Induction motor speed

[davelectronic]

Just a question about induction motor speed. Is it possible to slow a split phase induction motor, single phase motor ( refrigeration compressor) I know variable frequency drives can drive induction motor speeds, but is something like ballast a way to slow down a refrigeration compressor. I'm looking about half it's rated speed. Thanks for reading, any ideas appreciated.

[CaptDon]

Highly not recommended!!! Nearly any method to slow down a sealed induction motor compressor can and probably will result in the motor stalling under certain head pressure situations. The question arises, why in the world do you want to slow it down? There are 'dual inverter' refrigeration systems that control both compressor speed and condensor/evaporator fan speeds BUT they rely on intelligent speed feedback to overcome stalled and overloaded conditions. What will you save by slowing the compressor down? The efficiency of both the induction motor as well as the piston part of the compressor will drop at lower speeds. It takes more energy to drive the piston faster, however due to leakage past the piston rings and poorly sealing valves the pumping efficiency will also drop at lower speeds so one would expect a 'sweet spot' in a hump shaped speed vs. efficiency graph and hopefully the engineers put that sweet spot right at the top of the normal operating curve!! When you slow an induction motor by lowering the voltage many bad things happen which is why many refrigeration systems often die during summer 'brown out' conditions!!

[davelectronic]

Thank you for the reply, I almost knew you were going to say these things in your post. I was hoping for a "but" idea, and I guess it doesn't exist. It's for an airbrush compressor project. I've used these compressors before for the same projects, using a blow down valve to unload the compressor when it's filled the air receiver. I wanted to try and lower the compressors rating for a slower full of the air receiver. But reading your post, it's looking unlikely it can be done. I understand the stall if it hasn't got enough voltage and current, but was thinking I could keep a margin above the lower motor stalling limit.

[CaptDon]

If you do have a 'relieving type' pressure switch that unloads the compressor when the tank is full you could try a lower voltage since the compressor would be unloaded at start up. A variac (variable auto-transformer) of sufficient amperage to match the compressor draw could work or a transformer of perhaps 12 to 18 volts on the secondary wired as a 'buck transformer' could work. Only the current rating on the secondary will have to match the compressor current draw, the primary current would be around 20% of the secondary current. The buck voltage will subtract from the line voltage when wired in line correctly so the motor may see 96 to 100 volts. It would be hard to know what a proper voltage selection would be and the reason a variac may be a better choice. Perhaps a large storage tank would be better which would lengthen the time between cycling? As a side note, if the motor uses a capacitor or has a start winding it probably won't like running slow. It is possible and probable as the voltage goes down the current will go up due to a poor power factor which will cause the motor to run hotter because of increased (Isqr)R losses.

[Phil1977]

Just in case this information is helpful: Some newer compressor freezer boxes for 12V/24V DC operation feature a "brushless" DC motor inside the hermetically closed compressor housing. You could use a standard sensorless brushless DC controller to control its speed.

[davelectronic]

It's a mains rated split phase capacitor start induction motor. So DC wouldn't work, the compressor would be under no load with pressure as it would be completely unloaded. Only the spin up friction of the rotor. How would 12 to 18 Volts possibly start the compressor up ? The start up current at full rated voltage is 5 Amps, but settles down to much less than that once it's spinning. Thanks again for your replies.

[Doctorandus_P]

There are some triac based motor regulators, which can regulate some induction motors in steps. The Idea is to feed the motor with a half period, then skip a few half periods and give it a half period on the other side of the zero line.

Schematics for this are very simple and have been around for 30+ years. Advertised use is for fans and such (load of fans goes down dramatically with lower speeds). I never built such a motor controller myself, but my guess is it would not work very well with a heavier load on the motor.

[CaptDon]

I didn't say 12 to 18 volts would start the motor!!!! I said 12 to 18 volts wired as a buck voltage against the 120vac mains!! That 12 to 18 volts would SUBTRACT from the mains voltage being wired as a BUCK voltage. Forget it, I even stated it would provide something like 96 to 100 volts to the motor. 120 - 12    or 120 - 18  AND I stated the secondary current would have to match the draw of the motor. Very obviously to most is that the required primary current would be less by the factor of the turns ratio so you would be concerned when shopping for a transformer of the secondary current capability and desired buck voltage. None of this was obvious so don't go down the buck voltage road.

[Siwastaja]

Compressor motors, no way, you need a VFD (there are special types for single-phase motors and capacitor-run motors).

Certain induction motor types are designed to be slowed down by lowering equivalent voltage (via transformer, series capacitor, or triac dimmer). Mostly fan motors; they are designed to be crappier efficiency than normal, and the mechanical load caused by the fan goes down with speed so it works out.

