Brushless DC motors for rigid disk drives

George P. Gogue

G2 Consulting,
Beaverton, OR 97005


The use of brushless dc motors in rigid disk drives has become an important segment of the motor market. The flexibility and simplicity of design makes these motors ideal in an application where management of space is of great importance. The switching circuits of these motors add controllability to the performance and make a high degree of speed-control a possibility. The unique operating conditions in disk drives are reflected in the specifications describing these motors. In addition to the electrical and mechanical requirements, the materials used are strictly controlled and specified.

Brushless motors are selected for rigid disk drives for several specific reasons. The availability of a low-voltage dc supply in these drives is high among these reasons. Space limitations require the motors to be flexible in their diameter to axial length ratio and the brushless motor inherently allows that. It is also adaptable to accurate speed control (+/-0.1%) by virtue of the electronic circuitry required in these computer peripherals and, therefore, constitutes no substantial burden on the designer. Moreover, the technology involved in the motor switching circuit is compatible with the rest of the electronic circuitry.

Figure 1. EVOLUTION OF SPINDLE MOTOR FOR HARD DISK DRIVE

Figure 1 shows the two most popular designs currently in use. The cantilever design is the more conventional while the in-hub design is becoming the more widely used. In both designs, the disks are mounted on the hub (upper) side of the assembly. In the cantilever design the electric motor is on the other side (lower) of the mounting bracket, whereas in the in-hub design the electric motor is actually inside the hub.

The head-disk area (HDA) is isolated from the electric motor by a combination of mechanical and magnetic barriers called seals. These seals ensure that no contaminants migrate into the HDA.

The electric motor is designed with the rotating member on the outside, providing a support for the permanent magnets. The lamination stack and winding assembly constitute the stationary member at the core of the electric motor. The bearing assembly is situated more closely to the winding assembly in the in-hub design than in the cantilever design. The rotational forces in the cantilever design are generated in a different axial location from the bearings and are transferred from the electric motor to the hub via the shaft.

The permanent magnets mounted on the inside of the rotor-cup are ceramic or rare earth magnets. These materials are economically acceptable and at the same time produce as much magnetic flux as can be carried by the steel.

The winding consists of several coils of copper wire, wound to produce 2, 3 or 4 phases. The coils are usually wound on a slotted lamination stack of magnetic steel.

The only electronic components required inside the motor are 1, 2 or 3 sensors (depending on the type of switching) to produce position-dependent pulses essential for commutating the winding. These sensors are usually Hall sensors producing either sinusoidal or digital pulses.

The complete system of brushless motor and electronic driver is shown in the block diagram of Figure 2.

Figure 2

The electronic driver consists of both the logic circuit and power switching devices. The speed-control circuit is designed to operate either on pulse-width-modulation (PWM) or linear control. All the above circuits, however, require feedback signals from the motor and they are rotor-position dependent. Hall sensors in the vicinity of the rotating magnets provide the signals.

Types of switching circuit

A common situation facing the designer of these drives is which type of brushless motor switching is most suitable for the application. The main types are:

a) 1-phase switching

b) 3-phase switching

c) 4-phase switching

Each of the above types can be either unipolar or bipolar. This means that for the 1-phase switching either one winding or both windings are used at any one time. For the 3-phase switching it means either one winding or two windings are used at any one time. The 4-phase switching can be achieved either by switching one winding or two windings on at any one time. The selection of winding configuration is based on all of the following parameters:

a) The space, i.e. diameter and axial length, available for a motor capable of satisfying the output requirements.

b) The required starting torque at some or all of the possible starting positions.

c) The starting current available from the power supply and whether this current is controlled electronically or by the motor winding resistance.

d) The maximum number of power switching devices allowed as well as their type which determines the voltage drop across them.

e) The maximum tolerable torque ripple under running condition. However, the speed-control circuit is also important in minimizing this ripple.

f) The electrical and mechanical time constants required are based on switching and acceleration requirements. A high inductance can cause switching problems hence is specified by a low electrical time constant. A high motor moment of inertia can result in a long acceleration time if the mechanical time constant is not specified.

g) Non-repetitive runout of the hub can be affected by the type of switching circuit. The speed-control circuit is also a factor in limiting the runout to a low level.

h) The temperature rise of the motor and the switching circuit reflects directly on the temperature rise of the adjacent parts in the drive unit. Thus the limit on heat dissipation can affect the choice of winding resistance and continuous current and, hence, the type of winding.

i) The disk mounting configuration on the motor hub determines which type of motor winding is most suitable. The cantilever and in-hub type motors make use of the available space in different ways by having different diameter to length ratios.

