How to drive an induction motor on the cheap?

akio123008 · April 3, 2019

I'm planning to build an electric go-kart. I've already got the kart, now I just need to build the drive train. I was planning to get a brushless motor combined with an electronic speed controller to do this, which would be a fine solution, but then I spotted some nice AC induction motors in my dad's shed. He doesn't use them for anything, so I thought it'd be a nice idea to get one of those on the vehicle.

Here's my problem though; how would I drive such a motor? I know how 3 phase induction motors work, and how they're different from (synchronous) brushless motors, and that they can therefore not be driven using an ESC. Even if they could, these motors are designed to run on 230V per phase, meaning a normal ESC doesn't provide the right voltage. Normally induction motors are driven using VFD's but those are insanely expensive so that's not an option. Even a fixed frequency inverter to drive such a motor is hugely expensive. This leads me to my 2 questions:

1. Why the hell are ESCs (which are basically inverters/VFDs and use the same electronics) so incredibly cheap (even very high power ones) compared to VFDs and inverters?

2. Is there a more simple and cheap solution that I can use to drive an induction motor?

johnnyappleseed · April 4, 2019

I've worked with BLDC motors before, but only have a little knowledge about induction motors, so this is just speculation:

13 hours ago, akio123008 said:

1. Why the hell are ESCs (which are basically inverters/VFDs and use the same electronics) so incredibly cheap (even very high power ones) compared to VFDs and inverters?

Not disagreeing with you, just curious what price range/ power rating you are looking at? I've found powerful ESC's to be relatively expensive, at least compared to brushed DC motors when you get into the mid power to high power range. A group I'm a part of bought some older version of these because they were cheap compared to others of comperable specs (link).

A BLDC is pretty similar to an induction motor. I imagine some of the extra cost is the AC-DC rectification, though if you are looking at DC source VFC for running off batteries you wouldn't have this. I haven't examined them, but probably some real beefy capacitive filtering is involved for the rectification. Additionally you may have answered your own question about cost:

13 hours ago, akio123008 said:

Even if they could, these motors are designed to run on 230V per phase, meaning a normal ESC doesn't provide the right voltage.

I don't really know enough about induction motors to answer number 2.

bob345 · April 4, 2019

13 hours ago, akio123008 said:

I'm planning to build an electric go-kart. I've already got the kart, now I just need to build the drive train. I was planning to get a brushless motor combined with an electronic speed controller to do this, which would be a fine solution, but then I spotted some nice AC induction motors in my dad's shed. He doesn't use them for anything, so I thought it'd be a nice idea to get one of those on the vehicle.

Here's my problem though; how would I drive such a motor? I know how 3 phase induction motors work, and how they're different from (synchronous) brushless motors, and that they can therefore not be driven using an ESC. Even if they could, these motors are designed to run on 230V per phase, meaning a normal ESC doesn't provide the right voltage. Normally induction motors are driven using VFD's but those are insanely expensive so that's not an option. Even a fixed frequency inverter to drive such a motor is hugely expensive. This leads me to my 2 questions:

1. Why the hell are ESCs (which are basically inverters/VFDs and use the same electronics) so incredibly cheap (even very high power ones) compared to VFDs and inverters?

2. Is there a more simple and cheap solution that I can use to drive an induction motor?

The main cost difference come down to demand and cost of manufacturing the motors. AC induction motors are much more expensive to design and manufacture due to the sheer amount of EMF modeling and complex manufacturing processes needed produce an efficient rotor. VFD's are also MUCH more complicated pieces of gear than your run of the mill esc. which adds to the cost. The major difference i would say though is that esc's are made by the millions for the RC market so there is just so much more demand. ESC's are incredibly simple in design in comparison to a vfd as well. AC induction motors use primarily in industrial automation because of their accuracy and ease of control with a proper system.

akio123008 · April 4, 2019

5 hours ago, johnnyappleseed said:

Not disagreeing with you, just curious what price range/ power rating you are looking at? I've found powerful ESC's to be relatively expensive, at least compared to brushed DC motors when you get into the mid power to high power range. A group I'm a part of bought some older version of these because they were cheap compared to others of comperable specs (link).

On chinese websites you can get say a 100A 24V speed controller for like 30 USD. A VFD with the same power output would easily cost over 100.

5 hours ago, bob345 said:

The main cost difference come down to demand and cost of manufacturing the motors. AC induction motors are much more expensive to design and manufacture due to the sheer amount of EMF modeling and complex manufacturing processes needed produce an efficient rotor. VFD's are also MUCH more complicated pieces of gear than your run of the mill esc. which adds to the cost. The major difference i would say though is that esc's are made by the millions for the RC market so there is just so much more demand. ESC's are incredibly simple in design in comparison to a vfd as well. AC induction motors use primarily in industrial automation because of their accuracy and ease of control with a proper system.

AC induction motors aren't more expensive at all, they're actually much cheaper. That's why I've got a bunch of them right here ready to use. Synchronous (aka BLDC but I hate that term) motors that run on ESCs are more expensive because of the permanent magnets inside them, induction motors don't have those, making them actually very affordable. The problem is that the equipment to drive an induction motor is expensive. I'd say that's probably because of the pure sine wave that the typical VFD generates, compared to the simple trapezoidal waveform generated by an ESC. I'm not too sure about that though.