Motor power

Does a VFD mean the power of a motor is reduced e.g. 3/4 hp motor running 415v three phase v same motor wired delta 220v three phase from a VFD?

Power output is usually the same and can be confirmed by the motor plate.

Emgee

A motor will produce the same power in star and delta provided the correct voltage is applied for the configuration. Typically a motor running 415v in star will need about 240v in delta, there is often a bit of variation on these exact figures as the line voltage across Europe varies although we nominally agree on 230v this is rarely adhered to as the tolerance is fiddled to embrace 220v to 240v. The new building I worked in was set to 230v but 254v was in some older areas, I remember a nut feeder did not like the elevated voltage and we had to install a transformer for it. Although it may have been possible to adjust the tapping of the main transformer it would have caused chaos to a lot of other equipment.

Mike

That is what I thought but had what has turned out to be an idiot on Facebook saying the power is halved but was using power=V x I rather than P = √3 × pf × I × V. I think I should have realised when the √3 was questioned.

Wiring a motor Star or Delta does not affect the rated power output of the motor. It can affect its starting performance under load but that was not the question. The key point is that the current per phase is lower for the star connection than the delta.

With a VFD it i important to note that the mechanical output power is reduced when running the motor at lower speeds. This is because power is proportional to speed an torque and torque in a motor is proportional to current. You can't drive more current than the rating so power drops with speed. Conversly power increases with speed for the same torque / current.
Any motor being run on a VFD should be geared for the highest speed possible.
The limiting factor for this is the insulation of the windings. This is because the voltage must be boosted to mainntain the same current and torque at higher speeds.
Some motors desigined for VFDs have improved insulation to allow for this or are designed for maximum output at 400Hz and line voltage this gives higher output speed and a much smaller motor for the same output. Aircraft use 400Hz or higher to take advantage of the significant weight savings (transformers for 400Hz are much smaller too).

Robert G8RPI.

Fortunately the sort of VFD boxes folk of us are likely to get involved with generally don't have the continuous over voltage capability likely to damage a motor in decent condition. The sharp edges and spikes inevitable to VFD operation can be unkind to older insulation though.

In practice a non-issue although sometimes you just get unlucky.

Magnetic saturation is more of an issue. Many motors have a maximum frequency specification, typically in the 70 to 80 Hz region. When driven above this speed using the appropriate voltage for full power things go into saturation and power falls off. Even if unspecified this applies to all motors so the simple short hand of "constant power above rated frequency" no longer applies. You can still drive the motor considerably faster but power does drop off.

Not a worry for us. We generally need real power at lower speeds. High speeds mean small work, small drills or small cutters which simply can't take serious power.

Vector drive VFD boxes often have torque boost settings which overdrive the motor over part of the run up speed range to get more torque when starting heavier loads. Can be very helpful with compressors. I've cured a couple with iffy starting on rotary converters by swutching to VFD boxes.

Good brand vector drive inverters are remarkable beasts and do an excellent job of giving the motor just enough electricity to do its duties leading to cooler running and a kinder life. Except for those dratted spikes.

Clive

Edited By Clive Foster on 03/10/2020 18:33:20

One thing to note with VFDs and 3 phase motors is that the number of poles the stator has determines the natural RPM of the motor at 50Hz, pro rata if frequency changes. So nominal speed of a 2 pole motor is 3000 (actually slightly lower as some slipage is required for the magnetic field to drive the rotor round). A four pole nominally runs at 1500rpm and a 6 pole at 1000rpm. It is important to match the motor to its requirement, if you get a 2 pole motor and end up running the VFD at 20hz to get a useable speed at the workpiece you have the wrong motor.

The √3 is the ratio of 240V to 415V (approximate voltages). If you draw 3 equal lines from a single point with each line at 120° to the next the ratio of the line length to the end to end length is √3. The 3 lines like this is the star wired motor. This means that putting 415V across the end points will result in 240V across each winding. When the motor is set up as a delta wired motor the 240V across the terminals is now applied directly to each coil. This is why the 240/415V motors we home workshop users use with UK mains are wired in delta as the single phase supplied VFDs output is 240V across each terminal (unless you pay more for something different). If you have 3 phase as supplied by the mains it will have 415V between each pair of terminals so the motor gets wired in star configuration.

Small 3 phase motors often have direct on line (dol) starters but larger ones have star/delta starters to reduce inrush currents at start up. It just gets more complicated when things get bigger.

Martin C

Was it possible that the ‘idiot’ was trying to tell you the power was reduced to 50% if running star connected 415V motor from a 230V VFD, while still connected in star?

It won’t - it will be reduced to about 58%

He was trying to convince me running a three phase motor wired in delta via a 230v VDF would only give half the power and didn't even understand P = √3 × pf × I × V saying √3 is not used in three phase power calculations and he did boast he was chartered engineer come electrician. There's me just a marine engineer ( big ships) who know nothing, delete is a wonderful thing as I left him talking to himself