 advice should be sought for a separate control circuit.
10. What about other brake 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 the maximum power from a given motor frame size.