AC motors, asynchronous motors

This group of induction machines includes electrical machines whose mode of operation is based on a rotating magnetic field in the air gap between the stator and rotor. The most important and most commonly used machine in this group is the asynchronous AC induction motor with a squirrel cage design. It is characterized by the following features:

• A simple and robust design
• High operational reliability
• Low-maintenance operation
• A low price

In electrical drive technology the following electric motors are generally used:

• Asynchronous AC motors (squirrel-cage rotors, slip-ring rotors, torque motors)
• Asynchronous single-phase AC motors
• Asynchronous or synchronous servomotors
• DC motors

Since AC motors with frequency inverters provide better, simpler and more low-maintenance speed control, DC motors and AC motors with slip rings are becoming less and less relevant. Other types of AC asynchronous motor are only of marginal importance in drive engineering. As a result, they will not be dealt with in detail here.

If you combine an electric motor such as an AC motor with a gear unit you get a gearmotor. Regardless of the electrical principle of the motor, the way it is mounted on a gear unit is becoming especially important in terms of the mechanical design of the motor. SEW‑EURODRIVE uses specially adapted motors with this purpose in mind.

Rotor

In the slots of the rotor laminated core, there is an injected or inserted winding (usually made from aluminum and/or copper). Classically, one turn of the windung corresponds to one bar. This winding is short-circuited on both ends by rings made from the same material. The bars with the short-circuit rings are reminiscent of a cage. That is where the second common name for AC motors comes from: "the squirrel-cage motor."

Stator

The winding, which is encapsulated with synthetic resin, is inserted into the half-closed slot on the laminated stator core. The number and width of the coils are varied to achieve different numbers of poles (= speeds). Together with the motor housing, the laminated core forms the stator.

Endshields

The endshields are made from steel, gray cast iron, or die-cast aluminum and seal off the inside of the motor on the A-side and B-side. The constructive design when transitioning to the stator determines among other things the IP degree of protection of the motor.

Rotor shaft

The rotor-side laminated core is attached to a steel shaft. The two shaft ends pass through the endshield on both the A-side and B-side. The output shaft end is installed on the A-side (designed as a pinion shaft end for the gearmotor); the fan and its fan-cooling wings and/or supplementary systems such as mechanical brakes and encoders are installed on the B-side.

Motor housing

The motor housing can be produced from die-cast aluminum when the power rating is low to medium. However, the housing for all of the power classes above those is produced from gray cast iron and welded steel. A terminal box in which the stator winding ends are connected to a terminal block for the customer-side electrical connection is attached to the housing. Cooling fins enlarge the surface of the housing and also increase the emission of heat into the environment.

Fan, fan guard

A fan on the B-side shaft end is covered by a hood. This hood guides the air flow produced during rotation of the fan through the fins on the housing. As a rule, the fans are not independent of the direction of rotation of the rotor. An optional canopy prevents (small) parts from falling through the fan guard grid when the mounting position is vertical.

Bearings

The bearings in the A-side and B-side endshields mechanically connect the rotating parts to the stationary parts. Usually, deep groove ball bearings are used. Cylindrical roller bearings are rarely used. The bearing size depends on the forces and speeds that the relevant bearing has to absorb. Different types of sealing systems ensure that the required lubricating properties are maintained in the bearing and that oil and/or grease does not escape.

The symmetrical, three-phase winding system of the stator is connected to a three-phase current power system with the appropriate voltage and frequency. Sinusoidal currents of the same amplitude flow in each of the three winding phases. Each of the currents are temporally offset from each other by 120°. Since the winding phases are also spatially offset by 120°, the stator builds up a magnetic field that rotates with the frequency of the applied voltage.

This rotating magnetic field – or rotating field for short – induces an electrical voltage in the rotor winding or rotor bars. Short-circuit currents flow because the winding is short-circuited by the ring. Together with the rotating field, these currents build forces and produce a torque over the radius of the rotor that accelerates the rotor speed in the direction of the rotating field. The frequency of the voltage generated in the rotor drops as the speed of the rotor increases. This is because the difference between the rotating field speed and the rotor speed becomes smaller.

The induced voltages, which are now lower as a result, lead to lower currents in the rotor cage and therefore lower forces and torques. If the rotor were to turn at the same speed as the rotating field, it would rotate synchronously, no voltage would be induced, and the motor would not be able to develop any torque as a result. However, the load torque and friction torques in the bearings lead to a difference between the rotor speed and rotating field speed and this results in an equilibrium between the acceleration torque and load torque. The motor runs asynchronously.

