Very low torque at low speeds

Motor Drivers/Sensors
1

Hello everyone,

I am using the MP6532 (TSSOP) with Hall sensors and a 20 kHz PWM frequency. The PWM signal is generated by an MCU, which sends PWM and DIR signals to my board.

The MOSFETs I am using are 045N10N3, and each Rsense resistor has a value of 0.1 Ω (0R1).

The MCU firmware operates as follows: it starts by sending a 0% PWM duty cycle, then gradually increases the duty cycle with a ramp-up until reaching 100% speed. After that, the speed decreases with a ramp-down until the motor stops. The system then waits 2 seconds and repeats the cycle.

The motor rotates correctly, but a problem arises.

When the motor is stopped (while waiting to start rotating), I hold the shaft with my fingers. As soon as the motor begins to draw current, the motor does not rotate. I can easily prevent it from turning, which means the rotational force (mechanical torque) is extremely low.

I measured the current consumption by connecting a multimeter in series with the 48 V supply. When the motor reaches maximum speed, the multimeter reads 0.300 A.

I also connected a 100 µF / 63 V electrolytic capacitor in parallel with the 48 V supply.

The OCREF pin is connected to the VREG pin through a 100 kΩ resistor in order to disable the protection function.

Below are some technical specifications of the system:

  • Supply voltage (power supply and motor): 48 VDC

  • Motor power: 315 W

  • Motor current (no load): 0.8 A

  • Motor current (with load): 6 A

  • Number of poles: 4

  • Hall sensor pull-up resistors: 4.7 kΩ

Please help me understand why the torque is so low when the motor starts from standstill.

Also, please advise how to increase the starting torque.

Thank you in advance for your responses.

2

Hello, here are some suggestions, if the issue still persists.

  1. Physics: The torque is low because you implemented a simple voltage ramp and no feedback loop. Motor torque is proportional to the phase current (by motor constant Ki in Nm/Ampere), so low voltage→low current→low torque.
  2. Try to implement a PI speed control loop and use speed ramp, it helps to increase torque and start against higher load, friction, and braking fingers:). It makes difference if the controller + motor result in 1 Nm on the shaft instead of 10-20mNm at low voltage.
  3. (Some troubleshooting) You can try another combination of Hall encoder and motor phases connection. Sometimes PhA, PhB, PhC do not match to HallA, HallB, HallC, the motor rotates but doesn‘t bring its rated power. Noload speed and current will be far from the datasheet values, too.
  4. 0.1 Ohm current sensing resistors mean current limit with 5A amplitude, its not crucial but you won‘t reach 6A rated current.
  5. Compare PWM Signals at the outputs with your controller PWM at the driver input with an oscilloscope, for active phases the pulse width should match with the input. If the pulses at the output are shorter than it could be some issue with bootstrapping, limiting, dead time setting and so on.

Hope it helps, some feedback would be nice if you found the solution.