[davelectronic]

I misunderstood the buck transformer idea, I see what you mean now. A triac type dimmer is a light load for a cooling fan, and start up current wouldn't be that high compared to a fridge compressor load at start up. A single phase vfd would be good, but they are rare, three pase might be an option, but there expensive. Just an idea, would the primary of a MOT wired in series with the compressor work ? Given the MOT secondary has been removed. Just a thought. It would drop the voltage, but I don't know if it would produce enough current on start up.

[Siwastaja]

Dropping voltage in any way does not work with a compressor. Dropping voltage increases slip, decreases torque, decreases efficiency and finally the motor just pulls too much current and dies. You need to simultaneously drop the frequency with voltage, and this is complex, you need a VFD.

[BILLPOD]

How about one of these?...: https://www.amazon.com/PointZero-Airbrush-Compressor-Regulator-Water-Trap/dp/B004KNAHE2

[davelectronic]

Thank you for your replies, I can clearly see now there is no simple solution to control this and similar induction motors. A VFD seems it would work, but I don't know if you can run a single phase motor on a three phase VFD. I've not seen any single phase VFD's.

[Zero999]

Just in case this information is helpful: Some newer compressor freezer boxes for 12V/24V DC operation feature a "brushless" DC motor inside the hermetically closed compressor housing. You could use a standard sensorless brushless DC controller to control its speed.

I've seen those, but I assumed they just use the same type of induction motor as the mains ones, just wound for a lower voltage.

[Phil1977]

Just in case this information is helpful: Some newer compressor freezer boxes for 12V/24V DC operation feature a "brushless" DC motor inside the hermetically closed compressor housing. You could use a standard sensorless brushless DC controller to control its speed.

I've seen those, but I assumed they just use the same type of induction motor as the mains ones, just wound for a lower voltage.

I´ve thought the same, disassembled one and found a real 3-phase BLDC motor + controller inside. It´s the Vevor 15L box for slightly >€120,-

[Siwastaja]

Just in case this information is helpful: Some newer compressor freezer boxes for 12V/24V DC operation feature a "brushless" DC motor inside the hermetically closed compressor housing. You could use a standard sensorless brushless DC controller to control its speed.

I've seen those, but I assumed they just use the same type of induction motor as the mains ones, just wound for a lower voltage.

AFAIK, inverter compressors are always BLDC (in other words, a synchronous permanent magnet AC motor type), this would apply to inverter air conditioners as well as small 12V coolers. I don't think there is any advantage in using induction motor (+ induction motor VFD) for these designs over the usual BLDC, otherwise it would be used. Some old or non-mass-produced large machine (e.g. a ground source heat pump) could use a 3-phase induction motor compressor with an off-the-shelf industrial VFD, but that would be a special case.

If anything, BLDC sensorless FOC is way easier than induction motor sensorless FOC. I don't know if that's the reason though, control problems are "solve once, then replicate". Might be one of the historical reasons why the inverter compressor mass market went "directly" to BLDC, though.

[Phil1977]

I just remember that 10 years ago it was quite difficult to operate a compressor freezer from a battery. Even a sinewave-inverter with 10 times the power specification of the induction motor in usual freezers was struggling to provide the necessary inrush current, and then you needed a battery that was able to power such a big inverter.

The BLDC designs of the new boxes easily run on small batteries - at least for some time   

[Siwastaja]

I just remember that 10 years ago it was quite difficult to operate a compressor freezer from a battery. Even a sinewave-inverter with 10 times the power specification of the induction motor in usual freezers was struggling to provide the necessary inrush current, and then you needed a battery that was able to power such a big inverter.

It is the crappy nature of fixed frequency mains motors. They start at basically what is 0% efficiency, pulling massive inrush current.

The problem is with the same word "inverter" with two clearly different meanings. I would prefer to say "motor controller" or "variable frequency drive", but the compressor/air conditioning industry chose to say "inverter". A controller which uses DC voltage to rotate a brushless motor (so AC motor basically) has very little in common with an inverter which takes battery DC and outputs general purpose 120V or 230V 50Hz or 60Hz AC to be used with whatever random loads.

The main difference is scaling frequency and voltage to be suitable for the motor, based on measured or estimated RPM. Plus that in generic mains load, you need a voltage source with infinite current capability. When driving motor, you are creating a current source you want to and can control.

These two things make a huge difference for efficiency and completely removes even the concept of inrush current, as current is always in the inverter's direct control.

[DavidKo]

I would use some pressure flask, pressure switch to turn the compressor on/off and air pressure regulator like it is done in commercial products.