AC Motor Principles

AC three-phase motor used in a HEV

FIGURE. AC three-phase motor used in a HEV.

A few years ago, the automotive technician did not need to be concerned much about the operating principles of the AC motor. With the increased focus on HEVs and EVs, this is no longer an option since most of these vehicles use AC motors.

AC voltage gradually changes. It is rated at the RMS

FIGURE. AC voltage gradually changes. It is rated at the RMS.

AC voltage has a changing direction of current flow. However, this change does not occur immediately. Notice that the AC voltage sine wave indicates that in one cycle the voltage will be zero at three times. Also notice that as the current changes directions, it gradually builds up or falls in the other direction. The sine wave illustrates that the amount of current in an AC circuit always varies. The current rating is based on the average referred to as a root mean square (RMS) value.

AC Motor Construction

Like the DC motor, the AC motor uses a stator (field winding) and a rotor. Common types of AC motors are the synchronous motor and the induction motor. In both motor types, the stator comprises individual electromagnets that are either electrically connected to each other or connected in groups. The difference is in the rotor designs. AC motors can use either single-phase or three-phase AC current. Since the three-phase is the most common motor used in HEV and EV vehicles, we will focus our discussion on these.

Note: Three-phase AC voltage is commonly used in motors because it provides a smoother and more constant supply of power. Three-phase AC voltage is like having three independent AC power sources, which have the same amplitude and frequency but are 120 degrees out of phase with each other.

As in a DC motor, the movement of the rotor is the result of the repulsion and attraction of the magnetic poles. However, the way this works in an AC motor is very different. Because the current is alternating, the polarity in the windings constantly changes. The principle of operation for all three-phase motors is the rotating magnetic field. The rotor turns because it is pulled along by a rotating magnetic field in the stator. The stator is stationary and does not physically move. However, the magnetic field does move from pole to pole. There are three factors that cause the magnetic field to rotate. The first is the fact that the voltages in a three-phase system are 120 degree out of phase with each other. The second is the fact that the three voltages change polarity at regular intervals. Finally, the third factor is the arrangement of the stator windings around the inside of the motor.

The motor stator is energized with the three-phase AC voltage

FIGURE. The motor stator is energized with the three-phase AC voltage.

In Figure the stator is a two-pole three-phase motor. Two-pole means that there are two poles per phase. The motor is wired with three leads: L1, L2, and L3.

Note: The stator of AC motors do not have actual pole pieces as shown in Figure 6-48. They are illustrated to help understand how the rotating magnetic field is created in a three-phase motor.

The three AC sine waves are 120° apart. At any one time there are two voltages at the same polarity

FIGURE. The three AC sine waves are 120° apart. At any one time there are two voltages at the same polarity.

Each of the poles is wound in such a manner that when current flows through the winding they develop opposite magnetic polarities. All three windings are joined to form a wye connection for the stator. Since each phase reaches its peak at successively later times, the strongest point of the magnetic field in each winding is also in succession.

This succession of the magnetic fields is what creates the effect of the magnetic field continually moving around the stator.

Since the rotating magnetic field will rotate around the stator once for every cycle of the voltage in each phase, the field is rotating at the frequency of the source voltage. Remember that as the magnetic field moves, new magnetic polarities are present. As each polarity change is made, the poles of the rotor are attracted by the opposite poles on the stator. Therefore, as the magnetic field of the stator rotates, the rotor rotates with it. The speed with which the rotor turns depends on the number of windings and poles built into the motor, the frequency of the AC supply voltage, and the load on the rotor’s shaft. Frequency modulation (thus motor speed) can be altered by use of controllers.

Synchronous Motors

The speed at which the magnetic field rotates is called the synchronous speed. The two main factors determining the synchronous speed of the rotating magnetic field are the number of stator poles (per phase) and the frequency of the applied voltage. A synchronous motor operates at a constant speed, regardless of load. The speed of the rotor is equal to the synchronous speed.

The synchronous motor does not depend on induced current in the rotor to produce torque. The strength of the magnetic field determines the torque output of the rotor, while the speed of the rotor is determined by the frequency of the AC input to the stator.

Synchronous motors cannot be started by the applying of three-phase AC power to the stator. This is because when the AC voltage is applied to the stator windings, a high-speed rotating magnetic field is present immediately. The rotating magnetic field will pass the rotor so quickly that the rotor does not have time to start turning.

In order to start the motor, the rotor contains a squirrel-type winding made of heavy copper bars connected by copper rings. When voltage is first applied to the stator windings, the resulting rotating magnetic field cuts through the squirrel-cage bars. The cutting action of the field induces a current into the squirrel-cage. Since the squirrel cage is shorted, the low voltage that is induced into the squirrel-cage windings results in a relatively large current flow in the cage. This current flow produces a magnetic field within the rotor that is attracted to the rotating magnetic field of the stator. The result is the rotor begins to turn in the direction of rotation of the stator field.