The magnitude of this difference increases or decreases depending on the motor load but is never zero, because there is always friction in the bearings, even in no-load operation. If the load torque exceeds the maximum acceleration torque that can be produced by the motor, the motor "stalls" into an impermissible operating state that may lead to thermal damage.

The relative movement between the rotating field speed and mechanical speed that is required for the function is defined as the slip "s" and is specified as a percentage of the rotating field speed. Motors with a lower power rating can have a slip of 10 to 15 percent. AC motors with a higher power rating have approx. 2 to 5 percent slip.

The AC motor takes electrical power from the voltage supply system and converts it into mechanical power – that is, into speed and torque. If the motor were to operate without losses, the output mechanical power P_out would correspond to the input electrical power P_in.

However, losses also occur in AC motors, which is unavoidable whenever energy is converted: Copper losses P_Cu and bar losses P_Z occur when a current flows through a conductor. Iron losses P_Fe result from the remagnetization of the laminated core with a line frequency. Friction losses P_Rb result from the friction in the bearings and air losses result from using air for cooling. The copper, rod, iron and friction losses cause the motor to heat up. The efficiency of the machine is defined as the ratio between output and input power.

Due to legal regulations, more and more attention has been paid to using motors with higher efficiency levels over the past few years. Energy efficiency classes have been defined in corresponding normative agreements. Manufacturers have adopted these classes in their technical data. To reduce the significant losses caused by the machine, this has meant the following for the design of the electric motor:

• The increased use of copper in the motor winding ( P_Cu)
• Better sheet metal material (P_Fe)
• An optimized fan geometry (P_Rb)
• An energetically optimized bearing (P_Rb)

By recording the torques and current against the speed, you get the characteristic speed-torque characteristics of the AC motor. The motor follows this characteristic curve every time it is switched on until it reaches its stable operating point. The characteristic curves are influenced by the number of poles as well as the design and material of the rotor winding. Knowledge of these characteristic curves is particularly important for drives that are operated with counter-torques (e.g. hoists).

If the counter-torque of the driven machine is higher than the pull-up torque, the rotor speed becomes "stuck in the dip." The motor no longer reaches its nominal operating point (that is, the stable, thermally safe operating point). The motor even comes to a standstill if the counter-torque is greater than the starting torque. If a running drive is overloaded (e.g. an overloaded conveyor belt), its speed drops as the load increases. If the counter-torque exceeds the breakdown torque, the motor "stalls" and the speed slows to the pull-up speed or even to zero. All of these scenarios lead to extremely high currents in the rotor and stator, which means that both heat up very rapidly. This effect can lead to irreparable thermal damage to the motor – or "burning out" – if no suitable protection devices are in place.

The heat generated in an electrical current-carrying conductor depends on the resistance of the conductor and the magnitude of the current that it is carrying. Frequent switching on and starting up against a counter-torque place a very great thermal load on the AC motor. The permitted heating of the motor depends on the temperature of the surrounding cooling medium (e.g. air) and the thermal resistance of the insulation material in the winding.

The motors are allocated to thermal classes (which were earlier called "insulation classes") that govern the maximum permitted overtemperatures in the motors. A motor must be able to withstand sustained operation at an elevated temperature based on its rated power in the thermal class for which it was designed without suffering damage. With a maximum coolant temperature of 40° C, for example, the maximum permitted overtemperature in the thermal class 130 (B): dT = 80 K.

• The most simple operating mode involves applying a constant load torque. After a certain time, the motor reaches its thermal steady state condition as a result of the sustained load in the operating point. This operation is referred to as continuous duty S1.
• In short-time duty S2, the motor operates at a constant load for a certain time period (tB). The motor does not reach its thermal steady state condition during this time. This is followed by an idling time that must be long enough to allow the motor to return to the coolant temperature.
• In intermittent duty S3, the motor operates at a constant load for a certain time period (tB). The startup must not have an effect on the heating of the motor in this case. This is followed by a specific idling time (tSt). The relative cyclic duration factor (cdf) is specified in this operating mode. In the standard IEC 60034-1 IEC 60034-1 the proportion of the operating time in a cycle time (= operating time + idling time) of 10 minutes for illustrative purposes is specified.

Example: Operating mode S3/40% applies if the motor alternates between four minutes of running and six minutes switched off.

