VFD Link Question

[dereckbc]

Hey any of you guys got a good link on VFD's?

Got a Pet project. I want to buy another Electric Golf Cart and modify it to go faster, much faster

Anyway golf carts use a variety of motors depending on how old they are. Older ones use a Series Wound motor. Newer ones use shunt wound motors with regen braking. The really new ones use AC motors which I am not interested in right now.

What has me somewhat perplexed because motors are not my thing is I find controllers for all the motors and they all have different Current ratings for the same motor type. For example you can buy a 300 to 600 amp controller for a GE D392 36 volt motor. They are VFD types but something does not click in my head because the current is related to voltage and resistance. Not sure how you can push that many amps without altering the voltage. I know they use PWM, but that does not change the voltage?

So I want to understand a little bit more and need some input.

 

[LLSolutions]

If you want to use a VFD i think you're going to have to use an AC motor If you want to vary the frequency. Any control of the DC motor would probably just use good old resistance.

try this

http://www.joliettech.com/what_is_a_variable_frequency_drive.htm

 

[dereckbc]

If you want to use a VFD i think you're going to have to use an AC motor try this.

http://www.joliettech.com/what_is_a_variable_frequency_drive.htm

Thank you for the link and it helps, but someone correct me if I am wrong here, but VFD is not restricted to AC motors. I am more than happy to admit I do not Know It All after 30 years of tinkering.

VFD to my knowledge converts line AC to DC then changes to DC pulses of varying pulse width modulation (PWM). Doesn't matter if it is bipolar (AC) or DC. That is my Mystery with motors and controls.

I have a good grasp of PWM as it relates to telemetry and communications, but how the heck does it relate to current if the voltage of the pulses is set at say 48 volts and a set motor impedance of X. 36 or 48 volts can only push so many amps through X amount of impedance?

I understand AC motors are locked (synced) to the frequency applied which relates to silp, torque, and HP. But how in the heck do you push more amps than voltage and impedance dictates to get more current throug to improve torque with a fixed voltage and impedance?

In my limited capacity the only way to do it is to step up the voltage via Switch Mode Iversion of voltage and frequency.

I am missing something?

 

[Jraef]

Thank you for the link and it helps, but someone correct me if I am wrong here, but VFD is not restricted to AC motors.

Well, by strict definition, yes they ARE restricted to AC motors. VFD stands for Variable Frequency Drive, and when you have Frequency, it inherently means you have oscillation or in the case of electricity, alternation. But I know what you mean, it's just your semantics that is incorrect. A VSD, Variable Speed Drive, can mean either AC or DC, and DC drives can indeed be made with PWM output. The difference is that in a DC VSD, the PWM does not also alternate!

I know it doesn't answer your other questions, but I just wanted to clear that up.

 

[Besoeker]

Hey any of you guys got a good link on VFD's?

Got a Pet project. I want to buy another Electric Golf Cart and modify it to go faster, much faster

Anyway golf carts use a variety of motors depending on how old they are. Older ones use a Series Wound motor. Newer ones use shunt wound motors with regen braking. The really new ones use AC motors which I am not interested in right now.

What has me somewhat perplexed because motors are not my thing is I find controllers for all the motors and they all have different Current ratings for the same motor type. For example you can buy a 300 to 600 amp controller for a GE D392 36 volt motor. They are VFD types but something does not click in my head because the current is related to voltage and resistance. Not sure how you can push that many amps without altering the voltage. I know they use PWM, but that does not change the voltage?

So I want to understand a little bit more and need some input.

Assuming you're going to use a shunt would motor, there are two ways of adjusting the speed.
You can keep the shunt field fixed and vary the armature voltage. More volts gives more speed but, the motor is likely to flash over round the commutator if you use excessive voltage so there is limited scope for increasing the speed above the declared motor voltage.
Or, you can run the motor at rated armature and reduce the shunt field current. This will make the motor run faster but the available torque will be reduced - it's sometimes called constant horsepower mode. But, (and there had to be a but didn't there?) you might run into a problem with overspeeding the motor and risk a messy mechanical failure.

