Voltage limit, >51V quickly overheating

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imc
Posts: 2
Joined: 22 Jul 2017, 23:22
Location: SF

Voltage limit, >51V quickly overheating

Postby imc » 22 Jul 2017, 23:38

While doing initial testing with the VESC powered by a adjustable supply and connected to a small ebike hub motor I found the MOSFETs to quickly overheat (5-15sec) at low constant currents (<0.5A), when above 50V. This overheating appeared to be entirely voltage dependent.

Running at 40-50V it seems fine, although running hotter than I might expect at about 60-80 degrees even with very low motor currents.

Can someone explain the cause of this? Are additional heat sinks recommended when running above 50V? I had understood voltage limit to be 60V, and I expected overheating only while pushing current.

The motor was being run while configured with FOC using current control mode from BLDC Tool.

VESC from diyelectricskateboard.com
v4.12 Hardware (Latest)
v2.18 Firmware (Latest)

rew
Posts: 943
Joined: 25 Mar 2016, 12:29
Location: Delft, Netherlands.

Re: Voltage limit, >51V quickly overheating

Postby rew » 23 Jul 2017, 07:08

I had the same happen with my own design. This happened when the dead time configured in the CPU was set too low for my setup. That in turn depends on the FET and the FET-driver. So you should have the DRV8302 for the fet-driver and IRFS7530 for the FETs. If it's just an implementation of the VESC, then there is not much they can have done about that. The only thing I can think of is that They might have gotten a wrong gate resistor installed. Or fake FETs.... So what are your gate resistors?

pf26
Posts: 309
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Voltage limit, >51V quickly overheating

Postby pf26 » 23 Jul 2017, 09:32

I think 60 Volts is enough to be nearly sure to kill the VESC4.
Above 50V, depending on your cable length and motor inductance, most likely the FETs already start to receive voltage spikes beyond their avalanche voltage, which they can absorb, causing more heat to be released. I don't think it is safe to run above 50V, especially with regen, since the DRV chip max voltage limit is very near to the FETs avalanche voltage, and it is quite easy to burn the DRV in these conditions (AFAIK, the higher the temperature, the greater the risk).
You could try to add a voltage suppressor selectec to activate in the 56Volts region, and see if it gets warm instead of your Fets.. This would validate my point of view.

imc
Posts: 2
Joined: 22 Jul 2017, 23:22
Location: SF

Re: Voltage limit, >51V quickly overheating

Postby imc » 27 Jul 2017, 04:28

Thanks for the replies guys

- As far as I can tell the FETs are legit, and the gate resistors are correct.
- I'm not running with regen, this is a geared hub motor with roller clutch. Thanks for the suggestion about the voltage suppressor. I may try this later on but for now I'm going to move to 12s and try to finish the build!

I'll update here if I come back to investigate this in the future.

pf26
Posts: 309
Joined: 28 Mar 2016, 14:37
Location: FR Valence

Re: Voltage limit, >51V quickly overheating

Postby pf26 » 04 Aug 2017, 11:12

Roller clutch will prevent large amounts of regen. But I fear the motor inertia alone could cause enough regen to kill sensitive components, for instance if you decrease dutycycle reference within a small amount of time. A few joules is enough to raise the voltage significantly (just limitted by caps) and a fast rotating motor has quite a few joules of inertia.


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