A different comparison can be made between the three major types of switching circuit based on the ratios of minimum and average torque to the maximum torque. These ratios reflect on the starting torque obtainable from the motor as well as on the percentage torque ripple. Table 1 shows this comparison and also gives the number of switching devices in each circuit.

Table 1

The equations used in calculating the torque ratios in Table 1 are as follows:

In both Equations (1) and (2), m is the number of commutations per pole-pair.

Changing from unipolar to bipolar switching for any of the three major types may, however, require additional devices beyond the number given in the table. For example in the 1-phase motor, a fifth device of power rating equal to the other four is required. Equations (1) and (2) assume that the emf waveforms of the motors are sinusoidal. This, however, is not always the case. The values for the above 2 equations, given in the table are in fact lower than would be obtained from motors with trapezoidal emf waveforms and the percentage ripple is higher. This is why the trapezoidal waveforms are desired in brushless motor designs.

No account is taken in Table 1 of the effect of the reluctance torque on the total torque, and the ratios given are solely due to the electromagnetic torque. The reluctance torque is the bidirectional torque produced by the interaction of the magnets with the lamination stack, and the electromagnetic torque results from the interaction of the current-carrying winding with the magnetic field.

The requirement for reluctance torque varies with the type of winding configuration. A 1-phase motor requires a high reluctance torque to facilitate starting at the commutation instants when the electromagnetic torque is zero. Whereas in a 3-phase or 4-phase motor it is desirable to minimize the reluctance torque, resulting in an increase in the minimum torque.

Other types of switching circuit, not listed in Table 1, can be used with 3-phase and 6-phase delta-connected windings. Delta-connected windings generally have a low terminal resistance for equivalent values of back emf constant (V/krpm or V/rad/sec).

The majority of computer peripheral applications use 3-phase star-connected motors with 4 or 8 poles. The low torque ripple can be achieved with a reasonable cost of electronics compared to, for example, a 6-phase motor. The switching circuit used with any motor can be a combination of bipolar start and unipolar run. This results in a high starting torque from the motor or, alternatively, a smaller motor delivering the specified starting torque, whichever is more beneficial. Since the running condition of the motor is at very light load, the unipolar operation may be acceptable. The benefits would be in reducing the number of switching devices used and hence the total power dissipation.

Brushless motor performance

The performance of a brushless DC motor can best be described by its speed/torque characteristics. These are results of tests performed on the motor between no-load and stall conditions. Figure 3 shows typical speed/torque and current/torque characteristics of a brushless motor.

Figure 3

The linear variation of speed with torque is typical of brushless DC motors as seen from the following equation:

n = speed in krpm

V = supply voltage in Volts

= emf constant in V/krpm

= generated torque in oz-in

= motor terminal resistance in Ohms

= torque constant in oz-in/A

m = number of power switching devices ON

= saturation voltage across each switching device in Volts

Equation (3) does not include the effect of high current on the speed/torque curve known as armature reaction. The magnetic flux produced by the winding deforms the main magnetic field causing the torque to drop faster at high values of current. Equation (4) shows the linear relationship between torque and current, again ignoring the effect of armature reaction at high values of current.

where I = current to motor in A

A useful equation for the estimated required can be developed using Equations (3) and (4) and Figure 3. The unknown values in this Equation 5 can usually be assumed based on prior experience or published data.

where:

= no-load current in A

= generated torque at stall in oz-in

= no-load speed in krpm

An estimate of the required starting torque , can be made using Equation (6) at the desired acceleration time t.

= motor moment of inertia in

= load moment of inertia in

= angular acceleration in rad/

= friction torque of motor in oz-in

= load torque and stiction in oz-in

The angular acceleration can be calculated from the following equation:

where t is acceleration time in seconds to reach final speed n in rpm linearly.