Capacitor types for motor starting? - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: January 17, 2015, 08:43:57 am
content: Will any type of non-polarized capacitor work as a motor start capacitor as long as capacity, ambient temperature rating, and voltage are correct? Motors are Hurst brand, less than 1/10 HP, synchronous, reversible, 120 VAC, 60 Hz.Mike in California

author: SeanB
author_rank: Super Contributor
timestamp: January 18, 2015, 02:02:46 pm
content: No, get a proper motor start rated unit, they are designed to not fail as short circuit, but have a disconnect mechanism that breaks the terminals free in case of fault. Easy enough to get the required value from an electricla wholesaler or industrial supplier.If it is a synchronous unit it will not be a start capacitor in any case, but a motor run capacitor. 1/10HP will be a value of around 4uF, and you really want a 400VAC rated motor run capacitor there in any case. Available off the shelf ( I keep them at work for motors) in either a plastic or metal can, with either 1/4in spade connectors or wire leads.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 18, 2015, 04:45:12 pm
content: HVAC supply stores will have all sorts of motor capacitors. The run caps usually are 370V or 440V rated, both of which are fine for 120V applications. (Just go with the latter as the cost difference is small.)

author: calzap
author_rank: Frequent Contributor
timestamp: January 18, 2015, 07:28:37 pm
content: Thanks for the info Sean. My interpretation of the wiring diagram (see pic) was that the capacitor was a motor start cap. Then I found the description below on Hurst's website; it's in their catalog too. The motors that I have are 9 W, 300 rpm, direct drive. Hurst specifies 0.85 mfd rated for 250 V for these motors. Hurst normally supplies capacitors with their AC, but not DC, motors. For the DC motors, they give capacitance and voltage specs and simply warn against using polarized capacitors. I have not been able to determine the type of cap Hurst supplies with its AC motors. My motors are new but came w/o caps and were surplus from a company that makes check- and currency- sorting machines. I'll be using them to drive low-speed fans in insulated boxes used to age cheese and wine. Motors will be outside the boxes with the shaft extending inside with a fan blade on the end (and cage around the blade, of course).Mike in California-----------------------------------------------------------------------------------------------From Hurst website, 2015: Why Capacitor Start AC Synchronous Motors?All Permanent Magnet AC Synchronous motors manufactured at Hurst are of the capacitor start variation. These motors, which are classified in the sub-fractional group, find various applications in situations requiring frequent and prolonged starting periods. As the name suggests, these motors run at synchronous speed. The speed of a single phase AC synchronous motor can be determined using the formula Synchronous speed (in RPM) = 120f/p where f is the frequency of the power supply and p is the number of poles. For the same starting torque, the capacitor motor when compared to the split-phase motor requires half of the current for starting. The auxiliary winding of the capacitor motor has twice the number of turns of the split-phase AC motor. A split-phase AC motor basically has an inductive auxiliary winding. The lesser current in the auxiliary winding of the capacitor motor results in less copper loss and subsequently less heat generated by that motor. Because of the capacitor in the winding, the capacitor motor has the advantage of a greater phase shift between the current of the stator winding and that of the auxiliary winding. Phase shift is typically about 80% compared to 20% for the split-phase motor. The increased phase shift translates into easier starting for the capacitor start motor.

author: tautech
author_rank: Super Contributor
timestamp: January 18, 2015, 07:53:05 pm
content: Quote from: calzap on January 17, 2015, 08:43:57 amThere are 2 types: motor start & motor run.Both are self healing poly prop types.Some motors use both.The start types are for intermittent use, their duty cycle is spec'd in datasheets.These are normally used with motors that have centrifugal start switches and additional start windings.The run types are for continuous duty.

author: SeanB
author_rank: Super Contributor
timestamp: January 18, 2015, 07:55:29 pm
content: So it is a motor run capacitor, and a pretty low value one. At this low power you can get away just using 2 0.47uF 630VDC metal film capacitors in parallel as the current and voltage along with the power are quite low. That will do in this application and will last quite a long while.If you are looking to do this with a motor over around 200VA it will be best to use a proper motor run cap, as then the power in the capacitor will be higher than a plain box film capacitor can handle. Plain film will do up to around 100VA though, but you need to use a much higher voltage rating than you think, as the voltage on the capacitor will be at least 2 times the line voltage.

author: calexanian
author_rank: Super Contributor
timestamp: January 19, 2015, 12:58:15 am
content: You want the motor type caps in place. They are made to handle the condition of stalled rotor. here is a condition that exists between the inductance of the motor and the capacitor that in a stalled condition BAD THINGS HAPPEN!!!!!! The proper rated ones are as easy to find as falling off a rock. Most appliance repair shops and motor repair shops have, or can get them. Also they are commonly available online.