The brushless DC motor has a three-phase stator winding. The winding is single-voltage and is operated by the motor’s controller. It can’t operate without its controller.
The rotor has permanent magnets epoxied to its shaft. The magnets are reinforced with several wraps of high-strength fiberglass and epoxy tape.
Hall effect sensors are connected to the motor’s end bracket. A multipole magnet wheel is attached to the rotor shaft. The Hall sensor assembly has five to seven wires, which go to the controller in a separate conduit from the stator winding circuit. (The data carried by the control wires would be corrupted if they were in the same conduit as the stator leads.)
All brushless DC motors have ball or roller bearings. (It’s very important to monitor the condition of the bearings closely.) If a bearing breaks down, the rotor will be destroyed. Bearings in a brushless DC motor last longer than those in most other types of motors (because of the low shaft temperature). Permanent magnets eliminate the heat that results from a squirrel cage’s (magnetizing) winding current.
Operation of the Brushless DC Motor
The brushless DC motor operates from a variable-voltage variable-hertz speed controller. A Hall effect sensor (on the motor shaft) tells the controller the exact position of the magnets (relative to the stator poles). The rotating magnetic field is established when the controller alternately powers the stator’s three phases. TWO phases on and one phase off create a rotating magnetic field.
Three-Phase Servo Motors
Some servo motors operate much the same as the brushless DC motor. They have feedback devices called resolvers, encoders, tachometer generators, and Hall effect sensors, in combinations of two or more.
The servo motor’s controller tracks the rotor’s permanent magnets. It energizes the proper stator windings to position the shaft exactly as needed.
It can also control the exact speed of the motor.
To determine if a motor has permanent magnets, short two of its leads together and turn the shaft. Permanent magnets will cause resistance to rotation.
If the motor is disassembled, it is very important to put reference marks on the shaft and end bell. Marking the location of components simplifies reassembling.
If the alignment is not recorded, energize two winding leads with lowvoltage DC and controlled current (5 amperes or less). This will lock the rotor and provide a reference point for positioning the components. Most
servo controls compensate for some misalignment. The number of poles can also be determined this way.
These tiny motors and their controls will probably become throw-aways like other small motors.
Identifying the Three-Phase Induction Motor
Three-phase induction motors have many different connections.
Identification can be difficult, especially if there is no nameplate. Most of the connections and their function and application will be identified in this section.
Troubleshooting procedures found in Chapter 6 can be used on all the motors described in this section.
The number of leads a motor has can help identify its connection. The number can be from 3 to 18 or more. Most are divisible by 3, with the most common being the nine-lead dual-voltage motor.
Six-lead motors have many different variations and can be hard to identify. Wye-delta, constant-torque, constant-horsepower, and variabletorque motors are all six-lead motors (that use the numbers 1 through 6).
Connections encountered less frequently are two-speed one-winding, two-speed two-winding, part-winding start, wye-delta, 12-lead motors, dualvoltage multispeed, multimode, and European connections.
A motor with no nameplate presents a common identification problem. Sometimes a motor has been redesigned to a different horsepower, speed, or voltage (without the change being noted on the nameplate).
All motors should be tested using a current-limiting control, such as resistors. An unidentified motor should never have full power applied to it.
Nine-Lead Dual-Voltage Motors
A nine-lead dual-voltage motor will always be wye or delta connected.
The wye-connected nine-lead motor has three sets of two leads that light to each other and one set of three leads that light to each other.
The delta-connected nine-lead motor has three sets of three leads that light to each other. The multimode motor also has three sets of three leads that light to each other. There is, however, a big difference in the motor’s internal connections. This is explained under “Multimode Nine-Lead ThreePhase Motors,” earlier in the chapter. (This connection is found mainly in the oil field industry.)
Nine leads make it possible to operate a three-phase motor on either of two voltages. There is a 2-to-l difference between the high- and low-voltage
connections.
Nine-lead motors (both wye and delta) can also be connected partwinding start if they have the right style of winding. This is explained later in this section under “Concentric-Style Winding and Part-Winding Start Connection. ”
A variation of the nine-lead wye-connected motor involves a tenth lead. The tenth lead is used for joining the internal wye(s) when the wyeconnected motor is connected low voltage.
Troubleshooting these motors is covered in Chapter 6.
Three-Lead Motors
Three-lead three-phase induction motors are designed for one voltage and one speed. (Larger motors usually have three leads, because they are seldom designed for dual voltage.) Nine-lead motors (when rewound) can be changed to three leads if they operate on only one voltage; thus there are fewer connection problems than with nine leads. (The motor’s voltage rating should be stamped on the nameplate.)
Run the motor on low voltage (240 volts) with a current-limiting control if the voltage isn’t known. If it starts with much less inrush current than expected, it’s connected or designed for a higher voltage. The motor will have about one-fourth of its normal power and will start more slowly than normal.
It’s hard to determine the right voltage for small motors. They start quickly on low voltage—although connected for high voltage—because of their lightweight rotors. No-load amperes will be extremely low when run on low voltage.
The three-lead motor’s internal connection will be wye or delta. Both are tested with the same testing procedures found in Chapter 6.
Six-Lead Motors
When a motor has six leads, it can have many different connections. (Some examples are constant-torque, constant-horsepower, variable-torque, twospeed two-winding, part-winding start, and wye-delta connections.)
Six leads are sometimes used on single-speed induction motors, because there isn’t room between the motor’s core and shell for three larger leads. In this case, the leads of each phase should be secured in one lug so they stay together.
The data on the nameplate help to identify the six-lead motor’s connection. The number of circuits between leads is also helpful. The motor’s nameplate should have the horsepower, speed, and voltage. This information may be all that is needed to identify the motor’s connection. If the nameplate doesn’t give enough information, check the number of circuits between the leads. Circuit descriptions (in the following text) can then be used for identification.
A two-speed one-winding motor will have six leads that all show continuity to each other. They are numbered 1 through 6. Their nameplate will show different horsepower ratings (related to its design) but seldom will identify the motor’s type. They all have a 2-to-l speed rating and are oversized (for their horsepower).
Constant-torque motors have two horsepower ratings. (The high speed has twice as many horsepower as the low speed.) A constant-horsepower motor has the same horsepower rating for both speeds. Variable-torque motors have two horsepower ratings. (The low-speed horsepower is one-fourth that of the high-speed horsepower.)
The three connection schematics shown in Fig. 5.52 identify the types of connection used in these motors. (The constant-torque motor is the most common.)
Constant-Torque Connection
Connect the lead numbers (as shown on the constant-torque schematic for low speed) with T4, T5, and T6 open, and apply limited-current threephase power. If the motor runs well, connect it high speed. If it operates satisfactorily, run the motor with full rated voltage on both speeds.
Connected low speed, the motor may draw full or slightly above nameplate amperes. Connected high speed, it should draw approximately 50 percent less than the nameplate amperes, which would indicate that the motor is connected constant torque. This (constant-torque) motor has twice the horsepower on high speed that it has on low speed.
Variable-Torque Connection
Variable-torque and constant-torque motors use the same lead number combinations. If the motor’s power is especially low (on low speed),
FIGURE 5.52 (a) The constant-torque connection. (b) The variable-torque connection. (c) The constant-horsepower connection.