GB/T 21418-2008General specification for permanent magnet brushless motor system (English PDF)

3.1 Permanent magnet brushless motor system

A control system built around a permanent magnet brushless motor as the actuating element and formed on the basis of feedback information such as position, speed and torque.

The system comprises three parts: the permanent magnet brushless motor, the sensing and the driver (see Figure A.1 in Annex A).

The system has four basic modes of operation: open-loop running, torque control, speed control and position control (see Figures A.2 to A.5 in Annex A).

3.2 Driver

An electrical device which accepts control commands and is able to carry out torque, speed or rotor position control of the motor.

According to the way its control circuit and software are implemented, a driver may be divided into analogue control, mixed digital and analogue control and fully digital control; according to its drive method it may be divided into square wave drive and sine wave drive.

3.3 Permanent magnet brushless motor

A permanent magnet motor in which commutation or current control is carried out by an electronic circuit relying on rotor position information.

Permanent magnet brushless motors come in two forms, sine wave and square wave: a motor whose drive current is a square wave is normally called a permanent magnet brushless direct current motor, and a motor whose drive current is a sine wave is normally called a permanent magnet alternating current servo motor. According to the type of sensing used, motors may be divided into sensored motors and sensorless motors.

3.4 Sensor / sensing

The sensor or the technique used to detect the position, the speed or the current of a permanent magnet brushless motor. Sensors include proximity switches, photoelectric encoders, resolvers, Hall elements and current sensors.

3.5 Rated power

The maximum continuous output power of the system, the motor or the driver under specified conditions.

3.6 Peak torque

The maximum torque the motor is able to output under specified conditions. Working for a short time at the peak torque will not damage the motor or cause an unrecoverable loss of performance.

3.7 Peak current

The value of the line current when the motor outputs its peak torque under specified conditions. This current is a peak value when the motor runs on square wave and a root mean square value when it runs on sine wave.

3.8 Maximum continuous torque (rated torque)

The maximum torque the motor is able to output under specified conditions such that, running continuously at that torque, the temperature of the motor windings and the temperature of the power devices in the driver will not exceed their maximum permissible values and neither the motor nor the driver will be damaged.

3.9 Maximum continuous current of motor (rated current)

The value of the line current when the motor outputs its maximum continuous torque (rated torque) under specified conditions. This current is a peak value when the motor runs on square wave and a root mean square value when it runs on sine wave.

3.10 Static friction torque

The resisting torque that has to be overcome to start the rotor turning from any position when the motor is not energised.

3.11 Thermal resistance of motor

The impedance to heat flow between the motor windings and the motor casing.

3.12 No-load speed

The steady-state speed of the motor in the no-load condition under specified conditions.

3.13 Maximum permitted speed

The maximum design speed of the motor under the condition that the dielectric strength of the electrical insulation and the mechanical strength are assured.

3.14 Rated speed

The speed at which the motor outputs its maximum continuous torque while running at its rated power.

3.15 Torque constant

The average electromagnetic torque produced by the motor per unit of current supplied to it, under specified conditions.

3.16 Back EMF constant

The value of the line electromotive force induced in the armature winding per unit of speed when the motor windings are open circuit, under specified conditions.

3.17 Resistance

The direct current resistance of each phase winding of the motor at 20 degrees Celsius.

3.18 Inductance

The inductance across the stator winding when the motor is at rest.

3.19 Continuous duty zone

The working region of Figure 1 that lies within the maximum continuous torque, the maximum permitted speed and the rated speed (the unshaded region of the figure). It is the region bounded by the heating of the motor, by the mechanical strength under the effect of centrifugal force and by the limiting working conditions of the commutation or of the driver. Running inside this region, neither the motor nor the driver will exceed its maximum permissible temperature.

Note 1: the relation between the rated power P(N) in watts, the rated speed n(N) in revolutions per minute and the maximum continuous torque T(N) in newton metres is P(N) = T(max) x n(N) / (60 / 2 pi).

Note 2: motors fitted with other accessories such as oil seals or brakes should be derated in use.