The construction the rotor of a synchronous motor includes wound pole pieces that become electromagnets when DC voltage is applied to them. Hie excitation current can be applied to the rotor through sliprings or by a brushless exciter. As the rotor is accelerated to a speed of 95% of the speed of the rotating magnetic field, DC voltage is connected to the rotor through the sliprings on the rotor shaft or by a brushless exciter. The application of DC voltage to the rotor windings results in the creation of electromagnets. The electromagnetic field of the rotor is locked in step with the rotating magnetic field of the stator. The rotor will now turn at the same speed as the rotating magnetic field. Since the rotor is turning at the synchronous speed of the field, the cutting action between the stator field and the winding of the squirrel cage has ceased. This stops the induction of current flow in the squirrel cage. The speed of the rotor is locked to the speed of the rotating magnetic field even as different loads are applied.

Induction Motors

Concept of the induction motor

FIGURE. Concept of the induction motor.

An induction motor generates its own rotor current by induced voltage from the rotating magnetic field of the stator. The current is induced in the windings of the rotor as it cuts through the magnetic flux lines of the rotating stator field. Generally, the rotor windings are in the form of a squirrel cage. However, wound-rotor motors are constructed by winding three separate coils on the rotor 120 degree apart. The rotor will contain as many poles per phase as the stator winding. These coils are connected to three sliprings located on the rotor shaft so rushes can provide an external connection to the rotor.

When voltage is first applied to the stator windings, the rotor is not turning. To start the squirrel cage induction motor, the magnetic field of the stator cuts the rotor bars that induce a voltage into the cage bars. This induced voltage is the same frequency as the voltage applied to the stator. Since the rotor is stationary, maximum voltage is induced into the squire cage and causes current to flow through the cage’s bars. The current flow results in the production of a magnetic field around each bar.

The magnetic field of the rotor is attracted to the rotating magnetic field of the stator. The rotor begins to turn in the same direction as the rotating magnetic field. As the rotor increases in speed, the rotating magnetic field cuts the cage bars at a slower rate, resulting in less voltage being induced into the rotor. This also results in a reduction of rotor current. With the decrease in rotor current, the stator current also decreases. If the motor is operating without a load, the rotor continues to accelerate until it reaches a speed close to that of the rotating magnetic field. This means that when a squirrel-cage induction motor is first started, it has a current draw several times greater than its normal running current.

The inverter module controls the speed and direction of the AC motor

FIGURE. The inverter module controls the speed and direction of the AC motor.

If the rotor were to turn at the same speed as the rotating magnetic field, there would be no induced voltage in the rotor and, consequently, no rotor current. This means that an induction motor can never reach synchronous speed. If the motor is operated with no load, the rotor will accelerate until the torque developed is proportional to friction losses. As loads are applied to the motor, additional torque is required to overcome the load. The increase in load causes a reduction in rotor speed. This results in the rotating magnetic field cutting the cage bars at a faster rate. This in turn increases the induced voltage and current in the cage and produces a stronger magnetic field in the rotor, thus, more torque to be produced. The increased current flow in the rotor causes increased current flow in the stator. This is why motor current increases as load is added.

The difference between the synchronous speed and actual rotor speed is called slip. Slip is directly proportional to the load on the motor. When loads are on the rotor’s shaft, the rotor tends to slow and slip increases. The slip then induces more current in the rotor and the rotor turns with more torque, but at a slower speed and therefore produces less CEMF.

In HEVs and EVs, the direction of motor rotation will need to change to meet certain operating requirements. In a three-phase AC motor, the direction of rotation can be changed by simply reversing any two of its stator leads. This causes the direction of the rotating magnetic field to reverse.

An electronic controller is used to manage the flow of electricity from the H V battery pack to control the speed and direction of rotation of the electric motor(s). The intent of the driver is relayed to the controller by use of an accelerator position sensor. The controller monitors this signal plus other inputs regarding the operating conditions of the vehicle. Based on this inputs, the controller provides a duty cycle control of the voltage levels to the motor(s).

Boost and power transistors of the inverter module

FIGURE. Boost and power transistors of the inverter module.

If the HEV or EV uses AC motors, an inverter module is used to convert the DC voltage from the HV battery to a three-phase AC voltage for the motor. This conversion is done by using sets of power transistors. The transistors PWM the voltage while reversing polarity at a fixed frequency. The inverter module is usually a slave module to the hybrid control processor. Often the inverter module is called the motor control processor since it not only provides for current modification but also motor control.