IPM (interior permanent magnet) motor is an Alternating Current (AC) synchronous motor, where the permanent magnets are inserted inside the rotor, while in brushless dc motors the permanent magnets are mounted on the surface of the motor, as illustrated in Figure 1.
All RoboteQ BL products with new G4 MCU generation and firmware version v3.0 and later support the IPM motor over the whole operating range, employing maximum torque per ampere and field weakening control algorithms.
Figure 1. (a) IPM motor (b) brushless dc motor configurations
Below the basic structural and operating differences between the IPM and BLDC motor:
- Reduce the risk of a magnet being peeled off by centrifugal force
- Higher total torque (IPM motor produces both magnetic and reluctance torque due to the rotor magnetic circuit design)
- Higher speed range (IPM motor has higher field weakening capability, due to the insertion of the permanent magnets inside the rotor)
- Reduce the risk of Permanent Magnets demagnetization
- Higher efficiency, especially at high speed range (lower permanent magnet eddy current losses)
- The permanent magnets are mounted on the surface of the rotor
- PMs are glued (provokes aging due to centrifugal forces and heat) or banded
- Lower total torque (BLDC produces only magnetic alignment torque)
- Lower speed range (BLDC motor has lower field weakening capability, due to the PMs location at the airgap)
- Lower efficiency (higher PM eddy current losses)
The technological transition from conventional to more electric transportation means has been facilitated over the last years, due to the intensified awareness shift towards energy efficiency and environmentally friendly technology platforms. This tendency has been particularly evident in modern automotive and servo applications. Electric traction applications require high performance motors with high torque density, high efficiency over a wide speed operating range and high temporary overload capability, characteristics entirely fulfilled in IPM motor configuration. A representative example emphasizing the IPM motor advantages is shown in below video, presenting the new Tesla Model 3's traction motor configuration.
Typical IPM motor torque-speed and power-speed curves are described in the figures 2 and 3, consisting of Constant Torque Region (CTR) and Constant Power Region (CPR). It is evident that in the CPR region the torque reduction for the same motor current is low, especially for lower loading conditions, due to the increased reluctance torque added to the magnet torque.
Figure 2. Typical IPM motor torque-speed curve.
Figure 3. Typical IPM motor power-speed curve.
Constant Torque Region IPM motor control algorithm
For optimal efficiency during the whole IPM motor operating range the control technique employed for the CTR – low up to rated speed region is the Maximum Torque Per Ampere (MTPA), where both torque (Iq) and field weakening (Id) currents needed to be adjusted for each required torque demand as shown in figure 4. These calculations are implemented internally in the controller, by utilizing the IPM motor equivalent d-q axis (Park transformation) circuit equations. In CTR, the only absolute limit applied in the motor is the motor thermal current maximum limit, as shown in figure 5 and the current vector is below current limit and voltage limit loops.
Figure 4. IPM motor required Iq, Id with produced electromagnetic torque.
Figure 5. IPM motor MTPA d-q current control strategy on CTR - low speed region.
In order to effectively operate the IPM motors at MTPA curve, the following data required to be set during controller configuration from Roborun+ utility:
Or by sending the configuration command for Single Channel Controllers:
^LD cc nn
where cc = motor channel and nn = motor d-axis inductance (H) * 1000000
^LQ cc nn
where cc = motor channel and nn = motor q-axis inductance (H) * 1000000
^VK cc nn
where cc = motor channel and nn = peak value of motor induced phase to phase voltage amplitude (produced back-emf) per krpm speed * 1000
IPM motor operation at CTR with MTPA current control is supported for all BL products supporting sinusoidal commutation with firmware version 2.1 and later.
Above values are necessary in order to calculate the optimal Id, Iq current commands needed to be applied to FOC motor operation. Typically, these values are included in each IPM motor technical datasheet. Alternatively, the d-q axis motor inductances can be calculated from “Motor characterization” tool at Roborun+ utility (see Roborun+ Utility User Manual for more details).
Furthermore, the switching mode should be set to “Sinusoidal” and the FOC gains should be inserted according to the method described in “FOC Gains Determination & Tuning” chapter at Section 8 of Motor Controllers Manual, considering as phase inductance the d-axis inductance Ld for flux proportional gain and q-axis inductance Lq for torque proportional gain, respectively. It is noted that the IPM motor operation algorithm applies only at close loop operating modes, including FOC torque and flux gains. In all other cases, IPM motor can operate as a brushless dc motor.
Consider the IPM motor with the following characteristics provided from manufacturer:
R (phase resistance) = 85 mOhm
Ld (d-axis inductance) = 580 uH
Lq (q-axis inductance) = 850 uH
The FOC gains for 50 Hz current loop bandwidth (typical for F3 MCU products) are calculated as follows:
Torque proportional gain = D-axis inductance (H) * Bandwidth = 0.000850 * 314 = 0.267
Torque integral gain = Phase resistance (Ohm) * Bandwidth = 0.085 * 314 = 26.69
Flux proportional gain = D-axis inductance (H) * Bandwidth = 0.000580 * 314 = 0.182
Flux integral gain = Phase resistance (Ohm) * Bandwidth = 0.085 * 314 = 26.69
Constant Power Region IPM motor control algorithm
When the motor operates in the high speed region - CPR, the MTPA d-q axis motor currents should be appropriately regulated, in order to satisfy the voltage limitation defined from battery dc voltage and PWM modulation method utilized, in conjunction with the current limitation, as shown in Figure 6. Therefore, the field weakening Id current should be increased, in absolute value, at this mode of operation to weaken the permanent magnet field and thus, the produced back-EMF of the motor.
In the controller, the field weakening current command is calculated according to the IPM motor equivalent d-q axis (Park transformation) circuit equations and additional stator voltage PI regulator.
Figure 6. IPM motor MTPV-field weakening control strategy on CPR - high speed region.
All RoboteQ BL products with new G4 MCU generation and firmware version v3.0 and later support the automatic field weakening operation, permitting the operation of IPM motor in the CPR within voltage limit.
In order to effectively operate the IPM motors at field weakening - CPR, the following data required to be set during controller configuration from Roborun+ utility:
Or by sending the configuration command from Console tab:
^FWVR cc nn
where cc = motor channel and nn = Field Weakening Voltage Ratio(%) * 10
^MXPW cc nn
where cc = motor channel and nn = Maximum