Synchronous motors types and controls

There are many types of motors and control schemes but it's always difficult to understand which are the most suitable for every application. Let's clarify it and suggest the best combination for each of them.

synchronous

motor

Introduction

Which is the best way to control a 3-phase synchronous motor? This is a very complex question, with many possible answers depending on the type of motor to be driven and the required mission. Let's take a look to some of them and their typical applications.

Synchronous motors properties

Motor type	Motor Inductance	Switching Frequency	Comment
PMSM IPM and SPM	Tens to Hundreds of uH	High	➞ High switching frequency needed to reduce current ripple ➞ Motor losses are kept low at the expenses of inverter switching losses ➞ These motors are made to be reactive and are typically used in aerospace, automotive, smaller powertrain and actuator systems
PMSM IPM and SPM	mH	Low	➞Switching frequency can be chosen in the 1kHz-5kHz typical range ➞ The motor inductance is high enough to filter current ripple, so inverter exhibits lower switching losses ➞ These motors exhibit lower dynamic responses and are typically used in industrial, power conversion systems and railway applications
Reluctance	mH	Low	➞ Low switching frequency ➞ Motor losses are not so low ➞ Cheap ➞ Reliable and durable (no magnet) ➞ Low dependency from temperature ➞ Torque shows high ripple ➞ These motors are used in industrial and heavy loads systems

Suitable control methods

Motor type	Inductance	Switching Frequency	Operating speed	Control schemes
PMSM	Low [tens to hundreds uH]	High [10kHz-50kHz]	Low RPM [<5kRPM]	▶ SPWM ▶ SVPWM ▶ THinj-PWM ▶ FOC
	Low [tens to hundreds uH]	High [10kHz-50kHz]	High RPM [up to 50kRPM]	▶ SPWM ▶ SVPWM ▶ THinj-PWM ▶ FOC
	High [hundreds uH to tens mH]	Low [1kHz-10kHz]	Low RPM [<5kRPM]	▶ FOC ▶ Direct Torque Control ▶ Adaptive Control ▶ VVVF
	High [hundreds uH to tens mH]	Low [1kHz-10kHz]	High RPM [<10kRPM]	▶ FOC ▶ Direct Torque Control ▶ Adaptive Control ▶ Six-Step ▶ VVVF
Reluctance	High [tens to hundreds mH]	Low [1kHz-10kHz]	Low RPM [<5kRPM]	▶ SPWM ▶ SVPWM ▶ THinj-PWM ▶ FOC
Reluctance	High [tens to hundreds mH]	Low [1kHz-10kHz]	High RPM [<10kRPM]	▶ Direct Torque Control ▶ Adaptive Control ▶ VVVF ▶ VFD

Control schemes properties

Control Scheme	Description	Max phase voltage	Load step response	Efficiency	Applications
SPWM	Very simple scheme, based on triangle-sine PWM. Overmodulation not possible, so maximum power is limited. Required switching frequency is quite high to ensure proper control of the electrical machine. Motor losses are reduced (reduced phase current ripple) but inverter switching losses increase.	$$\frac{V_{DC}}{2}$$	Average to fast	Average	▶ Cheap electronics ▶ Consumer electronics
SVPWM	More advanced scheme, used as starting point in many other control methods. Overmodulation is possible, allowing thus higher output power and higher phase currents. Again, motor losses are kept low at expenses of inverter switching losses.	$$\frac{V_{DC}}{\sqrt{3}}$$	Average to fast	High	▶ Automotive ▶ Industrial ▶ Powertrain
DPWMx	More advanced scheme, used to reduce losses in the inverter and increase the overall system efficiency. It could introduce unwanted acoustic noise or vibrations since switching is periodically suspended during the electric period.	$$\frac{V_{DC}}{\sqrt{3}}$$	Fast	Very high	▶ Automotive ▶ Industrial ▶ Powertrain
FOC (Field Oriented Control)	Advanced control scheme. It controls the output torque by controlling current in dq-axis.	$$\frac{V_{DC}}{\sqrt{3}}$$	Fast	High	▶ Automotive ▶ Industrial ▶ Powertrain
DTC (Direct Torque Control)	Advanced control scheme. It controls the output torque by controlling current in dq-axis.	$$\frac{V_{DC}}{\sqrt{3}}$$	Fast	High	▶ Automotive ▶ Industrial ▶ Powertrain
Six-Step	Simple control scheme. It gives the maximum voltage to the motor phase. Inverter switching losses are reduced, whereas motor losses are kept low thanks to high motor inductance (current ripple is quite low). However, high acoustic and electromagnetic noise is generated, due to the lower switching frequency, so the higher control period. Torque exhibits an important ripple too.	$$V_{DC}$$	Average	High	▶ Industrial ▶ Railway ▶ Powertrain