My input is "don't try this at home".

 

[dereckbc]

Well, by strict definition, yes they ARE restricted to AC motors. VFD stands for Variable Frequency Drive, and when you have Frequency, it inherently means you have oscillation or in the case of electricity, alternation. But I know what you mean, it's just your semantics that is incorrect. A VSD, Variable Speed Drive, can mean either AC or DC, and DC drives can indeed be made with PWM output. The difference is that in a DC VSD, the PWM does not also alternate!

I know it doesn't answer your other questions, but I just wanted to clear that up.

Yes I knew that part, and you are correct I interchaged the terms, like ground and earth. My bad

I just don't quite understand if you have say a 48 VDC series wound motor rated at say 9 HP how the heck a contoller can push 500 amps through it. I am missing something.

Anyway I bought a used 2001 EZ-GO Fleet Series Golf cart for dirt cheap. The standard controller is 175 amps and a 2 HP motor that goes about 14 MPH. Ordered a upgrade motor and controller. Motor HP = 9.1 HP, and the Controller is 600 amps. Suppose to get me 30 mph. How or what the 600 amps is a mystery because a 48 volt motor rated at 9.1 HP comes out to 140 amps when applied to my protien computer. :-?

 

[Besoeker]

Anyway I bought a used 2001 EZ-GO Fleet Series Golf cart for dirt cheap. The standard controller is 175 amps and a 2 HP motor that goes about 14 MPH. Ordered a upgrade motor and controller. Motor HP = 9.1 HP, and the Controller is 600 amps. Suppose to get me 30 mph. How or what the 600 amps is a mystery because a 48 volt motor rated at 9.1 HP comes out to 140 amps when applied to my protien computer. :-?

Your protien (protein?) calculator didn't take into account efficiency or any short-term overload capability that the motor might have - it's common for traction type motors to have a high overload capacity. Maybe that could explain it.

How are you going to get the 30 mph? Is the new motor rated for double the rpm of the original or are you going to change gear ratios?
For that matter, will the original gearing and mechanical parts be capable of handling more than four times the power?

 

[dereckbc]

Your protien (protein?) calculator didn't take into account efficiency or any short-term overload capability that the motor might have - it's common for traction type motors to have a high overload capacity. Maybe that could explain it.

You are right I did not account for efficiency, just 6800 watts/48 volts. Can?t find the efficiency specs for the motor. I have a little better understanding than before after talking to a rep at Altrax the manufacture for the controller. The high current rating is for the 0 and low RPM torque for only a few brief seconds. Still not clear how it can surge to 400, 500, or 600 amps.

How are you going to get the 30 mph? Is the new motor rated for double the rpm of the original or are you going to change gear ratios?
For that matter, will the original gearing and mechanical parts be capable of handling more than four times the power?

Yes the new motor GE D398 is rated 48 volt/9.1 HP/ 6100 RPM. The motor in it right now is rated 36 volt/ 2.2 HP/ 3500 rpm. However golf cart manufactures restrict the speed/rpm via the controller. If I just change the controller alone, then I can get the full 3500 rpm out of the existing motor and squeeze another 20% torque and RPM out of it.

Yes the axle is made to take the strain of the added power. With the new motor I will have 50% more torque, and if I wanted I could put a lift kit on the cart which would allow for bigger wheel/tires and get even more speed if I desire.

The whole reason I am doing this is I live in a gated community with a private golf course. My home is about 1-1/2 miles from the course and I have to use the roads to get there. Speed limit is 35 around here with a lot of hills. I want a cart that can go faster than a stock cart and not have problems pulling the hills. With the motor/controller combo I am getting, I will have a speed switch Low/High. In low speed will boost the torque but limit the speed to 15. High speed will reduce low rpm torque but will not limit Rpm.

So I am in a learning curve of motors and controls, and just curious how the controller work. What I do know is the controller is a PWM type operating at 18 Khz..