Figure 1 - Diagram of the working zones. The figure plots torque in newton metres against speed in revolutions per minute, and marks the peak torque T(P), the maximum permitted speed n(max), the rated speed n(N) and the maximum continuous torque T(N).

3.20 Intermittent duty zone

The region of Figure 1 that lies below the peak torque and above the maximum continuous torque (the shaded region of Figure 1). Working for a short time in this region, the motor current is greater than the maximum continuous current but the motor windings will not be damaged within a certain time.

The permissible duration of the short-time overcurrent is determined by the thermal time constant of the windings.

3.21 System efficiency

The ratio of the output power of the motor to the active input power of the driver.

3.22 Driver efficiency

The ratio of the active output power of the driver to the active input power of the driver.

3.23 Motor efficiency

The ratio of the output power of the motor to the active input power of the motor.

3.24 Thermal time constant

The time required for the temperature rise of the motor windings to reach 63.2 per cent of its steady value, at constant power loss and under specified conditions.

3.25 Rated input voltage

The root mean square value of the voltage applied at the supply terminals of the driver under specified conditions.

3.26 Rated frequency

The frequency of the input alternating voltage.

3.27 Rated input current

The root mean square value of the input current of the driver when the driver delivers its rated output current, under specified conditions.

3.28 Rated continuous output current

The maximum root mean square current the driver is able to deliver continuously without exceeding the specified limits, under specified conditions.

3.29 Overload capability

The maximum root mean square current that can be delivered for a specified time without exceeding the specified limits, under specified conditions.

3.30 Electromagnetic compatibility (EMC)

The ability of the system, the driver or the motor to work correctly in a specified electromagnetic environment without producing electromagnetic disturbance that anything in that environment could not tolerate.

3.31 Closed loop control

Control in which, in order to bring the feedback variable to the reference variable, the system compares the feedback variable with the reference variable and uses the difference between the two to set the manipulated variable. According to which quantity is controlled it is divided into three kinds: position control, speed control and torque control.

3.32 Speed ripple

With the motor running in the steady state, if the maximum instantaneous speed is n(max) and the minimum is n(min), the speed ripple is (n(max) - n(min)) / (n(max) + n(min)) x 100 per cent. (1)

3.33 Torque ripple

With the motor running in the steady state, a constant load is applied to the motor and the output torque is measured continuously; if the maximum instantaneous torque is T(max) and the minimum is T(min), the torque ripple is (T(max) - T(min)) / (T(max) + T(min)) x 100 per cent. (2)

3.34 Speed difference between positive and negative directions

For a system with a closed speed loop, without changing the magnitude of the speed command and changing only the direction of rotation of the motor, the average speeds n(cw) and n(ccw) in the two directions are measured under no load and the speed difference between the positive and negative directions is calculated as the absolute value of (n(cw) - n(ccw)) divided by (n(cw) + n(ccw)), multiplied by 100 per cent. (3)

3.35 Speed regulated ratio

With the system at rated speed and no load, and with only the supply voltage varying, or only the ambient temperature varying, or only the load varying, the change in the average speed of the motor expressed as a percentage of the rated speed is called respectively the speed regulation for voltage variation, the speed regulation for temperature variation and the speed regulation for load variation.

3.36 Speed ratio

The ratio of the lowest no-load speed n(min) at which the system still meets the specified speed regulation and the specified torque ripple (or speed ripple) to the rated speed n(N) is called the speed ratio D, so that D = n(min) / n(N). (4)

3.37 Overshoot

For a step response, the maximum instantaneous deviation of the output variable from its final steady value, usually expressed as a percentage of the difference between the final steady value and the initial steady value. See Figure 2.

3.38 Response time following a step change of reference input

The system input is taken from zero to a positive step corresponding to n(N), and the time from the start of the step signal until the speed first reaches 0.9 n(N) is measured (Figure 2); the system input is then taken from the value corresponding to n(N) down to zero as a negative step, and the time from the start of the step signal until the speed first reaches 0.1 n(N) is measured. The times specified in these positive and negative step processes are called the speed response time following a step change of reference input.