2.5HP DC Motor Controller Back EMF Issue

Thread Starter

Ri5ux

Joined Sep 5, 2017
42
Hi, I'm currently building a motor controller for a 2.5HP DC Motor that runs at 6700 RPMs @ 130V DC. This motor was pulled from a treadmill, and I'm currently in the process of building an electric go-kart, which the motor will be used in. I'm controlling the polarity of the motor using a relay, and controlling the speed of the motor via PWM signal, with N-Channel MOSFETs. I am using 3x RJH2009 MOSFETs to drive the motor. I'm also using an ER506 Superfast recovery diode as a snubber. At 64V DC input to the motor, there are no noticeable EMF spikes measured at the positive terminal of the battery (96V, 2.5Ah Lead Acid Battery); however, when moving up to 72V, EMF spikes of up to 1000+VDC leak through... this is obviously a problem, and it's obvious my diode is not sufficient. I feel a bit limited in terms of diode options at this point. I tried an HVR-1X-3 high voltage, ultra fast recovery diode pulled from a microwave, which supports up to 12,000V Peak, but the EMF spikes worsened, appearing at only 48V input.

I'm looking for a way to eradicate the EMF and work my way up to 96VDC input to the motor. I've seen others use a capacitor and low resistance resistor in parallel, but coming across a capacitor that can handle voltage spikes of 1000+ V also seems difficult. I'm by no means an expert, and am still learning and doing this as a hobby, so please go easy on me. I'm always willing to learn.

Here's a rough schematic of what I have set up, let me know if more information is needed, and I will definitely provide it :)
 

AlbertHall

Joined Jun 4, 2014
8,184
I can find no data on the RJH2009 MOSFET. Is that the correct number?
The HVR-1X-3 is a standard recovery diode not ultra-fast.
 

AlbertHall

Joined Jun 4, 2014
8,184
The MOSFET maximum voltage is a mere 200V. If it survived at the higher supply voltages I don't see how there can be 1000V spikes. What did you use to measure these spikes?
 

Thread Starter

Ri5ux

Joined Sep 5, 2017
42
The MOSFET maximum voltage is a mere 200V. If it survived at the higher supply voltages I don't see how there can be 1000V spikes. What did you use to measure these spikes?
I'm using a standard multimeter, I know I should be using an oscilloscope to measure these, but I don't have one capable of measuring voltage that high. The spikes don't appear to last long, but I of course have no way of knowing that for sure. After seeing two spikes of over 1000V, I shut down the circuit, but the MOSFETs still function on lower voltages with constant voltage. It appears to me that the spikes were short enough not to do permanent damage to the MOSFETs. Also, the reason I say 1000+V is because the multimeter drops out after that, so there are possibly even higher spikes, cant imagine they'd be too much higher though.
 

AlbertHall

Joined Jun 4, 2014
8,184
You have a very special meter if it reliably measure fast spikes. Don't believe those readings. The only way to check is with an oscilloscope.
 

Thread Starter

Ri5ux

Joined Sep 5, 2017
42
You have a very special meter if it reliably measure fast spikes. Don't believe those readings. The only way to check is with an oscilloscope.
So you're thinking it's over-exaggerating the spikes perhaps? I'll see if I can get my hands on an oscilloscope and get back to you.
 

crutschow

Joined Mar 14, 2008
23,140
It's the spikes across the MOSFET drain to source that's critical, not those seen at the battery.
 

Alec_t

Joined Sep 17, 2013
10,267
As a side issue, have you considered a way to prevent the relay state being switched while the FET is conducting? Suddenly reversing a 2.5HP motor without first bringing it to a halt is a recipe for disaster.
 

Thread Starter

Ri5ux

Joined Sep 5, 2017
42
As a side issue, have you considered a way to prevent the relay state being switched while the FET is conducting? Suddenly reversing a 2.5HP motor without first bringing it to a halt is a recipe for disaster.
Yes, I have, I am going to prevent the motor from spinning if it's currently spinning. The "controller" for this is arduino based.

It's the spikes across the MOSFET drain to source that's critical, not those seen at the battery.
I agree there, but it's important for there to be no spikes on the battery as well, as I have a LM7805 voltage regulator in the circuit, and I can't have high voltage spikes being fed into that and the rest of the circuitry.
 
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Thread Starter

Ri5ux

Joined Sep 5, 2017
42
An LM7805 can't operate from a 96V battery. :eek:
I won't be operating it on 96V. The battery has 16 "packs" with 3 cells per pack. 6V per pack. I have the LM7805 connected to the first two packs of the battery. In other words, I'm running it on 12V... however those nasty voltage spikes I mentioned also appear there.

I've decided I'm going to set up a voltage divider and monitor the voltage using the arduino, since I can't currently get a real oscilloscope.
 
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Thread Starter

Ri5ux

Joined Sep 5, 2017
42
Oddly, the voltage spikes stopped... The multi-meter isn't acting the way it was previously anymore. I took off two of the RJK2009 MOSFETs, they must have been causing it. Also checked by monitoring the voltage using the arduino with a voltage divider, it stayed within 3 volts range.
 
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Thread Starter

Ri5ux

Joined Sep 5, 2017
42
Just figured I'd update the status on this. The voltage spikes were still there. I bought an oscilloscope a few months ago and ironed out all of the issues. It is currently working as expected. Added a better snubber circuit. Changed the power supply situation out as well. No longer using the 7805 voltage regulator. I'm using a switch mode power supply which I've modified to run on the 96V batteries. Basically completely restarted the entire project. Now using a 300A 600V IGBT I picked up for cheap.
 
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