Types Of Inverter

Kazuya SHIRAHATA

Oriental Motor Co., Ltd. offers a wide variety of speed control motors. Our speed control motor packages include the motor, the driver (controller), and a potentiometer which allows for easy speed control adjustment. There are three speed control motor product groups. The "AC speed control motor unit" that uses the most popular single-phase capacitor-run induction motor, the small and highly efficient "Brushless DC motor unit", and the "Inverter unit" that combines a three-phase induction motor with a small inverter. This article explains the structure, principle of speed control and the features of each product group, and our standard products are introduced.

1. Introduction
A large number of motors are being used for general purposes in our surroundings from house-hold equipment to machine tools in industrial facilities. The electric motor is now a necessary and indispensable source of power in many industries. The function and the performance required for these motors are wide-ranging. When focusing attention on the speed control segment of the motor market, servo and stepping motors control their speed with a pulse train, while the induction motor and the brushless DC motor control speed with an external resistor and/or DC voltage.
This article explains the structure, the speed control principle, and the features of the following three product groups that can control the speed relatively easily by using an analog input.

• AC speed control motor
• Brushless DC motor unit
• Inverter unit

2. Speed control methods of the various speed control motors
The output control method of a speed control circuit can be divided roughly into two groups: phase control and inverter control, which make up the product groups shown in Fig. 1.

Fig. 1 Classification of speed control motors

2.1. AC speed control motors

2.1.1. Construction of motor
As shown in Fig. 2, the construction of the single phase and three-phase induction motors includes a stator where the primary winding is wound and a basket-shaped, solid aluminum die cast rotor. The rotor is low-cost because the structure is simple and does not use a magnet.

Fig. 2 Construction of induction motor

When the speed of this motor is to be controlled, a tacho-generator is used to detect the speed and is attached to the motor as shown in Fig. 3. The tacho-generator is made of a magnet connected directly to the motor shaft and a stator coil that detects the magnetic poles, and generates an AC voltage at 12 cycles per revolution. Since this voltage and frequency increase with a rise of the rotational speed, the rotational speed of the motor is controlled based on this signal.

Fig. 3 AC speed control motor system

2.1.2. Principle of speed control
Rotational speed N of an induction motor can be shown by the expression (1). When the voltage applied to the motor is increased and decreased, the slip s changes, then the rotational speed N will change.
N= 120·f ·(1-s)/P · · · · · · · · · · (1)

N: Rotational speed [r/min]
F: Frequency 〔Hz〕
P: Number of poles of a motor
S: Slip

In the case of an induction motor as shown in Fig. 4, a stable range and an unstable range exists in the Rotational Speed - Torque curve. Since it is impossible to reliably operate in the unstable range, simple voltage control (open loop control) is limited to controlling the speed in a narrow range like, N1~N3 in Fig. 5. To make it possible to operate reliably even in the above-mentioned unstable range, it is necessary to detect the rotational speed of the motor and use a voltage control mechanism (closed-loop control) that reduces the speed error when compared to a set value.

Fig. 4 Rotational speed - torque characteristics of induction motors

Fig. 5 Simple voltage control

Available voltage control methods include control by a transformer or by phase control. Fig. 6 shows when voltage is controlled by using a transformer. This method is not so easy to do with an AC speed control motor. Alternately, the AC voltage can be adjusted by setting the ON/OFF time of every half cycle of the AC voltage (50 or 60Hz) applied to the motor using a switching element (thyristor or triac) that can directly turn on and off the AC voltage as shown Fig. 7 and Fig. 8. Speed control is obtained by the phase control method by controlling the r.m.s. value of the AC voltage.

Fig. 6 Voltage change by transformer

Fig. 7 Voltage change by phase control

Fig. 8 Triac control circuit

This AC speed control method can provide steady speed control by closed-loop phase control even in the unstable range.
Fig. 9 shows the configuration of the speed control system for an AC speed control motor in a block diagram.

Fig. 9 Block diagram of AC speed control motor system