DRV8256: The motor heats up when using PWM to control the speed

Hi Cherry,

Cherry Zhou said:

while when the duty cycle is within 20% or 100%, the temperature of the motor is normal which is as the same as when driving a motor directly with a DC voltage

To clarify, are you saying that when the duty cycle is equal to 20% or 100%, the motor temperature is normal? 

Cherry Zhou said:

1) Is the temperature rise of the motor with a PWM signal duty cycle between 30% and 90% ok?

As soon as there is PWM switching, there will be switching losses as well as I2R losses that will cause high motor temperatures. When the duty cycle is close to DC (~100%), the switching losses disappear and only I2R losses mainly contribute to temperature rise.

Cherry Zhou said:

3) Is there any way to cool down?

I would try changing the decay modes as a start. Due to the way the different decay modes operates, some decay modes generate more PWM switching to regulate current and can lead to higher switching losses. 

Regards,

Pablo Armet

Hi Cherry,

Cherry Zhou said:

The DRV8256P is used to drive a 24-V DC motor with a maximum current of 2A, the chip's IN1 uses a PWM signal, 50 kHz, and in2 is normally low. Itrip is set to 2.5A and Toff is designed for 24us, slow decay mode.

When the duty cycle of the PWM signal is between 30% and 90%, the motor is hot enough with a temperature rise of 50 degrees C, while when the duty cycle is within 20% or 100%, the temperature of the motor is normal which is as the same as when driving a motor directly with a DC voltage.

So the motor is in speed control, and is there any load on the motor (pump or fan application)? With driving load, is it hard to think a full 24v applied to the motor (100%PWM) and it runs cooler than at 40%pwm, as there is much more I^2R loss in the motor wiring at 100% than 40%, regardless switching loss in the iron lamination. Just to be sure, are you talking about the heat on the driver chip or the motor body?

Btw, why driving with 50khz pwm instead of 20khz for less switching loss? Need 50Khz because the motor has very low inductance winding? R and L of motor?

Brian

Hi Pablo and Brian,

Thanks for your support.

Pablo Armet said:

To clarify, are you saying that when the duty cycle is equal to 20% or 100%, the motor temperature is normal? 

When the duty cycle is less than 20% OR equal to 100%, the motor temperature is normal. 

Pablo Armet said:

I would try changing the decay modes as a start. Due to the way the different decay modes operates, some decay modes generate more PWM switching to regulate current and can lead to higher switching losses. 

The customer has tried all decay modes with little change. And then they decided to use slow decay mode. 

Brian Dang said:

So the motor is in speed control, and is there any load on the motor (pump or fan application)? With driving load, is it hard to think a full 24v applied to the motor (100%PWM) and it runs cooler than at 40%pwm, as there is much more I^2R loss in the motor wiring at 100% than 40%, regardless switching loss in the iron lamination.

Temperature measured with motor at idle speed and without any load.

Is the decay current controlled?

Brian Dang said:

Just to be sure, are you talking about the heat on the driver chip or the motor body?

The heat generated by the drive chip is normal. The case here is the heat on the motor body, which usually starts to get hot after 20 minutes. After two hours of testing with a thermocouple, the temperature on the motor has reached 70 degrees.

Brian Dang said:

Btw, why driving with 50khz pwm instead of 20khz for less switching loss? Need 50Khz because the motor has very low inductance winding? R and L of motor?

The 50 kHz is used due to the 1k, 2k, 10k, 20k, 50k,100k test comparison of the drive frequency, it was found that the motor temperature had the longest rise time at 50kHz, so it was decided to drive the frequency at 50kHz.

R of the motor is approximately 1.8 ohms, L is not specified and is measured to be approximately 900uH @1kHZ.

Thanks and regards,

Cherry

Hi Cherry,

Cherry Zhou said:

The customer has tried all decay modes with little change. And then they decided to use slow decay mode. 

Because these decay modes only work if the Itrip current limit trigger. Since your motor is running no load, so I don't think it ever triggered the current limit at the stable speed.

Cherry Zhou said:

The DRV8256P is used to drive a 24-V DC motor with a maximum current of 2A,

Cherry Zhou said:

2) What is the possible cause?

3) Is there any way to cool down?

Could you help check this case? Thanks.

Why do you want to use 8256P with PWM mode for this application instead of 8256E with Phase/Enable mode?

The PWM mode will cause more heat lost on the motor for this reason: at every low time (0v) pwm cycle on IN1 input, with IN1 and IN2 = 0v, the motor is braked causing short circuit across the motor wires, and this cause high current in the motor to generate heat as I^2R, according to Table 7-3.

With PH/EN mode, with pwm input to EN pin, when the pwm signal is low, it causes the FETs in high Z  and so low current flow through the motor and so less heat generated.

With PH/EN mode, I think you should try with 20khz PWM and I think it uses less current than 50khz.

Brian

Hi Pablo and Brian,

Pablo Armet said:

If the motor driver temperature is normal, the root cause might be on the motor itself. I would test the motor temperature at 20kHz again and measure the temperature. As Brian mentioned, all signs point to heat due to switching losses. 

Pablo Armet said:

There are other ways to slow down the rise time. adding a simple RC circuit can help slow down the output edge rise. If motor temperature is desirable at 20kHz, you can place an RC filter to increase  rise time.

The customer would try to verify these 2 cases.

Brian Dang said:

Because these decay modes only work if the Itrip current limit trigger. Since your motor is running no load, so I don't think it ever triggered the current limit at the stable speed.

The ITRIP is set at 2.5A and although the motor is running unloaded and operates at very little current, the current when the motor is started will exceed 2.5A. Will this trigger decay mode? 

Brian Dang said:

Why do you want to use 8256P with PWM mode for this application instead of 8256E with Phase/Enable mode?

The DRV8256P was selected based on the customer's need for the motor to be in a brake state when it is stopped, as shown in the spec table 7-3, rows 2 and 5, which are not seen in table 7-2. 

Thanks and regards,

Cherry

Hello Cherry,

Cherry Zhou said:

The ITRIP is set at 2.5A and although the motor is running unloaded and operates at very little current, the current when the motor is started will exceed 2.5A. Will this trigger decay mode? 

Yes but only for the short duration of the inrush current. 

Update us once the customer tests the proposed solutions.

Regards,

Pablo Armet