The permitted switching frequency specifies how often a motor can be switched on in an hour without thermally overloading it. It is dependent on the following:

• The mass moments of inertia to be accelerated
• The static load
• The type of braking
• The duration of the run-up
• The ambient temperature
• The cyclic duration factor

The permitted starting frequency of a motor can be increased by the following measures:

• Increasing the thermal class
• Selecting the next larger motor
• Fitting a forced cooling fan
• Changing the gear unit ratio and therefore the inertia ratios
• By choosing a different type of braking

AC motors can be operated at different speeds by switching of windings or parts of windings. Different numbers of poles result from inserting several windings into the stator slots or by reversing the direction of current flow in individual parts of the winding. In the case of separate windings, the power for each pole number is less than half of the power of a single-speed motor of the same size.

Pole-changing AC gearmotors are used as travel drives, for example. The travel speed is high during operation with low numbers of poles. The low-speed winding is switched to for positioning. Due to inertia, the motor initially keeps turning at a high speed during the changeover. The AC motor operates as a generator during this phase and slows down. The kinetic energy is converted into electrical energy and fed back into the supply system. The large torque step caused by the changeover is a disadvantage. However, appropriate circuit measures can be taken to reduce this.

Current developments in low-cost inverter technology promote the technological replacement use of pole-changing motors by single-speed, frequency inverter controlled motors in many applications.

A single-phase motor is a good option when in your applications

• no high starting or start-up torque is required,
• the motors are connected to a single-phase AC supply system,
• and a relatively low power (<= 2.2 kW) is used.

Typical application examples include ventilators, pumps, and compressors. There are two fundamental design differences here:

On the one hand, the classic asynchronous AC motor is connected only to one phase and the neutral conductor. The third connection is produced through a phase shift using a capacitor. Since the capacitor can generate only a 90° phase offset and not a 120° phase offset, this type of single-phase motor is usually rated only with two thirds of the power of a comparable AC motor.

The second way to build a single-phase motor involves technical adjustments to the winding. Instead of the three-phase winding, only two phases are implemented, one as the main phase and one as the auxiliary phase. The coils, which are spatially offset by 90°, are also supplied with current by a capacitor with a temporal 90° offset, which produces the rotating field. The unequal current ratios of the main winding and auxiliary winding also usually only allow for two thirds of the power of an AC motor of the same size. Typical motors for single-phase operation include capacitor motors, shaded pole motors and starting motors, which do not include capacitors.

The SEW‑EURODRIVE range includes both types of single-phase motor design – The DRK.. motors. Both are supplied with an integrated running capacitor. Since this capacitor is housed directly in the terminal box, interfering contours are avoided. With a running capacitor, approx. 45 to 50 percent of the nominal torque is available for start-up.

For customers who require a higher starting torque of up to 150 % of the rated torque, SEW-EURODRIVE can supply the capacitance values of the starting capacitors required for this purpose, which are available from well-stocked specialist dealers.

Torque motors are special design AC motors with squirrel-cage rotors. By design, they are rated so that their current consumption is only high enough to ensure that they do not cause themselves irreparable thermal damage when the speed is 0. This feature is helpful, for instance, when opening doors and point-setting or in press dies, for when a position has been reached and must be safely maintained by an electric motor.

Another common operating mode is countercurrent braking operation: An external load is capable of turning the rotor against the direction of rotation of the rotating field. The rotating field "slows down" the speed and withdraws regenerative energy from the system, which is fed into the supply system – similar to rotary braking without mechanical braking work.

SEW‑EURODRIVE offers the DRM../DR2M.. together with 12-pole torque motors which are thermally designed for long-term use with the rated torque in an idle state. SEW‑EURODRIVE torque motors are suitable for a variety of different requirements and speeds and are available with up to three rated torques, depending on the operating mode.

If you are using electric motors in areas where there is a risk of explosion (as per Directive 2014/34/EU (ATEX)), specific preventive measures must be taken on the drives. SEW‑EURODRIVE offers a number of different designs with this in mind based on the area and region of use.

SEW‑EURODRIVE offers the LSPM motor range for applications that are operated directly from the supply system and also require a synchronous speed or have this characteristic sensorless on a simple inverter. LSPM is the abbreviation of "Line Start Permanent Magnet." The LSPM motor is an AC asynchronous motor with additional permanent magnetsin the rotor. It runs asynchronously, synchronizes with the operating frequency, and runs in synchronous mode from then on without slip synchronously to the mains frequency. Motor technology that opens up new, flexible application possibilities in drive technology, e.g. the transfer of loads without a drop in speed.

These compact hybrid motors do not incur any rotor losses during operation and are characterized by their high efficiency. Energy saving classes up to IE4 are achieved.

The size of a DR..J motor with LSPM technology is two stages smaller in comparison to a series motor with the same power and energy efficiency class. Motors of the same size, on the other hand, achieve an efficiency class two times better than that of asynchronous motors.